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In cosmology, the Big Bang theory is currently the dominant theory about the
early development and current shape of the universe.
Extrapolating the history of the universe backwards using current physical
models leads to a gravitational singularity, at which all distances become
zero and temperatures and pressures become infinite. What this means is
unclear, and most physicists believe that this result is due to our limited
understanding of the laws of physics with regard to this type of situation,
and in particular, the lack of a theory of quantum gravity.
Based on measurements of the expansion of the universe using type I
supernova, measurements of the lumpiness of the cosmic microwave background,
and measurements of the correlation function of galaxies, it is currently
believed that the big bang occurred 13.7 ± 0.2 billion years ago. The fact
that these three separate measurements of completely different things are
all consistent with each other is considered strong evidence for the model.
The universe as we know it was initially almost uniformly filled with energy
and extremely hot. As the distances in the universe rapidly grew, the
temperature dropped, leading to the creation of the known forces of physics,
elementary particles, and eventually hydrogen and helium atoms in a process
called Big bang nucleosynthesis.
Over time, the slightly denser regions of the almost, but not quite,
uniformly distributed matter were pulled together by gravity into clumps,
forming gas clouds, stars, galaxies, and the other astronomical structures
seen today. The details of how the process of galaxy formation occurred
depends on the type of matter in the universe, and the three competing
pictures of how this occurred are known as cold dark matter, hot dark
matter, and baryonic matter. These three models have be tested through
computer simulations and observations of galactic correlation functions.
It is at present unknown whether the singularity of spacetime described
above is a physical reality or just a mathematical extrapolation of general
relativity beyond its limits of applicability. The resolution of this
question has to wait until a confirmed theory of quantum gravity is available.
In general relativity, one usually talks about spacetime and cannot cleanly
separate space from time. In the Big Bang theory, this difficulty does not
arise; Weyl's postulate is assumed and time can be unambiguously measured at
any point as the "time since the Big Bang".
The Big Bang was not an explosion of matter moving outward to fill an empty
universe. Instead, it involved the rapid growth of the universe itself.
Because of this, the distance (in the sense of comoving distance) between
far removed galaxies increases faster than the speed of light. This does not
violate the laws of special relativity, a theory which is physically valid
only as a local theory. It states, among other things, that matter and
information cannot travel through space faster than the speed of light, and
it is empirically invalid for global space-time concepts (because it ignores
History of the theory
In 1927, the Belgian priest Georges Lema”tre was the first to propose that
the universe began with the explosion of a "primeval atom". Earlier, in
1918, the Strasbourg astronomer Wirtz had measured a systematic redshift of
certain "nebulae", and called this the K-correction, but he wasn't aware of
the cosmological implications, nor that the supposed nebulae were actually
galaxies outside our own Milky Way.
Einstein's theory of general relativity developed during this time had the
result that the universe could not remain static, a result that he himself
considered wrong, and which he attempted to fix by adding a cosmological
constant which did not fix the problem. Applying general relativity to
cosmology was done by Friedman whose equations describe the
In the 1930s, Edwin Hubble found experimental evidence to help justify
Lema”tre's theory. Again using redshift measurements, Hubble determined that
distant galaxies are receding in every direction at speeds (relative to the
Earth) directly proportional to their distance, a fact now known as Hubble's law.
Since galaxies were receding, this suggested two possibilities. One,
proposed by George Gamow, was that the universe began a finite time in the
past and has been expanding ever since. The other was Fred Hoyle's steady
state model in which new matter would be created as the galaxies moved away
from each other and that the universe at one point in time would look
roughly like any other point in time. For a number of years the support for
these two opposing theories was evenly divided.
In the intervening period however, all observational evidence gathered has
provided overwhelming support for the Big Bang theory, and since the
mid-1960s it has been regarded as the best available theory of the origin
and evolution of the cosmos, and virtually all theoretical work in cosmology
involves extensions and refinements to the basic big bang theory. Much of
the current work in cosmology includes understanding how galaxies form
within the context of the big bang, understanding what happened at the big
bang, and reconciling observations with the basic theory.
Over the decades a number of weaknesses have been identified in the big bang
theory, but these have thus far all been addressed by extensions and
refinements such as cosmic inflation. As of 2003, there are no weaknesses in
the big bang theory which are regarded as fatal by most or even a large
minority of cosmologists. However, there remain small numbers of who still
support non-standard cosmologies in which the big bang is considered incorrect.
