The Big Bang.

The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe
According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded
rapidly. This rapid expansion caused the Universe to cool and resulted in its present continuously expanding
state. According to the most recent measurements and observations, the Big Bang occurred approximately 13.75
billion years ago which is thus considered the age of the Universe. After its initial expansion from a
singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles,
including protons, neutrons, and electrons. While protons and neutrons combined to form the first atomic nuclei
only a few minutes after the Big Bang, it would take thousands of years for electrons to combine with them and
create electrically neutral atoms. The first element produced was hydrogen, along with traces of helium and
lithium. Giant clouds of these primordial elements would coalesce through gravity to form stars and galaxies,
and the heavier elements would be synthesized either within stars or during supernovae.

The Big Bang is a well-tested scientific theory and is widely accepted within the scientific community. It
offers a comprehensive explanation for a broad range of observed phenomena. Since its conception, abundant
evidence has been uncovered in support of the model. The core ideas of the Big Bang—the expansion, the early
hot state, the formation of helium, and the formation of galaxies—are derived from many observations that are
independent from any cosmological model; these include the abundance of light elements, the cosmic microwave
background, large scale structure, and the Hubble diagram for Type Ia supernovae. As the distance between galaxy
clusters is increasing today, it can be inferred that everything was closer together in the past. This idea has
been considered in detail back in time to extreme densities and temperatures , and large particle accelerators
have been built to experiment in such conditions, resulting in further development of the model. On the other
hand, these accelerators have limited capabilities to probe into such high energy regimes. There is little
evidence regarding the absolute earliest instant of the expansion. Thus, the Big Bang theory cannot and does not
provide any explanation for such an initial condition; rather, it describes and explains the general evolution
of the universe going forward from that point on.

Timeline of the Big Bang.

Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite
density and temperature at a finite time in the past. This singularity signals the breakdown of general
relativity. How closely we can extrapolate towards the singularity is debated—certainly no closer than the end
of the Planck epoch. This singularity is sometimes called "the Big Bang", but the term can also refer to the
early hot, dense phase itself, which can be considered the "birth" of our Universe. Based on measurements of
the expansion using Type 1a supernovae, measurements of temperature fluctuations in the cosmic microwave
background, and measurements of the correlation function of galaxies, the Universe has a calculated age of 13.75
± 0.11 billion years. The agreement of these three independent measurements strongly supports the LCDM
(lambda CDM) model that describes in detail the contents of the Universe.

The earliest phases of the Big Bang are subject to much speculation. In the most common models the Universe was
filled homogeneously and isotropically with an incredibly high energy density and huge temperatures and pressures
and was very rapidly expanding and cooling. Approximately 10-37 seconds into the expansion, a phase transition
caused a cosmic inflation, during which the Universe grew. After inflation stopped, the Universe consisted of a
quark-gluon plasma, as well as all other elementary particles. 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
antileptons—of 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 grow in size 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. After about 10-11 seconds, the picture becomes less speculative,
since particle energies drop to values that can be attained in particle physibs experiments. 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 now no longer high enough
to create new proton–antiproton pairs (similarly for neutrons–antineutrons), so a mass annihilation immediately
followed, leaving just one in 1010 of the original protons and neutrons, 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 was about that
of air, neutrons combined with protons to form the Universe's deuterium and helium nuclei in a process called
Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei. As the Universe cooled, the rest
mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000
years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter
and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background
radiation..

Over a long period of time, the slightly denser regions of the nearly 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, warm dark matter, hot
dark matter, and baryonic matter. The best measurements available (from WMAP) show that the data is well-fit by
a Lambda-CDM model in which dark matter is assumed to be cold (warm dark matter is ruled out by early
reionization), and is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter
makes up about 4.6%. In an "extended model" which includes hot dark matter in the form of neutrinos, then if
the "physical baryon density" Wbh2 is estimated at about 0.023 (this is different from the 'baryon density'
Wb expressed as a fraction of the total matter/energy density, which as noted above is about 0.046), and the 
corresponding cold dark matter density Wch2 is about 0.11, the corresponding neutrino density Wvh2 is estimated
to be less than 0.0062.

Independent lines of evidence from Type 1a supernovae and the CMB imply that the Universe today is dominated by
a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations
suggest 73% of the total energy density of today's Universe is in this form. When the Universe was very young,
it was likely infused with dark energy, but with less space and everything closer together, gravity had the
upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion,
the growing abundance of dark energy caused the Expansion of the universe to slowly begin to accelerate. Dark
energy in its simplest formulation takes the form of the cosmological constant term in Einstein’s field equations
of general relativity, but its composition and mechanism are unknown and, more generally, the details of its
equation of state and relationship with the Standard Model of particle physics continue to be investigated both
observationally and theoretically.

