Physical cosmology is the study of the largest-scale structures and dynamics of the
Universe and is concerned with fundamental questions about its origin, structure, evolution, and ultimate fate.
[1] For most of human history, it was a branch of
metaphysics and
religion.
Cosmology as a
science originated with the
Copernican principle, which implies that celestial bodies obey identical
physical laws to those on Earth, and
Newtonian mechanics, which first allowed us to understand those physical laws.
Physical cosmology, as it is now understood, began with the development in 1915 of
Albert Einstein's
general theory of relativity, followed by major observational discoveries in the 1920s: first,
Edwin Hubble discovered that the universe contains a huge number of external
galaxies beyond our own
Milky Way; then, work by
Vesto Slipher and others showed that the universe is
expanding. These advances made it possible to speculate about the
origin of the universe, and allowed the establishment of the
Big Bang Theory, by
Georges Lemaitre, as the leading cosmological model. A few researchers still advocate a handful of
alternative cosmologies;
[2] however, most cosmologists agree that the Big Bang theory explains the observations better.
Dramatic advances in observational cosmology since the 1990s, including the
cosmic microwave background, distant
supernovae and galaxy
redshift surveys, have led to the development of a
standard model of cosmology. This model requires the universe to contain large amounts of
dark matter and
dark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.
[3]
Cosmology draws heavily on the work of many disparate areas of research in theoretical and applied
physics. Areas relevant to cosmology include
particle physics experiments and
theory, theoretical and observational
astrophysics,
general relativity,
quantum mechanics, and
plasma physics.
Subject history
Modern cosmology developed along tandem tracks of theory and observation. In 1916, Albert Einstein published his theory of
general relativity, which provided a unified description of gravity as a geometric property of space and time.
[4] At the time, Einstein believed in a
static universe, but found that his original formulation of the theory did not permit it.
[5] This is because masses distributed throughout the universe gravitationally attract, and move toward each other over time.
[6] However, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this
cosmological constant to his field equations in order to force them to model a static universe.
[7] However, this so-called Einstein model is unstable to small perturbations—it will eventually start to
expand or contract.
[5] The Einstein model describes a static universe; space is finite and unbounded (analogous to the surface of a sphere, which has a finite area but no edges). It was later realized that Einstein's model was just one of a larger set of possibilities, all of which were consistent with general relativity and the cosmological principle. The cosmological solutions of general relativity were found by
Alexander Friedmann in the early 1920s.
[8] His equations describe the
Friedmann–Lemaître–Robertson–Walker universe, which may expand or contract, and whose geometry may be open, flat, or closed.
In the 1910s,
Vesto Slipher (and later
Carl Wilhelm Wirtz) interpreted the
red shift of
spiral nebulae as a
Doppler shift that indicated they were receding from Earth. However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to its
angular size, but a physical size must be assumed to do this. Another method is to measure the
brightness of an object and assume an intrinsic
luminosity, from which the distance may be determined using the
inverse square law. Due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own
Milky Way, nor did they speculate about the cosmological implications. In 1927, the
Belgian Roman Catholic priest Georges Lemaître independently derived the Friedmann–Lemaître–Robertson–Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval
atom"—which was later called the
Big Bang. In 1929,
Edwin Hubble provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of
Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance. This fact is now known as
Hubble's law, though the numerical factor Hubble found relating recessional velocity and distance was off by a factor of ten, due to not knowing about the types of Cepheid variables.
Given the
cosmological principle, Hubble's law suggested that the universe was expanding. Two primary explanations were proposed for the expansion. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other explanation was
Fred Hoyle's
steady state model in which new matter is created as the galaxies move away from each other. In this model, the universe is roughly the same at any point in time.
For a number of years, support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model, and since the precise measurements of the cosmic microwave background by the
Cosmic Background Explorer in the early 1990s, few cosmologists have seriously proposed other theories of the origin and evolution of the cosmos. One consequence of this is that in standard general relativity, the universe began with a
singularity, as demonstrated by
Roger Penrose and
Stephen Hawking in the 1960s.
