Big Bang

The Big Bang is a physical theory that explains how the universe expanded from a very early state that was extremely dense and hot. Many cosmological models use this idea to explain several things. These include the amounts of light elements, the cosmic microwave background (CMB) radiation, the redshift of galaxies, and the large-scale structure of the universe.

The universe looks very even in every direction. This leads to the horizon and flatness problems. Scientists explain these with cosmic inflation, a very fast expansion that happened in the earliest moments. Careful measurements of how fast the universe is expanding suggest that cosmic inflation happened about 13.787±0.02 billion years ago. This is considered the age of the universe. A wide range of evidence strongly supports the Big Bang model, which is now widely accepted.

Using the known laws of physics, scientists can run the expansion backward in time. This shows a very hot and dense early universe. The first version of the model pointed back to a Big Bang singularity. The modern view includes inflation, which started the hot Big Bang. As the universe cooled, subatomic particles and later atoms could form. These early elements were mostly hydrogen, with some helium and lithium. They came together because of gravity, with help from dark matter, to form early stars and galaxies. Measurements of supernovae show that the expansion of the universe is accelerating. Scientists link this to an idea called dark energy.

The idea of an expanding universe was introduced in 1922 by the physicist Alexander Friedmann, using the mathematics of the Friedmann equations. The earliest observation of an expanding universe is known as Hubble's law. It was published in 1929 by physicist Edwin Hubble. He found that galaxies move away from Earth faster the farther away they are. Independent of Friedmann's work and Hubble's observations, physicist Georges Lemaître proposed in 1931 that the universe came from a "primeval atom." This was the start of the modern idea of the Big Bang. In 1964, the CMB was discovered by accident. Later measurements supported the Big Bang models. By the late 1960s, most cosmologists were convinced that the competing steady-state model was wrong.

Some parts of the universe are still not fully explained by the Big Bang models. These include the unequal amounts of matter and antimatter, known as baryon asymmetry. They also include the detailed nature of dark matter around galaxies, and the origin of dark energy.

For other uses, see Big Bang (disambiguation) and Big Bang Theory (disambiguation).

Features of the models

Big Bang models rest on three main ideas. The first is that physical laws are the same everywhere. The second is the cosmological principle, and the third is that the universe's matter can be modeled as a perfect fluid. The first idea is one of the basic ideas behind the theory of relativity. The cosmological principle says that on very large scales, the universe is homogeneous and isotropic. This means it looks the same in every direction from any place. A perfect fluid has no viscosity, and its pressure is related to its density. Scientists first took these ideas as starting points, but they have since tested them. Observations show that the fine-structure constant has hardly changed over most of the universe's age. General relativity, the main physical law behind these models, has passed strict tests in the Solar System and among binary stars. The cosmological principle has been confirmed to a high level of accuracy using the temperature of the CMB. Across the scale of the CMB horizon, the universe was measured to be uniform, with an upper limit on the order of 10% unevenness, as of 1995.

Main article: Expansion of the universe

The cosmological principle makes the equations of general relativity much simpler. This gives the Friedmann–Lemaître–Robertson–Walker metric, which describes the shape of the universe. It also gives the Friedmann equations, which show how that shape changes over time. At this level, the only key value is the mass–energy density. This means the geometry of the universe and its expansion follow directly from its density. In Big Bang cosmology, mass–energy density controls the shape and change of the universe. Using astronomy, thermodynamics, and particle physics, cosmologists have worked out how this density has changed over the universe's life. Today, luminous matter, such as stars and planets, makes up less than 5% of the density. Dark matter accounts for 27%, and dark energy makes up the remaining 68%.

Main article: Cosmological horizon

An important feature of Big Bang spacetime is the presence of particle horizons. The universe has a limited age, and light travels at a limited speed. So some events in the past may not yet have had time for their light to reach Earth. This sets a past horizon, which limits the most distant objects we can see. Space is expanding, so distant objects are moving away faster and faster. Light sent out today may never catch up to them. This creates a future horizon, which limits the events we will be able to influence. Which types of horizon exist depends on the details of the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. Some early processes in the universe happened too slowly to reach thermodynamic equilibrium. Others were fast enough to reach thermalization. Scientists check this by comparing the rate of a process with the Hubble parameter.

