hypothesis of the formation of the universe

Origins of the universe, explained

The most popular theory of our universe's origin centers on a cosmic cataclysm unmatched in all of history—the big bang.

The best-supported theory of our universe's origin centers on an event known as the big bang. This theory was born of the observation that other galaxies are moving away from our own at great speed in all directions, as if they had all been propelled by an ancient explosive force.

A Belgian priest named Georges Lemaître first suggested the big bang theory in the 1920s, when he theorized that the universe began from a single primordial atom. The idea received major boosts from Edwin Hubble's observations that galaxies are speeding away from us in all directions, as well as from the 1960s discovery of cosmic microwave radiation—interpreted as echoes of the big bang—by Arno Penzias and Robert Wilson.

Further work has helped clarify the big bang's tempo. Here’s the theory: In the first 10^-43 seconds of its existence, the universe was very compact, less than a million billion billionth the size of a single atom. It's thought that at such an incomprehensibly dense, energetic state, the four fundamental forces—gravity, electromagnetism, and the strong and weak nuclear forces—were forged into a single force, but our current theories haven't yet figured out how a single, unified force would work. To pull this off, we'd need to know how gravity works on the subatomic scale, but we currently don't.

It's also thought that the extremely close quarters allowed the universe's very first particles to mix, mingle, and settle into roughly the same temperature. Then, in an unimaginably small fraction of a second, all that matter and energy expanded outward more or less evenly, with tiny variations provided by fluctuations on the quantum scale. That model of breakneck expansion, called inflation, may explain why the universe has such an even temperature and distribution of matter.

After inflation, the universe continued to expand but at a much slower rate. It's still unclear what exactly powered inflation.

Aftermath of cosmic inflation

As time passed and matter cooled, more diverse kinds of particles began to form, and they eventually condensed into the stars and galaxies of our present universe.

By the time the universe was a billionth of a second old, the universe had cooled down enough for the four fundamental forces to separate from one another. The universe's fundamental particles also formed. It was still so hot, though, that these particles hadn't yet assembled into many of the subatomic particles we have today, such as the proton. As the universe kept expanding, this piping-hot primordial soup—called the quark-gluon plasma—continued to cool. Some particle colliders, such as CERN's Large Hadron Collider , are powerful enough to re-create the quark-gluon plasma.

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Radiation in the early universe was so intense that colliding photons could form pairs of particles made of matter and antimatter, which is like regular matter in every way except with the opposite electrical charge. It's thought that the early universe contained equal amounts of matter and antimatter. But as the universe cooled, photons no longer packed enough punch to make matter-antimatter pairs. So like an extreme game of musical chairs, many particles of matter and antimatter paired off and annihilated one another.

Somehow, some excess matter survived—and it's now the stuff that people, planets, and galaxies are made of. Our existence is a clear sign that the laws of nature treat matter and antimatter slightly differently. Researchers have experimentally observed this rule imbalance, called CP violation , in action. Physicists are still trying to figure out exactly how matter won out in the early universe.

Building atoms

Within the universe's first second, it was cool enough for the remaining matter to coalesce into protons and neutrons, the familiar particles that make up atoms' nuclei. And after the first three minutes, the protons and neutrons had assembled into hydrogen and helium nuclei. By mass, hydrogen was 75 percent of the early universe's matter, and helium was 25 percent. The abundance of helium is a key prediction of big bang theory, and it's been confirmed by scientific observations.

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Despite having atomic nuclei, the young universe was still too hot for electrons to settle in around them to form stable atoms. The universe's matter remained an electrically charged fog that was so dense, light had a hard time bouncing its way through. It would take another 380,000 years or so for the universe to cool down enough for neutral atoms to form—a pivotal moment called recombination. The cooler universe made it transparent for the first time, which let the photons rattling around within it finally zip through unimpeded.

We still see this primordial afterglow today as cosmic microwave background radiation , which is found throughout the universe. The radiation is similar to that used to transmit TV signals via antennae. But it is the oldest radiation known and may hold many secrets about the universe's earliest moments.

From the first stars to today

There wasn't a single star in the universe until about 180 million years after the big bang. It took that long for gravity to gather clouds of hydrogen and forge them into stars. Many physicists think that vast clouds of dark matter , a still-unknown material that outweighs visible matter by more than five to one, provided a gravitational scaffold for the first galaxies and stars.

Once the universe's first stars ignited , the light they unleashed packed enough punch to once again strip electrons from neutral atoms, a key chapter of the universe called reionization. In February 2018, an Australian team announced that they may have detected signs of this “cosmic dawn.” By 400 million years after the big bang , the first galaxies were born. In the billions of years since, stars, galaxies, and clusters of galaxies have formed and re-formed—eventually yielding our home galaxy, the Milky Way, and our cosmic home, the solar system.

Even now the universe is expanding , and to astronomers' surprise, the pace of expansion is accelerating. It's thought that this acceleration is driven by a force that repels gravity called dark energy . We still don't know what dark energy is, but it’s thought that it makes up 68 percent of the universe's total matter and energy. Dark matter makes up another 27 percent. In essence, all the matter you've ever seen—from your first love to the stars overhead—makes up less than five percent of the universe.

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May 21, 2013

12 min read

Origin of the Universe

Cosmologists are closing in on the ultimate processes that created and shaped the universe

By Michael S. Turner

The universe is big in both space and time and, for much of humankind's history, was beyond the reach of our instruments and our minds. That changed dramatically in the 20th century. The advances were driven equally by powerful ideas—from Einstein's general relativity to modern theories of the elementary particles—and powerful instruments—from the 100- and 200-inch reflectors that George Ellery Hale built, which took us beyond our Milky Way galaxy, to the Hubble Space Telescope, which has taken us back to the birth of galaxies. Over the past 30 years the pace of progress has accelerated with the realization that dark matter is not made of ordinary atoms, the discovery of dark energy, and the dawning of bold ideas such as cosmic inflation and the multiverse.

The universe of 100 years ago was simple: eternal, unchanging, consisting of a single galaxy, containing a few million visible stars. The picture today is more complete and much richer. The cosmos began 13.7 billion years ago with the big bang. A fraction of a second after the beginning, the universe was a hot, formless soup of the most elementary particles, quarks and leptons. As it expanded and cooled, layer on layer of structure developed: neutrons and protons, atomic nuclei, atoms, stars, galaxies, clusters of galaxies, and finally superclusters. The observable part of the universe is now inhabited by 100 billion galaxies, each containing 100 billion stars and probably a similar number of planets. Galaxies themselves are held together by the gravity of the mysterious dark matter. The universe continues to expand and indeed does so at an accelerating pace, driven by dark energy, an even more mysterious form of energy whose gravitational force repels rather than attracts.

The overarching theme in our universe's story is the evolution from the simplicity of the quark soup to the complexity we see today in galaxies, stars, planets and life. These features emerged one by one over billions of years, guided by the basic laws of physics. In our journey back to the beginning of creation, cosmologists first travel through the well-established history of the universe back to the first microsecond; then to within 10

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−34 second of the beginning, for which ideas are well formed but the evidence is not yet firm; and finally to the earliest moments of creation, for which our ideas are still just speculation. Although the ultimate origin of the universe still lies beyond our grasp, we have tantalizing conjectures, including the notion of the multiverse, whereby the universe comprises an infinite number of disconnected subuniverses.

Expanding Universe

Using the 100-inch Hooker telescope on Mount Wilson in 1924, Edwin Hubble showed that fuzzy nebulae, studied and speculated about for several hundred years, were galaxies just like our own—thereby enlarging the known universe by 100 billion. A few years later he showed that galaxies are moving apart from one another in a regular pattern described by a mathematical relation now known as Hubble's law, according to which galaxies that are farther away are moving faster. It is Hubble's law, played back in time, that points to a big bang 13.7 billion years ago.

Hubble's law found ready interpretation within general relativity: space itself is expanding, and galaxies are being carried along for the ride [ see box on opposite page ]. Light, too, is being stretched, or redshifted—a process that saps its energy, so that the universe cools as it expands. Cosmic expansion provides the narrative for understanding how today's universe came to be. As cosmologists imagine rewinding the clock, the universe becomes denser, hotter, more extreme and simpler. In exploring the beginning, we also probe the inner workings of nature by taking advantage of an accelerator more powerful than any built on Earth—the big bang itself.

By looking out into space with telescopes, astronomers peer back in time—and the larger the telescope, the farther back they peer. The light from distant galaxies reveals an earlier epoch, and the amount this light has redshifted indicates how much the universe has grown in the intervening years. The current record holder has a redshift of more than 10, representing a time when the universe was less than one-eleventh its present size and only a few hundred million years old. Telescopes such as the Hubble Space Telescope and the 10-meter Keck telescopes on Mauna Kea routinely take us back to the epoch when galaxies like ours were forming, a few billion years after the big bang. Light from even earlier times is so strongly redshifted that astronomers must look for it in the infrared and radio bands. Telescopes such as the planned James Webb Space Telescope, a 6.5-meter infrared telescope, and the Atacama Large Millimeter Array (ALMA), a network of 66 radio dishes already operating in northern Chile, can take us back to the birth of the very first stars and galaxies.

Computer simulations say that those stars and galaxies emerged when the universe was about 100 million years old. Before then, the universe went through a time called the “dark ages,” when it was almost pitch-black. Space was filled with a featureless gruel, five parts dark matter and one part hydrogen and helium, that thinned out as the universe expanded. Matter was slightly uneven in density, and gravity acted to amplify these density variations: denser regions expanded more slowly than less dense ones did. By 100 million years the densest regions did not merely expand more slowly but actually started to collapse. Such regions contained about one million solar masses of material each. They were the first gravitationally bound objects in the cosmos.

Dark matter accounted for the bulk of their mass but was, as its name suggests, unable to emit or absorb light. So it remained in an extended cloud. Hydrogen and helium gas, on the other hand, emitted light, lost energy and became concentrated in the center of the cloud. Eventually it collapsed all the way down to stars. These first stars were much more massive than today's—hundreds of solar masses. They lived very short lives before exploding and leaving behind the first heavy elements. Over the next billion years or so the force of gravity assembled these million-solar-mass clouds into the first galaxies.

Radiation emitted from primordial hydrogen clouds, which were greatly redshifted by the expansion, should be detectable by giant arrays of radio antennas with a total collecting area of up to one square kilometer. When built, these arrays will watch as the first generation of stars and galaxies ionize the hydrogen and bring the dark ages to an end.

Faint Glow of a Hot Beginning

Beyond the dark ages is the glow of the hot big bang at a redshift of 1,100. This radiation has been redshifted from visible light (a red-orange glow) beyond even the infrared to microwaves. What we see from that time is a wall of microwave radiation filling the sky—the cosmic microwave background radiation (CMB), discovered in 1964 by Arno Penzias and Robert Wilson. It provides a glimpse of the universe at the tender age of 380,000 years, the period when atoms formed. Before then, the universe was a nearly uniform soup of atomic nuclei, electrons and photons. As it cooled to a temperature of about 3,000 kelvins, the nuclei and electrons came together to form atoms. Photons ceased to scatter off electrons and streamed across space unhindered, revealing the universe at a simpler time before the existence of stars and galaxies.

In 1992 NASA's Cosmic Background Explorer satellite discovered that the intensity of the CMB has slight variations—about 0.001 percent—reflecting a slight lumpiness in the distribution of matter. The degree of primordial lumpiness was enough to act as seeds for the galaxies and larger structures that would later emerge from the action of gravity. The pattern of these variations in the CMB across the sky also encodes basic properties of the universe, such as its overall density and composition, as well as hints about its earliest moments; the careful study of these variations has revealed much about the universe [ see illustration on page 41 ].

