The Big Bang is a physical theory that describes how the universe expanded from an early state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, the redshift of galaxies and the large-scale structure of the universe. The observed uniformity of the universe, which leads to the horizon and flatness problems, is explained through cosmic inflation: a phase of accelerated expansion during the earliest stages. Detailed measurements of the expansion rate of the universe place the cosmic inflation at an estimated billion years ago, which is considered the age of the universe. A wide range of empirical evidence strongly favors the Big Bang model, which is now widely accepted.
Extrapolating this cosmic expansion backward in time using the known laws of physics, the models describe an extraordinarily hot and dense primordial universe. The original Big Bang model extrapolated the universe back towards a Big Bang singularity. However, the modern view incorporates inflationary cosmology, whereby the universe emerged out of inflation in a supercooled state. This was followed by a reheating event that converted the energy that drove inflation into a hot and dense plasma, initiating the hot Big Bang. As the universe expanded, it cooled sufficiently to allow the formation of subatomic particles, and later atoms. These primordial elements—mostly hydrogen, with some helium and lithium—then coalesced under the force of gravity aided by dark matter, forming early stars and galaxies. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to a concept called dark energy.
The concept of an expanding universe was introduced by the physicist Alexander Friedmann in 1922 with the mathematical derivation of the Friedmann equations. The earliest empirical observation of an expanding universe is known as Hubble's law, published in work by physicist Edwin Hubble in 1929, which discerned that galaxies are moving away from Earth at a rate that accelerates proportionally with distance. Independent of Friedmann's work, and independent of Hubble's observations, in 1931 physicist Georges Lemaître proposed that the universe emerged from a "primeval atom", introducing the modern notion of the Big Bang. In 1964, the CMB was accidentally discovered. Over the next few years measurements showed this radiation to be uniform over directions in the sky and the shape of the energy versus intensity curve, both consistent with the Big Bang models of high temperatures and densities in the distant past. By the late 1960s most cosmologists were convinced that competing steady-state model of cosmic evolution was incorrect.
There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models. These include the unequal abundances of matter and antimatter known as baryon asymmetry, the detailed nature of dark matter surrounding galaxies, and the origin of dark energy.
Features of the models Assumptions Big Bang cosmology models depend on three major assumptions: the universality of physical laws, the cosmological principle, and that the matter content can be modeled as a perfect fluid. 22.1 Introduction to the standard big-bang model The universality of physical laws is one of the underlying principles of the theory of relativity. The cosmological principle states that on large scales the universe is homogeneous and isotropic—appearing the same in all directions regardless of location. A perfect fluid has no viscosity; the pressure of a perfect fluid is proportional to its density.
These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that the largest possible deviation of the fine-structure constant over much of the age of the universe is of order 10−5. The key physical law behind these models, general relativity has passed stringent tests on the scale of the Solar System and binary stars. The cosmological principle has been confirmed to a level of 10−5 via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound on the order of 10% inhomogeneity, as of 1995.
Expansion prediction
The cosmological principle dramatically simplifies the equations of general relativity, giving the Friedmann–Lemaître–Robertson–Walker metric to describe the geometry of the universe and, with the assumption of a perfect fluid, the Friedmann equations giving the time dependence of that geometry. The only parameter at this level of description is the mass-energy density: the geometry of the universe and its expansion is a direct consequence of its density. All of the major features of Big Bang cosmology are related to these results.
Mass-energy density
In Big Bang cosmology, the mass–energy density controls the shape and evolution of the universe. By combining astronomical observations with known laws of thermodynamics and particle physics, cosmologists have worked out the components of the density over the lifespan of the universe. In the current universe, luminous matter, the stars, planets, and so on makes up less than 5% of the density. Dark matter accounts for 27% and dark energy the remaining 68%.
Horizons
An important feature of the Big Bang spacetime is the presence of particle horizons. Since the universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach earth. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted today may never "catch up" to very distant objects. This defines a future horizon, which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the Friedmann–Lemaître–Robertson–Walker (FLRW) metric that describes the expansion of the universe.
The understanding of the universe back to very early times suggests that there is a past horizon, though in practice an observer's view is also limited by the opacity of the universe at early times. Therefore, that view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well.
Thermalization Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate thermodynamic equilibrium. Others were fast enough to reach thermalization. The parameter usually used to find out whether a process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the Hubble parameter. The larger the ratio, the more time particles had to thermalize before they were too far away from each other.
Timeline
Big Bang models propose a universe that has been expanding and cooling from a very hot and very compact state. Modern theories are based on a brief preceding era of inflation.
Inflation
Cosmological inflation is an extremely brief period of time, a billion, trillion times shorter than a trillionth of a second, during which the universe doubled in size more than 80 times. Inflation seeded what would later become the large-scale structure of the universe from microscopic quantum fluctuations that were "frozen in" during inflation.
Inflation supercooled the universe to a negligible temperature and density. Any particles that may have been present before inflation began would have been diluted to extremely low concentrations. After inflation ended, the universe underwent a period of reheating, which transferred the energy that was responsible for inflation into a hot and dense thermal plasma of primarily Standard Model particles. The exact mechanism that drove reheating is unknown, but many models have been put forward. It is also unknown what the temperature after reheating was, although it must have been between 1011 and 1029 K.
Hot Big Bang
After reheating temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point, an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptonsof the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present universe.
The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10−12 seconds.
After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass annihilation immediately followed, leaving just one in 108 of the original matter particles and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos).
A few minutes into the expansion, when the temperature was about a billion kelvin and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis (BBN). Most protons remained uncombined as hydrogen nuclei.
As the universe cooled, the rest energy density of matter came to gravitationally dominate over that of the photon and neutrino radiation at a time of about 50,000 years. At a time of about 380,000 years, the universe cooled enough that electrons and nuclei combined into neutral atoms (mostly hydrogen) in an event called recombination. This process made the previously opaque universe transparent, and the photons that last scattered during this epoch comprise the cosmic microwave background.
Structure formation
After the recombination epoch, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter (CDM), warm dark matter, hot dark matter, and baryonic matter. The best measurements available, from the Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter is assumed to be cold. This CDM is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. (See Table 8.)
Cosmic acceleration