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The Big Bang - overview of the Big Bang
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Big Bang
In physical cosmology, the term Big Bang has two related meanings. It is
a cosmological model in which the universe has been expanding for around
13.7 billion years (13.7 Ga), starting from a tremendously dense and hot
state. The term is also used in a narrower sense to describe the
fundamental 'fireball' that erupted at or close to time t=0 in the
history of the universe.
Observational evidence for the Big Bang includes the analysis of the
spectrum of light from galaxies, which reveal a shift towards longer
wavelengths proportional to each galaxy's distance in a relationship
described by Hubble's law. Combined with the assumption that observers
located anywhere in the universe would make similar observations (the
Copernican principle), this suggests that space itself is expanding.
Extrapolation of this expansion back in time yields a state in the
distant past in which the universe was in a state of immense density and
temperature. This hot, dense state is the key premise of the Big Bang.
Theoretical support for the Big Bang comes from mathematical models,
called Friedmann models, which show that a Big Bang is consistent with
general relativity and with the cosmological principle, which states
that the properties of the universe should be independent of position or
orientation.
The theory of Big Bang nucleosynthesis predicts the rates at which
various light elements are created in models of the early universe and
gives results that are generally consistent with observations. The Big
Bang model also predicts the cosmic microwave background radiation (CMB),
a background of weak microwave radiation filling the whole universe. The
discovery of the CMB in 1964 led to general acceptance among physicists
that the Big Bang is the best model for the origin and evolution of the
universe.
History
The Big Bang theory developed from observations of the structure of the
universe and from theoretical considerations. In 1912 Vesto Slipher
measured the first Doppler shift of a "spiral nebula" (spiral nebula is
the obsolete term for spiral galaxies), and soon discovered that almost
all such nebulae were receding from Earth. He did not grasp the
cosmological implications of this fact, and indeed at the time it was
highly controversial whether or not these nebulae were "island
universes" outside our Milky Way.[1] Ten years later, Alexander
Friedmann, a Russian cosmologist and mathematician, derived the
Friedmann equations from Albert Einstein's equations of general
relativity, showing that the universe might be expanding in contrast to
the static universe model advocated by Einstein.[2] In 1924, Edwin
Hubble's measurement of the great distance to the nearest spiral nebulae
showed that these systems were indeed other galaxies. Independently
deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian
Roman Catholic priest, predicted that the recession of the nebulae was
due to the expansion of the universe.[3] In 1931 Lemaître went further
and suggested that the universe began as a simple "primeval atom",
echoing previous speculations about the cosmic egg origin of the
universe.[4]
Starting in 1924, Hubble painstakingly developed a series of distance
indicators, the forerunner of the cosmic distance ladder, using the 100
inch Hooker telescope at Mount Wilson Observatory. This allowed him to
estimate distances to galaxies whose redshifts had already been
measured, mostly by Slipher. In 1929, Hubble discovered a correlation
between distance and recession velocity—now known as Hubble's law.[5][6]
Lemaître had already shown that this was expected, given the
cosmological principle.[7]
During the 1930s other ideas were proposed as non-standard cosmologies
to explain Hubble's observations, including the Milne model,[8] the
oscillatory universe (originally suggested by Friedmann, but advocated
by Einstein and Richard Tolman)[9] and Fritz Zwicky's tired light
hypothesis.[10]
After World War II, two distinct possibilities emerged. One was Fred
Hoyle's steady state model, whereby new matter would be created as the
universe seemed to expand. In this model, the universe is roughly the
same at any point in time.[11] The other was Lemaître's Big Bang theory,
advocated and developed by George Gamow, who introduced big bang
nucleosynthesis[12] and whose associates, Ralph Alpher and Robert
Herman, predicted the CMB.[13] It is an irony that it was Hoyle who
coined the name that would come to be applied to Lemaître's theory,
referring to it sarcastically as "this big bang idea" during a radio
broadcast.[14] For a while, support was split between these two
theories. Eventually, the observational evidence, most notably from
radio source counts, began to favor the latter. The discovery of the
cosmic microwave background radiation in 1964[15] secured the Big Bang
as the best theory of the origin and evolution of the cosmos. Much of
the current work in cosmology includes understanding how galaxies form
in the context of the Big Bang, understanding the physics of the
universe at earlier and earlier times, and reconciling observations with
the basic theory.
Huge strides in Big Bang cosmology have been made since the late 1990s
as a result of major advances in telescope technology as well as the
analysis of copious data from satellites such as COBE,[16] the Hubble
Space Telescope and WMAP.[17] Cosmologists now have fairly precise
measurement of many of the parameters of the Big Bang model, and have
made the unexpected discovery that the expansion of the universe appears
to be accelerating (see dark energy).