As we roll a movie of the universe's evolution back from that point, we see the primordial plasma becoming ever hotter and denser. Prior to about 100,000 years, the energy density of radiation exceeded that of matter, which kept matter from clumping. Therefore, this time marks the beginning of gravitational assembly of all the structure seen in the universe today. Still further back, when the universe was less than a second old, atomic nuclei had yet to form; only their constituent particles—namely, protons and neutrons—existed. Nuclei emerged when the universe was seconds old and the temperatures and densities were just right for nuclear reactions. This process of big bang nucleosynthesis produced only the lightest elements in the periodic table: a lot of helium (about 25 percent of the atoms in the universe by mass) and smaller amounts of lithium and the isotopes deuterium and helium 3. The rest of the plasma (about 75 percent) stayed in the form of protons that would eventually become hydrogen atoms. All the rest of the elements in the periodic table formed billions of years later in stars and stellar explosions.

Nucleosynthesis theory accurately predicts the abundances of elements and isotopes measured in the most primeval samples of the universe—namely, the oldest stars and high-redshift gas clouds. The abundance of deuterium, which is very sensitive to the density of atoms in the universe, plays a special role: its measured value implies that ordinary matter amounts to 4.5 ± 0.1 percent of the total energy density. (The remainder is dark matter and dark energy.) This estimate agrees precisely with the composition that has been gleaned from the analysis of the CMB. This correspondence is a great triumph. That these two very different measures, one based on nuclear physics when the universe was a second old and the other based on atomic physics when the universe was 380,000 years old, agree is a strong check not just on our model of how the cosmos evolved but on all of modern physics.

Answers in the Quark Soup

Earlier than a microsecond, even protons and neutrons could not exist and the universe was a soup of nature's basic building blocks: quarks, leptons, and the force carriers (photons, the W and Z bosons, and gluons). We can be confident that the quark soup existed because experiments at particle accelerators have re-created similar conditions here on Earth today.

To explore this epoch, cosmologists rely not on bigger and better telescopes but also on powerful ideas from particle physics. The development of the Standard Model of particle physics 30 years ago has led to bold speculations, including string theory, about how the seemingly disparate fundamental particles and forces are unified. As it turns out, these new ideas have implications for cosmology that are as important as the original idea of the hot big bang. They hint at deep and unexpected connections between the world of the very big and of the very small. Answers to three key questions—the nature of dark matter, the asymmetry between matter and antimatter, and the origin of the lumpy quark soup itself—have been starting to emerge.

It now appears that the early quark soup phase was the birthplace of dark matter. The identity of dark matter remains unclear, but its existence is very well established. Our galaxy and every other galaxy, as well as clusters of galaxies, are held together by the gravity of unseen dark matter. Whatever the dark matter is, it must interact weakly with ordinary matter; otherwise it would have shown itself in other ways. Attempts to find a unifying framework for the forces and particles of nature have led to the prediction of stable or long-lived particles that might constitute dark matter. Some of these hypothetical particles would be present today as remnants of the quark soup phase in the correct numbers to be the dark matter and could even be detected.

One candidate is the called the neutralino, the lightest of a putative new class of particles that are heavier counterparts of the known particles. The neutralino is thought to have a mass between 100 and 1,000 times that of the proton, just within the reach of experiments now under way at the Large Hadron Collider at CERN near Geneva. Physicists have also built ultrasensitive underground detectors, as well as satellite and balloon-borne varieties, to look for this particle or the by-products of its interactions.

A second candidate is the axion, a superlightweight particle about one-trillionth the mass of the electron. Its existence is hinted at by subtleties that the Standard Model predicts in the behavior of quarks. Efforts to detect it exploit the fact that in a very strong magnetic field, an axion can transform into a photon. Both neutralinos and axions have the important property that they are, in a specific technical sense, “cold.” Although they formed under broiling hot conditions, they were slow-moving and thus easily clumped into galaxies.

The early quark soup phase probably also holds the secret to why the universe today contains mostly matter rather than both matter and antimatter. Physicists think the universe originally had equal amounts of each, but at some point it developed a slight excess of matter—about one extra quark for every billion antiquarks. This imbalance ensured that enough quarks would survive annihilation with antiquarks as the universe expanded and cooled. More than 40 years ago accelerator experiments revealed that the laws of physics are ever so slightly biased in favor of matter, and in a still to be understood series of particle interactions very early on, this slight bias led to the creation of the quark excess.

The quark soup itself is thought to have arisen at an extremely early time—perhaps 10

−34 second after the big bang in a burst of cosmic expansion known as inflation. This burst, driven by the energy of a new field (thought to be distantly related to the recently discovered Higgs field) called the inflaton, would explain such basic properties of the cosmos as its general uniformity and the lumpiness that seeded galaxies and other structures in the universe. As the inflaton field decayed away, it released its remaining energy into quarks and other particles, thereby creating the heat of the big bang and the quark soup itself.

Inflation leads to a profound connection between the quarks and the cosmos: quantum fluctuations in the inflaton field on the subatomic scale get blown up to astrophysical size by the rapid expansion and become the seeds for all the structure we see today. In other words, the pattern seen on the CMB sky is a giant image of the subatomic world. Observations of the CMB agree with this prediction, providing the strongest evidence that inflation or something like it occurred very early in the history of the universe.

Birth of the Universe

As cosmologists try to go even further to understand the beginning of the universe itself, our ideas become less firm. Einstein's general theory of relativity has provided the theoretical foundation for a century of progress in our understanding of the evolution of the universe. Because the general theory of relativity does not incorporate quantum theory, the other pillar of contemporary physics, it cannot be relied upon to address the very earliest moments of creation when quantum gravity effects should have been important. The discipline's greatest challenge is to develop a quantum theory of gravity, with which we will be able to address the so-called Planck era prior to about 10

−43 second, when spacetime itself was taking shape.

Tentative attempts at a unified theory have led to some remarkable speculations about our very beginnings. String theory, for example, predicts the existence of additional dimensions of space and possibly other universes floating in that larger space. What we call the big bang may have been the collision of our universe with another. The marriage of string theory with the concept of inflation has led to perhaps the boldest idea yet, that of a multiverse—namely, that the universe comprises an infinite number of disconnected pieces, each with its own local laws of physics.

The multiverse concept, which is still in its infancy, turns on two key theoretical findings. First, the equations describing inflation strongly suggest that if inflation happened once, it should happen again and again, with an infinite number of inflationary regions created over time. Nothing can travel between these regions, so they have no effect on one another. Second, string theory suggests that these regions have different physical parameters, such as the number of spatial dimensions and the kinds of stable particles.

The idea of the multiverse provides novel answers to two of the biggest questions in all of science: what happened before the big bang and why the laws of physics are as they are (Albert Einstein's famous musing about “whether God had any choice” about the laws). The multiverse makes moot the question of what happened before the big bang because there were an infinite number of big bang beginnings, each triggered by its own burst of inflation. Likewise, Einstein's question is pushed aside: within the infinity of universes, all possibilities for the laws of physics have been tried, so there is no particular reason for the laws that govern our universe.

Cosmologists have mixed feelings about the multiverse. If the disconnected subuniverses are truly incommunicado, we cannot hope to test their existence; they seem to lie beyond the realm of science. Part of me wants to scream, One universe at a time, please! On the other hand, the multiverse solves various conceptual problems. If correct, it will make Hubble's enlargement of the universe by a mere factor of 100 billion and Copernicus's banishment of Earth from the center of the universe in the 16th century seem like small advances in the understanding of our place in the cosmos.

Modern cosmology has humbled us. We are made of protons, neutrons and electrons, which together account for only 4.5 percent of the universe, and we exist only because of subtle connections between the very small and the very large. Events guided by the microscopic laws of physics allowed matter to dominate over antimatter, generated the lumpiness that seeded galaxies, filled space with dark matter particles that provide the gravitational infrastructure, and ensured that dark matter could build galaxies before dark energy became significant and the expansion began to accelerate [ see box above ]. At the same time, cosmology by its very nature is arrogant. The idea that we can understand something as vast in both space and time as our universe is, on the face of it, preposterous. This strange mix of humility and arrogance has gotten us pretty far in the past century in advancing our understanding of the present universe and its origin. I am bullish on further progress in the coming years, and I firmly believe we are living in a golden age of cosmology.

Cosmic History

The Universe’s History

The origin, evolution, and nature of the universe have fascinated and confounded humankind for centuries. New ideas and major discoveries made during the 20th century transformed cosmology – the term for the way we conceptualize and study the universe – although much remains unknown. Here is the history of the universe according to cosmologists’ current theories.

Cosmic Inflation

Around 13.8 billion years ago, the universe expanded faster than the speed of light for a fraction of a second, a period called cosmic inflation. Scientists aren’t sure what came before inflation or what powered it. It’s possible that energy during this period was just part of the fabric of space-time. Cosmologists think inflation explains many aspects of the universe we observe today, like its flatness, or lack of curvature, on the largest scales. Inflation may have also magnified density differences that naturally occur on space’s smallest, quantum-level scales, which eventually helped form the universe’s large-scale structures.

Big Bang and Nucleosynthesis

When cosmic inflation stopped, the energy driving it transferred to matter and light – the big bang. One second after the big bang, the universe consisted of an extremely hot (18 billion degrees Fahrenheit or 10 billion degrees Celsius) primordial soup of light and particles. In the following minutes, an era called nucleosynthesis, protons and neutrons collided and produced the earliest elements – hydrogen, helium, and traces of lithium and beryllium. After five minutes, most of today’s helium had formed, and the universe had expanded and cooled enough that further element formation stopped. At this point, though, the universe was still too hot for the atomic nuclei of these elements to catch electrons and form complete atoms. The cosmos was opaque because a vast number of electrons created a sort of fog that scattered light.

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Recombination

Around 380,000 years after the big bang, the universe had cooled enough that atomic nuclei could capture electrons, a period astronomers call the epoch of recombination. This had two major effects on the cosmos. First, with most electrons now bound into atoms, there were no longer enough free ones to completely scatter light, and the cosmic fog cleared. The universe became transparent, and for the first time, light could freely travel over great distances. Second, the formation of these first atoms produced its own light. This glow, still detectable today, is called the cosmic microwave background. It is the oldest light we can observe in the universe.

After the cosmic microwave background, the universe again became opaque at shorter wavelengths due to the absorbing effects of all those hydrogen atoms. For the next 200 million years the universe remained dark. There were no stars to shine. The cosmos at this point consisted of a sea of hydrogen atoms, helium, and trace amounts of heavier elements.

First Stars

Gas was not uniformly distributed throughout the universe. Cooler areas of space were lumpier, with denser clouds of gas. As these clumps grew more massive, their gravity attracted additional matter. As they became denser, and more compact, the centers of these clumps became hotter – hot enough eventually that nuclear fusion occurred in their centers. These were the first stars. They were 30 to 300 times more massive than our Sun and millions of times brighter. Over several hundred million years, the first stars collected into the first galaxies.

Reionization

At first, starlight couldn’t travel far because it was scattered by the relatively dense gas surrounding the first stars. Gradually, the ultraviolet light emitted by these stars broke down, or ionized, hydrogen atoms in the gas into their constituent electrons and protons. As this reionization progressed, starlight traveled farther, breaking up more and more hydrogen atoms. By the time the universe was 1 billion years old, stars and galaxies had transformed nearly all this gas, making the universe transparent to light as we see it today.