Timeline

Main article: Chronology of the universe

Big Bang models propose that the universe has been growing bigger and cooler from a very hot and very small start. Modern ideas are based on a brief early period called inflation, when the universe became about 80 times larger in a very short time. Inflation helped create the large-scale structure of the universe from tiny quantum fluctuations. After inflation ended, the universe became hot and dense, filled with a plasma of Standard Model particles. As the universe cooled, baryogenesis left a little more matter than antimatter, which is why matter exists today. A few minutes in, Big Bang nucleosynthesis formed the first deuterium and helium nuclei. About 380,000 years later, recombination made the universe clear enough for light to travel freely. That light became the cosmic microwave background.

After the recombination era, slightly denser regions pulled in nearby matter. These regions grew into gas clouds, stars, and galaxies. How this happened depends on the kinds of matter in the universe, such as cold dark matter, warm dark matter, hot dark matter, and baryonic matter. This process is explained in more detail in Structure formation.

Evidence shows that today the universe is mostly made of dark energy. This is a mysterious kind of energy that seems to spread through all of space. Early on, gravity slowed the expansion, but after billions of years, the universe began to speed up. This is described in Accelerating expansion of the universe. What happened in the very first moments of the universe is still one of the greatest unsolved problems in physics.

Concept history

Main article: History of the Big Bang theory

See also: Timeline of cosmological theories

The name "Big Bang" did not become popular until the 1970s. Astronomer Fred Hoyle first used the term in a 1949 BBC Third Programme radio broadcast. He used it to compare the idea with his own steady-state model. Hoyle said the name was just a catchy picture, not an insult.

Early ideas came from watching the sky. In 1912, Vesto Slipher found that most spiral nebulae were moving away from Earth. In 1924, Edwin Hubble showed that these nebulae were other galaxies. In 1929, he found a link between how far away a galaxy is and how fast it moves. Georges Lemaître suggested that the universe was growing bigger. He also suggested that, looking back in time, all of its matter was once packed into a single point.

After World War II, there were two main ideas. One was the steady-state model. The other was Lemaître's idea of an expanding universe, which was developed by George Gamow. Over time, evidence pointed more toward the Big Bang. In 1964, scientists discovered the cosmic microwave background, and this made the Big Bang the best explanation for how the universe began and changed.

The Big Bang model was first traced back to a Big Bang singularity, an early point of very high heat and density. In 1981, Alan Guth proposed a very fast early growth of the universe called "inflation." This idea helped explain some puzzles, such as why the universe looks so even and flat. Since the late 1990s, better telescopes and satellites have helped scientists measure things more carefully. They also found that the universe's expansion seems to be speeding up.

Observational evidence

Main articles: Hubble's law and Expansion of the universe

See also: Distance measures (cosmology) and Scale factor (cosmology)

Big Bang models help explain many things that scientists have seen, including the amounts of light elements, the cosmic microwave background, large-scale structure, and Hubble's law. The first clue was that the universe is getting bigger. We can tell because galaxies appear to move away from us. Scientists also found the cosmic microwave background and measured the amounts of light elements made by Big Bang nucleosynthesis. Newer clues come from the study of galaxy formation and evolution and the way large-scale cosmic structures are spread out. Together, these are sometimes called the "four pillars" of the Big Bang models. Distant quasars and galaxies also appear to move away from us, and the farther away they are, the faster they seem to move. The cosmic microwave background looks almost the same in every direction. It was discovered in 1964 by Arno Penzias and Robert Wilson.

Some parts of modern Big Bang models use strange ideas that have not yet been seen in laboratory experiments. Dark matter is the idea that scientists are studying most actively right now. There are other puzzles too, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter. Scientists are also very interested in Hubble tension, which is a disagreement between different ways of measuring how fast the universe is growing. Dark energy is another big area of study, but it is not yet clear whether it can be detected directly. These are still unsolved problems in physics.

Problems and related issues in physics

See also: List of unsolved problems in physics

Like any scientific theory, the Big Bang models have raised puzzles. Some puzzles have been solved, but others are still open. Solving some puzzles has also led to new questions. The horizon problem, the magnetic monopole problem, and the flatness problem are mostly solved by inflation theory. However, the details of inflation are still not understood. Many scientists, including some of its founders, say it has been shown to be wrong. Astrophysicists and cosmologists are still studying these puzzles.