Overview
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.[18] This singularity signals the breakdown of
general relativity. How closely we can extrapolate towards the
singularity is debated—certainly not earlier than the Planck epoch. The
early hot, dense phase is itself referred to as "the Big Bang",[19] and
is 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
13.7 ± 0.2 billion years.[20] 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 high energy density, huge temperatures
and pressures, and was very rapidly expanding and cooling. Approximately
10−35 seconds into the expansion, a phase transition caused a cosmic
inflation, during which the universe grew exponentially.[21] After
inflation stopped, the universe consisted of a quark-gluon plasma, as
well as all other elementary particles.[22] 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
anti-leptons—of the order of 1 part in 30 million. This resulted in the
predominance of matter over antimatter in the present universe.[23]
The universe continued to grow in size and fall in temperature (and
hence the typical energy of each particles was decreasing). Symmetry
breaking phase transitions put the fundamental forces of physics and the
parameters of elementary particles into their present form.[24] 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 Kelvins 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.[25] 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 380,000 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.[26]
Over 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 three
possible types of matter are known as cold dark matter, hot dark matter
and baryonic matter. The best measurements available (from WMAP) show
that the dominant form of matter in the universe is cold dark matter.
The other two types of matter make up less than 20% of the matter in the
universe.[17]
The universe today appears to be dominated by a mysterious form of
energy known as dark energy. Approximately 70% of the total energy
density of today's universe is in this form. This dark energy causes the
expansion of the universe to accelerate, observed as a slower than
expected expansion at very large distances. Dark energy in its simplest
formulation takes the form of a cosmological constant term in Einstein's
field equations of general relativity, but its composition is 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.[7]
All these observations can be explained by the ΛCDM model of cosmology,
which is a mathematical model of the Big Bang with six free parameters.
As noted above, there is no compelling physical model for the first
10−11 seconds of the universe. To resolve the paradox of the initial
singularity, a theory of quantum gravitation is needed. Understanding
this period of the history of the universe is one of the greatest
unsolved problems in physics.
Theoretical underpinnings
The Big Bang theory depends on two major assumptions:
1. The universality of physical laws
2. The cosmological principle—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.[27] 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.[28]
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.[29] The
universe has been measured to be homogeneous on the largest scales at
the 10% level.[30]
FLRW metric
General relativity describes spacetime by a metric, which determines the
distances that separate nearby points. The points themselves (galaxies,
stars, etc.) are specified using a coordinate chart or "grid" that is
laid down over all spacetime. The cosmological principle implies that
the metric should be homogeneous and isotropic on large scales, which
uniquely singles out the Friedmann-Lemaître-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.[31]
The Big Bang is not an explosion of matter moving outward to fill an
empty universe. Instead, space itself expands with time 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.
Horizons
An important feature of the Big Bang spacetime is the presence of
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 had
time to reach us. This places a limit—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 by us 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 FRW model that describes our
universe. Our understanding of the universe back to very early times
suggests that there was a past horizon, though in practice our view is
limited by the opacity of the universe at early times. If the expansion
of the universe continues to accelerate, there is a future horizon as
well.[32]
Observational evidence
The earliest and most direct kinds of observational evidence (sometimes
called the three pillars of the Big Bang theory) are the Hubble-type
expansion seen in the redshifts of galaxies, the detailed measurements
of the cosmic microwave background, and the abundance of light elements
(see Big Bang nucleosynthesis). Many other lines of evidence now support
the picture, notably various properties of the large-scale structure of
the cosmos which are predicted to occur due to gravitational growth of
structure in the standard Big Bang theory.
Hubble's law expansion
Observations of distant galaxies and quasars show that these objects are
redshifted—the light emitted from them has been shifted to longer
wavelengths. This can be seen by taking a frequency spectrum of an
object and matching the spectroscopic pattern of emission lines or
absorption lines corresponding to atoms of the chemical elements
interacting with the light. From this analysis, a redshift can be
measured. If this is interpreted as a Doppler shift, the recessional
velocity of the object can be calculated. For some galaxies, it is then
possible to estimate distances via the cosmic distance ladder. When the
recessional velocities are plotted against these distances, a linear
relationship known as Hubble's law is observed:[5]
v = H_0 D \,
where
v is the recessional velocity of the galaxy or other distant object
D is the distance to the object and
H0 is Hubble's constant, measured to be (70 +2.4/-3.2) km/s/Mpc by the
WMAP probe.[20]
Hubble's law has two possible explanations. Either we are at the center
of an explosion of galaxies—which is untenable given the Copernican
principle—or the universe is uniformly expanding everywhere. This
universal expansion was considered mathematically in the context of
general relativity well before Hubble made his analysis and
observations, and it remains the cornerstone of the Big Bang theory as
developed by Friedmann, Lemaître, Robertson and Walker.