For many years, scientists thought the universe’s current expansion was slowing down. But in fact, cosmic expansion is speeding up. In 1998, astronomers found that certain supernovae, bright stellar explosions, were fainter than expected. They concluded this could only happen if the supernovae had moved farther away, at a faster rate than predicted.

Scientists suspect a mysterious substance they call dark energy is accelerating expansion. Future research may yield new surprises, but cosmologists suggest it’s likely the universe will continue to expand forever.

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The early universe

All matter in the universe was formed in one explosive event 13.7 billion years ago – the Big Bang

The Big Bang

In 1929 the American astronomer Edwin Hubble discovered that the distances to far-away galaxies were proportional to their redshifts. Redshift occurs when a light source moves away from its observer: the light's apparent wavelength is stretched via the Doppler effect towards the red part of the spectrum. Hubble’s observation implied that distant galaxies were moving away from us, as the furthest galaxies had the fastest apparent velocities. If galaxies are moving away from us, reasoned Hubble, then at some time in the past, they must have been clustered close together.

Hubble’s discovery was the first observational support for Georges Lemaître’s Big Bang theory of the universe, proposed in 1927. Lemaître proposed that the universe expanded explosively from an extremely dense and hot state, and continues to expand today. Subsequent calculations have dated this Big Bang to approximately 13.7 billion years ago. In 1998 two teams of astronomers working independently at Berkeley, California observed that supernovae – exploding stars – were moving away from Earth at an accelerating rate. This earned them the Nobel prize in physics in 2011 . Physicists had assumed that matter in the universe would slow its rate of expansion; gravity would eventually cause the universe to fall back on its centre. Though the Big Bang theory cannot describe what the conditions were at the very beginning of the universe, it can help physicists describe the earliest moments after the start of the expansion.

In the first moments after the Big Bang, the universe was extremely hot and dense. As the universe cooled, conditions became just right to give rise to the building blocks of matter – the quarks and electrons of which we are all made. A few millionths of a second later, quarks aggregated to produce protons and neutrons. Within minutes, these protons and neutrons combined into nuclei. As the universe continued to expand and cool, things began to happen more slowly. It took 380,000 years for electrons to be trapped in orbits around nuclei, forming the first atoms. These were mainly helium and hydrogen, which are still by far the most abundant elements in the universe. Present observations suggest that the first stars formed from clouds of gas around 150–200 million years after the Big Bang. Heavier atoms such as carbon, oxygen and iron, have since been continuously produced in the hearts of stars and catapulted throughout the universe in spectacular stellar explosions called supernovae.

But stars and galaxies do not tell the whole story. Astronomical and physical calculations suggest that the visible universe is only a tiny amount (4%) of what the universe is actually made of. A very large fraction of the universe, in fact 26%, is made of an unknown type of matter called " dark matter ". Unlike stars and galaxies, dark matter does not emit any light or electromagnetic radiation of any kind, so that we can detect it only through its gravitational effects. 

An even more mysterious form of energy called “dark energy” accounts for about 70% of the mass-energy content of the universe. Even less is known about it than dark matter. This idea stems from the observation that all galaxies seems to be receding from each other at an accelerating pace, implying that some invisible extra energy is at work.

The Origin, History, Evolution & Future of the Universe

SPECIAL REPORT: Our universe is both ancient and vast, and expanding out farther and faster every day. This accelerating universe, the dark energy that seems to be behind it, and other puzzles like the exact nature of the Big Bang and the early evolution of the universe are among the great puzzles of cosmology .

There was a time when scientists thought Earth was at the center of the universe. As late as the 1920s, we did not realize that our galaxy was just one of many in a vast universe. Only later did we recognize that the other galaxies were running away from us — in every direction — at ever greater speeds. Likewise in recent decades, our understanding of the universe has accelerated.

In this 8-part series of stories, videos, pictures and infographics, SPACE.com looks at some of the most amazing revelations  about the universe  and the enduring enigmas still to be solved.

Video Show: A Blueprint for the Universe The universe is filled with stars, galaxies, planets and more, plus a veritable buffet of invisible stuff like dark matter astronomers have yet to see. But scientists have pinned down some of the major ingredients of our universe. Take a look at star stuff and more in this video show.

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Images: Peering Back to the Big Bang & Early Universe Our universe is 13.7 billion years old, but astronomers are peering deep into its  history and are getting a greater understanding of how the first stars formed, and how the earliest galaxies came together. See images, illustrations and diagrams of the universe from now back to the Big Bang.

The Universe: Big Bang to Now in 10 Easy Steps The widely accepted theory for the origin and evolution of the universe is the Big Bang model, which states that the universe began as an incredibly hot, dense point roughly 13.7 billion years ago. Here's a breakdown of the Big Bang to now in 10 easy steps.

The History & Structure of the Universe (Infographic Gallery) Tour the universe's 13.7-billion-year history, from the Big Bang to planet Earth today, in this SPACE.com infographic series.

The Big Bang: What Really Happened at Our Universe's Birth? Big Bang theory holds that our universe began 13.7 billion years ago, in a massive expansion that blew space up like a balloon. Here's a brief rundown of what astronomers think happened.

The Universe's Dark Ages: How Our Cosmos Survived The dark ages of the universe — an era of darkness that existed before the first stars and galaxies — mostly remain a mystery because there is so little of it to see, but scientists intensely desire to shed light on them in order to learn secrets about how the universe came into being.

The Universe Today: What It All Looks Like Now     In the 1920s, astronomer Georges Lemaître proposed what became known as the Big Bang theory, which is the most widely accepted model to explain the formation of the universe.

Endless Void or Big Crunch: How Will the Universe End? Not only are scientists unsure how the universe will end, they aren't even sure it will end at all.

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Origins of the Universe 101

How old is the universe, and how did it begin? Throughout history, countless myths and scientific theories have tried to explain the universe's origins. The most widely accepted explanation is the big bang theory. Learn about the explosion that started it all and how the universe grew from the size of an atom to encompass everything in existence today.

Earth Science, Astronomy

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8.1: Origin of the Universe

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• Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
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The universe appears to have an infinite number of galaxies and solar systems and our solar system occupies a small section of this vast entirety. The origins of the universe and solar system set the context for conceptualizing the Earth’s origin and early history.

Big-Bang Theory

The mysterious details of events prior to and during the origin of the universe are subject to great scientific debate. The prevailing idea about how the universe was created is called the big-bang theory . Although the ideas behind the big-bang theory feel almost mystical, they are supported by Einstein’s theory of general relativity [ 1 ]. Other scientific evidence, grounded in empirical observations, supports the big-bang theory.

The big-bang theory proposes the universe was formed from an infinitely dense and hot core of the material. The bang in the title suggests there was an explosive, outward expansion of all matter and space that created atoms. Spectroscopy confirms that hydrogen makes up about 74% of all matter in the universe. Since its creation, the universe has been expanding for 13.8 billion years and recent observations suggest the rate of this expansion is increasing [ 2 ].

Spectroscopy

Spectroscopy is the investigation and measurement of spectra produced when materials interact with or emit electromagnetic radiation. Spectra is the plural for spectrum which is a particular wavelength from the electromagnetic spectrum . Common spectra include the different colors of visible light, X-rays, ultraviolet waves, microwaves, and radio waves. Each beam of light is a unique mixture of wavelengths that combine across the spectrum to make the color we see. The light wavelengths are created or absorbed inside atoms, and each wavelength signature matches a specific element. Even white light from the Sun, which seems like an uninterrupted continuum of wavelengths, has gaps in some wavelengths. The gaps correspond to elements present in the Earth’s atmosphere that act as filters for specific wavelengths. These missing wavelengths were famously observed by Joseph von Fraunhofer (1787–1826) in the early 1800s [ 3 ], but it took decades before scientists were able to relate the missing wavelengths to atmospheric filtering. Spectroscopy shows that the Sun is mostly made of hydrogen and helium. Applying this process to light from distant stars, scientists can calculate the abundance of elements in a specific star and visible universe as a whole. Also, this spectroscopic information can be used as an interstellar speedometer.

The Doppler effect is the same process that changes the pitch of the sound of an approaching car or ambulance from high to low as it passes. When an object emits waves, such as light or sound, while moving toward an observer, the wavelengths get compressed. In sound, this results in a shift to a higher pitch. When an object moves away from an observer, the wavelengths are extended, producing a lower-pitched sound. The Doppler effect is used on light emitted from stars and galaxies to determine their speed and direction of travel. Scientists, including Vesto Slipher (1875–1696) [ 6 ] and Edwin Hubble (1889–1953) [ 7 ], examined galaxies both near and far and found that almost all galaxies outside of our galaxy are moving away from each other, and us. Because the light wavelengths of receding objects are extended, visible light is shifted toward the red end of the spectrum, called a redshift . In addition, Hubble noticed that galaxies that were farther away from Earth also had a greater amount of redshift, and thus, the faster they are traveling away from us. The only way to reconcile this information is to deduce the universe is still expanding. Hubble’s observation forms the basis of the big-bang theory.

Cosmic Microwave Background Radiation

Another strong indication of the big-bang is cosmic microwave background radiation . Cosmic radiation was accidentally discovered by Arno Penzias (1933–) and Robert Woodrow Wilson (1936–) [ 8 ] when they were trying to eliminate background noise from a communication satellite. They discovered very faint traces of energy or heat that are omnipresent across the universe. This energy was left behind from the big bang, like an echo.

Stellar Evolution

Astronomers think the big bang created lighter elements, mostly hydrogen and smaller amounts of elements helium, lithium, and beryllium. Another process must be responsible for creating the other 90 heavier elements. The current model of stellar evolution explains the origins of these heavier elements.

Birth of a Star

Stars start their lives as elements floating in cold, spinning clouds of gas and dust known as nebulas . Gravitational attraction or perhaps a nearby stellar explosion causes the elements to condense and spin into a disk shape. In the center of this disk shape, a new star is born under the force of gravity. The spinning whirlpool concentrates material in the center, and the increasing gravitational forces collect even more mass. Eventually, the immensely concentrated mass of material reaches a critical point of such intense heat and pressure it initiates fusion.

Fusion is not a chemical reaction. Fusion is a nuclear reaction in which two or more nuclei, the centers of atoms, are forced together and combine creating a new larger atom. This reaction gives off a tremendous amount of energy, usually as light and solar radiation. An element such as hydrogen combines or fuses with other hydrogen atoms in the core of a star to become a new element, in this case, helium. Another product of this process is energy, such as solar radiation that leaves the Sun and comes to the Earth as light and heat. Fusion is a steady and predictable process, which is why we call this the main phase of a star’s life. During its main phase, a star turns hydrogen into helium. Since most stars contain plentiful amounts of hydrogen, the main phase may last billions of years, during which their size and energy output remains relatively steady.

The giant phase in a star’s life occurs when the star runs out of hydrogen for fusion. If a star is large enough, it has sufficient heat and pressure to start fusing helium into heavier elements. This style of fusion is more energetic and the higher energy and temperature expand the star to a larger size and brightness. This giant phase is predicted to happen to our Sun in another few billion years, growing the radius of the Sun to Earth’s orbit, which will render life impossible. The mass of a star during its main phase is the primary factor in determining how it will evolve. If the star has enough mass and reaches a point at which the primary fusion element, such as helium, is exhausted, fusion continues using new, heavier elements. This occurs over and over in very large stars, forming progressively heavier elements like carbon and oxygen. Eventually, fusion reaches its limit as it forms iron and nickel. This progression explains the abundance of iron and nickel in rocky objects, like Earth, within the solar system. At this point, any further fusion absorbs energy instead of giving it off, which is the beginning of the end of the star’s life [ 9 ].

