In cosmology, the Big Bang is the scientific theory that describes the early development and shape of the universe. The central idea is that the
theory of general relativity can be combined with the
observations on the largest scales of galaxies receding from each other to extrapolate the conditions of the universe back or forward in time. A natural consequence of the Big Bang is that in the past the universe had a higher temperature and a higher density. The
term "Big Bang" is used both in a narrow sense to refer to a point in time when the observed expansion of the universe (Hubble's law) began, and in a more general sense to refer to the prevailing
cosmological paradigm explaining the origin and evolution of the universe.
The term "Big Bang" was coined in 1949 by Fred Hoyle during a BBC radio program, The Nature of Things; the text
was published in 1950. Hoyle did not subscribe to the theory and intended to mock the
concept.
One consequence of the Big Bang is that the conditions of today's universe are different from the conditions in the past or in
the future. From this model, George Gamow in 1948 was able to predict that there should be
evidence for a Big Bang in a phenomenon that would later be called the cosmic microwave background radiation (CMB). The CMB was discovered in the
1960s and served as a confirmation of the Big Bang theory over its chief rival, the
steady state theory.
According to the Big Bang, 13.7 billion (13.7 × 109) years ago the universe was in an incredibly dense state with
huge temperatures and pressures. There is no compelling physical model for the first 10-33 seconds of the universe.
Einstein's theory of gravity predicts a gravitational
singularity where densities become infinite. To resolve this paradox, a theory of quantum gravity is needed. Understanding this period of the history of the universe is one of the greatest
unsolved problems in physics.
History of the theory
In 1927, the Belgian Jesuit priest Georges Lemaître was the first
to propose that the universe began with the "explosion" of a "primeval atom". Earlier, in
1918, the Strasbourg astronomer Carl Wilhelm 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.
Albert Einstein's theory of general relativity, developed during this time, admitted no static solutions (that is to say, the
universe had to be either expanding or shrinking), a result that he himself considered wrong, and which he tried to fix by adding
a cosmological constant. Applying general relativity to
cosmology was first done by Alexander Friedmann whose
equations describe the Friedmann-Lemaître-Robertson-Walker universe.
In 1929, Edwin Hubble provided an
observational basis for Lemaître's theory. Having, in 1913, already determined that most
spiral nebulae (what would later be determined to be galaxies) were receding from
Earth, Hubble combined this with distance measurements determined by observing Cepheid variable stars in distant galaxies to discover that the galaxies are receding in every direction
at speeds (relative to the Earth) directly proportional to their distance. This fact is now known as Hubble's law (see Edwin Hubble: Mariner of the Nebulae by Edward
Christianson).
Given the cosmological principle, receding galaxies
suggested two opposing possibilities. One, advocated and developed by George Gamow, was that the universe emerged from an
extremely hot, dense state a finite time in the past, and has been expanding ever since. The other possibility was Fred Hoyle's steady state
model in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly
the same at any point in time. For a number of years the support for these theories was evenly divided.
In the intervening years, the observational evidence supported the idea that the universe evolved from a hot dense state.
Since the discovery of the cosmic microwave background in 1965 it has been regarded as the
best theory of the origin and evolution of the cosmos. Before the late 1960s, many
cosmologists thought the infinitely dense singularity found in Friedmann's
cosmological model was a mathematical over-idealization, and that the universe was contracting before entering the hot dense
state and starting to expand again. This is Richard Tolman's oscillating universe. In the sixties, Stephen Hawking and others demonstrated that this idea was unworkable, and
the singularity is an essential feature of Einstein's gravity. This led the majority of cosmologists to accept the Big Bang, in
which the universe we observe began a finite time ago.
Virtually all theoretical work in cosmology now involves extensions and refinements to the basic Big Bang theory. Much of the
current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding what happened at
the Big Bang, and reconciling observations with the basic theory.
Huge advances in Big Bang cosmology were made in the late 1990s and the early 21st century as a result of major advances in telescope technology in combination with large amounts of satellite data such as that from COBE, the Hubble space telescope and
WMAP. These data have allowed cosmologists to calculate many of the parameters of the Big
Bang to a new level of precision and led to the unexpected discovery that the expansion of the universe appears to be accelerating.
See also: Timeline of cosmology
Descriptive overview
Based on measurements of the expansion of the universe using Type Ia
supernovae, measurements of the lumpiness of the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a measured
age of 13.7 ± 0.2
billion years. The fact that these three independent measurements are consistent is considered strong evidence for the
so-called concordance model that describes the detail nature of the
contents of the universe.