The redshift of galaxies
By analyzing the light from distant galaxies, one notices that the shape of
the light's spectrum is very similar, but the whole spectrum is shifted
towards longer wavelengths for more distant galaxies. This suggests that the
galaxies are moving away from us, resulting in an effect akin to the Doppler
effect called redshift.
A (now) major aspect of the Big Bang hypothesis was the prediction in the
1940s of cosmic microwave background radiation or CMBR. The theory proposed
that, as all the mass/energy of the universe emerged from the primordial
explosion, the initial density of the universe was incredibly high, and
hence the temperature of the universe must have been extremely hot (as
matter gets hotter when compressed to a higher density). The initial
temperature of the universe was so high that matter (as we know it) could
not exist, as the subatomic particles would have been too energetic to
aggregate into atoms.
However, as the universe was expanding it would also have cooled down. As
the temperature of the universe fell, matter could form from the primordial
plasma. The theory predicted that at some stage (currently reckoned to be
around 500,000 years after the beginning), this plasma would thin out
sufficiently to permit photons to be set free from the attraction of the
other matter, and travel through the constantly expanding reaches of space.
The process that produced this blast of free energy is known as photon decoupling.
Based on this premise, the theory predicted that this massive blast of
radiation should have left some traces in the cosmos, and would have a
number of properties. Essentially it says that as the universe was extremely
hot at one point, it should still be a little bit warm even today, and
calculations predicted a residual temperature of about 3 Kelvin (3 degrees
Celsius above absolute zero). Additionally, as the radiation was produced
simultaneously, the traces of it should be uniform or isotropic. Another
prediction was that as these photons are subject to the expansion of space,
their wavelengths would have been "stretched" or red-shifted. A critical
further prediction was that the further away one looks, the hotter the
universe should appear to be (as looking further away corresponds to looking
backwards in time), and at some extremely distant point the radiation in the
universe should be so thick as to become opaque.
At the time they were made, the predictions of the Big Bang theory regarding
CMBR were largely ignored, simply because they remained unverifiable due to
inadequate technology for nearly 20 years.
However, in 1964, Arno Penzias and Robert Wilson conducted a series of
diagnostic observations using a new microwave receiver owned by Bell
Laboratories (which was designed for normal telephone communications) and
accidentally discovered the cosmic background radiation originally predicted
by Gamow. This observation was later confirmed by the Peebles group at
Princeton University, who were themselves trying to construct a microwave
antenna with a ruby maser to detect the CMBR when Penzias and Wilson "ran
across" it. It was not until Penzias and Wilson consulted with the Peebles
group that they understood what it was they had detected. Penzias and Wilson
published their findings jointly with the Peebles group in the Astrophysical Journal.
Their discovery provided substantial confirmation of almost every aspect of
the CMBR predictions, and overwhelmingly swayed the balance of opinion in
favour of the Big Bang hypothesis. Penzias and Wilson were awarded the Nobel
Prize for this discovery.
In 1989, NASA launched the Cosmic Background Explorer satellite (COBE), and
the initial findings (released in 1990) were consistent with the Big Bang
theory's predictions regarding CMBR, finding a local residual temperature of
2.726 K, determining that the CMBR was generally isotropic, and confirming
the "haze" effect as distance increased. During the 1990s, CMBR data was
studied further to see if small anisotropies predicted by the big bang would
be observed. They were found in the late 1990s. In early 2003 the results of
the Wilkenson Microwave Anisotropy satellite (WMAP) were analysed giving the
most accurate cosmological values we have to date. This satellite also
disproved several specific inflationary models, but the results were
consistent with the inflation theory in general.
Abundance of primordial elements
Using the Big Bang model it is possible to calculate the concentration of
helium 4, helium 3, deuterium and lithium 7 in the universe. All the
abundances depend on a single parameter, the ratio of photons to baryons.
Measurements of primordial abundances for all four isotopes are consistent
with a unique value of that parameter (see big bang nucleosynthesis.) Steady
State theories fail to account for the abundance of deuterium in the cosmos,
because deuterium easily undergoes nuclear fusion in stars and there are no
known astrophysical processes other than the Big Bang itself that can
produce it in large quantities. Hence the fact that deuterium is not a rare
component of the universe suggests that the universe has a finite age.
Distribution of quasars
Quasars are predicted to only be possible in the early stages of a dynamic
cosmos by the Big Bang theory, and observational evidence supports this, as
quasar populations become denser the further away one looks.