All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the LCDM
model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity.
As noted above, there is no well-supported model describing the action prior to 10−15 seconds or so. Apparently
a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras
in the history of the Universe is currently one of the greatest unsolved problems in physics.

FLRW metric.
General relativity describes spacetime by a metric, which determines the distances that separate nearby points.
The points, which can be galaxies, stars, or other objects, themselves are specified using acoordinate chart
or "grid" that is laid down over all spacetime. The cosmological principle implies that the metric should be
homogenous and isotropic on large scales, which uniquely singles out the Friedmann-Lematre-Robertson-Walker
metric (FLRW metric). This metric contains a scale factor, which describes how the size of the Universe changes
with time. This enables a convenient choice of a coordinate system to be made, called comoving coordinates. In
this coordinate system the grid expands along with the Universe, and objects that are moving only due to the
expansion of the Universe remain at fixed points on the grid. While their coordinate distance (comoving distance)
remains constant, the physical distance between two such comoving points expands proportionally with the scale
factor of the Universe.

The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead, space itself
expands with time everywhere and increases the physical distance between two comoving points. Because the FLRW
metric assumes a uniform distribution of mass and energy, it applies to our Universe only on large scales—local
concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the 
large-scale expansion of space.

Underlying assumptions.

The Big Bang theory depends on two major assumptions: the universality of physical laws and the cosmological
principle. The cosmological principle states that on large scales the Universe is homogeneous and isotropic.
These ideas were initially taken as postulates, but today there are efforts to test each of them. For example,
the first assumption has been tested by observations showing that largest possible deviation of the fine
structure constant over much of the age of the universe is of order 10−5. Also, general relativity has passed
stringent tests on the scale of the Solar System and binary stars while extrapolation to cosmological scales has
been validated by the empirical successes of various aspects of the Big Bang theory.

If the large-scale Universe appears isotropic as viewed from Earth, the cosmological principle can be derived
from the simpler Copernican principle, which states that there is no preferred (or special) observer or vantage
point. To this end, the cosmological principle has been confirmed to a level of 10−5 via observations of the CMB
The Universe has been measured to be homogeneous on the largest scales at the 10% level.

Dark energy.

Measurements of the redshift-magnitude  relation for type1a supernovae indicate that the expansion of the
Universe has been accelerating since the Universe was about half its present age. To explain this acceleration,
general relativity requires that much of the energy in the Universe consists of a component with large negative
presure, dubbed "dark energy". Dark energy, though speculative, solves numerous problems. Measurements of the
cosmic microwave background indicate that the Universe is very nearly spatially flat, and therefore according to
general relativity the Universe must have almost exactly the critical density of mass/energy. But the mass
density of the Universe can be measured from its gravitational clustering, and is found to have only about 30%
of the critical density. Since theory suggests that dark energy does not cluster in the usual way it is the best
explanation for the "missing" energy density. Dark energy also helps to explain two geometrical measures of the
overall curvature of the Universe, one using the frequency ofgravitational lenses, and the other using the
characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is believed to be a property of vacuum energy, but the exact nature and existence of dark
energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant
and quintessence. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark
energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos. According to theory, the energy density
in matter decreases with the expansion of the Universe, but the dark energy density remains constant (or nearly
so) as the Universe expands. Therefore matter made up a larger fraction of the total energy of the Universe in
the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes
even more dominant.

Dark matter.

During the 1970s and 1980s, various observations showed that there is not sufficient visible matter in the
Universe to account for the apparent strength of gravitational forces within and between galaxies. This led to
the idea that up to 90% of the matter in the Universe is dark matter that does not emit light or interact with
normal baryonic matter. In addition, the assumption that the Universe is mostly normal matter led to predictions
that were strongly inconsistent with observations. In particular, the Universe today is far more lumpy and
contains far less deuterium than can be accounted for without dark matter. While dark matter has always been
controversial, it is inferred by various observations: the anisotropies in the CMB, galaxy cluster velocity
dispersions, large-scale structure distributions, gravitational lensing studies, and X-ray measurements of galaxy
clusters.

Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter
particles have been observed in laboratories. Many particle physics candidates for dark matter have been
proposed, and several projects to detect them directly are underway.



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