SOPHIA OF WISDOM III - CAROLINE E. KENNEDY
THIS IS THE WAY IT HAPPENED IN THE BEGINNING BUT DUE TO PERSONS USING TO MUCH MAGIC AND
TRYING TO RECREATE WHAT WAS ALREADY CREATED THE BIG BANG THEORY HAPPENS EVERYDAY
The Hubble Ultra Deep Field, or HUDF, is an image
of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 3, 2003 through January 16, 2004. It is the deepest image of the universe ever taken in visible light, looking back (to when the universe is thought to have been 800 million years old) more than
13 billion years ago. The HUDF contains an estimated 10,000 galaxies. The patch of sky in which the galaxies reside (just one-tenth the diameter of the full moon as viewed from Earth) was chosen because it had a low density of bright stars in the near-field. Although most of
the targets visible in the Hubble image can also be seen at infrared wavelengths by ground-based telescopes, Hubble is the only instrument which can make observations
of these distant targets at visible wavelengths. Located southwest of Orion in the Southern-Hemisphere constellation Fornax at right ascension 3h 32m 40.0s, declination -27° 47' 29" (J2000), the image covers 36.7 square arcminutes. This is smaller than a 1 mm by 1 mm square of paper held 1 meter away, and equal to roughly
one thirteen-millionth of the total area of the sky. The image is oriented such that the upper left corner points toward north
(-46.4°) on the celestial sphere. The star near the center of the field is USNO-A2.0 0600-01400432 with apparent magnitude of 18.95.
In total, the image required 800 exposures taken over the course of 400 Hubble orbits around
Earth. The total amount of exposure time was 11.3 days for the ACS and 4.5 days for the NICMOS.
According to the Big Bang theory, the universe has a finite age, so we might expect very distant (and hence very young)
galaxies to look different from the typical older galaxies we see today. This is indeed seen in the HUDF, although some argue that the difference is partly a result of the unusual wavelength used for the HUDF (corresponding to ultraviolet light from the rest-frame of the most distant galaxies). The Hubble Ultra Deep Field also shows more evidence for galaxy formation and merging than in local studies,
as expected for the early universe .
SOPHIA OF WISDOM III - CAROLINE E. KENNEDY
The Big Bang
is a cosmological model of the universe that has become well supported by several independent observations. After Edwin Hubble discovered that galactic distances were generally proportional to their redshifts in 1929, this observation was taken to indicate that the universe is expanding. If the universe is seen to be expanding today, then it must have been smaller, denser, and hotter in the past. This
idea has been considered in detail all the way back to extreme densities and temperatures, and the resulting conclusions have
been found to conform very closely to what is observed.
Ironically, the term 'Big Bang' was first coined by Fred Hoyle in a derisory statement seeking to belittle the credibility of the theory that he did not believe
to be true. However, the discovery of the cosmic microwave background in 1964 was taken as almost undeniable support for the Big
Analysis of the spectrum of light from distant galaxies reveals a shift towards longer wavelengths proportional to each galaxy's distance in a relationship described by Hubble's law, which is taken to indicate that the universe is undergoing a continuous expansion. Furthermore,
the cosmic microwave background radiation discovered in 1964 provides strong evidence that due to the expansion, the universe has naturally
cooled from an extremely hot, dense initial state. The discovery of the cosmic microwave background led to almost universal
acceptance among physicists, astronomers, and astrophysicists that the Big Bang describes the evolution of the universe quite
well, at least in its broad outline.
Further evidence supporting the Big Bang model comes from the relative proportion of light
elements in the universe. The observed abundances of hydrogen and helium throughout the cosmos closely match the calculated
predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes
of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.
However, there are mysteries of the universe that are not explained by the Big Bang model alone.
For example, a region of the universe 12 billion lightyears distant in one direction appears little different than a region
12 billion lightyears distant in the opposite direction. But since the universe is 'only' around 13.7 billion years old, it
would appear these regions could never have been causally connected. How, then, can they be so similar? Alan Guth's 1981 theory of cosmic inflation, a short, sudden burst of extreme exponential expansion in the very early universe, provided
an explanation for this horizon problem and several of the features unaccounted for by the original Big Bang model. The successor to
Guth's original theory has found some circumstantial support, but it is not yet nearly as well supported as the Big Bang model.
- See also: Timeline of cosmology and History of astronomy
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. 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. 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 physicist and Roman Catholic priest, predicted that the recession of the nebulae was
due to the expansion of the universe.
In 1931 Lemaître went further and suggested that the evident expansion in forward time required that the universe
contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the
universe into a single point, a "primeval atom", at a point in time before which time and space did not exist. As such, at this point, the fabric
of time and space had not yet come into existence. This perhaps echoed previous speculations about the cosmic egg origin of the universe.
Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner
of the cosmic distance ladder, using the 100-inch (2,500 mm) 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. Lemaître had already shown that this was expected, given the Cosmological Principle.
Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang.
During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model, the oscillatory universe (originally suggested by Friedmann, but advocated by Einstein and Richard Tolman) and Fritz Zwicky's tired light hypothesis.
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. The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background (CMB). 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 as "this big bang idea" in derision during a 1950 BBC radio broadcast. 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 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, the Hubble Space Telescope and WMAP. 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.