Main article: Baryon asymmetry

Scientists do not yet know why the universe has more matter than antimatter. They generally think the young, very hot universe had equal amounts of both. They believe these amounts destroyed each other, leaving only the matter we see today.

Main article: Dark energy

Measurements of type Ia supernovae show that the universe's expansion has been speeding up. Scientists suggest that a mysterious form of energy called "dark energy" may explain this. Its exact nature is still one of the great mysteries of the Big Bang.

Main article: Dark matter

Observations from the 1970s and 1980s showed that there is not enough visible matter to explain the strength of gravity within and between galaxies. This led scientists to the idea of dark matter. Dark matter does not give off light, and it may make up much of the universe's matter. Scientists have not yet seen dark matter particles directly. However, they can see its effects through gravity.

Misconceptions

Some people also misunderstand the Big Bang model, which is related to confusion about the nature of cosmic expansion. One common mistake is thinking that the Big Bang model fully explains the origin of the universe. It only describes the universe when it was very hot and dense. Inflation happened before the Big Bang. The Big Bang began only after the energy that drove inflation turned into a hot plasma during reheating.^{: 8}

Another mistake is about the speeds in Hubble's law. These are not speeds in the usual sense, and they are not related to the spatial parts of 4-velocities. So it is not surprising that, under Hubble's law, galaxies beyond the Hubble distance seem to move away faster than the speed of light. These speeds do not mean that anything can travel faster-than-light. The Big Bang was also not an explosion of one point into empty space. Expansion happens everywhere, not from any single point. For this reason, the name Everywhere Stretch has been suggested. It shows that every point in the universe expands because of general relativity.^{: 109}

Implications

Scientists can only predict the future of the universe for a limited time. That time is much longer than the universe's current age, but after that, the ideas are more guesswork. The origin of the universe is also still a matter of conjecture. Lemaître called the early hot, dense state the "primeval atom", and Gamow called it "ylem". Some models of inflation suggest that certain combinations of laws of nature are much more likely than others. This may partly explain why the universe is fairly stable. Another idea is a multiverse, where every possible universe exists. Only stable ones could allow thinking beings to appear. In a flat universe, gravitational potential energy balances other forms of energy, so no extra energy needs to be created. Current theories do not work at the very start of the universe, so scientists need a theory of quantum gravity. Some approaches, such as the Wheeler–DeWitt equation, suggest that time may be an emergent property. If so, time may not have existed before the Big Bang. Since nothing is known about what came before inflation, there is much speculation about "cosmogony". One idea is that quantum fluctuations caused the Big Bang. This had a very small chance of happening. However, according to the totalitarian principle, even the most unlikely event will eventually happen.

Other ideas include Emergent Universe models. These have a quiet past before inflation, which is somewhat like old ideas of a cosmic egg and the world's birth out of primordial chaos. Models of a finite spacetime include the Hartle–Hawking no-boundary condition. Brane cosmology models link inflation to moving branes in string theory. The ekpyrotic model describes the Big Bang as a collision of branes. The cyclic model says these collisions happen again and again. Eternal inflation suggests that inflation ends at random points. Each of these points forms a bubble universe with its own big bang.

Before scientists discovered dark energy, they thought a dense enough universe might collapse in a Big Crunch. A less dense universe might keep expanding forever. In that case, stars would burn out and leave white dwarfs, neutron stars, and black holes. Temperatures would slowly drop toward absolute zero in a Big Freeze. If protons are unstable, matter would disappear. Black holes would eventually fade away through Hawking radiation. The entropy of the universe would keep rising until no useful energy was left. This idea is known as heat death. Today, the universe is expanding faster and faster. This means more of the visible universe will pass beyond an event horizon. The final result is not known. Some theories of phantom dark energy suggest that everything could eventually be torn apart in a Big Rip.

Main article: Ultimate fate of the universe

The Big Bang also has a major connection to religion and philosophy. It is one of the most lively areas of discussion between science and religion. Some people believe the Big Bang suggests a creator. Others argue that Big Bang cosmology makes the idea of a creator unnecessary.

Main article: Religious interpretations of the Big Bang theory