The early universe was filled homogeneously and isotropically with a very high energy
density. Approximately 10-35 seconds after the Planck epoch,
the universe expanded exponentially during a period called
cosmic inflation. After inflation stopped, the material components
of the universe were in the form of a quark-gluon plasma where
the constituent particles were all moving relativistically. By an as yet
unknown process, baryogenesis occurred producing the observed asymmetry between matter and antimatter. As the universe grew in size, the temperature dropped, leading to further
symmetry breaking processes that manifested themselves as the
known forces of physics, elementary particles, and later allowed for the formation of the universe's hydrogen and helium atoms in
a process called Big Bang nucleosynthesis. As the
universe cooled, matter gradually stopped moving relativistically and its rest
mass energy density came to gravitationally dominate over radiation. After about 300,000 years the radiation decoupled from the atoms and continued through space
largely unimpeded. This relic radiation is the cosmic microwave background.
Over time, the slightly denser regions of the nearly uniformly distributed matter gravitationally grew into even denser
regions, forming gas clouds, stars, galaxies, and the other astronomical structures
observable today. The details of this process are dependent on the amount and type of matter in the universe. The three possible
types 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 in the form of cold dark matter. The other two types of matter make up less
than 20% of the matter in the universe.
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 component
of the universe's composition has the property of causing the expansion of the
universe to deviate from a linear velocity-distance relationship by causing spacetime to expand faster than expected at very large distances. Dark energy takes the form of a cosmological constant term in Einstein's field equations of general relativity, but
the details of its equation of state and relationship with the
standard model of particle physics continue to be investigated both
observationally and theoretically.
See also: Timeline of the Big Bang
Theoretical underpinnings
As it stands today, the Big Bang is dependent on three assumptions:
- The universality of physical laws
- The cosmological principle
- The Copernican principle
When first developed, these ideas were simply taken as postulates, but today there are efforts underway to test each of them.
The universality of physical laws has been tested to the level that the largest deviation of physical constants over the age of the universe can be is of
order 10-5. The isotropy of the universe that defines the Cosmological
Principle has been tested to a level of 10-5 and the universe has been measured to be homogenous on the largest scales
to the 10% level. There are efforts currently underway to test the Copernican Principle by means of looking at the interaction of
clusters of galaxies and the CMB through the Sunyaev-Zeldovich Effect to a level of 1% accuracy.
The Big Bang theory uses Weyl's postulate to unambiguously
measure time at any point as the "time since the Planck epoch". Measurements in this system rely on conformal
coordinates in which so-called comoving distances and
conformal times remove the expansion of the universe from consideration of spacetime measurements. In such a coordinate system, objects moving with the cosmological flow are always the
same comoving distance away and the horizon or limit of the
universe is set by the conformal time.
The Big Bang is therefore not an explosion of matter moving outward to fill an empty universe; it is spacetime itself that is
expanding. It is this expansion that causes the physical distance between any two fixed points in our universe to increase.
Objects that are bound together (for example, by gravity) do not expand with
spacetime's expansion because the physical laws that govern them are assumed to be uniform and independent of the metric expansion. Moreover, the expansion of the universe on today's local scales
is so small that any dependence of physical laws on the expansion is unmeasurable by current techniques.
Evidence
It is generally stated that there are three observational pillars that support the Big Bang theory of cosmology. These 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. Additionally, the observed
correlation function of
large scale structure in the
universe fits well with standard Big Bang theory.
Hubble law expansion
Observations of distant galaxies and quasars show that these objects are redshifted, meaning that the light emitted from
them has been proportionately shifted to longer wavelengths. This is seen by taking a spectrum of the objects and then matching the spectroscopic
pattern of emission or absorption lines corresponding to atoms of the elements interacting with the radiation. From this analysis, a measured redshift can be
determined which is explained by a recessional velocity corresponding to a Doppler shift for the radiation. When the recessional velocities are plotted against the distances to the
objects, a linear relationship, known as the Hubble Law, is observed:
v = H0 D
where v is the recessional velocity, D is the distance to the object and H0 is the Hubble
constant measured to be 71 ± 4 km/s/Mpc by the WMAP probe.