Death of a Star

The death of a star can range from spectacular to other-worldly (see figure). Stars like the Sun form a planetary nebula, which comes from the collapse of the star’s outer layers in an event like the implosion of a building. In the tug-of-war between gravity’s inward pull and fusion’s outward push, gravity instantly takes over when fusion ends, with the outer gasses puffing away to form a nebula. More massive stars do this as well but with a more energetic collapse, which starts another type of energy release mixed with element creation known as a supernova. In a supernova , the collapse of the core suddenly halts, creating a massive outward-propagating shock wave. A supernova is the most energetic explosion in the universe short of the big bang. The energy release is so significant the ensuing fusion can make every element up through uranium [ 10 ].

The death of the star can result in the creation of white dwarfs, neutron stars, or black holes. Following their deaths, stars like the Sun turn into white dwarfs.

White dwarfs are hot star embers, formed by packing most of a dying star’s mass into a small and dense object about the size of Earth. Larger stars may explode in a supernova that packs their mass even tighter to become neutron stars. Neutron stars are so dense that protons combine with electrons to form neutrons. The largest stars collapse their mass even further, becoming objects so dense that light cannot escape their gravitational grasp. These are the infamous black holes and the details of the physics of what occurs in them are still up for debate.

1. Einstein A (1917) Cosmological Reflections on the General Relativity Theory. Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften (Berlin), Seite 142-152 142–152

2. Perlmutter S, Aldering G, Goldhaber G (1999) Measurements of Omega and Lambda from 42 high-redshift supernovae. Astrophys J 517:565–586

3. Fraunhofer J (1817) Bestimmung des Brechungs-und des Farbenzerstreungs-Vermögens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernröhre. Ann Phys 56:264–313. https://doi.org/10.1002/andp.18170560706

6. Slipher VM (1913) The radial velocity of the Andromeda Nebula. Lowell Observatory Bulletin 2:56–57

7. Hubble E (1929) A relation between distance and radial velocity among extra-galactic nebulae. Proc Natl Acad Sci U S A 15:168–173

8. Penzias AA, Wilson RW (1965) A Measurement of Excess Antenna Temperature at 4080 Mc/s. Astrophys J 142:419–421

9. Salaris M, Cassisi S (2005) Evolution of stars and stellar populations. John Wiley & Sons

10. Timmes FX, Woosley SE, Weaver TA (1995) Galactic chemical evolution: Hydrogen through zinc. The Astrophysical journal Supplement series 98:617–658

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• 25 March 2024

How did the Big Bang get its name? Here’s the real story

• Helge Kragh 0

Helge Kragh is a historian of science at the University of Copenhagen.

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Today 'Big Bang' is a household phrase, used even by people who have no idea of how the Universe was born some 14 billion years ago. Credit: Henning Dalhoff/SPL

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“Words are like harpoons,” UK physicist and astronomer Fred Hoyle told an interviewer in 1995. “Once they go in, they are very hard to pull out.” Hoyle, then 80 years old, was referring to the term Big Bang, which he had coined on 28 March 1949 to describe the origin of the Universe. Today, it is a household phrase, known to and routinely used by people who have no idea of how the Universe was born some 14 billion years ago. Ironically, Hoyle deeply disliked the idea of a Big Bang and remained, until his death in 2001, a staunch critic of mainstream Big Bang cosmology.

Several misconceptions linger concerning the origin and impact of the popular term. One is whether Hoyle introduced the nickname to ridicule or denigrate the small community of cosmologists who thought that the Universe had a violent beginning — a hypothesis that then seemed irrational. Another is that this group adopted ‘Big Bang’ eagerly, and it then migrated to other sciences and to everyday language. In reality, for decades, scientists ignored the catchy phrase, even as it spread in more-popular contexts.

This new map of the Universe suggests dark matter shaped the cosmos

The first cosmological theory of the Big Bang type dates back to 1931, when Belgian physicist and Catholic priest Georges Lemaître proposed a model based on the radioactive explosion of what he called a “primeval atom” at a fixed time in the past. He conceived that this primordial object was highly radioactive and so dense that it comprised all the matter, space and energy of the entire Universe. From the original explosion caused by radioactive decay, stars and galaxies would eventually form, he reasoned. Lemaître spoke metaphorically of his model as a “fireworks theory” of the Universe, the fireworks consisting of the decay products of the initial explosion.

However, Big Bang cosmology in its modern meaning — that the Universe was created in a flash of energy and has expanded and cooled down since — took off only in the late 1940s, with a series of papers by the Soviet–US nuclear physicist George Gamow and his US associates Ralph Alpher and Robert Herman. Gamow hypothesized that the early Universe must have been so hot and dense that it was filled with a primordial soup of radiation and nuclear particles, namely neutrons and protons. Under such conditions, those particles would gradually come together to form atomic nuclei as the temperature cooled. By following the thermonuclear processes that would have taken place in this fiery young Universe, Gamow and his collaborators tried to calculate the present abundance of chemical elements in an influential 1948 paper 1 .

Competing ideas

The same year, a radically different picture of the Universe was announced by Hoyle and Austrian-born cosmologists Hermann Bondi and Thomas Gold. Their steady-state theory assumed that, on a large scale, the Universe had always looked the same and would always do so, for eternity. According to Gamow, the idea of an ‘early Universe’ and an ‘old Universe’ were meaningless in a steady-state cosmology that posited a Universe with no beginning or end.

Over the next two decades, an epic controversy between these two incompatible systems evolved. It is often portrayed as a fight between the Big Bang theory and the steady-state theory, or even personalized as a battle between Gamow and Hoyle. But this is a misrepresentation.

Soviet–US nuclear physicist George Gamow was an early proponent of Big Bang cosmology. Credit: Bettmann/Getty

Both parties, and most other physicists of the time, accepted that the Universe was expanding — as US astronomer Edwin Hubble demonstrated in the late 1920s by observing that most galaxies are rushing away from our own. But the idea that is so familiar today, of the Universe beginning at one point in time, was widely seen as irrational. After all, how could the cause of the original explosion be explained, given that time only came into existence with it? In fact, Gamow’s theory of the early Universe played almost no part in this debate.

Rather, a bigger question at the time was whether the Universe was evolving in accordance with German physicist Albert Einstein’s general theory of relativity, which predicted that it was either expanding or contracting, not steady. Although Einstein’s theory doesn’t require a Big Bang, it does imply that the Universe looked different in the past than it does now. And an ever-expanding Universe does not necessarily entail the beginning of time. An expanding Universe could have blown up from a smaller precursor, Lemaître suggested in 1927.

An apt but innocent phrase

On 28 March 1949, Hoyle — a well-known popularizer of science — gave a radio talk to the BBC Third Programme, in which he contrasted these two views of the Universe. He referred to “the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past”. This lecture was indeed the origin of the cosmological term ‘Big Bang’. A transcript of the talk was reproduced in full in the BBC’s The Listener magazine, and Hoyle mentioned it in his 1950 book The Nature of the Universe , which was based on a series of BBC broadcasts he made earlier the same year.

How dwarf galaxies lit up the Universe after the Big Bang

Although Hoyle resolutely dismissed the idea of a sudden origin of the Universe as unacceptable on both scientific and philosophical grounds, he later said that he did not mean it in ridiculing or mocking terms, such as was often stated. None of the few cosmologists in favour of the exploding Universe, such as Lemaître and Gamow, was offended by the term. Hoyle later explained that he needed visual metaphors in his broadcast to get across technical points to the public, and the casual coining of ‘Big Bang’ was one of them. He did not mean it to be derogatory or, for that matter, of any importance.

Hoyle’s ‘Big Bang’ was a new term as far as cosmology was concerned, but it was not in general contexts. The word ‘bang’ often refers to an ordinary explosion, say, of gunpowder, and a big bang might simply mean a very large and noisy explosion, something similar to Lemaître’s fireworks. And indeed, before March 1949, there were examples in the scientific literature of meteorologists and geophysicists using the term in their publications. Whereas they referred to real explosions, Hoyle’s Big Bang was purely metaphorical, in that he did not actually think that the Universe originated in a blast.

The Big Bang was not a big deal

For the next two decades, the catchy term that Hoyle had coined was largely ignored by physicists and astronomers. Lemaître never used ‘Big Bang’ and Gamow used it only once in his numerous publications on cosmology. One might think that at least Hoyle took it seriously and promoted his coinage, but he returned to it only in 1965, after a silence of 16 years. It took until 1957 before ‘Big Bang’ appeared in a research publication 2 , namely in a paper on the formation of elements in stars in Scientific Monthly by the US nuclear physicist William Fowler, a close collaborator of Hoyle and a future Nobel laureate.

How Einstein built on the past to make his breakthroughs

Before 1965, the cosmological Big Bang seems to have been referenced just a few dozen times, mostly in popular-science literature. I have counted 34 sources that mentioned the name and, of these, 23 are of a popular or general nature, 7 are scientific papers and 4 are philosophical studies. The authors include 16 people from the United States, 7 from the United Kingdom, one from Germany and one from Australia. None of the scientific papers appeared in astronomy journals.

Among those that used the term for the origin of the Universe was the US philosopher Norwood Russell Hanson, who in 1963 coined his own word for advocates of what he called the ‘Disneyoid picture’ of the cosmic explosion. He called them ‘big bangers’, a term which still can be found in the popular literature — in which the ultimate big banger is sometimes identified as God.

A popular misnomer

A watershed moment in the history of modern cosmology soon followed. In 1965, US physicists Arno Penzias and Robert Wilson’s report of the discovery of the cosmic microwave background — a faint bath of radio waves coming from all over the sky — was understood as a fossil remnant of radiation from the hot cosmic past. “Signals Imply a ‘Big Bang’ Universe” announced the New York Times on 21 May 1965. The Universe did indeed have a baby phase, as was suggested by Gamow and Lemaître. The cosmological battle had effectively come to an end, with the steady-state theory as the loser and the Big Bang theory emerging as a paradigm in cosmological research. Yet, for a while, physicists and astronomers hesitated to embrace Hoyle’s term.

Work by US physicists Arno Penzias and Robert Wilson vindicated the Big Bang theory. Credit: Bettmann/Getty

It took until March 1966 for the name to turn up in a Nature research article 3 . The Web of Science database lists only 11 scientific papers in the period 1965–69 with the name in their titles, followed by 30 papers in 1970–74 and 42 in 1975–79. Cosmology textbooks published in the early 1970s showed no unity with regard to the nomenclature. Some authors included the term Big Bang, some mentioned it only in passing and others avoided it altogether. They preferred to speak of the ‘standard model’ or the ‘theory of the hot universe’, instead of the undignified and admittedly misleading Big Bang metaphor.

Nonetheless, by the 1980s, the misnomer had become firmly entrenched in the literature and in common speech. The phrase has been adopted in many languages other than English, including French ( théorie du Big Bang ), Italian ( teoria del Big Bang ) and Swedish ( Big Bang teorin ). Germans have constructed their own version, namely Urknall , meaning ‘the original bang’, a word that is close to the Dutch oerknal . Later attempts to replace Hoyle’s term with alternative and more-appropriate names have failed miserably.

The many faces of the metaphor

By the 1990s, ‘Big Bang’ had migrated to commercial, political and artistic uses. During the 1950s and 1960s, the term frequently alluded to the danger of nuclear warfare as it did in UK playwright John Osborne’s play Look Back in Anger, first performed in 1956. The association of nuclear weapons and the explosive origin of the Universe can be found as early as 1948, before Hoyle coined his term. As its popularity increased, ‘Big Bang’ began being used to express a forceful beginning or radical change of almost any kind — such as the Bristol Sessions, a series of recording sessions in 1927, being referred to as the ‘Big Bang’ of modern country music.