An alternative view to extend the Big Bang model, suggesting the universe had no beginning or singularity and the age of the universe is infinite, has been presented.
[12][13][14]
Energy of the cosmos
Light
chemical elements, primarily
hydrogen and
helium, were created in the
Big Bang process
(see Nucleosynthesis). The small atomic nuclei combined into larger atomic nuclei to form heavier elements such as
iron and
nickel, which are more stable
(see Nuclear fusion). This caused a
later energy release. Such reactions of nuclear particles inside
stars continue to contribute to
sudden energy releases, such as in
nova stars. Gravitational collapse of matter into
black holes is also thought to power the most energetic processes, generally seen at the centers of galaxies
(see Quasar and Active galaxy).
Cosmologists cannot explain all cosmic phenomena exactly, such as those related to the
accelerating expansion of the universe, using conventional
forms of energy. Instead, cosmologists propose a new form of energy called
dark energy that permeates all space.
[15] One hypothesis is that dark energy is the energy of
virtual particles, which are believed to exist in a vacuum due to the
uncertainty principle.
There is no clear way to define the total energy in the universe using the most widely accepted theory of gravity,
general relativity. Therefore, it remains controversial whether the total energy is conserved in an expanding universe. For instance, each
photon that travels through intergalactic space loses energy due to the
redshift effect. This energy is not obviously transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law of
conservation of energy.
[16]
Thermodynamics of the universe is a field of study that explores which form of energy dominates the cosmos –
relativistic particles which are referred to as
radiation, or non-relativistic particles referred to as matter. Relativistic particles are particles whose
rest mass is zero or negligible compared to their
kinetic energy, and so move at the speed of light or very close to it; non-relativistic particles have much higher rest mass than their energy and so move much slower than the speed of light.
As the universe expands, both matter and radiation in it become diluted. However, the
energy densities of radiation and matter dilute at different rates. As a particular volume expands, mass energy density is changed only by the increase in volume, but the energy density of radiation is changed both by the increase in volume and by the increase in the
wavelength of the
photons that make it up. Thus the energy of radiation becomes a smaller part of the universe's total energy than that of matter as it expands. The very early universe is said to have been 'radiation dominated' and radiation controlled the deceleration of expansion. Later, as the average energy per photon becomes roughly 10
eV and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'. The intermediate case is not treated well
analytically. As the expansion of the universe continues, matter dilutes even further and the
cosmological constant becomes dominant, leading to an acceleration in the universe's expansion.
History of the universe
The history of the universe is a central issue in cosmology. The history of the universe is divided into different periods called epochs, according to the dominant forces and processes in each period. The standard cosmological model is known as the
Lambda-CDM model.
Equations of motion
The
equations of motion governing the universe as a whole are derived from
general relativity with a small, positive
cosmological constant.
[17] The solution is an expanding universe; due to this expansion, the radiation and matter in the universe cool down and become diluted. At first, the expansion is slowed down by
gravitation attracting the
radiation and matter in the universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. In our universe this happened billions of years ago.
Particle physics in cosmology
Particle physics is important to the behavior of the early universe, because the early universe was so hot that the average energy density was very high. Because of this,
scattering processes and
decay of unstable particles are important in cosmology.
As a rule of thumb, a scattering or a decay process is cosmologically important in a certain cosmological epoch if the time scale describing that process is smaller than, or comparable to, the time scale of the expansion of the universe. The time scale that describes the expansion of the universe is
with
being the
Hubble constant, which itself actually varies with time. The expansion timescale
is roughly equal to the age of the universe at that time.
Timeline of the Big Bang
Observations suggest that the universe began around 13.8 billion years ago.
