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This does not necessarily mean the universe was physically small at the Big Bang, although that is indeed one of the possibilities. Going forward, this provides a model of our universe which matches all current physical observations extremely closely. This initial period of the Universe's chronology is called the " Big Bang ". The standard model of cosmology does not attempt to explain why the universe began to exist; it explains only how the universe physically developed once that moment happened.

We interpret the singularity from the FLRW metric as meaning that current theories are inadequate to describe what actually happened at the start of the Big Bang itself.

It is widely believed that a correct theory of quantum gravity may allow a more correct description of that event, but no such theory has yet been developed. After that moment, all distances throughout the universe began to increase from perhaps zero because the FLRW metric itself changed over time, affecting distances between all non-bound objects everywhere. For this reason we say that the Big Bang "happened everywhere".

During the very earliest moments of cosmic time, the energies and conditions were so extreme that our current knowledge can only suggest possibilities, so our current knowledge may turn out to be incorrect. To give one example, eternal inflation theories propose that inflation lasts forever throughout most of the universe, making the notion of "N seconds since Big Bang" ill-defined. Therefore the earliest stages are an active area of research and based on ideas which are still speculative and subject to modification as scientific knowledge improves. The inflationary period marks a specific period when a very rapid change in scale occurred, but does not mean that it stayed the same at other times.

More precisely, during inflation, the expansion accelerated; then, after inflation and for about 9. Initially, the universe was inconceivably hot and dense. It has cooled over time, which eventually allowed the forces, particles and structures we see around us to manifest as they do today. The Planck epoch is an era in traditional non-inflationary Big Bang cosmology immediately after the event which began our known universe.

During this epoch, the temperature and average energies within the universe were so high that everyday subatomic particles could not form, and even the four fundamental forces that shape our universe— electromagnetism , gravitation , weak nuclear interaction , and strong nuclear interaction —were combined and formed one fundamental force. Little is understood about physics at this temperature; different hypotheses propose different scenarios.

Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory relies on the theory of general relativity , which is thought to break down for this epoch due to quantum effects. Models that aim to describe the universe and physics during the Planck epoch are generally speculative and fall under the umbrella of " New Physics ".

Examples include the Hartle—Hawking initial state , string landscape , string gas cosmology , and the ekpyrotic universe. As the universe expanded and cooled, it crossed transition temperatures at which forces separated from each other. These phase transitions can be visualised as similar to condensation and freezing phase transitions of ordinary matter. This is not apparent in everyday life, because it only happens at far higher temperatures than we usually see in our present universe.

These phase transitions in the universe's fundamental forces are believed to be caused by a phenomenon of quantum fields called " symmetry breaking ". In everyday terms, as the universe cools, it becomes possible for the quantum fields that create the forces and particles around us, to settle at lower energy levels and with higher levels of stability.


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In doing so, they completely shift how they interact. Forces and interactions arise due to these fields, so the universe can behave very differently above and below a phase transition. For example, in a later epoch, a side effect of one phase transition is that suddenly, many particles that had no mass at all acquire a mass they begin to interact differently with the Higgs field , and a single force begins to manifest as two separate forces.

Assuming that nature is described by a Grand Unified Theory , the grand unification epoch began with a phase transitions of this kind, when gravitation separated from the universal combined gauge force. This caused two forces to now exist: gravity, and an electrostrong interaction.

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There is no hard evidence yet, that such a combined force existed, but many physicists believe it did. The physics of this electrostrong interaction would be described by a so-called grand unified theory GUT.

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The grand unification epoch ended with a second phase transition, as the electrostrong interaction in turn separated, and began to manifest as two separate interactions, called the strong and electroweak interactions. Depending on how epochs are defined, and the model being followed, the electroweak epoch may be considered to start before or after the inflationary epoch.

In some models it is described as including the inflationary epoch. The electroweak interaction will also separate later, dividing into the electromagnetic and weak interactions. The exact point where electrostrong symmetry was broken is not certain, because of the very high energies of this event. At this point of the very early universe, the metric that defines distance within space, suddenly and very rapidly changed in scale , leaving the early universe at least 10 78 times its previous volume and possibly much more.

This change is known as inflation. Although light and objects within spacetime cannot travel faster than the speed of light , in this case it was the metric governing the size and geometry of spacetime itself that changed in scale. Changes to the metric are not limited by the speed of light.