In the United Kingdom, the term was widely used for a major transformation of the London Stock Exchange in 1986. “After the Big Bang tomorrow, the City will never be the same again,” wrote Sunday Express Magazine on 26 October that year. That use spread to the United States. In 1987, the linguistic journal American Speech included ‘Big Bang’ in its list of new words and defined ‘big banger’ as “one involved with the Big Bang on the London Stock Exchange”.

Today, searching online for the ‘Big Bang theory’ directs you first not to cosmology, but to a popular US sitcom. Seventy-five years on, the name that Hoyle so casually coined has indeed metamorphosed into a harpoon-like word: very hard to pull out once in.

Nature 627 , 726-728 (2024)

doi: https://doi.org/10.1038/d41586-024-00894-z

Gamow, G. Nature 162 , 680–682 (1948).

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Fowler, W. A. Sci. Mon. 84 , 84–100 (1957).

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Hawking S. W. & Tayler, R. J. Nature 209 , 1278–1279 (1966).

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Stars, Galaxies, and the Universe

The universe, lesson objectives.

• Explain the evidence for an expanding universe.
• Describe the formation of the universe according to the Big Bang Theory.
• Define dark matter and dark energy.
• Big Bang Theory
• dark energy
• dark matter
• Doppler Effect

Introduction

The study of the universe is called cosmology . Cosmologists study the structure and changes in the present universe. The universe contains all of the star systems, galaxies, gas and dust, plus all the matter and energy that exists now, that existed in the past, and that will exist in the future. The universe includes all of space and time.

Evolution of Human Understanding of the Universe

What did the ancient Greeks recognize as the universe? In their model, the universe contained Earth at the center, the Sun, the Moon, five planets, and a sphere to which all the stars were attached. This idea held for many centuries until Galileo’s telescope helped allow people to recognize that Earth is not the center of the universe. They also found out that there are many more stars than were visible to the naked eye. All of those stars were in the Milky Way Galaxy.

In the early 20th century, an astronomer named Edwin Hubble Figure below discovered that what scientists called the Andromeda Nebula was actually over 2 million light years away — many times farther than the farthest distances that had ever been measured. Hubble realized that many of the objects that astronomers called nebulas were not actually clouds of gas, but were collections of millions or billions of stars — what we now call galaxies.

KQED: Nobel Laureate George Smoot and the Origin of the Universe

George Smoot, a scientist at Lawrence Berkeley National Lab, shared the 2006 Nobel Prize in Physics for his work on the origin of the universe. Using background radiation detected by the Cosmic Background Explorer Satellite (COBE), Smoot was able to make a picture of the universe when it was 12 hours old. Learn more at: http://science.kqed.org/quest/video/nobel-laureate-george-smoot-and-the-origin-of-the-universe/ .

Dark Matter and Dark Energy

The Big Bang theory is still the best scientific model we have for explaining the formation of the universe and many lines of evidence support it. However, recent discoveries continue to shake up our understanding of the universe. Astronomers and other scientists are now wrestling with some unanswered questions about what the universe is made of and why it is expanding. A lot of what cosmologists do is create mathematical models and computer simulations to account for these unknown phenomena.

Dark Matter

The things we observe in space are objects that emit some type of electromagnetic radiation. However, scientists think that matter that emits light makes up only a small part of the matter in the universe. The rest of the matter, about 80%, is dark matter.

Dark matter emits no electromagnetic radiation so we can’t observe it directly. However, astronomers know that dark matter exists because its gravity affects the motion of objects around it. When astronomers measure how spiral galaxies rotate, they find that the outside edges of a galaxy rotate at the same speed as parts closer to the center. This can only be explained if there is a lot more matter in the galaxy than they can see.

Gravitational lensing occurs when light is bent from a very distant bright source around a super-massive object ( Figure below ). To explain strong gravitational lensing, more matter than is observed must be present.

Lesson Summary

• The universe contains all the matter and energy that exists now, that existed in the past, and that will exist in the future. The universe also includes all of space and time.
• Redshift is a shift of element lines toward the red end of the spectrum. Redshift occurs when the source of light is moving away from the observer.
• Light from almost every galaxy is redshifted. The farther away a galaxy is, the more its light is redshifted, and the faster it is moving away from us.
• The redshift of galaxies means that the universe is expanding.
• The universe was squeezed into a very small volume and then exploded in the Big Bang theory about 13.7 billion years ago.
• Recent evidence shows that there is a lot of matter in the universe that we cannot detect directly. This matter is called dark matter.
• The rate of the expansion of the universe is increasing. The cause of this increase is unknown; one possible explanation involves a new form of energy called dark energy.

Review Questions

1. What is redshift, and what causes it to occur? What does redshift indicate?

2. What is Hubble’s law?

3. What is the cosmological theory of the formation of the universe called?

4. How old is the universe, according to the Big Bang theory?

5. Describe two different possibilities for the nature of dark matter.

6. What makes scientists believe that dark matter exists?

7. What observation caused astronomers to propose the existence of dark energy?

Further Reading / Supplemental Links

• The science of dark matter: http://cdms.berkeley.edu/Education/DMpages/index.shtml
• More about cosmology: http://stardate.org/resources/btss/cosmology/
• The Big Bang: http://hurricanes.nasa.gov/universe/science/bang.html

Points to Consider

• The expansion of the universe is sometimes modeled using a balloon with dots marked on it, as described earlier in the lesson. In what ways is this a good model, and it what ways does it not correctly represent the expanding universe? Can you think of a different way to model the expansion of the universe?
• The Big Bang theory is currently the most widely accepted scientific theory for how the universe formed. What is another explanation of how the universe could have formed? Is your explanation one that a scientist would accept?
• Earth Science for High School. Provided by : CK-12. Located at : http://www.ck12.org/book/CK-12-Earth-Science-For-High-School/ . License : CC BY-NC: Attribution-NonCommercial

Origin Of The Universe: 8 Different Theories

How did the Universe we know come into existence? And how do we explain its origin? These are some of the questions cosmologists and physicists have been trying to unravel for decades.

Undoubtedly, every piece of evidence and data collected over the years by cosmologists points toward the possibility that it all might have started with a ‘big bang.’ But what if there is more?

Table of Contents

What Is The Big Bang Theory? A Brief Introduction

In 1927, Belgian astronomer Georges Lemaitre proposed the theory of an expanding universe (later confirmed by Edwin Hubble). He theorized that an expanding universe could be traced back to a singular point, which he termed the “primeval atom,” back in time. It laid the foundation for the modern Big Bang theory.

The Big Bang Theory is an explanation, based mostly on mathematical models, of how and when the Universe came into existence.

The cosmological model of the Universe described in the Big Bang theory explains how it initially expanded from a state of infinite density and temperature, known as the primordial (or gravitational) singularity.

This expansion was followed by cosmic inflation and a massive temperature drop. During this phase, the Universe ballooned at a much faster rate than the speed of light (by a factor of 10 26 ).

Subsequently, the Universe was reheated to a point where elementary particles (quarks, leptons, and so on) before a gradual decrease in temperature (and density) led to the formation of the first protons and neutrons .

A few minutes into the expansion, protons and neutrons combine to form primordial hydrogen and helium-4 nuclei. The estimated radius of the observable Universe during this phase was 300 light-years. The earliest stars and galaxies appeared about 400 million years after the event.

A crucial piece of the Big Bang model is the cosmic microwave background (CMB), which is the electromagnetic radiation left from the time when the Universe was in its infancy. CMB remains the most definitive proof of the Big Bang.

While the theory remains widely accepted across the scientific spectrum, a few alternative explanations — such as steady-state Universe and eternal inflation, have gained attraction over the years.

Below, we have discussed seven of the most popular alternatives of the Big Bang, explaining the origin of the Universe.

8. Quantum Fluctuation Theory 

According to quantum mechanics, particles and antiparticles can spontaneously appear and annihilate in empty space. While doing so, they create temporary fluctuations in energy called vacuum fluctuations. 

The quantum fluctuation theory proposes that our universe might have formed from one of those vacuum fluctuations. Image a very small space where an energy fluctuation occurs. Instead of quickly annihilating, this fluctuation could potentially increase and develop something more substantial. 

This theory suggests that the initial fluctuation expanded rapidly, which eventually led to the birth of our universe. This rapid expansion can be associated with comic inflation. 

As our universe expanded and cooled, the energy from the initial fluctuation might have transformed into particles and antiparticles. These particles would ultimately become the building blocks of matter throughout the cosmos.

Although the concept seems intriguing, it is just a theoretical framework. The precise details of how our universe formed from a quantum fluctuation remain a subject of active research. Scientists continue to study and refine this theory to get deeper insights into the origin of the universe. 

Reference Sources –

Spontaneous creation of the universe from nothing, arXiv :1404.1207

Massive galaxy clusters hint at primordial quantum diffusion, Physics Review Letters  

7. Theory of Eternal Inflation

The concept of the inflationary universe was first introduced by cosmologist Alan Guth in 1979 to explain why the Universe is flat, something that was missing from the original Big Bang theory.

Though Guth’s idea of inflation explains the flat Universe, it creates a scenario that prevents the Universe from escaping that inflation . If this were the case, reheating of the Universe wouldn’t have taken place, and neither would the formation of stars and galaxies.

This particular problem was solved by Andreas Albrecht and Paul Steinhardt in their “new inflation” model. They argued that rapid inflation of the Universe happened just for a few seconds before ceasing. It demonstrated how the Universe can go through rapid inflation and still end up getting heated.

Based on the previous works of Steinhardt and Alexander Vilenkin, Andrei Linde, a professor at Stanford University, proposed an alternative to Guth’s inflation theory called chaotic inflation or ‘eternal inflation theory.

The theory argues that the inflationary phase of the Universe goes on forever; it didn’t end for the Universe as a whole. In other words, cosmic inflation continues in some parts of the Universe and ceases in others. This leads to a multiverse scenario, wherein space is broken into bubbles. It’s like a universe inside a universe.

In a multiverse, different universes may have different laws of nature and physics at work. So, instead of a single expanding cosmos, our Universe might be an inflationary multiverse with many small universes with varying properties.

However, Paul Steinhardt believes that his ‘new inflation’ theory doesn’t lead to or predict anything and argues that the multiverse notion is a “fatal flaw” and unnatural.

Eternal inflation and its implications, arXiv :hep-th/0702178

Inflationary paradigm in trouble after Planck2013, arXiv :1304.2785

6. Conformal Cyclic Model

The conformal cyclic cosmological (CCC) model speculates that the Universe goes through repeated cycles of the Big Bang and subsequent expansions. The general idea is that the ‘Big Bang’ was not the beginning of the Universe but rather a transition phase. It was developed by renowned theoretical physicist and mathematician Roger Penrose.

The theory suggests that the universe goes through a series of cycles, each involving a Big Bang followed by expansion, contraction, and another Big Bang. These cycles are infinite, which means the universe goes through this process repeatedly with no end. 

This concept is often compared with a spring oscillation, where the universe expands and contracts periodically. 

Unlike the standard Big Bang theory that postulates a singular beginning of the universe, the Cyclic Universe theory avoids the singularity problem by suggesting that our universe had no initial singularity but has always existed in this cyclical pattern. 

As a basis for his model, Penrose used multiple FLRW (Friedmann–Lemaître–Robertson–Walker) metric sequences. He argued that the conformal boundary of one FLRW sequence could be attached to the boundary of another.