Cosmic microwave background radiation
One feature of the Big Bang theory was the prediction of the cosmic microwave background radiation or CMB. As the early universe cooled off due to
the expansion, the universe's temperature would fall below 3000 K. Above this
temperature, electrons and protons are separate, making the universe opaque to light. Below 3000 K, atoms form, allowing light to
pass freely through the gas of the universe. This is known as photon
decoupling.
The radiation from this region will travel unimpeded for the remainder of the lifetime of the universe, becoming redshifted
because of the Hubble expansion. This results in a redshift of the uniformly distributed blackbody spectrum of the 3000 K to 3 K. It is observed at every point in the universe to come from all
directions of space.
In 1964, Arno Penzias and Robert Wilson, while conducting a series of diagnostic
observations using a new microwave receiver owned by Bell Laboratories, discovered the cosmic background radiation. Their
discovery provided substantial confirmation of the general CMB predictions, and pitched the balance of opinion in favor of the
Big Bang hypothesis. Penzias and Wilson were awarded the 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 theory's predictions regarding CMB, finding a local residual temperature of 2.726 K and determining that the CMB was
isotropic to an accuracy of 10-5. During the 1990s, CMB data was studied
further to see if small anisotropies predicted by the Big Bang theory would be observed. They were found in 2000 by the Boomerang experiment.
In early 2003 the results of the Wilkinson Microwave
Anisotropy satellite (WMAP) were analyzed, 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. The abundances predicted are about 25 percent for 4He, a
2H/H ratio of about 10-3, a 3He/H of about 10-4 and a 7Li/H abundance of
about 10-9.
Measurements of primordial abundances for all four isotopes are consistent with a
unique value of that parameter and the fact that the measured abundances are in the same range as the predicted ones is
considered strong evidence for the Big Bang. There is no obvious reason outside of the Big Bang that, for example, the universe
should have more helium than deuterium or more deuterium than 3He.
Galactic evolution and quasar distribution
The details of the distribution of galaxies and quasars are both constraints and confirmations of current
theory. The finite age of the universe at earlier times means that galaxy evolution is closely tied to the cosmology of the universe. The types and
distribution of galaxies appears to change markedly over time, evolving by means of the Boltzmann Equation. Observations reveal a time-dependent relationship of the galaxy and quasar distributions, star formation
histories, and the type and size of the largest-scale structures in the universe (superclusters). These observations are in statistical agreement with simulations. They are well explained by
the Big Bang theory and help constrain model parameters.
Standard problems
Historically, a number of problems have arisen within the Big Bang theory. Some of them are today mainly of historical
interest, and have been avoided 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 they can be addressed through refinements of the
theory. Some detractors of the Big Bang cite these problems as ad hoc modifications and
addenda to the theory. Most often attacked are the parts of standard cosmology that include dark matter, dark energy, and cosmic inflation. These are strongly suggested by observations of the
cosmic microwave background, large scale structure and type IA supernovae, but remain at the frontiers of inquiry in physics. There is not yet a consensus on the
particle physics origin of dark matter, dark energy and inflation. While their gravitational effects are understood observationally and theoretically, they have not yet
been incorporated into the standard model of particle physics in an accepted way.
There are a small number of proponents of non-standard
cosmologies who believe that there was no Big Bang at all. While some aspects of standard cosmology are inadequately
explained in the standard model, most physicists accept that the close
agreement between Big Bang theory and observation have firmly established all the basic parts of the theory.
What follows is a short list of standard Big Bang "problems" and puzzles:
The horizon problem
The horizon problem results from the premise that
information cannot travel faster than light, and hence two
regions of space which are separated by a greater distance than the speed of light multiplied by the age of the universe cannot
be in causal contact. The observed isotropy of the
cosmic microwave background (CMB) is
problematic in this regard, because the horizon size at that time
corresponds to a size that is about 2 degrees on the sky. If the universe has had the same expansion history since the Planck epoch, there is no mechanism to allow for these regions to have the same
temperature.
This apparent inconsistency is resolved by inflationary
theory in which a homogeneous and isotropic scalar energy field dominates the universe at a time 10-35 seconds
after the Planck epoch. During inflation, the universe undergoes exponential expansion, and regions in causal contact expand past
each other's horizons. Heisenberg's uncertainty principle predicts that there would be quantum thermal fluctuations during the inflationary phase,
which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the universe. After
inflation, the universe expands by means of a Hubble Law, and regions that were
out of causal contact come back into the horizon. This explains the observed isotropy of the CMB. Inflation predicted that the
primordial fluctuations are nearly scale invariant and Gaussian which has been accurately confirmed by measurements of the CMB.