The theory requires the relation v = HD to hold at all times, where D is
the proper distance, v = dD / dt, and v, H, and D all vary as the
universe expands (hence we write H0 to denote the present-day Hubble
"constant"). For distances much smaller than the size of the observable
universe, the Hubble redshift can be thought of as the Doppler shift
corresponding to the recession velocity v. However, the redshift is not
a true Doppler shift, but rather the result of the expansion of the
universe between the time the light was emitted and the time that it was
detected.[33]
Cosmic microwave background radiation
During first few days of the universe, the universe was in full thermal
equilibrium, with photons being continually emitted and absorbed, giving
the radiation a blackbody spectrum. As the universe expanded, it cooled
to a temperature at which photons could no longer be created or
destroyed. The temperature was still high enough for electrons and
nuclei to remain unbound, however, and photons were constantly
"reflected" from these free electrons through a process called Thomson
scattering. Because of this repeated scattering, the early universe was
opaque to light.
When the temperature fell to a few thousand Kelvin, electrons and nuclei
began to combine to form atoms, a process known as recombination. Since
photons scatter infrequently from neutral atoms, radiation decoupled
from matter when nearly all the electrons had recombined, at the epoch
of last scattering, 380,000 years after the Big Bang. These photons make
up the CMB that is observed today, and the observed pattern of
fluctuations in the CMB is a direct picture of the universe at this
early epoch. The energy of photons was subsequently redshifted by the
expansion of the universe, which preserved the blackbody spectrum but
caused its temperature to fall, meaning that the photons now fall into
the microwave region of the electromagnetic spectrum. The radiation is
thought to be observable at every point in the universe, and comes from
all directions with (almost) the same intensity.
In 1964, Arno Penzias and Robert Wilson accidentally discovered the
cosmic background radiation while conducting diagnostic observations
using a new microwave receiver owned by Bell Laboratories.[15] Their
discovery provided substantial confirmation of the general CMB
predictions—the radiation was found to be isotropic and consistent with
a blackbody spectrum of about 3 K—and it pitched the balance of opinion
in favor of the Big Bang hypothesis. Penzias and Wilson were awarded a
Nobel Prize for their discovery.
In 1989, NASA launched the Cosmic Background Explorer satellite (COBE),
and the initial findings, released in 1990, were consistent with the Big
Bang's predictions regarding the CMB. COBE found a residual temperature
of 2.726 K and in 1992 detected for the first time the fluctuations
(anisotropies) in the CMB, at a level of about one part in 105.[16] John
C. Mather and George Smoot were awarded Nobels for their leadership in
this work. During the following decade, CMB anisotropies were further
investigated by a large number of ground-based and balloon experiments.
In 2000–2001, several experiments, most notably BOOMERanG, found the
universe to be almost geometrically flat by measuring the typical
angular size (the size on the sky) of the anisotropies. (See shape of
the universe.)
In early 2003, the first results of the Wilkinson Microwave Anisotropy
satellite (WMAP) were released, yielding what were at the time the most
accurate values for some of the cosmological parameters. This satellite
also disproved several specific cosmic inflation models, but the results
were consistent with the inflation theory in general.[20] This satellite
is still gathering data. Another satellite will be launched within the
next few years, the Planck Surveyor, which will provide even more
accurate measurements of the CMB anisotropies. Many other ground- and
balloon-based experiments are also currently running; see Cosmic
microwave background experiments.
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 as ratios
to the amount of ordinary hydrogen, H.[25] All the abundances depend on
a single parameter, the ratio of photons to baryons, which itself can be
calculated independently from the detailed structure of CMB
fluctuations. The ratios predicted (by mass, not by number) are about
0.25 for 4He/H, about 10-3 for 2H/H, about 10-4 for 3He/H and about 10-9
for 7Li/H.[25]
The measured abundances all agree at least roughly with those predicted
from a single value of the baryon-to-photon ratio. The agreement is
excellent for deuterium, close but formally discrepant for 4He, and a
factor of two off for 7Li; in the latter two cases there are substantial
systematic uncertainties. Nonetheless, the general consistency with
abundances predicted by BBN is strong evidence for the Big Bang, as the
theory is the only known explanation for the relative abundances of
light elements, and it is virtually impossible to "tune" the Big Bang to
produce much more or less than 20–30% helium.[34] Indeed there is no
obvious reason outside of the Big Bang that, for example, the young
universe (i.e., before star formation, as determined by studying matter
supposedly free of stellar nucleosynthesis products) should have more
helium than deuterium or more deuterium than 3He, and in constant
ratios, too.