The FLRW metric is the closest approximation of the nature of the Universe and a part of the Lambda-CDM model . Each sequence begins with a big bang, followed by inflation and subsequent expansion.

The cyclic or oscillating model, wherein the Universe reiterates over and over in an indefinite cycle, first came into the spotlight in the 1930s, when Albert Einstein investigated the idea of an ‘everlasting’ universe. He considered that after reaching a certain point, the Universe starts collapsing and ends with a Big Crunch before going through the Big Bounce.

Right now, there are four different variations of the cyclic model of the Universe, one of which is the Conformal Cyclic Cosmology.

Read: Does Universe Iterate Through Infinite Numbers of Big Bangs?

This theory has been studied extensively, but it faces some serious challenges in terms of observational evidence. One of the major challenges has been detecting remnants of past cycles in our current universe. 

5. Black Hole Mirage

A study conducted by a group of researchers in 2013 speculated that our Universe might have originated from the debris spewed out of a collapsed four-dimensional star or a black hole.

According to the cosmologists associated with the research, one of the limitations of the Big Bang theory is to explain the temperature equilibrium found in the Universe.

While most scientists concur that the inflationary theory gives an adequate explanation of how a small patch with uniform temperature would rapidly expand to become the Universe we observe today, the group found it implausible due to the chaotic nature of the Big Bang.

To solve this problem, the team proposed a model of the cosmos, in which our three-dimensional Universe is a membrane and is floating inside a four-dimensional ‘bulk universe.’

They argued that if the 4-D ‘bulk universe’ has 4-D stars, it’s likely they will collapse into 4-D black holes. These 4-D black would have a 3-D event horizon (just like the 3-D ones have a 2-D event horizon ), which they named ‘hypersphere.’

Read:  11 Biggest Unsolved Mysteries in Physics

When the team simulated the collapse of a 4-D star, they discovered that the ejected debris from the dying star was likely to cast a 3-D membrane around that 3-D event horizon. Our Universe might be one such membrane.

The ‘4-D black hole’ model of the cosmos does explain why the temperature is almost uniform throughout the Universe. It may also give valuable insights into exactly what triggered the cosmic inflation a few seconds after its genesis.

However, a recent observation by ESA’s Planck satellite has uncovered small variations in the cosmic microwave background (CMB) temperature. These satellite readings differ from the proposed model by about four percent.

4. Plasma Universe Theory

Our current understanding of the Universe is mostly influenced by gravity, specifically Einstein’s General Theory of Relativity, through which cosmologists explain the nature of the Universe. Coincidentally, just like most other things, an alternative to gravity has also been entertained by scientists over the years.

The plasma cosmology (or plasma universe theory) speculates that electromagnetic forces and plasma play a much more important role in the Universe than gravity.

Although the approach has many different flavors, the basic idea remains the same: every astronomical body, including the sun, stars, and galaxies, results from some electrical process.

The first prominent plasma universe theory was proposed by Nobel laureate Hannes Alfvén in the 1960s. He was later joined by Swedish theoretical physicist Oskar Klein to develop the Alfvén–Klein model .

The model is built around the assumption that the Universe sustains equal amounts of matter and antimatter (that’s not the case according to modern particle physics). The boundaries of these two regions are marked with cosmic electromagnetic fields. And thus, interactions between the two would produce plasma, which Alfvén named ‘ambiplasma.’

According to the theory, such plasma would form large sections of matter and antimatter throughout the Universe. Furthermore, it theorized that our current location in the cosmos must be in a section where the matter is much more abundant than the antimatter – hence solving the matter-antimatter asymmetry problem.

Read:  Could Life Form In a Two-Dimensional Universe?

3. Slow Freeze Theory

Decades of mathematical modeling and research have led cosmologists to a valid conclusion that our Universe started from a single point of infinite density and temperature called the singularity. The subsequent expansion of the cosmos allowed it to cool, which led to the formation of galaxies, stars, and other astronomical objects.

However, as we know, the standard Big Bang model has not gone unchallenged, and one such challenging theory was proposed by Christof Wetterich, a professor at Germany’s Heidelberg University.

Wetterich argued that the Universe we know today might have actually started as cold and sparse, awakened from a long freeze. Over time, the fundamental particles in the early Universe became heavier while the gravitational constant decreased.

Furthermore, he explained that if masses of the particles have been increasing, radiation from the early Universe could make space appear hotter and move away from each other even if it wasn’t the case.

The basic idea of Wetterich’s Slow Freeze cosmic model is that the Universe has no beginning and no future. Instead of a hot Big Bang, the theory advocates for a cold and slowly evolving Universe.

According to Wetterich, the theory explains density fluctuations in the early Universe (primordial fluctuations) and why our current cosmos is dominated by dark energy.

Read: All Interesting Facts About Black Holes and White Holes

2. Hindu Cosmology

Religion and science have been the best of enemies since at least the time of Copernicus and Galileo. There is perhaps no room for science when we talk about religion and vice-versa. However, there is one religion whose cosmological beliefs sit well with the current model of the Universe.

Creation theories in Hindu mythology are widely considered one of the most ancient and significant of all other religious counterparts.

Over the years, prominent physicists and cosmologists, including Carl Sagan and Niels Bohr, have admired Hindu cosmological beliefs for its close similarity with the timelines in the standard cosmological model of the Universe.

According to Hindu mythology, the Universe follows an infinite cyclic model. It means that our current Universe will be replaced by an endless number of universes. Each iteration of the Universe is divided into two phases, ‘Kalpa’ (or the day of Brahma) and ‘pralaya’ (the night of Brahma), and each is 4.32 billion years long.

According to Hindu mythology, the age of the Universe (8.64 billion years) is more than the currently estimated age of the solar system.

1. Steady State Universe

The Steady-State model asserts that the observable Universe remains the same at any place and time. In the Universe, which is forever expanding, matter is continuously created to fill the space.

The idea of the steady-state theory was first proposed in 1948 by cosmologists Hermann Bondi, Fred Hoyle, and Thomas Gold. It was derived from the perfect cosmological principle, which itself states that the Universe is the same no matter where you look, and it will always be the same.

According to the model, galaxies and other large astronomical bodies near us should appear similar to those that are far away. However, the Big Bang tells us that distant galaxies should look younger than those at close proximity (when observed from the Earth) since light takes much longer to reach us.

The Steady-State theory gained widespread popularity in the early and mid-20th century. However, by the 1960s, it was mostly discarded by the scientific community in favor of the Big Bang after the discovery of the cosmic microwave background.

Interestingly, according to a 1931 manuscript that was discovered by researchers in 2014, Albert Einstein was working on an alternative to the Big Bang theory. It was identical to Fred Hoyle’s Steady State model, proposing that the universe has expanded steadily. However, the idea was shortly abandoned by Einstein.

More to Know

How old is the universe.

The universe is nearly 14 billion years old (13.78 billion, to be exact). Scientists have reached this conclusion after extensive studies of the cosmic microwave background and analyzing data from the Plack Space Observatory, WMAP, and other space probes.

However, a team of researchers in 2019 calculated the age of the universe to be a couple of billion years younger than the age predicted by the Plank study. The team used the movement of galaxies and stars to estimate how fast the universe is expanding. A higher expansion rate means that the universe reached its current size faster, and thus, it must be younger.

To put an end to the discrepancy, an international team of astronomers analyzed the data from the Atacama Cosmology Telescope (ACT) in Chile in 2020 to find out the approximate age of the universe.

The team determined the age of the universe to be 13.77 billion years , give or take 40 million years, which is in line with the estimate from the Plank research team.

What is the Age of the Sun?

The estimated age of the Sun is about 4.56 billion years. How did they reach this number? Well, it is a combination of nuclear cosmochronology and simulations of the stellar evolution model.

8 Biggest Black Holes In The Universe | As Per Their Solar Masses

15 Brightest Stars In The Sky | Based On Apparent Magnitude

Different Types of Galaxies In The Universe

Varun Kumar

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In reference to the big bang, you never explain where all the material in the universe derived from. No matter what thoughtful possibility, Where did all the material derive from? It cannot come out of nothingness. Where is your explanation. What are you saying that I already said that? Is that any excuse for not being able to explain?

Its easier and more plausible that : (a) The universe has ALWAYS existed and WILL always exist. And that : (b) Is always subject to change dispite long periods of static in some regions. (c) contains an amount of energy which is infinite and dispite chemical interactions, remains the same. And (d) is composed of elements and particles, atoms and sub-atomic particles which can be known and are limited thou thier by-products may be unlimited. AND : (e) Life and living beings almost d i dont happen at all and so is unlikely to exist elsewhere at least anywhere near where we are. (f) 99.9 % of space is freezing cold and 0.001% is way hotter than any life form could ever tolerate. And percentages of infinite amounts are abstract. (g) the human mind and brain is the most complex thing in the known universe. (h) the universe is mainly harmless. (i) always take a towel !

Just came across your comment. I have searched for the words to extract your very questions, not really knowing just how to express such inquiries. I now have reference material to go forward. Yours. LOL.

Thanks for putting the words in my mouth. Brilliantly said.

do you have evidence about the theory of Steady State??

I’ve read it all, and I can see a preference for the big bank theory, in some version or other. What I cannot seem to find, in any writings, is a definition of the place at which the bang started, be it a point, line, or plane. If the universe is expanding, there are a lot of theories about it. But no one seems to be able to tell me from where?

If the Big Bang theory is correct and if the red-shift/ blue-shift theory is correct, they would lend credence to the idea of all expanding or outward flow of stars and galaxies all being red ( going away) vs blue ( coming in towards us). Under this scenario – trajectories at various galaxies and stars could be calculated to determine a point of origin of where the Big Bang started from. Is that spot then to be found void of all matter …? To date … blank spaces seem to hold much intrigue following Hubble’s highly successful 12 day synchronized stare into one of those many blank spaces, and most astronomers do not come away disappointed upon seeing developed plates of those blank spots. Every cubic light-year is chocked full of matter. So with this theory one could say …”bye – bye Big Bang theory”…

What Are the Theories of the Universe?

Humans have pondered the beginning of the universe since our species evolved. Generations of people have looked towards the sky as a source of amazement, religion, and wonder. Thankfully, over the last several centuries , scientists from around the world have begun to piece together empirical data to support a variety of hypotheses about how all of life as we know it began. Today, let’s take a closer look at an age-old question: what are the theories of the universe?

The Big Bang Theory

The most robust, well-supported theory as to the origins of the universe is the Big Bang Theory. A Belgian priest, Georges Lemaître , first suggested the idea of big bang theory in the 1920s. Since then, Einstein’s theory of relativity and modern science has lent credibility to this developing theory.

Big Bang Assumptions

Before we can get into specifics, let’s understand a few basic assumptions concerning the Big Bang Theory. Each of the following points is assumed to be true in the universe, and this notion is part of the foundation upon which the Big Bang rests. 

• The universe is constant . This one is first for a reason; it’s important. Our modeling and understanding of the world hinge on the idea that physical properties are the same everywhere. For example, we assume gravity , electricity, magnetism, and light all behave the same way, even in far off places in the galaxy and universe. 
• The universe is homogenous . Secondly, we assume that the universe is roughly the same in all directions. You can think of these like shovelfuls of dirt. Some scoops might have big rocks, some might have worms, and some may have more clay than others. But, in the long term, every 100 shovelfuls will be roughly the same in composition.
• The universe is not centered around us. Physicists refer to this notion as the “ privileged location .” This means that earth is somewhere in the universe, but we really have no idea where it is in relation to the “edge” (more on that later).
• The universe has a beginning. All matter and energy that has ever been and that will ever be were created during the Big Bang. No new material or energy has been created since then.

