THE BIG BANG THEORY
The Big Bang theory describes
how the Universe began in a rapid expansion about 13.7 billion years ago and
has evolved since that time. It is thought that all of space was created in
this first moment.
Since the 1940s, when the
modern form of the theory took shape, scientists have detected radiation from
the early Universe with radio telescopes and satellites and named it cosmic
microwave background radiation (CMB). The CMB, which is formed of microwaves
and radio waves, is considered important evidence in support of the Big Bang
because it matches theorists' predictions.
The Big Bang theory is the prevailing
cosmological model that explains 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.
Georges
Lemaître first proposed what would become the Big Bang theory in what he called
his "hypothesis of the primeval atom." Over time, scientists would
build on his initial ideas to form the modern synthesis. The framework for the
Big Bang model relies on Albert Einstein's general relativity and on
simplifying assumptions such as homogeneity and isotropy of space. The
governing equations had been formulated by Alexander Friedmann. In 1929, Edwin
Hubble discovered that the distances to far away galaxies were generally
proportional to their redshifts—an idea originally suggested by Lemaître in
1927. Hubble's observation was taken to indicate that all very distant galaxies
and clusters have an apparent velocity directly away from our vantage point:
the farther away, the higher the apparent velocity.
While
the scientific community was once divided between supporters of the Big Bang
and those of alternative cosmological models, most scientists became convinced
that some version of the Big Bang scenario best fit observations after the
discovery of the cosmic microwave background radiation in 1964, and especially
when its spectrum (i.e., the amount of radiation measured at each wavelength)
was found to match that of thermal radiation from a black body. Since then,
astrophysicists have formulated further hypotheses to account for some
discrepancies that have arisen within the model.
Timeline of the Big Bang
Extrapolation of the expansion of the Universe
backwards in time using general
relativity yields an
infinite density and temperatureat 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,[notes
1] which can be
considered the "birth" of our Universe. Based on measurements of the
expansion using Type Ia 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.[15] The
agreement of these three independent measurements strongly supports the ΛCDM 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 highenergy density and huge temperatures and pressures and was very rapidly expanding and
cooling. Approximately 10−37 seconds
into the expansion, a phase transition caused acosmic inflation, during which the Universe
grew exponentially.[16] 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 andleptons 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 physics 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
(one thousand million; 109; SI prefix giga-) 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 hydrogennuclei.
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 thecosmic 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" Ωbh2 is estimated at about 0.023 (this is
different from the 'baryon density' Ωb 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 Ωch2 is about 0.11, the corresponding
neutrino density Ωvh2 is
estimated to be less than 0.0062.[15]
Independent
lines of evidence from Type Ia 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 ΛCDM 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.
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.
Speculative physics beyond Big Bang theory
While the Big Bang model is well established in
cosmology, it is likely to be refined in the future. Little is known about the
earliest moments of the Universe's history. The equations of classical general
relativity indicate asingularity at the origin of cosmic time, although
this conclusion depends on several assumptions. Moreover, general relativity
must break down before the Universe reaches the Planck
temperature, and a correct treatment ofquantum gravity may avoid the would-be singularity.
Some
proposals, each of which entails untested hypotheses, are:
§
Models including the Hartle–Hawking
no-boundary condition in
which the whole of space-time is finite; the Big Bang does represent the limit
of time, but without the need for a singularity.
§
Big Bang lattice model states that the Universe at the moment of
the Big Bang consists of an infinite lattice offermions which is smeared over the fundamental
domain so it has both
rotational, translational, and gauge symmetry. The symmetry is the largest
symmetry possible and hence the lowest entropy of any state.
§
Brane cosmology models in which inflation is due to
the movement of branes in string theory; the pre-Big Bang model; the ekpyrotic model, in which the Big Bang is the
result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model
in which collisions occur periodically. In the latter model the Big Bang was
preceded by a Big Crunch and
the Universe endlessly cycles from one process to the other.
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