Galactic evolution and distribution
Detailed observations of the morphology and distribution of galaxies and
quasars provide strong evidence for the Big Bang. A combination of
observations and theory suggest that the first quasars and galaxies
formed about a billion years after the Big Bang, and since then larger
structures have been forming, such as galaxy clusters and superclusters.
Populations of stars have been aging and evolving, so that distant
galaxies (which are observed as they were in the early universe) appear
very different from nearby galaxies (observed in a more recent state).
Moreover, galaxies that formed relatively recently appear markedly
different from galaxies formed at similar distances but shortly after
the Big Bang. These observations are strong arguments against the
steady-state model. Observations of star formation, galaxy and quasar
distributions and larger structures agree well with Big Bang simulations
of the formation of structure in the universe and are helping to
complete details of the theory.[35]
Other lines of evidence
After some controversy, the age of universe as estimated from the Hubble
expansion and the CMB is now in good agreement with (i.e. slightly
larger than) the ages of the oldest stars, both as measured by applying
the theory of stellar evolution to globular clusters and through
radiometric dating of individual Population II stars.
The prediction that the CMB temperature was higher in the past has been
experimentally supported by observations of temperature-sensitive
emission lines in gas clouds at high redshift. This prediction also
implies that the amplitude of the Sunyaev-Zel'dovich effect in clusters
of galaxies does not depend directly on redshift; this seems to be
roughly true, but unfortunately the amplitude does depend on cluster
properties which do change substantially over cosmic time, so a precise
test is impossible.
Features, issues and problems
While very few researchers now doubt the Big Bang occurred, the
scientific community was once divided between supporters of the Big Bang
and those of alternative cosmological models. Throughout the historical
development of the subject, problems with the Big Bang theory were posed
in the context of a scientific controversy regarding which model could
best describe the cosmological observations (see the history section
above). With the overwhelming consensus in the community today
supporting the Big Bang model, many of these problems are remembered as
being mainly of historical interest; the solutions to them have been
obtained either through modifications to the theory or as the result of
better observations. Other issues, such as the cuspy halo problem and
the dwarf galaxy problem of cold dark matter, are not considered to be
fatal as it is anticipated that they can be solved through further
refinements of the theory.
The core ideas of the Big Bang—the expansion, the early hot state, the
formation of helium, the formation of galaxies—are derived from many
independent observations including Big Bang nucleosynthesis, the cosmic
microwave background, large scale structure and Type Ia supernovae, and
can hardly be doubted as important and real features of our universe.
Precise modern models of the Big Bang appeal to various exotic physical
phenomena that have not yet been observed in terrestrial laboratory
experiments or incorporated into the Standard Model of particle physics.
Of these features, dark energy and dark matter are the most secure,
while inflation and baryogenesis remain speculative: they provide
satisfying explanations for important features of the early universe,
but could be replaced by alternative ideas without affecting the rest of
the theory.[36] Explanations for such phenomena remain at the frontiers
of inquiry in physics.
The following is a short list of Big Bang "problems" and puzzles:
Horizon problem
The horizon problem results from the premise that information cannot
travel faster than light. In a universe of finite age, this sets a
limit—the particle horizon—on the separation of any two regions of space
that are in causal contact.[37] The observed isotropy of the CMB is
problematic in this regard: if the universe had been dominated by
radiation or matter at all times up to the epoch of last scattering, the
particle horizon at that time would correspond to about 2 degrees on the
sky. There would then be no mechanism to cause these regions to have the
same temperature.
A resolution to this apparent inconsistency is offered by inflationary
theory in which a homogeneous and isotropic scalar energy field
dominates the universe at some very early period (before baryogenesis).
During inflation, the universe undergoes exponential expansion, and the
particle horizon expands much more rapidly than previously assumed, so
that regions presently on opposite sides of the observable universe are
well inside each other's particle horizon. The observed isotropy of the
CMB then follows from the fact that this larger region was in causal
contact before the beginning of inflation.
Heisenberg's uncertainty principle predicts that during the inflationary
phase there would be quantum thermal fluctuations, which would be
magnified to cosmic scale. These fluctuations serve as the seeds of all
current structure in the universe. Inflation predicts that the
primordial fluctuations are nearly scale invariant and Gaussian, which
has been accurately confirmed by measurements of the CMB.
Flatness/oldness problem
The overall geometry of the universe is determined by whether the Omega
cosmological parameter is less than, equal to or greater than 1. From
top to bottom: a closed universe with positive curvature, a hyperbolic
universe with negative curvature and a flat universe with zero
curvature.
The overall geometry of the universe is determined by whether the Omega
cosmological parameter is less than, equal to or greater than 1. From
top to bottom: a closed universe with positive curvature, a hyperbolic
universe with negative curvature and a flat universe with zero
curvature.