There is good evidence that this happened, and it is widely accepted that it did take place. But the exact reasons why it happened are still being explored. So a range of models exist that explain why and how it took place - it is not yet clear which explanation is correct. In several of the more prominent models, it is thought to have been triggered by the separation of the strong and electroweak interactions which ended the grand unification epoch. One of the theoretical products of this phase transition was a scalar field called the inflaton field.

As this field settled into its lowest energy state throughout the universe, it generated an enormous repulsive force that led to a rapid expansion of the metric that defines space itself. Inflation explains several observed properties of the current universe that are otherwise difficult to account for, including explaining how today's universe has ended up so exceedingly homogeneous similar on a very large scale, even though it was highly disordered in its earliest stages.

The rapid expansion of space meant that elementary particles remaining from the grand unification epoch were now distributed very thinly across the universe. However, the huge potential energy of the inflation field was released at the end of the inflationary epoch, as the inflaton field decayed into other particles, known as "reheating".

This heating effect led to the universe being repopulated with a dense, hot mixture of quarks, anti-quarks and gluons. In other models, reheating is often considered to mark the start of the electroweak epoch , and some theories, such as warm inflation , avoid a reheating phase entirely. Therefore, in inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation.

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After inflation ended, the universe continued to expand, but at a much slower rate. About 4 billion years ago the expansion gradually began to speed up again. This is believed to be due to dark energy becoming dominant in the universe's large-scale behavior. It is still expanding today. On March 17, , astrophysicists of the BICEP2 collaboration announced the detection of inflationary gravitational waves in the B-mode power spectrum which was interpreted as clear experimental evidence for the theory of inflation. As the universe's temperature continued to fall below a certain very high energy level, a third symmetry breaking occurs.

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So far as we currently know, it was the final symmetry breaking event in the formation of our universe. It is believed that below some energies unknown yet, [21] [ verification needed ] the Higgs field spontaneously acquires a vacuum expectation value. When this happens, it breaks electroweak gauge symmetry. This has two related effects:. After electroweak symmetry breaking, the fundamental interactions we know of — gravitation , electromagnetism , the strong interaction and the weak interaction — have all taken their present forms, and fundamental particles have their expected masses, but the temperature of the universe is still too high to allow the stable formation of many particles we now see in the universe, so there are no protons or neutrons , and therefore no atoms , atomic nuclei , or molecules.

More exactly, any composite particles that form by chance, almost immediately break up again due to the extreme energies. If supersymmetry is a property of our universe, then it must be broken at an energy that is no lower than 1 TeV , the electroweak scale.

In Search of the Big Bang

The masses of particles and their superpartners would then no longer be equal. This very high energy could explain why no superpartners of known particles have ever been observed. After cosmic inflation ends, the universe is filled with a hot quark—gluon plasma , the remains of reheating. From this point onwards the physics of the early universe is much better understood, and the energies involved in the Quark epoch are directly accessible in particle physics experiments and other detectors.

This was the period in the evolution of the early universe immediately after electroweak symmetry breaking , when the fundamental interactions of gravitation , electromagnetism , the strong interaction and the weak interaction had taken their present forms, but the temperature of the universe was still too high to allow quarks to bind together to form hadrons. During the quark epoch the universe was filled with a dense, hot quark—gluon plasma , containing quarks, leptons and their antiparticles.

Collisions between particles were too energetic to allow quarks to combine into mesons or baryons. Baryons are subatomic particles such as protons and neutrons , that are composed of three quarks. It would be expected that both baryons, and particles known as antibaryons would have formed in equal numbers. However, this does not seem to be what happened — as far as we know, the universe was left with far more baryons than antibaryons.

In fact, almost no antibaryons are observed in nature. It is not clear how this came about.


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  • Any explanation for this phenomenon must allow the Sakharov conditions related to baryogenesis to have been satisfied at some time after the end of cosmological inflation. Current particle physics suggests asymmetries under which these conditions would be met, but these asymmetries appear to be too small to account for the observed baryon-antibaryon asymmetry of the universe. The quark—gluon plasma that composes the universe cools until hadrons , including baryons such as protons and neutrons , can form. A comparatively small residue of hadrons remained at about 1 second of cosmic time, when this epoch ended.