[18] Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot that
particles had energies higher than those currently accessible in
particle accelerators on Earth. Therefore, while the basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses. Following this, in the early universe, the evolution of the universe proceeded according to known
high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the
cosmic microwave background was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first
stars and
quasars, and ultimately galaxies,
clusters of galaxies and
superclusters formed. The future of the universe is not yet firmly known, but according to the
ΛCDM model it will continue expanding forever.
Areas of study
Below, some of the most active areas of inquiry in cosmology are described, in roughly chronological order. This does not include all of the Big Bang cosmology, which is presented in
Timeline of the Big Bang.
Very early universe
The early, hot universe appears to be well explained by the Big Bang from roughly 10
−33 seconds onwards. But there are several
problems. One is that there is no compelling reason, using current particle physics, for the universe to be
flat, homogeneous, and
isotropic (see the cosmological principle). Moreover,
grand unified theories of particle physics suggest that there should be
magnetic monopoles in the universe, which have not been found. These problems are resolved by a brief period of
cosmic inflation, which drives the universe to
flatness, smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles. The physical model behind cosmic inflation is extremely simple, but it has not yet been confirmed by particle physics, and there are difficult problems reconciling inflation and
quantum field theory. Some cosmologists think that
string theory and
brane cosmology will provide an alternative to inflation.
Another major problem in cosmology is what caused the universe to contain far more matter than
antimatter. Cosmologists can observationally deduce that the universe is not split into regions of matter and antimatter. If it were, there would be
X-rays and
gamma rays produced as a result of
annihilation, but this is not observed. Therefore, some process in the early universe must have created a small excess of matter over antimatter, and this (currently not understood) process is called
baryogenesis. Three required conditions for baryogenesis were derived by
Andrei Sakharov in 1967, and requires a violation of the particle physics
symmetry, called
CP-symmetry, between matter and antimatter. However, particle accelerators measure too small a violation of CP-symmetry to account for the baryon asymmetry. Cosmologists and particle physicists look for additional violations of the CP-symmetry in the early universe that might account for the baryon asymmetry.
Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and
experiment, rather than through observations of the universe.
Big bang nucleosynthesis
Big Bang nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when the universe was about three minutes old and its
temperature dropped below that at which
nuclear fusion could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced. Starting from
hydrogen ions (
protons), it principally produced
deuterium,
helium-4, and
lithium. Other elements were produced in only trace abundances. The basic theory of nucleosynthesis was developed in 1948 by
George Gamow,
Ralph Asher Alpher, and
Robert Herman. It was used for many years as a probe of physics at the time of the Big Bang, as the theory of Big Bang nucleosynthesis connects the abundances of primordial light elements with the features of the early universe. Specifically, it can be used to test the
equivalence principle, to probe
dark matter, and test
neutrino physics. Some cosmologists have proposed that Big Bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.
Cosmic microwave background
The cosmic microwave background is radiation left over from
decoupling after the epoch of
recombination when neutral
atoms first formed. At this point, radiation produced in the Big Bang stopped
Thomson scattering from charged ions. The radiation, first observed in 1965 by
Arno Penzias and
Robert Woodrow Wilson, has a perfect thermal
black-body spectrum. It has a temperature of 2.7
kelvins today and is isotropic to one part in 10
5.
Cosmological perturbation theory, which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular
power spectrum of the radiation, and it has been measured by the recent satellite experiments (
COBE and
WMAP) and many ground and balloon-based experiments (such as
Degree Angular Scale Interferometer,
Cosmic Background Imager, and
Boomerang). One of the goals of these efforts is to measure the basic parameters of the
Lambda-CDM model with increasing accuracy, as well as to test the predictions of the Big Bang model and look for new physics. The recent measurements made by WMAP, for example, have placed limits on the neutrino masses.