The flatness problem (also known as the oldness problem) is an
observational problem associated with a
Friedmann-Lemaître-Robertson-Walker metric.[37] The universe may have
positive, negative or zero spatial curvature depending on its total
energy density. Curvature is negative if its density is less than the
critical density, positive if greater, and zero at the critical density,
in which case space is said to be "flat". The problem is that any small
departure from the critical density grows with time, and yet the
universe today remains very close to flat.[38] Given that a "natural"
timescale for departure from flatness might be the Planck time, 10−43
seconds, the fact that the universe has reached neither a Heat Death nor
a Big Crunch after billions of years requires some explanation. For
instance, even at the relatively "late" age of a few minutes (the time
of nucleosynthesis), the universe must have been within one part in 1014
of the critical density, or it would not exist as it does today.[39]
A resolution to this problem is offered by inflationary theory. During
the inflationary period, spacetime expanded to such an extent that its
curvature would have been smoothed out. Thus, it is believed that
inflation drove the universe to a very nearly spatially flat state, with
almost exactly the critical density.
Magnetic monopoles
The magnetic monopole objection was raised in the late 1970s. Grand
unification theories predicted topological defects in space that would
manifest as magnetic monopoles. These objects would be produced
efficiently in the hot early universe, resulting in a density much
higher than is consistent with observations, given that searches have
never found any monopoles. This problem is also resolved by cosmic
inflation, which removes all point defects from the observable universe
in the same way that it drives the geometry to flatness.[37]
Baryon asymmetry
It is not yet understood why the universe has more matter than
antimatter.[23] It is generally assumed that when the universe was young
and very hot, it was in statistical equilibrium and contained equal
numbers of baryons and anti-baryons. However, observations suggest that
the universe, including its most distant parts, is made almost entirely
of matter. An unknown process called "baryogenesis" created the
asymmetry. For baryogenesis to occur, the Sakharov conditions must be
satisfied. These require that baryon number is not conserved, that
C-symmetry and CP-symmetry are violated and that the universe depart
from thermodynamic equilibrium.[40] All these conditions occur in the
Standard Model, but the effect is not strong enough to explain the
present baryon asymmetry.
Globular cluster age
In the mid-1990s, observations of globular clusters appeared to be
inconsistent with the Big Bang. Computer simulations that matched the
observations of the stellar populations of globular clusters suggested
that they were about 15 billion years old, which conflicted with the
13.7-billion-year age of the universe. This issue was generally resolved
in the late 1990s when new computer simulations, which included the
effects of mass loss due to stellar winds, indicated a much younger age
for globular clusters.[41] There still remain some questions as to how
accurately the ages of the clusters are measured, but it is clear that
these objects are some of the oldest in the universe.
Dark matter
During the 1970s and 1980s, various observations (notably of galactic
rotation curves) 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 was initially
controversial, it is now indicated by numerous observations: the
anisotropies in the CMB, galaxy cluster velocity dispersions,
large-scale structure distributions, gravitational lensing studies, and
X-ray measurements of galaxy clusters.[42]
The evidence for dark matter comes from its gravitational influence on
other matter, and 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.[43]
Dark energy
Measurements of the redshift–magnitude relation for Type Ia supernovae
have revealed 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 pressure, dubbed
"dark energy". Dark energy is indicated by several other lines of
evidence. 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.[7] Since dark energy does not cluster
in the usual way it is the best explanation for the "missing" energy
density. Dark energy is also required by two geometrical measures of the
overall curvature of the universe, one using the frequency of
gravitational lenses, and the other using the characteristic pattern of
the large-scale structure as a cosmic "ruler".
Negative pressure is a property of vacuum energy, but the exact nature
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 2006, which combined data from the CMB and
other sources, indicate that the universe today is 74% dark energy, 22%
dark matter, and 4% regular matter.[17] 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.
In the best current model of the Big Bang, dark energy is explained by
the presence of a cosmological constant in the theory of General
Relativity. However, the size of the constant that properly explains
dark energy is surprisingly small relative to naive estimates based on
ideas about quantum gravity. Distinguishing between the cosmological
constant and other explanations of dark energy is an active area of
current research.
The future according to the Big Bang theory
Before observations of dark energy, cosmologists considered two
scenarios for the future of the universe. If the mass density of the
universe was greater than the critical density, then the universe would
reach a maximum size and then begin to collapse. It would become denser
and hotter again, ending with a state that was similar to that in which
it started—a Big Crunch.[32] Alternatively, if the density in the
universe was equal to or below the critical density, the expansion would
slow down, but never stop. Star formation would cease as all the
interstellar gas in each galaxy is consumed; stars would burn out
leaving white dwarfs, neutron stars, and black holes. Very gradually,
collisions between these would result in mass accumulating into larger
and larger black holes. The average temperature of the universe would
asymptotically approach absolute zero—a Big Freeze. Moreover, if the
proton was unstable, then baryonic matter would disappear, leaving only
radiation and black holes. Eventually, black holes would evaporate. The
entropy of the universe would increase to the point where no organized
form of energy could be extracted from it, a scenario known as heat
death.