Flatness
The flatness problem is an observational problem that
results from considerations of the geometry associated
with Friedmann-Lemaître-Robertson-Walker metric. In general, the universe can have three
different kinds of geometries: hyperbolic geometry, Euclidean geometry, or elliptic geometry. Each one of these geometries is tied directly to the critical density of the universe, the hyperbolic corresponding to less than the critical density,
elliptic corresponding to greater than the critical density, and Euclidean corresponding to exactly equal to the critical
density. The universe is measured to be required to be within one part in 1015 of the critical density in its earliest
stages. Any deviation more than that would have caused either a Heat Death or a
Big Crunch and the universe would not exist as it does today.
The resolution to this problem is again offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that any
residual curvature associated with it would have been completely smoothed out to
a high degree of precision. Thus, the universe is driven to be flat by inflation.
Magnetic monopoles
The magnetic monopole problem was an objection that was
raised in the late-1970s. Grand unification theories predicted point defects in space that would manifest as magnetic monopoles, and the density of these monopoles was much higher than what could be accounted for.
This problem is also resolvable by the addition of cosmic inflation
which removes all point defects from the observable universe in the same way that the geometry is driven to flat.
Missing matter
During the 1970s and 1980s various
observations (notably of galactic rotation curves)
showed that there was 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 non-baryonic dark matter. In addition, assuming that
the universe was mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the
universe is far less lumpy and contains far less deuterium than can be accounted
for without dark matter. While dark matter was initially controversial, it is
now a widely accepted part of standard cosmology due to observations in the anisotropies in the CMB, galaxy cluster velocity dispersions, large scale structure distributions,
gravitational lensing studies, and x-ray measurements from galaxy clusters. Dark matter
particles have only been detected through their gravitational signatures, and have not yet been observed in laboratories.
However, there are many particle physics candidates for dark matter, and several projects to detect them are underway.
Dark energy
In the 1990s, detailed measurements of the mass density of the universe revealed a value that was 30% that of the critical density. For the universe to be flat, as is indicated by measurements of the cosmic microwave background, this would have meant
that fully 70% of the energy density of the universe was left unaccounted for. Measurements of Type Ia supernovae reveal that the universe is undergoing a non-linear acceleration of the Hubble Law expansion of
the universe. General relativity requires that this additional
70% be made up by an energy component with large negative
pressure. The nature of the so-called dark energy remains one of
the great mysteries of the Big Bang. Possible candidates include a scalar cosmological constant and quintessence. Observations to help understand this are ongoing.
Globular cluster age
A certain set of observations were made in the mid-1990s involving the ages of
globular clusters that were found to be inconsistent with
the Big Bang. Computer simulations of 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 with other new
computer simulations which included the effects of mass loss due to stellar
winds indicated a much younger age for globular clusters. 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.
The future according to the Big Bang theory
In the past, before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the
mass density of the universe is above the critical density, then the universe would reach a maximum size and begin to collapse in a Big Crunch. In this scenario, the universe would become denser and hotter again, ending
with a state that was similar to that in which it started. Alternatively, if the mass
density in the universe were equal to or below the critical
density, the expansion would slow down, but never stop. New star formation would cease as the universe grows less dense. The
average temperature of the Universe would asymptotically approach absolute
zero. 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 also known as heat death. Moreover, if proton decay exists,
then hydrogen, the predominant form of baryonic matter in the universe today, would disappear, leaving only radiation.
Modern observations of accelerated expansion have led
cosmologists to the Lambda-CDM model of the universe. This model
contains dark energy, in the form of a cosmological constant. This energy causes more and more of the presently visible universe to
pass beyond our horizon and out of contact with us. It is not known what
will happen after this. The cosmological constant 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 so-called phantom energy theories suggest that ultimately galaxy clusters and eventually galaxies themselves will
be torn apart by the ever increasing expansion in a so-called Big Rip.
See also Ultimate fate of the
universe.
Speculative physics beyond the Big Bang
There remains the possibility that the Big Bang will be developed in the future and in particular, we might learn something
about inflation or whatever came immediately before the Big Bang.
It might be the case that there are parts of the universe well beyond what can be observed in principle. In the case of inflation
this is required: exponential expansion has pushed large regions of space beyond our observable horizon. It may be possible to deduce what happened when we better understand physics at very high
energy scales. Speculations about this tend to involve theories of quantum gravity.