Newer experiments, such as
QUIET and the
Atacama Cosmology Telescope, are trying to measure the
polarization of the cosmic microwave background. These measurements are expected to provide further confirmation of the theory as well as information about cosmic inflation, and the so-called secondary anisotropies, such as the
Sunyaev-Zel'dovich effect and
Sachs-Wolfe effect, which are caused by interaction between
galaxies and
clusters with the cosmic microwave background.
On 17 March 2014, astronomers at the
Harvard–Smithsonian Center for Astrophysics announced the apparent detection of
gravitational waves, which, if confirmed, may provide strong evidence for
inflation and the
Big Bang.
[9][10][11][19] However, on 19 June 2014, lowered confidence in confirming the
cosmic inflation findings was reported.
[20][21][22]
Formation and evolution of large-scale structure
Understanding the formation and evolution of the largest and earliest structures (i.e.,
quasars,
galaxies,
clusters and
superclusters) is one of the largest efforts in cosmology. Cosmologists study a model of
hierarchical structure formation in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling. One way to study structure in the universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the universe and measure the matter
power spectrum. This is the approach of the
Sloan Digital Sky Survey and the
2dF Galaxy Redshift Survey.
Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into
filaments, superclusters and
voids. Most simulations contain only non-baryonic
cold dark matter, which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy.
Other, complementary observations to measure the distribution of matter in the distant universe and to probe
reionization include:
- The Lyman-alpha forest, which allows cosmologists to measure the distribution of neutral atomic hydrogen gas in the early universe, by measuring the absorption of light from distant quasars by the gas.
- The 21 centimeter absorption line of neutral atomic hydrogen also provides a sensitive test of cosmology
- Weak lensing, the distortion of a distant image by gravitational lensing due to dark matter.
These will help cosmologists settle the question of when and how structure formed in the universe.
Dark matter
Evidence from
Big Bang nucleosynthesis, the
cosmic microwave background and structure formation suggests that about 23% of the mass of the universe consists of non-baryonic dark matter, whereas only 4% consists of visible,
baryonic matter. The gravitational effects of dark matter are well understood, as it behaves like a cold,
non-radiative fluid that forms
haloes around galaxies. Dark matter has never been detected in the laboratory, and the particle physics nature of dark matter remains completely unknown. Without observational constraints, there are a number of candidates, such as a stable
supersymmetric particle, a
weakly interacting massive particle, an
axion, and a
massive compact halo object. Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations (
MOND) or an effect from
brane cosmology.
Dark energy
If the universe is
flat, there must be an additional component making up 73% (in addition to the 23% dark matter and 4% baryons) of the energy density of the universe. This is called dark energy. In order not to interfere with Big Bang nucleosynthesis and the cosmic microwave background, it must not cluster in haloes like baryons and dark matter. There is strong observational evidence for dark energy, as the total energy density of the universe is known through constraints on the flatness of the universe, but the amount of clustering matter is tightly measured, and is much less than this. The case for dark energy was strengthened in 1999, when measurements demonstrated that the expansion of the universe has begun to gradually accelerate.
Apart from its density and its clustering properties, nothing is known about dark energy.
Quantum field theory predicts a
cosmological constant (CC) much like dark energy, but 120
orders of magnitude larger than that observed.
Steven Weinberg and a number of string theorists
(see string landscape) have invoked the 'weak
anthropic principle': i.e. the reason that physicists observe a universe with such a small cosmological constant is that no physicists (or any life) could exist in a universe with a larger cosmological constant. Many cosmologists find this an unsatisfying explanation: perhaps because while the weak anthropic principle is self-evident (given that living observers exist, there must be at least one universe with a cosmological constant which allows for life to exist) it does not attempt to explain the context of that universe. For example, the weak anthropic principle alone does not distinguish between:
- Only one universe will ever exist and there is some underlying principle that constrains the CC to the value we observe.
- Only one universe will ever exist and although there is no underlying principle fixing the CC, we got lucky.
- Lots of universes exist (simultaneously or serially) with a range of CC values, and of course ours is one of the life-supporting ones.
