Timeline of the Big Bang
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This timeline of the Big Bang describes the events that have occurred and will occur according to the scientific theory of the Big Bang. Observations suggest that the universe as we know it began around 13.7 billion years ago. Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth. Therefore, while the basic features of this epoch have been worked out in the big bang theory, the details are largely based on educated guesses. Following this, in the early universe, the evolution of the universe proceeded according to known high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted. Finally, the epoch of structure formation began, when matter first started to aggregate into the first stars and quasars, and ultimately galaxies, clusters of galaxies and superclusters formed. The future of the universe is not yet firmly known.
The very early universe
All our understanding of the very early universe is very speculative. No accelerator experiments currently probe sufficiently high energies to provide insight into this period. Scenarios differ radically. Some ideas include the Hartle-Hawking initial state, string landscape, brane inflation, string gas cosmology, and the ekpyrotic universe. Some of these ideas are mutually compatible, others are not.
The Planck epoch – 10-43 seconds
If supersymmetry is correct, then at this time the four fundamental forces – electromagnetism, weak nuclear force, strong nuclear force and gravity – all have the same strength, and are unified into one fundamental force. Little else is known about this epoch, although different theories make different predictions. Einstein's theory of general relativity predicts a gravitational singularity before this time. Physicists hope that speculative theories of quantum gravity, such as string theory and loop quantum gravity, will eventually lead to a better understanding of the singularity, or even allow us to calculate what came before.
The Grand Unification Epoch – 10-33 seconds
As the universe expands and cools from the Planck epoch, gravity begins to separate from the fundamental gauge interactions: electromagnetism and the strong and weak nuclear forces. Physics at this scale may be described by a grand unified theory in which the gauge group of the Standard Model is embedded in a much larger group, which is broken to produce the observed forces of nature. Eventually, the grand unification is broken as the strong nuclear force separates from the electroweak force. This should produce magnetic monopoles.
Cosmic inflation
- Main article: Cosmic inflation
The temperature, and therefore the time, at which cosmic inflation occurs is not known. During inflation, the universe is flattened and the universe enters a homogeneous and isotropic rapidly expanding phase in which the seeds of structure formation are laid down in the form of a primordial spectrum of nearly-scale-invariant fluctuations.
Reheating
During reheating, the exponential expansion that occurred during inflation ceases and the potential energy of the inflaton field decays into a hot, relativistic plasma of particles. If grand unification is a feature of our universe, then cosmic inflation must occur during or after the grand unification symmetry is broken, otherwise magnetic monopoles would be seen in the visible universe. At this point, the universe is dominated by radiation.
Baryogenesis
No known physics can explain the fact that there are so many more baryons in the universe than antibaryons. In order for this to be explained, the Sakharov conditions must be met at some time after inflation. There are hints that this is possible in known physics and from studying grand unified theories, but the full picture is not known.
The early universe
At this time, the universe is filled with a quark-gluon plasma.
The electroweak epoch – 10-12 s
In electroweak symmetry breaking, all the fundamental particles are believed to acquire a mass via the Higgs mechanism in which the Higgs boson acquires a vacuum expectation value. At this time, neutrinos decouple and begin travelling freely through space. This cosmic neutrino background, while unlikely to ever be observed in detail, is analogous to the cosmic microwave background that was emitted much later.
Supersymmetry breaking
If supersymmetry is a property of our universe, then it must be broken at an energy as low as 1 TeV, the electroweak symmetry scale. The masses of particles and their superpartners would then no longer be equal, which could explain why no superpartners of known particles have ever been observed.
The hadron epoch – 10-6 s–10-2 s
The quark-gluon plasma which composes the universe cools until hadrons, including baryons such as protons and neutrons, can form.
Nucleosynthesis – 1 s
At this time, the universe is cool enough that atomic nuclei can begin to form. Protons (hydrogen ions) and neutrons begin to combine into atomic nuclei. At the end of nucleosynthesis, the universe has cooled to the point where nuclear fusion stops. At this time, there are about three times more hydrogen ions as helium-4 nuclei and only trace quantities of other nuclei.
Matter domination – 70,000 years
At this time, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) are equal. The Jeans length, which determines the smallest structures that can form (due to competition between gravitational attraction and pressure effects) begins to fall and perturbations, instead of being wiped out by radiation free-streaming, can begin to grow in amplitude.
Recombination – 500,000 years
Hydrogen and helium atoms begin to form and the density of the universe falls. These cause the photons to decouple from matter, and matter and radiation begins to evolve independently. Most importantly, this means that the photons that compose the cosmic microwave background are all emitted during this epoch.
Dark ages
In this epoch, very few atoms are ionized, so the only radiation emitted is the 21 cm spin line of neutral hydrogen. There is currently an observational effort underway to detect this faint radiation, as it is in principle an even more powerful tool than the cosmic microwave background for studying the early universe.
Structure formation
Structure formation in the big bang model proceeds hierarchically, with smaller structures forming before larger ones. The first structures to form are quasars, which are thought to be bright, early active galaxies and population III stars. Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory: that is, all structures could be understood as small deviations from a perfect homogeneous universe. This is computationally relatively easy to study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulations with billions of particles.
Reionization
The first quasars form from gravitational collapse. The intense radiation they emit reionizes the surrounding universe. From this point on, most of the universe is composed of plasma.
Formation of stars
The first stars, most likely Population III stars, form and start the process of turning the light elements that were formed in the Big Bang (hydrogen, helium and lithium) into heavier elements.
Formation of galaxies
Large volumes of matter collapse to form a galaxy. Population II stars are formed early on in this process, with Population I stars formed later.
Formation of groups, clusters and superclusters
Gravitational attraction pulls galaxies towards each other to form groups, clusters and superclusters.
Formation of the solar system
Finally, objects on the scale of our solar system form.
Today
Since the expansion of the universe appears to be accelerating, superclusters are likely to be the largest structures that will ever form in the universe. The present accelerated expansion prevents any more inflationary structures entering the horizon and prevents new gravitationally bound structures from forming.
Ultimate fate of the universe
Like the very early universe, advances in fundamental physics are required before it will be possible to know the ultimate fate of the universe with any certainty. Below are some of the main possibilities.
Heat death
This scenario is generally considered to be the most likely, as it occurs if the universe continues expanding as it has been. The universe approaches a highly entropic state, in which galaxies collapse into black holes which subsequently evaporate via Hawking radiation. In this case, the universe will indefinitely consist solely of a bath of uniform radiation.
Big crunch
If the energy density dark energy becomes negative or the universe is closed, then it is possible that the expansion of the universe will reverse and the universe will contract towards a hot, dense state, analogous to a time-reverse of the big bang. This is often proposed as part of an oscillatory universe scenario, such as the cyclic model.
Big rip
This scenario is possible only if the energy density of dark energy actually increases without limit over time. Such dark energy is called phantom energy and is unlike any known kind of energy. In this case, the expansion rate of the universe will increase without limit. Gravitationally bound systems, such as clusters of galaxies, galaxies, the solar system and ultimately the Earth will be torn apart. Eventually the expansion will be so rapid as to overcome the electromagnetic forces holding people, molecules and atoms together. Finally even atomic nuclei will be torn apart and the universe as we know it will end in an unusual kind of gravitational singularity.
Vacuum metastability disaster
If our universe is in a very long lived false vacuum, it is possible that the universe will tunnel into a lower energy state. In this case, all structures will be destroyed instantaneously, without any forewarning.