Basics of the Big Bang

The Big Bang is the leading theory as to the origins of the universe as we know it. It describes the mechanism by which everything we know started as a small singularity that ballooned into the earth, solar system, galaxy, and universe. The easiest way to understand this theory is through a timeline, so let’s dig in.

• 1 second. During the first second, the temperature around the big bang was about 5.5 billion degrees celsius (10 billion Fahrenheit). There would have been nothing to see at this point, though. According to NASA , “the free electrons would have caused light (photons) to scatter the way sunlight scatters from the water droplets in clouds.”
• 3 seconds. The initial explosion contained all the necessary subatomic particles for atoms and molecules: neutrons, protons, and electrons. The first basic elements form at this point: hydrogen, helium, and lithium. 
• 380,000 years. For the first time, light emerges into the universe. This radiation (light) is referred to as the cosmic microwave background. First predicted to exist in 1948 by Ralph Alpher , it is a signature mark of the Big Bang. This background of microwaves can still be observed today, and it used to estimate the age of the universe. 
• 300 million years. We’re jumping forward a bit here. As the initial burst of atoms and gas expands, gravity starts to become a relevant factor. Pockets of different densities of gas give birth to stars and collections of stars start to form galaxies. 
• ~9 billion years. Our sun forms. The universe is roughly 14 billion years old, and our sun is approximately 4.6 billion years old. 

Steady State Universe

The steady state universe hypothesis breaks one of the key Big Bang Theory assumptions. The steady state hypothesis states that matter and energy are being created continuously, steadily. First theorized in the 1920s by Sir James Jeans, the theory imagines a universe without a real beginning or end. 

In the steady state view, the universe has always been expanding and creating matter, and it will continue to do so. Although the theory has been revised and updated throughout the middle of the 20th century, an overwhelming amount of contradictory evidence supports the notion that the steady state hypothesis is largely false . 

Level II Multiverse

The multiverse concept is complicated . And, that may still be an understatement. One of the driving factors leading to the development of this theory is the seemingly perfect nature of physics in our universe. Light, gravity, physics… they all seem to work together perfectly to allow life to exist in our universe. This can be viewed as a major coincidence or an inevitability given a large number of trials. 

The multiverse concept postulates that multiple universes exist, simultaneously, and they each have different physical constants. For example, maybe a universe 2.0 (or 3.0 or 18.0 or 821.0) exists along with ours where light travels at a different speed. Changing this speed changes an exceptionally large number of other universal constants, and thus, everything we know about our universe.

Correct Hypothesis?

So, where does that leave us? What is the “correct” theory for the origin of the universe? By a large margin, the Big Bang is the most well-supported, evidence-based theory. That being said, new technology and new instruments allow us to gather different data every decade, so we will have to see what the future holds!

What other theories of the universe have you heard of? How do you think this all got here?

[Featured image by FelixMittermeier via Pixabay ]

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The Origin of the Universe, Earth, and Life

The term "evolution" usually refers to the biological evolution of living things. But the processes by which planets, stars, galaxies, and the universe form and change over time are also types of "evolution." In all of these cases there is change over time, although the processes involved are quite different.

In the late 1920s the American astronomer Edwin Hubble made a very interesting and important discovery. Hubble made observations that he interpreted as showing that distant stars and galaxies are receding from Earth in every direction. Moreover, the velocities of recession increase in proportion with distance, a discovery that has been confirmed by numerous and repeated measurements since Hubble's time. The implication of these findings is that the universe is expanding.

Hubble's hypothesis of an expanding universe leads to certain deductions. One is that the universe was more condensed at a previous time. From this deduction came the suggestion that all the currently observed matter and energy in the universe were initially condensed in a very small and infinitely hot mass. A huge explosion, known as the Big Bang, then sent matter and energy expanding in all directions.

This Big Bang hypothesis led to more testable deductions. One such deduction was that the temperature in deep space today should be several degrees above absolute zero. Observations showed this deduction to be correct. In fact, the Cosmic Microwave Background Explorer (COBE) satellite launched in 1991 confirmed that the background radiation field has exactly the spectrum predicted by a Big Bang origin for the universe.

As the universe expanded, according to current scientific understanding, matter collected into clouds that began to condense and rotate, forming the forerunners of galaxies. Within galaxies, including our own Milky Way galaxy, changes in pressure caused gas and dust to form distinct clouds. In some of these clouds, where there was sufficient mass and the right forces, gravitational attraction caused the cloud to collapse. If the mass of material in the cloud was sufficiently compressed, nuclear reactions began and a star was born.

Some proportion of stars, including our sun, formed in the middle of a flattened spinning disk of material. In the case of our sun, the gas and dust within this disk collided and aggregated into small grains, and the grains formed into larger bodies called planetesimals ("very small planets"), some of which reached diameters of several hundred kilometers. In successive stages these planetesimals coalesced into the nine planets and their numerous satellites. The rocky planets, including Earth, were near the sun, and the gaseous planets were in more distant orbits.

The ages of the universe, our galaxy, the solar system, and Earth can be estimated using modem scientific methods. The age of the universe can be derived from the observed relationship between the velocities of and the distances separating the galaxies. The velocities of distant galaxies can be measured very accurately, but the measurement of distances is more uncertain. Over the past few decades, measurements of the Hubble expansion have led to estimated ages for the universe of between 7 billion and 20 billion years, with the most recent and best measurements within the range of 10 billion to 15 billion years.

A disk of dust and gas, appearing as a dark band in this Hubble Space Telescope photograph, bisects a glowing nebula around a very young star in the constellation Taurus. Similar disks can be seen around other nearby stars and are thought to provide the (more...)

The age of the Milky Way galaxy has been calculated in two ways. One involves studying the observed stages of evolution of different-sized stars in globular clusters. Globular clusters occur in a faint halo surrounding the center of the Galaxy, with each cluster containing from a hundred thousand to a million stars. The very low amounts of elements heavier than hydrogen and helium in these stars indicate that they must have formed early in the history of the Galaxy, before large amounts of heavy elements were created inside the initial generations of stars and later distributed into the interstellar medium through supernova explosions (the Big Bang itself created primarily hydrogen and helium atoms). Estimates of the ages of the stars in globular clusters fall within the range of 11 billion to 16 billion years.

A second method for estimating the age of our galaxy is based on the present abundances of several long-lived radioactive elements in the solar system. Their abundances are set by their rates of production and distribution through exploding supernovas. According to these calculations, the age of our galaxy is between 9 billion and 16 billion years. Thus, both ways of estimating the age of the Milky Way galaxy agree with each other, and they also are consistent with the independently derived estimate for the age of the universe.

Radioactive elements occurring naturally in rocks and minerals also provide a means of estimating the age of the solar system and Earth. Several of these elements decay with half lives between 700 million and more than 100 billion years (the half life of an element is the time it takes for half of the element to decay radioactively into another element). Using these time-keepers, it is calculated that meteorites, which are fragments of asteroids, formed between 4.53 billion and 4.58 billion years ago (asteroids are small "planetoids" that revolve around the sun and are remnants of the solar nebula that gave rise to the sun and planets). The same radioactive time-keepers applied to the three oldest lunar samples returned to Earth by the Apollo astronauts yield ages between 4.4 billion and 4.5 billion years, providing minimum estimates for the time since the formation of the moon.

The oldest known rocks on Earth occur in northwestern Canada (3.96 billion years), but well-studied rocks nearly as old are also found in other parts of the world. In Western Australia, zircon crystals encased within younger rocks have ages as old as 4.3 billion years, making these tiny crystals the oldest materials so far found on Earth.

The best estimates of Earth's age are obtained by calculating the time required for development of the observed lead isotopes in Earth's oldest lead ores. These estimates yield 4.54 billion years as the age of Earth and of meteorites, and hence of the solar system.

The origins of life cannot be dated as precisely, but there is evidence that bacteria-like organisms lived on Earth 3.5 billion years ago, and they may have existed even earlier, when the first solid crust formed, almost 4 billion years ago. These early organisms must have been simpler than the organisms living today. Furthermore, before the earliest organisms there must have been structures that one would not call "alive" but that are now components of living things. Today, all living organisms store and transmit hereditary information using two kinds of molecules: DNA and RNA. Each of these molecules is in turn composed of four kinds of subunits known as nucleotides. The sequences of nucleotides in particular lengths of DNA or RNA, known as genes, direct the construction of molecules known as proteins, which in turn catalyze biochemical reactions, provide structural components for organisms, and perform many of the other functions on which life depends. Proteins consist of chains of subunits known as amino acids. The sequence of nucleotides in DNA and RNA therefore determines the sequence of amino acids in proteins; this is a central mechanism in all of biology.

Experiments conducted under conditions intended to resemble those present on primitive Earth have resulted in the production of some of the chemical components of proteins, DNA, and RNA. Some of these molecules also have been detected in meteorites from outer space and in interstellar space by astronomers using radio-telescopes. Scientists have concluded that the "building blocks of life" could have been available early in Earth's history.

An important new research avenue has opened with the discovery that certain molecules made of RNA, called ribozymes, can act as catalysts in modem cells. It previously had been thought that only proteins could serve as the catalysts required to carry out specific biochemical functions. Thus, in the early prebiotic world, RNA molecules could have been "autocatalytic"—that is, they could have replicated themselves well before there were any protein catalysts (called enzymes).

Laboratory experiments demonstrate that replicating autocatalytic RNA molecules undergo spontaneous changes and that the variants of RNA molecules with the greatest autocatalytic activity come to prevail in their environments. Some scientists favor the hypothesis that there was an early "RNA world," and they are testing models that lead from RNA to the synthesis of simple DNA and protein molecules. These assemblages of molecules eventually could have become packaged within membranes, thus making up "protocells"—early versions of very simple cells.

For those who are studying the origin of life, the question is no longer whether life could have originated by chemical processes involving nonbiological components. The question instead has become which of many pathways might have been followed to produce the first cells.

Will we ever be able to identify the path of chemical evolution that succeeded in initiating life on Earth? Scientists are designing experiments and speculating about how early Earth could have provided a hospitable site for the segregation of molecules in units that might have been the first living systems. The recent speculation includes the possibility that the first living cells might have arisen on Mars, seeding Earth via the many meteorites that are known to travel from Mars to our planet.

Of course, even if a living cell were to be made in the laboratory, it would not prove that nature followed the same pathway billions of years ago. But it is the job of science to provide plausible natural explanations for natural phenomena. The study of the origin of life is a very active research area in which important progress is being made, although the consensus among scientists is that none of the current hypotheses has thus far been confirmed. The history of science shows that seemingly intractable problems like this one may become amenable to solution later, as a result of advances in theory, instrumentation, or the discovery of new facts.

Creationist Views of the Origin of the Universe, Earth, and Life

Many religious persons, including many scientists, hold that God created the universe and the various processes driving physical and biological evolution and that these processes then resulted in the creation of galaxies, our solar system, and life on Earth. This belief, which sometimes is termed "theistic evolution," is not in disagreement with scientific explanations of evolution. Indeed, it reflects the remarkable and inspiring character of the physical universe revealed by cosmology, paleontology, molecular biology, and many other scientific disciplines.

The advocates of "creation science" hold a variety of viewpoints. Some claim that Earth and the universe are relatively young, perhaps only 6,000 to 10,000 years old. These individuals often believe that the present physical form of Earth can be explained by "catastrophism," including a worldwide flood, and that all living things (including humans) were created miraculously, essentially in the forms we now find them.

Other advocates of creation science are willing to accept that Earth, the planets, and the stars may have existed for millions of years. But they argue that the various types of organisms, and especially humans, could only have come about with supernatural intervention, because they show "intelligent design."