Other possible explanations for dark energy include
quintessence or a modification of gravity on the largest scales.
The effect on cosmology of the dark energy that these models describe is given by the dark energy's
equation of state, which varies depending upon the theory. The nature of dark energy is one of the most challenging problems in cosmology.
A better understanding of dark energy is likely to solve the problem of the
ultimate fate of the universe. In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than
superclusters from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until a
big rip, or whether it will eventually reverse.
Other areas of inquiry
Cosmologists also study:
is the Ricci scalar in two dimensions, 
the dilaton field, and 
the string constant. The indices 
range over 1,2, and 
over 
, where D the dimension of the target space. A further antisymmetric field could be added. This is generally considered when one wants this action generating a potential for inflation.[3] Otherwise, a generic potential is inserted by hand, as well as a cosmological constant.
is just the anomaly of the bosonic string theory in a flat spacetime. But here there are further terms that can grant a compensation of the anomaly also when 
, and from this cosmological models of a pre-big bang scenario can be constructed. Indeed, this low energy equations can be obtained from the following action:
is a constant that can always be changed by redefining the dilaton field. One can also rewrite this action in a more familiar form by redefining the fields (Einstein frame) as
one can write
is the Newton constant in D dimensions and 
the corresponding Planck mass. When setting 
in this action, the conditions for inflation are not fulfilled unless a potential or antisymmetric term is added to the string action,[3] in which case power-law inflation is possible.![S_2=\frac{1}{4\pi\alpha'}\int d^2z\sqrt{\gamma}\left[\gamma^{ab}G_{\mu\nu}(X)\partial_aX^\mu\partial_bX^\nu+\alpha'\ ^{(2)}R\Phi(X)\right],](http://upload.wikimedia.org/math/f/7/9/f793416dbff80ca9c204b3a128050741.png)
is the Ricci scalar in two dimensions, 
the dilaton field, and 
the string constant. The indices 
range over 1,2, and 
over 
, where D the dimension of the target space. A further antisymmetric field could be added. This is generally considered when one wants this action generating a potential for inflation.[3] Otherwise, a generic potential is inserted by hand, as well as a cosmological constant.
is just the anomaly of the bosonic string theory in a flat spacetime. But here there are further terms that can grant a compensation of the anomaly also when 
, and from this cosmological models of a pre-big bang scenario can be constructed. Indeed, this low energy equations can be obtained from the following action:![S=\frac{1}{2\kappa_0^2}\int d^Dx\sqrt{-G}e^{-2\Phi}\left[-\frac{2(D-26)}{3\alpha'}+R+4\partial_\mu\Phi\partial^\mu\Phi+O(\alpha')\right],](http://upload.wikimedia.org/math/5/6/c/56c3fa1a05ead707048c2be1327e879f.png)
is a constant that can always be changed by redefining the dilaton field. One can also rewrite this action in a more familiar form by redefining the fields (Einstein frame) as
one can write![S=\frac{1}{2\kappa^2}\int d^Dx\sqrt{-g}\left[-\frac{2(D-26)}{3\alpha'}e^{\frac{4\tilde\Phi}{D-2}}+\tilde R-\frac{4}{D-2}\partial_\mu\tilde\Phi\partial^\mu\tilde\Phi+O(\alpha')\right],](http://upload.wikimedia.org/math/f/9/a/f9a4e2c5f50d53936f76183c181b5dd0.png)
![\tilde R=e^{-2\omega}[R-(D-1)\nabla^2\omega-(D-2)(D-1)\partial_\mu\omega\partial^\mu\omega].](http://upload.wikimedia.org/math/7/d/c/7dc7271fcdf1fe4622f5fb8c77052b58.png)
is the Newton constant in D dimensions and 
the corresponding Planck mass. When setting 
in this action, the conditions for inflation are not fulfilled unless a potential or antisymmetric term is added to the string action,[3] in which case power-law inflation is possible.
with 
being the 