Timeline of the Big Bang
- See also: Timeline of the Big Bang
Extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity
is debated—certainly not earlier than the Planck epoch. The early hot, dense phase is itself referred to as "the Big Bang", 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 13.73 ± 0.12 billion years old. 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. After inflation stopped, the universe consisted of a quark-gluon plasma, as well as all other elementary particles. Temperatures were so high that the random motions of particles were at relativistic speeds, and particle-antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown
reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and anti-leptons—of the order of 1 part in 30 million. This resulted in
the predominance of matter over antimatter in the present universe.
The universe continued to grow in size and fall in temperature, hence the typical energy of
each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values
that can be attained in particle physics experiments. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess
of baryons over antibaryons. The temperature was now no longer high enough to create new proton-antiproton pairs (similarly
for neutrons-antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original
protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons.
After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy
density of the universe was dominated by photons (with a minor contribution from neutrinos).
A few minutes into the expansion, when the temperature was about a billion (one thousand million;
109; SI prefix giga) Kelvin and the density was about that of air, neutrons combined with protons to form the universe's
deuterium and helium nuclei in a process called Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This
relic radiation is known as the cosmic microwave background radiation.
The Hubble Ultra Deep Field showcases galaxies from an ancient era when the universe was younger, denser, and warmer according
to the Big Bang theory.
Over a long period of time, the slightly denser regions of the nearly uniformly distributed
matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process
depend on the amount and type of matter in the universe. The 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 18% of the matter in the universe..
Independent lines of evidence from Type Ia supernovae and the CMB imply the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 72% of the total energy density
of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less
space and everything closer together, gravity had the upper hand, and it was slowly braking the expansion. But eventually,
after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the
cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the
details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.
All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General
Relativity. As noted above, there is no well supported model describing the action prior to 10−15 seconds
or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the universe
is currently one of the greatest unsolved problems in physics.
Big bang theory assumptions
The Big Bang theory depends on two major assumptions: the universality of physical laws, and the Cosmological Principle. The cosmological principle states that on large scales the universe is homogeneous and isotropic.
These ideas were initially taken as postulates, but today there are efforts to test each of
them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the universe is of order 10−5. Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales
has been validated by the empirical successes of various aspects of the Big Bang theory.
If the large-scale universe appears isotropic as viewed from Earth, the cosmological principle
can be derived from the simpler Copernican Principle, which states that there is no preferred (or special) observer or vantage point. To this end,
the cosmological principle has been confirmed to a level of 10−5 via observations of the CMB. The universe has been measured to be homogeneous on the largest scales at the 10% level.
General relativity describes spacetime by a metric, which determines the distances that separate nearby points. The points, which can be galaxies,
stars, or other objects, themselves are specified using 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.
The Big Bang is not an explosion of matter moving outward to fill an empty universe. Instead,
space itself expands with time everywhere and increases the physical distance between two comoving points. Because
the FLRW metric assumes a uniform distribution of mass and energy, it applies to our universe only on large scales—local
concentrations of matter such as our galaxy are gravitationally bound and as such do not experience the large-scale expansion
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 or 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 FLRW
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.
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 been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark energy and dark matter are considered 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. Explanations for such phenomena remain at the frontiers of inquiry in physics.
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. 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.
The flatness problem (also known as the oldness problem) is an observational problem associated
with a Friedmann-Lemaître-Robertson-Walker metric. 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. 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.
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.
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.
A resolution to the horizon, flatness, and magnetic monopole problems alternative to cosmic
inflation is offered by the Weyl curvature hypothesis.
It is not yet understood why the universe has more matter than antimatter. 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. 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. 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.
A pie chart indicating the proportional composition of different energy-density components of the universe,
according to the best ΛCDM model fits. Roughly ninety-five percent is in the exotic forms of dark matter and dark energy
During the 1970s and 1980s, various observations showed that there is not sufficient visible
matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led
to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions
that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less
deuterium than can be accounted for without dark matter. While dark matter 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.
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
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. 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 2008, which combined data from the CMB and other sources, indicate
that the universe today is 72% dark energy, 23% dark matter, 4.6% regular matter and less then 1% of neutrinos. 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
In the ΛCDM, the best current model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the general theory of 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 were 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. Alternatively, if the density in the universe were 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 were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black
holes would evaporate by emitting Hawking radiation. 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 expands and cools. 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.[
Speculative physics beyond the Big Bang
A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic
expansion of the metric
seen on the left.
Image from WMAP
press release, 2006.
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 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 no-boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but
without the need for a singularity.
- brane cosmology models in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.
- 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.
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, the origins of reality, 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. This perception was enhanced by the fact that Georges Lemaître, who put the theory forth, was a Roman Catholic priest.
Some arguments for the Big Bang model include the fact that the universe is continually expanding, as well as evidence that the universe was in a very hot state long ago. The model is often used
in conjunction with the theory of evolution, inferring that not only life, but all of the universe, has evolved. Although
widely supported by atheists, many theists also endorse the theory, as long as it can co-exist along with their Creator theory.
Those questioning the Big Bang theory often try to disprove it through a series of questions,
each asking what caused something, leading to "what caused the Big Bang", with the rhetorical answer being the "uncaused cause", a God. Claimed scientific chinks in the Bang theory's armor are that of the above-mentioned flat/oldness
problem and the horizon problem, although they deal more with miscalculations of the model rather than there not being one
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