In this booklet, both these "Young Earth" and "Old Earth" views are referred to as "creationism" or "special creation."

There are no valid scientific data or calculations to substantiate the belief that Earth was created just a few thousand years ago. This document has summarized the vast amount of evidence for the great age of the universe, our galaxy, the solar system, and Earth from astronomy, astrophysics, nuclear physics, geology, geochemistry, and geophysics. Independent scientific methods consistently give an age for Earth and the solar system of about 5 billion years, and an age for our galaxy and the universe that is two to three times greater. These conclusions make the origin of the universe as a whole intelligible, lend coherence to many different branches of science, and form the core conclusions of a remarkable body of knowledge about the origins and behavior of the physical world.

Nor is there any evidence that the entire geological record, with its orderly succession of fossils, is the product of a single universal flood that occurred a few thousand years ago, lasted a little longer than a year, and covered the highest mountains to a depth of several meters. On the contrary, intertidal and terrestrial deposits demonstrate that at no recorded time in the past has the entire planet been under water. Moreover, a universal flood of sufficient magnitude to form the sedimentary rocks seen today, which together are many kilometers thick, would require a volume of water far greater than has ever existed on and in Earth, at least since the formation of the first known solid crust about 4 billion years ago. The belief that Earth's sediments, with their fossils, were deposited in an orderly sequence in a year's time defies all geological observations and physical principles concerning sedimentation rates and possible quantities of suspended solid matter.

Geologists have constructed a detailed history of sediment deposition that links particular bodies of rock in the crust of Earth to particular environments and processes. If petroleum geologists could find more oil and gas by interpreting the record of sedimentary rocks as having resulted from a single flood, they would certainly favor the idea of such a flood, but they do not. Instead, these practical workers agree with academic geologists about the nature of depositional environments and geological time. Petroleum geologists have been pioneers in the recognition of fossil deposits that were formed over millions of years in such environments as meandering rivers, deltas, sandy barrier beaches, and coral reefs.

The example of petroleum geology demonstrates one of the great strengths of science. By using knowledge of the natural world to predict the consequences of our actions, science makes it possible to solve problems and create opportunities using technology. The detailed knowledge required to sustain our civilization could only have been derived through scientific investigation.

The arguments of creationists are not driven by evidence that can be observed in the natural world. Special creation or supernatural intervention is not subjectable to meaningful tests, which require predicting plausible results and then checking these results through observation and experimentation. Indeed, claims of "special creation" reverse the scientific process. The explanation is seen as unalterable, and evidence is sought only to support a particular conclusion by whatever means possible.

• Cite this Page National Academy of Sciences (US). Science and Creationism: A View from the National Academy of Sciences: Second Edition. Washington (DC): National Academies Press (US); 1999. The Origin of the Universe, Earth, and Life.
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The sun was born when a dense gas cloud collapsed, 4.6 billion years ago

Associate Professor, Department of Earth Sciences, Carleton University

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Hanika Rizo receives funding from NSERC.

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While the upcoming total solar eclipse is a special moment to reflect on our place in the universe, scientists have been studying the birth of the sun and the formation of our solar system for a long time.

Our solar system today is mainly composed of a central star — the sun — along with an inner solar system with rocky planets, and an outer solar system with gas and ice giant planets. However, it hasn’t always been that way.

How was the sun formed?

Our solar system formed from the gravitational collapse of a “dense” giant molecular cloud of gas and dust, composed mainly of hydrogen, a bit of helium, and about one per cent of heavier elements. After the cloud collapsed, the majority of the mass concentrated onto the centre, creating our sun.

The star continued to contract until it reached its final size and density. Hydrogen fusion ignited the sun’s core, causing the star to emit light and heat.

Around the sun, the leftovers — about 0.5 to one per cent of the mass of the sun — created a protoplanetary disk, where planets subsequently formed.

Protoplanetary disks in the process of making planets are not just theory — they have actually been observed, such as the disk around HL Tauri , a young star with rings and gaps that are likely signs of forming planets.

We have a pretty good idea of when that collapse took place in our solar system because we can analyze the first (or oldest) solids that condensed out from the protoplanetary disk gas. This detailed analysis is only possible in our solar system, since we cannot directly collect material from other solar systems.

These solid fragments, called calcium-aluminum rich inclusions (CAIs), have been found in some of the oldest meteorites, and age-dated to 4,567.3 million years . This is when our solar system came into being, and provides the age for the birth of our sun.

Element factories

Very dense molecular clouds can collapse due to their own gravity . However, the collapse of our protosolar nebula was likely triggered by the perturbation from the passing shock wave of an exploding massive star, called a supernova . This shock wave compressed enough of the molecular cloud to start collapsing it, and form a central star and a planetary disk around it.

The evidence for this hypothesis is found in the isotope composition of some chemical elements in pre-solar grains. Pre-solar grains are tiny silicon-carbide minerals (under a micrometre in size), and can be found in parts per million quantities in some meteorites. These pre-solar grains have isotope compositions that cannot be explained by chemical or physical processes occurring in our solar system, and are better explained by these grains forming elsewhere .

The isotope composition of pre-solar grains implies that, after the supernova, these grains travelled into space, and they got trapped into our molecular cloud, which then collapsed, keeping those grains inside the meteorites that we study today.

How much older is the sun than the Earth?

The age of 4,567 million years found for the CAIs is often used as the age of the Earth. However, after the formation of CAIs, it likely took tens to a few hundreds of millions of years for Earth to form. Although we have determined the age of our solar system very precisely, debates still persist regarding the age of our own planet Earth.

The challenge comes from the fact that the Earth is an active planet, and is very efficient at recycling and reworking its oldest rocks, resetting their geochronological information.

More than 98 per cent of the proto-Earth’s mass might have been already melded together by the time a giant impact hit the proto-Earth . That giant impact added the remaining two per cent to Earth, and also led to the formation of our moon .

The giant impact, occurring somewhere between 70 to 120 million years after the CAIs formation, could provide the best determination for the age of the Earth. Independent age estimates can also be obtained from estimating the timing of Earth’s magma ocean solidification, a consequence of the moon-forming giant impact.

Studies attempting to determine the timing of magma ocean solidification provide ages between 100 and 150 million years after the birth of the sun .

The upcoming total solar eclipse is an opportunity for everyone to appreciate the wonders of our solar system, which took about 4.6 billion years to evolve.

It is truly a cosmic coincidence that total solar eclipses can be seen on Earth : the sun happens to be about 400 times larger than the moon, which is 400 times closer than the sun.

If you were on Mars or Venus, you would not be so lucky as to witness this phenomenon!

Johanna Teske of the Carnegie Institution for Science contributed to writing this article. She is a staff scientist, and researches the compositions of exoplanets.

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3 Body Problem: Is the Universe Really a ‘Dark Forest’ Full of Hostile Aliens in Hiding?

We have no good reason to believe that aliens have ever contacted Earth. Sure, there are conspiracy theories, and some rather strange reports about harm to cattle , but nothing credible. Physicist Enrico Fermi found this odd. His formulation of the puzzle, proposed in the 1950s and now known as the Fermi Paradox , is still key to the search for extraterrestrial life (SETI) and messaging by sending signals into space (METI).

The Earth is about 4.5 billion years old, and life is at least 3.5 billion years old. The paradox states that, given the scale of the universe, favorable conditions for life are likely to have occurred many, many times. So where is everyone? We have good reasons to believe that there must be life out there, but nobody has come to call.

This is an issue that the character Ye Wenjie wrestles with in the first episode of Netflix’s 3 Body Problem . Working at a radio observatory, she does finally receive a message from a member of an alien civilization—telling her they are a pacifist and urging her not to respond to the message or Earth will be attacked.

The series will ultimately offer a detailed, elegant solution to the Fermi Paradox, but we will have to wait until the second season.

Or you can read the second book in Cixin Liu’s series, The Dark Forest . Without spoilers, the explanation set out in the books runs as follows: “The universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound .”

Ultimately, everybody is hiding from everyone else. Differential rates of technological progress make an ongoing balance of power impossible, leaving the most rapidly progressing civilizations in a position to wipe out anyone else.

In this ever-threatening environment, those who play the survival game best are the ones who survive longest. We have joined a game which has been going on before our arrival, and the strategy that everyone has learned is to hide. Nobody who knows the game will be foolish enough to contact anyone—or to respond to a message.

Liu has depicted what he calls “the worst of all possible universes,”  continuing a trend within Chinese science fiction. He is not saying that our universe is an actual dark forest, with one survival strategy of silence and predation prevailing everywhere, but that such a universe is possible and interesting.

Liu’s dark forest theory is also sufficiently plausible to have reinforced a trend in the scientific discussion in the west—away from worries about mutual incomprehensibility, and towards concerns about direct threat.

We can see its potential influence in the protocol for what to do on first contact that was proposed in 2020 by the prominent astrobiologists Kelly Smith and John Traphagan. “First, do nothing,” they conclude, because doing something could lead to disaster .

In the case of alien contact, Earth should be notified using pre-established signaling rather than anything improvised, they argue. And we should avoid doing anything that might disclose information about who we are. Defensive behavior would show our familiarity with conflict, so that would not be a good idea. Returning messages would give away the location of Earth—also a bad idea.

Again, the Smith and Traphagan thought is not that the dark forest theory is correct. Benevolent aliens really could be out there. The thought is simply that first contact would involve a high civilization-level risk.

This is different from assumptions from a great deal of Russian literature about space of the Soviet era , which suggested that advanced civilizations would necessarily have progressed beyond conflict, and would therefore share a comradely attitude. This no longer seems to be regarded as a plausible guide to protocols for contact.

Misinterpreting Darwin

The interesting thing is that the dark forest theory is almost certainly wrong. Or at least, it is wrong in our universe. It sets up a scenario in which there is a Darwinian process of natural selection, a competition for survival.

Charles Darwin’s account of competition for survival is evidence-based. By contrast, we have absolutely no evidence about alien behavior, or about competition within or between other civilizations. This makes for entertaining guesswork rather than good science, even if we accept the idea that natural selection could operate at group level , at the level of civilizations.

Even if you were to assume the universe did operate in accordance with Darwinian evolution, the argument is questionable. No actual forest is like the dark one. They are noisy places where co-evolution occurs .

Creatures evolve together, in mutual interdependence, and not alone. Parasites depend upon hosts, flowers depend upon birds for pollination. Every creature in a forest depends upon insects. Mutual connection does lead to encounters which are nasty, brutish and short, but it also takes other forms. That is how forests in our world work.

Interestingly, Liu acknowledges this interdependence as a counterpoint to the dark forest theory. The viewer, and the reader, are told repeatedly that “in nature, nothing exists alone”—a quote from Rachel Carson’s Silent Spring (1962). This is a text which tells us that bugs can be our friends and not our enemies.

In Liu’s story, this is used to explain why some humans immediately go over to the side of the aliens, and why the urge to make contact is so strong, in spite of all the risks. Ye Wenjie ultimately replies to the alien warning.

The Carson allusions do not reinstate the old Russian idea that aliens will be advanced and therefore comradely. But they do help to paint a more varied and realistic picture than the dark forest theory.

For this reason, the dark forest solution to the Fermi Paradox is unconvincing. The fact that we do not hear anyone is just as likely to indicate that they are too far off, or we are listening in all the wrong ways, or else that there is no forest and nothing else to be heard.

This article is republished from The Conversation under a Creative Commons license. Read the original article .

Image Credit: ESO /A. Ghizzi Panizza ( www.albertoghizzipanizza.com )

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