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.
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; 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.
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. At the time, Einstein believed
in a static universe, but found that his
original formulation of the theory did
not permit it. This is because masses
distributed throughout the universe
gravitationally attract, and move toward
each other over time. 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.
However, this so-called Einstein model
is unstable to small perturbations—it
will eventually start to expand or
contract. The Einstein model describes a
static universe; space is finite and
unbounded. 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.
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 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.
Energy of the cosmos
Light chemical elements, primarily
hydrogen and helium, were created in the
Big Bang process. The small atomic
nuclei combined into larger atomic
nuclei to form heavier elements such as
iron and nickel, which are more stable.
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.
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. 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.
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. 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.
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.
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 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 Theory=
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,
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.
Standard model of Big Bang cosmology
The ΛCDM or Lambda-CDM model is a
parametrization of the Big Bang
cosmological model in which the universe
contains a cosmological constant,
denoted by Lambda, associated with dark
energy, and cold dark matter. It is
frequently referred to as the standard
model of Big Bang cosmology.
= 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 105.
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 and
many ground and balloon-based
experiments. 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.
However, on 19 June 2014, lowered
confidence in confirming the cosmic
inflation findings was reported.
= Formation and evolution of large-scale
structure=
Understanding the formation and
evolution of the largest and earliest
structures 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 or an
effect from brane cosmology.
= Dark energy=
If the universe is flat, there must be
an additional component making up 73% 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 much
like dark energy, but 120 orders of
magnitude larger than that observed.
Steven Weinberg and a number of string
theorists 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 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 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 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:
Whether primordial black holes were
formed in our universe, and what
happened to them.
The GZK cutoff for high-energy cosmic
rays, and whether it signals a failure
of special relativity at high energies
The equivalence principle, whether or
not Einstein's general theory of
relativity is the correct theory of
gravitation, and if the fundamental laws
of physics are the same everywhere in
the universe.
The increasing complexity of universal
structures, an example being the
progressively greater energy rate
density. 
See also
References
Further reading
= Popular=
Brian Greene. The Fabric of the Cosmos.
Penguin Books Ltd. ISBN 0-14-101111-4. 
Alan Guth. The Inflationary Universe:
The Quest for a New Theory of Cosmic
Origins. Random House. ISBN
0-224-04448-6. 
Hawking, Stephen W.. A Brief History of
Time: From the Big Bang to Black Holes.
Bantam Books, Inc. ISBN 0-553-38016-8. 
Hawking, Stephen W.. The Universe in a
Nutshell. Bantam Books, Inc. ISBN
0-553-80202-X. 
Ostriker, Jeremiah P.; Mitton, Simon.
Heart of Darkness: Unraveling the
mysteries of the invisible Universe.
Princeton, NJ: Princeton University
Press. ISBN 978-0-691-13430-7. 
Simon Singh. Big Bang: The Origin of the
Universe. Fourth Estate. ISBN
0-00-716221-9. 
Steven Weinberg [First published 1978].
The First Three Minutes. Basic Books.
ISBN 0-465-02437-8. 
= Textbooks=
Cheng, Ta-Pei. Relativity, Gravitation
and Cosmology: a Basic Introduction.
Oxford and New York: Oxford University
Press. ISBN 0-19-852957-0.  Introductory
cosmology and general relativity without
the full tensor apparatus, deferred
until the last part of the book.
Dodelson, Scott. Modern Cosmology.
Academic Press. ISBN 0-12-219141-2.  An
introductory text, released slightly
before the WMAP results.
Grøn, Øyvind; Hervik, Sigbjørn.
Einstein's General Theory of Relativity
with Modern Applications in Cosmology.
New York: Springer. ISBN
978-0-387-69199-2. 
Harrison, Edward. Cosmology: the science
of the universe. Cambridge University
Press. ISBN 0-521-66148-X.  For
undergraduates; mathematically gentle
with a strong historical focus.
Kutner, Marc. Astronomy: A Physical
Perspective. Cambridge University Press.
ISBN 0-521-52927-1.  An introductory
astronomy text.
Kolb, Edward; Michael Turner. The Early
Universe. Addison-Wesley. ISBN
0-201-11604-9.  The classic reference
for researchers.
Liddle, Andrew. An Introduction to
Modern Cosmology. John Wiley. ISBN
0-470-84835-9.  Cosmology without
general relativity.
Liddle, Andrew; David Lyth. Cosmological
Inflation and Large-Scale Structure.
Cambridge. ISBN 0-521-57598-2.  An
introduction to cosmology with a
thorough discussion of inflation.
Mukhanov, Viatcheslav. Physical
Foundations of Cosmology. Cambridge
University Press. ISBN 0-521-56398-4. 
Padmanabhan, T.. Structure formation in
the universe. Cambridge University
Press. ISBN 0-521-42486-0.  Discusses
the formation of large-scale structures
in detail.
Peacock, John. Cosmological Physics.
Cambridge University Press. ISBN
0-521-42270-1.  An introduction
including more on general relativity and
quantum field theory than most.
Peebles, P. J. E.. Principles of
Physical Cosmology. Princeton University
Press. ISBN 0-691-01933-9.  Strong
historical focus.
Peebles, P. J. E.. The Large-Scale
Structure of the Universe. Princeton
University Press. ISBN 0-691-08240-5. 
The classic work on large-scale
structure and correlation functions.
Rees, Martin. New Perspectives in
Astrophysical Cosmology. Cambridge
University Press. ISBN 0-521-64544-1. 
Weinberg, Steven. Gravitation and
Cosmology. John Wiley. ISBN
0-471-92567-5.  A standard reference for
the mathematical formalism.
Weinberg, Steven. Cosmology. Oxford
University Press. ISBN 0-19-852682-2. 
Benjamin Gal-Or, "Cosmology, Physics and
Philosophy", Springer Verlag, 1981,
1983, 1987, ISBN 0-387-90581-2, ISBN
0-387-96526-2.
External links
= From groups=
Cambridge Cosmology- from Cambridge
University
Cosmology 101 - from the NASA WMAP group
Center for Cosmological Physics.
University of Chicago, Chicago.
Origins, Nova Online - Provided by PBS.
= From individuals=
Gale, George, "Cosmology: Methodological
Debates in the 1930s and 1940s", The
Stanford Encyclopedia of Philosophy,
Edward N. Zalta
Madore, Barry F., "Level 5 : A
Knowledgebase for Extragalactic
Astronomy and Cosmology". Caltech and
Carnegie. Pasadena, California, USA.
Tyler, Pat, and Phil Newman "Beyond
Einstein". Laboratory for High Energy
Astrophysics NASA Goddard Space Flight
Center.
Wright, Ned. "Cosmology tutorial and
FAQ". Division of Astronomy &
Astrophysics, UCLA.
George Musser. "Four Keys to Cosmology".
Scientific American. Retrieved 22 March
2015. 
Cliff Burgess; Fernando Quevedo. "The
Great Cosmic Roller-Coaster Ride".
Scientific American. pp. 52–59. Could
cosmic inflation be a sign that our
universe is embedded in a far vaster
realm?
