The Big Bang Theory
The very early universe
All our understanding of the very early universe (cosmogony) is 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
Up to 10-43 seconds after the Big Bang
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, so they are possibly unified
into one fundamental force. Little is known about this epoch, although different theories make different
predictions. Einstein's theory of general relativity predicts a gravitational singularity before this time, but
under these conditions the theory is expected to break down due to quantum effects. Physicists hope that
proposed theories of quantum gravity, such as string theory and loop quantum gravity, will eventually
lead to a better understanding of that epoch.
The Grand unification epoch
Between 10-43 seconds and 10-35 seconds after the Big Bang
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.
The inflationary epoch
Between 10-35seconds and 10-32seconds after the Big Bang
The temperature, and therefore the time, at which cosmic inflation occurs is not known for certain.
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. Some energy from photons becomes virtual quarks and
hyperons, but these particles decay quickly. One scenario suggests that prior to cosmic inflation, the
universe was cold and empty, and the immense heat and energy associated with the early stages of the
big bang was created through the phase change associated with the end of inflation.
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
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broken, otherwise magnetic monopoles would be seen in the visible universe. At this point, the universe is
dominated by radiation; quarks, electrons and neutrinos form.
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
After cosmic inflation ends, the universe is filled with a quark-gluon plasma. From this point onwards the
physics of the early universe is better understood, and less speculative.
The electroweak epoch
Between 10-32 seconds and 10-12 seconds after the Big Bang
The temperature of the universe is high enough to merge electromagnetism and the weak interaction into
a single electroweak interaction. Particle interactions are energetic enough to create large numbers of
exotic particles, including W and Z bosons and Higgs bosons.
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 quark epoch
Between 10-12 seconds and 10-6 seconds after the Big Bang
In electroweak symmetry breaking, at the end of the electroweak epoch, all the fundamental particles are
believed to acquire a mass via the Higgs mechanism in which the Higgs boson acquires a vacuum
expectation value. The fundamental interactions of gravitation, electromagnetism, the strong interaction
and the weak interaction have now taken their present forms, but the temperature of the universe is still
too high to allow quarks to bind together to form hadrons.
The hadron epoch
Between 10-6 seconds and 1 second after the Big Bang
The quark-gluon plasma which composes the universe cools until hadrons, including baryons such as
protons and neutrons, can form. At approximately 1 second after the Big Bang 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.
The lepton epoch
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The majority of hadrons and anti-hadrons annihilate each other at the end of the hadron epoch, leaving
leptons and anti-leptons dominating the mass of the universe. Approximately 3 seconds after the Big Bang
the temperature of the universe falls to the point where new lepton/anti-lepton pairs are no longer
created and most leptons and anti-leptons are eliminated in annihilation reactions, leaving a small residue
of leptons.
The photon epoch
Between 3 seconds and 380,000 years after the Big Bang
After most leptons and anti-leptons are annihilated at the end of the lepton epoch the energy of the
universe is dominated by photons. These photons are still interacting frequently with charged protons,
electrons and (eventually) nuclei, and continue to do so for the next 300,000 years.
Nucleosynthesis
Between 100 seconds and 300 seconds after the Big Bang
During the photon epoch the temperature of the universe falls to the point where atomic nuclei can begin
to form. Protons (hydrogen ions) and neutrons begin to combine into atomic nuclei in the process of
nuclear fusion. However, nucleosynthesis only lasts for about three minutes, after which time the
temperature and density of the universe has fallen to the point where nuclear fusion cannot continue. At
this time, there are about three times more hydrogen ions than 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: 300,000 years
Hydrogen and helium atoms begin to form and the density of the universe falls. During recombination
decoupling occurs, causing the photons to evolve independently from the matter. Most importantly, this
means that the photons that compose the cosmic microwave background are a picture of the universe
during this epoch.
WMAP data shows the microwave background radiation variations
throughout the Universe from our perspective, though the actual
variations are much smoother than the diagram suggests
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.
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Structure formation
The Hubble Ultra Deep Fields often showcase galaxies from an ancient era that tell us
what the early Stelliferous Age was like.
Another Hubble image shows an infant galaxy forming nearby, which means this
happened very recently on the cosmological timescale. This is evidence that the
Universe is not quite finished with galaxy formation yet.
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, 8 billion years
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Finally, objects on the scale of our solar system form. Our sun is a late-generation star, incorporating the
debris from many generations of earlier stars, and formed roughly 5 billion years ago, or roughly 8 to 9
billion years after the big bang.
Today, 13.7 billion years
The best current data estimates the age of the universe today as 13.7 billion years since the big bang.
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.
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