Modern observations of accelerated expansion imply that more and more of
the currently visible universe will pass beyond our event horizon and
out of contact with us. The eventual result is not known. The ΛCDM model
of the universe contains dark energy in the form of a cosmological
constant. This theory suggests that only gravitationally bound systems,
such as galaxies, would remain together, and they too would be subject
to heat death, as the universe cools and expands. Other explanations of
dark energy—so-called phantom energy theories—suggest that ultimately
galaxy clusters, stars, planets, atoms, nuclei and matter itself will be
torn apart by the ever-increasing expansion in a so-called Big Rip.[44]
Speculative physics beyond the Big Bang
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 Penrose-Hawking singularity theorems[45]
require the existence of a singularity at the beginning of cosmic time.
However, these theorems assume that general relativity is correct, but
general relativity must break down before the universe reaches the
Planck temperature, and a correct treatment of quantum gravity may avoid
the singularity.
There may also be parts of the universe well beyond what can be observed
in principle. If inflation occurred this is likely, for exponential
expansion would push large regions of space beyond our observable
horizon.
Some proposals, each of which entails untested hypotheses, are:
* models including the Hartle-Hawking 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.[46]
* brane cosmology models[47] 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.[48]
* chaotic inflation, in which inflation events start here and there in a
random quantum-gravity foam, each leading to a bubble universe expanding
from its own big bang.[49]
Proposals in the last two categories see the Big Bang as an event in a
much larger and older universe, or multiverse, and not the literal
beginning.
Philosophical and religious interpretations
The Big Bang is a scientific theory, and as such stands or falls by its
agreement with observations. But as a theory which addresses, or at
least seems to address, creation itself, it has always been entangled
with theological and philosophical implications. In the 1920s and '30s
almost every major cosmologist preferred an eternal universe, and
several complained that the beginning of time implied by the Big Bang
imported religious concepts into physics; this objection was later
repeated by supporters of the steady state theory.[50] This perception
was enhanced by the fact that the theory's inventor, Georges Lemaître,
was a Roman Catholic priest. Lemaître himself always insisted that as a
physical theory, the Big Bang has no religious implications; and yet the
congruence between his scientific and religious beliefs is apparent in
his famous description of the beginning of the universe as "a day
without yesterday"—alluding to the creation account in Genesis. George
Gamow had no compunction in describing the graphs of conditions in the
Big Bang as "divine creation curves", and sent a copy of his book The
Creation of the Universe to the pope; yet even he favoured an
oscillating model in which the Big Bang was not a literal beginning. To
this day, many people's reactions to the Big Bang theory, both positive
and negative, are influenced by how well it can be harmonised with their
religious and philosophical world views.
Some interpretations of the Big Bang theory go beyond science, and some
purport to explain the cause of the Big Bang itself (first cause). These
views have been criticized by some naturalist philosophers as being
modern creation myths. Some people believe that the Big Bang theory is
inconsistent with traditional views of creation such as that in Genesis,
for example, while others, like astronomer and old Earth creationist
Hugh Ross, believe that the Big Bang theory lends support to the idea of
creation ex nihilo.[51]
A number of Christian and traditional Jewish sources have accepted the
Big Bang as a possible description of the origin of the universe,
interpreting it to allow for a philosophical first cause. Pope Pius XII
was an enthusiastic proponent of the Big Bang even before the theory was
scientifically well-established,[52][53] and consequently the Roman
Catholic Church has been a prominent advocate for the idea that creation
ex nihilo can be interpreted as consistent with the Big Bang. This view
is shared by many religious Jews in all branches of rabbinic Judaism.
Some groups contend the Big Bang is also consistent with the teaching of
creation according to Kabbalah. [54]
Some modern Islamic scholars[55] believe that the Qur'an parallels the
Big Bang in its account of creation, described as follows: "Do not the
unbelievers see that the heavens and the earth were joined together as
one unit of creation, before We clove them asunder?" (Ch:21,Ver:30). The
claim has also been made that the Qur'an describes an expanding
universe: "The heaven, We have built it with power. And verily, We are
expanding it." (Ch:51,Ver:47).[56] Parallels with the Big Crunch and an
oscillating universe have also been suggested: "On the day when We will
roll up the heavens like the rolling up of the scroll for writings, as
We originated the first creation, (so) We shall reproduce it; a promise
(binding on Us); surely We will bring it about." (Ch:21,Ver:104).