Some proposals are:
- chaotic inflation
- brane cosmology models, including the ekpyrotic model in which the Big Bang is the result of a collision between branes
- oscillatory universe which holds that the early
universe's the hot, dense state matches on to a contracting universe similar to ours. This yields a universe with an infinite
number of big bangs and big crunches. The cyclic extension of the ekpyrotic
model is a modern version of such a scenario.
- models including the Hartle-Hawking boundary condition
in which the whole of space-time is finite
Some of these scenarios are qualitatively compatible with one another. Each involves untested hypotheses.
Philosophical and religious interpretations
Philosophically, there are a number of interpretations of the Big Bang theory that are entirely speculative or
extra-scientific. Some of these ideas purport to explain the cause of the Big Bang itself (first cause), and have been criticized by some naturalist philosophers as being modern creation myths. Some people believe that the Big Bang theory lends support to traditional views of creation,
for example as given in Genesis, while others believe that all Big Bang theories are
inconsistent with such views.
The Big Bang as a scientific theory is not associated with any religion. While certain fundamentalist interpretations of religions conflict with the history of the universe as put forth by the
Big Bang, there are also more liberal interpretations that do not.
The following is a list of various religious interpretations of the Big Bang theory:
- A number of Christian apologists, and the Roman Catholic
Church in particular, have accepted the Big Bang as a description of the origin of the universe, interpreting it to allow for
a philosophical first cause.
- Students of Kabbalah, deism and other
non-anthropomorphic faiths concord with the Big Bang theory, notably the theory of "divine retraction" (Tzimtzum), as explained by Jewish Scholar Moses
Maimonides. Similarly, Pandeists, who believe that an initially sentient God
designed and then transformed himself into the non-sentient universe, often identify the Big Bang as the moment of
transformation.
- Some modern Islamic scholars believe that the Qur'an parallels the Big Bang in its account of creation, described as follows: "the heavens and the earth were
joined together as one unit, before We clove them asunder" (21:30). The Qur'an also appears to describe an expanding universe:
"The heavens, We have built them with power. And verily, We are expanding it" (51:47).
- Certain theistic branches of Hinduism, such as the Vaishnava-traditions, conceive of a theory
of creation with similarities to the theory of the Big Bang. The Hindu-mythos, narrated for example in the third book of the
Bhagavata Purana (primarily, chapters 10 and 26), describes a primordial state which bursts forth as the Great Vishnu glances over it, transforming into the active state of the sum-total of matter
("prakriti").
- Buddhism has a concept of a universe that has no creation event per se. The Big
Bang, however, is not seen to be in conflict with this since there are ways to get an eternal universe within the paradigm. A
number of popular Zen philosophers were intrigued, in particular, by the concept of the
oscillating universe.
- Cosmology, astrophysics and astronomy
- Physics topics
- Cosmic microwave background radiation
- Observational experiments
- Atomic chemical elements
- Lists
- Humor
- Big Bang used in other Fiction
External links and references
Big Bang overviews
Beyond the Big Bang
History
- D'Agnese, Joseph, "The last Big Bang man left standing, physicist Ralph Alpher
devised Big Bang Theory of universe (http://www.findarticles.com/p/articles/mi_m1511/is_7_20/ai_55030837)". Discover, July,
1999.
- Felder, Gary, "The Expanding Universe (http://www.ncsu.edu/felder-public/kenny/papers/cosmo.html)".
- Links to sample text and reviews: Big Bang by Simon Singh (http://www.321books.co.uk/reviews/big-bang-simon-singh.htm)
- John C Mather and John Boslough 1996, The very first light : the true
inside story of the scientific journey back to the dawn of the universe. ISBN 0-465-01575-1 p.300: LeMaitre, Annals
of the Scientific Society of Brussels 47A (1927):41 - GRT implies universe had to
be expanding. But Einstein brushed him off in the same year. LeMaitre's note was translated in Monthly Notice of the Royal
Astronomical Society (1931):483-490.
- See also LeMaitre, Nature 128(1931) suppl.:704. with a reference to the primeval atom.
- See review article by Ralph Alpher and Robert Herman Physics Today Aug 1988 pp24-34 which references
- Alpher 1948 Phys Rev D 74,1737
- Alpher and Herman 1948 Phys Rev D 74,1577
- Alpher Herman and Gamow 1948 Nature 162,774
Research articles
These are generally full of technical language, but sometimes with introductions in plain English.
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