One piece of evidence for the Big Bang model is that it resolves Olbers'
paradox of why the sky is black at night.
Weaknesses and criticisms of the Big Bang Theory
One weakness of the Big Bang theory is the obvious question of how the Big
Bang occurred. The difficulty of answering this question lies with the
absence of a theory of quantum gravity. As one goes back in time, the
temperature and the pressures increase to the point where the physical laws
governing the behavior of matter are unknown. It is hoped that as we
understand these laws that we will better be able to answer the question of
what happened "before" the Big Bang.
During the 1970s, observations were made that - assuming that all of the
matter within the universe could be seen - created problems for the Big Bang
theory, as it seemed to underestimate the amount of deuterium in the
universe and lead to a universe that was much more "lumpy" than observed.
These problems are resolved if one assumes that most of the matter in the
universe is not visible, and this assumption seems to be consistent with
observations that suggest that much of the universe consists of dark matter.
The effects that dark matter has on big bang calculations generally do not
depend on the detailed properties of the dark matter. The main property of
dark matter which influences cosmology is whether the dark matter consists
of particles that are heavy and hence are moving slowly, thereby creating
cold dark matter, or whether it consists of particles are are light and
hence are moving quickly, thereby creating hot dark matter, or whether the
dark matter consists of ordinary matter which is baryonic matter.
The future according to the Big Bang theory
All the matter in the universe is gravitationally attracted to other matter
which is within the observable horizon (defined by the age of the universe).
This should cause the expansion rate of the universe to slow down over time.
Exactly how much matter exists in any given volume, relative to how large
the horizon is and how fast the universe is currently expanding can lead to
one of three scenarios:
The Big Crunch
If the gravitational attraction of all the matter in the observable horizon
is high enough, then it could stop the expansion of the universe, and then
reverse it. The universe would then contract, in about the same time as the
expansion took. Eventually, all matter and energy would be compressed back
into a gravitational singularity. It is impossible to ask what would happen
after this, as time would stop in this singularity as well.
The Big Freeze
If the gravitational attraction of all the matter in the observable horizon
is low enough, then the expansion will never stop. As the matter disperses
into ever greater and greater volumes, new star formation would drop off.
The average temperature of the Universe would asymptotically approach
absolute zero, and the Universe would become very still and quiet.
Eventually, all the protons would decay, the black holes would evaporate,
and the Universe would consist of dispersed subatomic particles. The Big
Freeze is also known as the heat death of the universe.
If the gravitational attraction of all the matter in the observable horizon
is just right, then the expansion of the universe will asymptotically
approach zero. The temperature of the universe would asymptotically approach
a stable value slightly above absolute zero. Entropy would increase, and the
end result (with protons decaying) would be similar to the Big Freeze.
One extremely puzzling recent discovery comes from observations of type I
supernovae which allow one to better calculate the distance to galaxies,
from observations of the cosmic microwave background, from gravitational
lensing, and from the use of large length scale statistics of the
distributions of galaxies and quasars as standard rulers for measuring
distances. It appears that the expansion of the universe is accelerating, an
observation which astrophysicists are currently trying to understand (see
accelerating universe). The currently favored approach is to reintroduce a
non-zero cosmological constant into Einstein's equations of General
Relativity, and adjust the numerical value of that constant to match the
observed acceleration. This is akin to postulating a repelling "dark energy"
also called quintessence.
Big Bang theory vs. religion
When the Big Bang theory was originally proposed, it was rejected by most
scientists and enthusiastically embraced by the pope, because it seemed to
point to a creation event. While most scientists nowadays view the Big Bang
theory as the best explanation of the available evidence, and the Catholic
Church still accepts it, some conservative Christians (usually
Fundamentalists) oppose it because the age of the universe is far higher
than the one calculated from a literal reading of the book of Genesis the
Bible. Many ways have been proposed to reconcile the two including denying
the fundamentalist reading of Genesis or denying the correctness of the age
of the universe.
One common attempt at reconciling the two ages is by arguing that day does
not literally mean 24 hours. One example of this is Gerald Schroeder, who
claims that his calculations confirm a relativistic correspondence between
the measured age of the universe and the six days of creation described in
See also: Estimates of the date of Creation - creationism - creation myths
Origin of the term
The term "Big Bang" was coined by Fred Hoyle in a BBC radio program in the
early 1950s; Hoyle did not subscribe to the theory and intended to mock the concept.