Notes and references
1. ^ Slipher, V. M.. "The radial velocity of the Andromeda nebula".
Lowell Observatory Bulletin 1: 56–57.
Slipher, V. M.. "Spectrographic observations of nebulae". Popular
Astronomy 23: 21–24.
2. ^ Friedman, A (1922). "Über die Krümmung des Raumes". Z. Phys. 10:
377–386. (German) (English translation in: Friedman, A (1999). "On the
Curvature of Space". General Relativity and Gravitation 31: 1991–2000.
DOI:10.1023/A:1026751225741. )
3. ^ Lemaître, G. (1927). "Un Univers homogène de masse constante et de
rayon croissant rendant compte de la vitesse radiale des nébuleuses
extragalactiques". Annals of the Scientific Society of Brussels 47A: 41.
(French) Translated in: (1931) "Expansion of the universe, A homogeneous
universe of constant mass and growing radius accounting for the radial
velocity of extragalactic nebulae". Monthly Notices of the Royal
Astronomical Society 91: 483–490.
4. ^ Lemaître, G. (1931). "The evolution of the universe: discussion".
Nature 128: suppl.: 704.
5. ^ a b Edwin Hubble (1929). "A relation between distance and radial
velocity among extra-galactic nebulae". Proceedings of the National
Academy of Sciences 15: 168–173.
6. ^ E. Christianson (1995). Edwin Hubble: Mariner of the Nebulae.
Farrar Straus & Giroux. ISBN 0374146608.
7. ^ a b c P. J. E. Peebles and Bharat Ratra (2003). "The cosmological
constant and dark energy". Reviews of Modern Physics 75: 559–606.
DOI:10.1103/RevModPhys.75.559. arXiv:astro-ph/0207347.
8. ^ E. A. Milne (1935). Relativity, Gravitation and World Structure.
Oxford University Press.
9. ^ R. C. Tolman (1934). Relativity, Thermodynamics, and Cosmology.
Oxford: Clarendon Press. LCCN 340-32023. Reissued (1987) New York: Dover
ISBN 0-486-65383-8.
10. ^ Zwicky, F (1929). "On the Red Shift of Spectral Lines through
Interstellar Space". Proceedings of the National Academy of Sciences 15:
773–779. Full article (PDF)
11. ^ Hoyle, Fred (1948). "A New Model for the Expanding universe".
Monthly Notices of the Royal Astronomical Society 108: 372.
12. ^ R. A. Alpher, H. Bethe, G. Gamow (1948). "The Origin of Chemical
Elements". Physical Review 73: 803.
13. ^ R. A. Alpher and R. Herman (1948). "Evolution of the Universe".
Nature 162: 774.
14. ^ Simon Singh. Big Bang. Retrieved on 2007-05-28.
15. ^ a b A. A. Penzias and R. W. Wilson (1965). "A Measurement of
Excess Antenna Temperature at 4080 Mc/s". Astrophysical Journal 142:
419.
16. ^ a b Boggess, N.W., et al. (COBE collaboration) (1992). "The COBE
Mission: Its Design and Performance Two Years after the launch".
Astrophysical Journal 397: 420, Preprint No. 92-02. DOI:10.1086/171797.
17. ^ a b c D. N. Spergel et al. (WMAP collaboration) (2006). "Wilkinson
Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for
Cosmology". Retrieved on 2007-05-27.
18. ^ S. W. Hawking and G. F. R. Ellis (1973). The large-scale structure
of space-time. Cambridge: Cambridge University Press. ISBN
0-521-20016-4.
19. ^ There is no consensus about how long the Big Bang phase lasted:
for some writers this denotes only the initial singularity, for others
the whole history of the universe. Usually at least the first few
minutes, during which helium is synthesised, are said to occur "during
the Big Bang".
20. ^ a b c Spergel, D. N.; et al. (2003). "First-Year Wilkinson
Microwave Anisotropy Probe (WMAP) Observations: Determination of
Cosmological Parameters". The Astrophysical Journal Supplement Series
148: 175—194. DOI:10.1086/377226.
21. ^ Guth, Alan H. (1998). The Inflationary Universe: Quest for a New
Theory of Cosmic Origins. Vintage. ISBN 978-0099959502.
22. ^ Schewe, Phil, and Ben Stein (2005). "An Ocean of Quarks". Physics
News Update, American Institute of Physics 728 (#1). Retrieved on
2007-05-27.
23. ^ a b Kolb and Turner (1988), chapter 6
24. ^ Kolb and Turner (1988), chapter 7
25. ^ a b c Kolb and Turner (1988), chapter 4
26. ^ Peacock (1999), chapter 9
27. ^ Ivanchik, A. V.; A. Y. Potekhin and D. A. Varshalovich (1999).
"The fine-structure constant: a new observational limit on its
cosmological variation and some theoretical consequences". Astronomy and
Astrophysics 343: 439.
28. ^ Detailed information of and references for tests of general
relativity are given at Tests of general relativity.
29. ^ This ignores the dipole anisotropy at a level of 0.1% due to the
peculiar velocity of the solar system through the radiation field.
30. ^ Goodman, J. (1995). "Geocentrism reexamined". Physical Review D
52: 1821. DOI:10.1103/PhysRevD.52.1821.
31. ^ d'Inverno, Ray (1992). Introducing Einstein's Relativity. Oxford:
Oxford University Press. ISBN 0-19-859686-3. Chapter 23
32. ^ a b Kolb and Turner (1988), chapter 3
33. ^ Peacock (1999), chapter 3
34. ^ Steigman, Gary. "Primordial Nucleosynthesis: Successes And
Challenges". arXiv:astro-ph/0511534.
35. ^ E. Bertschinger (2001). "Cosmological perturbation theory and
structure formation". arXiv:astro-ph/0101009.
Edmund Bertschinger (1998). "Simulations of structure formation in the
universe". Annual Review of Astronomy and Astrophysics 36: 599–654.
36. ^ If inflation is true, baryogenesis must have occurred, but not
vice versa.
37. ^ a b c Kolb and Turner (1988), chapter 8
38. ^ Strictly, dark energy in the form of a cosmological constant
drives the universe towards a flat state; but our universe remained
close to flat for several billion years, before the dark energy density
became significant.
39. ^ R. H. Dicke and P. J. E. Peebles. "The big bang cosmology —
enigmas and nostrums". S. W. Hawking and W. Israel (eds) General
Relativity: an Einstein centenary survey: 504–517, Cambridge University
Press.
40. ^ A. D., Sakharov (1967). "Violation of CP invariance, C asymmetry
and baryon asymmetry of the universe". Pisma Zh. Eksp. Teor. Fiz. 5: 32.
(Russian) Translated in JETP Lett. 5, 24 (1967).
41. ^ Navabi, A. A.; N. Riazi (2003). "Is the Age Problem Resolved?".
Journal of Astrophysics and Astronomy 24: 3.
42. ^ Keel, Bill. Galaxies and the Universe lecture notes - Dark Matter.
Retrieved on 2007-05-28.
43. ^ Yao, W. M.; et al. (2006). "Review of Particle Physics". J. Phys.
G: Nucl. Part. Phys. 33: 1–1232. DOI:10.1088/0954-3899/33/1/001. Chapter
22].
44. ^ (2003) "Phantom Energy and Cosmic Doomsday". Phys.Rev.Lett. 91:
071301. arXiv:astro-ph/0302506.
45. ^ Hawking, Stephen; and Ellis, G. F. R. (1973). The Large Scale
Structure of Space-Time. Cambridge: Cambridge University Press. ISBN
0-521-09906-4.
46. ^ J. Hartle and S. W. Hawking (1983). "Wave function of the
universe". Phys. Rev. D 28: 2960.
47. ^ Langlois, David (2002). "Brane cosmology: an introduction".
arXiv:hep-th/0209261.
48. ^ Linde, Andre (2002). "Inflationary Theory versus Ekpyrotic/Cyclic
Scenario". arXiv:hep-th/0205259.
49. ^ A. Linde (1986). "Eternal chaotic inflation". Mod. Phys. Lett. A1.
A. Linde (1986). "Eternally existing self-reproducing chaotic
inflationary universe". Phys. Lett. B175.
50. ^ Kragh, Helge (1996). Cosmology and Controversy. Princeton
University Press. ISBN 069100546X.
51. ^ Ross, Hugh. Putting the Big Bang to the Test. Retrieved on
2006-09-19.
52. ^ Pius XII (1952). "Modern Science and the Existence of God". The
Catholic Mind 49: 182–192.
53. ^ Lemaître protested, objecting to religious endorsement of any
scientific theory, even his own. See Kragh (1996): 258.
54. ^ The Kabbalah Centre. Adam and Atom. Retrieved on 2006-11-12.
55. ^ Ahmad, Mirza Tahir (1987). Revelation, Rationality, Knowledge &
Truth. Islam International Publications Ltd. ISBN 1-85372-640-0. Part 4,
chapter 5: The Quran and Cosmology
56. ^ There are more prosaic translations of this verse which do not
suggest an expanding universe including "And the heaven, We raised it
high with power, and We are Makers of the vast extent." and "With power
and skill did We construct the Firmament: for it is We Who create the
vastness of space". However, Islamic scholars generally consider only
the original Arabic text to be authoritative, and many state that the
original Arabic text indeed indicates an expanding universe.
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