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. 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 Lemaître, 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. 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).
However, this so-called Einstein model is
unstable to small perturbations—it will
eventually start to expand or contract. 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 (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.
== Energy of the cosmos ==
The lightest chemical elements, primarily
hydrogen and helium, were created during the
Big Bang through the process of nucleosynthesis.
In a sequence of stellar nucleosynthesis reactions,
smaller atomic nuclei are then combined into
larger atomic nuclei, ultimately forming stable
iron group elements such as iron and nickel,
which have the highest nuclear binding energies.
The net process results in a later energy
release, meaning subsequent to the Big Bang.
Such reactions of nuclear particles can lead
to sudden energy releases from cataclysmic
variable stars such as novae. Gravitational
collapse of matter into black holes also powers
the most energetic processes, generally seen
in the nuclear regions of galaxies, forming
quasars and active 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 just the vacuum energy, a component
of empty space that is associated with the
virtual particles that exist 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 ===
Within the standard cosmological model, 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 ===
During the earliest moments of the universe
the average energy density was very high,
making knowledge of particle physics critical
to understanding this environment. Hence,
scattering processes and decay of unstable
elementary particles are important for cosmological
models of this period.
As a rule of thumb, a scattering or a decay
process is cosmologically important in a certain
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
1
/
H
{\displaystyle 1/H}
with
H
{\displaystyle H}
being the Hubble parameter, which varies with
time. The expansion timescale
1
/
H
{\displaystyle 1/H}
is roughly equal to the age of the universe
at each point in 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 (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 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 (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.
==== Standard model of Big Bang cosmology
====
The ΛCDM (Lambda cold dark matter) or Lambda-CDM
model is a parametrization of the Big Bang
cosmological model in which the universe contains
a cosmological constant, denoted by Lambda
(Greek Λ), associated with dark energy, and
cold dark matter (abbreviated CDM). 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 (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
results of 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
of the BICEP2 Collaboration announced the
apparent detection of B-mode polarization
of the CMB, considered to be evidence of primordial
gravitational waves that are predicted by
the theory of inflation to occur during the
earliest phase of the Big Bang. However, later
that year the Planck collaboration provided
a more accurate measurement of cosmic dust,
concluding that the B-mode signal from dust
is the same strength as that reported from
BICEP2. On January 30, 2015, a joint analysis
of BICEP2 and Planck data was published and
the European Space Agency announced that the
signal can be entirely attributed to interstellar
dust in the Milky Way.
=== 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, structure formation,
and galaxy rotation curves 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, a gravitationally-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, lead to
a big freeze, or follow some other scenario.
=== Gravitational waves ===
Gravitational waves are ripples in the curvature
of spacetime that propagate as waves at the
speed of light, generated in certain gravitational
interactions that propagate outward from their
source. Gravitational-wave astronomy is an
emerging branch of observational astronomy
which aims to use gravitational waves to collect
observational data about sources of detectable
gravitational waves such as binary star systems
composed of white dwarfs, neutron stars, and
black holes; and events such as supernovae,
and the formation of the early universe shortly
after the Big Bang.In 2016, the LIGO Scientific
Collaboration and Virgo Collaboration teams
announced that they had made the first observation
of gravitational waves, originating from a
pair of merging black holes using the Advanced
LIGO detectors. On June 15, 2016, a second
detection of gravitational waves from coalescing
black holes was announced. Besides LIGO, many
other gravitational-wave observatories (detectors)
are under construction.
=== Other areas of inquiry ===
Cosmologists also study:
Whether primordial black holes were formed
in our universe, and what happened to them.
Detection of cosmic rays with energies above
the GZK cutoff, 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 (2005). The Fabric of the Cosmos.
Penguin Books Ltd. ISBN 978-0-14-101111-0.
Alan Guth (1997). The Inflationary Universe:
The Quest for a New Theory of Cosmic Origins.
Random House. ISBN 978-0-224-04448-6.
Hawking, Stephen W. (1988). A Brief History
of Time: From the Big Bang to Black Holes.
Bantam Books, Inc. ISBN 978-0-553-38016-3.
Hawking, Stephen W. (2001). The Universe in
a Nutshell. Bantam Books, Inc. ISBN 978-0-553-80202-3.
Ostriker, Jeremiah P.; Mitton, Simon (2013).
Heart of Darkness: Unraveling the mysteries
of the invisible Universe. Princeton, NJ:
Princeton University Press. ISBN 978-0-691-13430-7.
Simon Singh (2005). Big Bang: The Origin of
the Universe. Fourth Estate. ISBN 978-0-00-716221-5.
Steven Weinberg (1993) [First published 1978].
The First Three Minutes. Basic Books. ISBN
978-0-465-02437-7.
=== Textbooks ===
Cheng, Ta-Pei (2005). Relativity, Gravitation
and Cosmology: a Basic Introduction. Oxford
and New York: Oxford University Press. ISBN
978-0-19-852957-6. Introductory cosmology
and general relativity without the full tensor
apparatus, deferred until the last part of
the book.
Dodelson, Scott (2003). Modern Cosmology.
Academic Press. ISBN 978-0-12-219141-1. An
introductory text, released slightly before
the WMAP results.
Grøn, Øyvind; Hervik, Sigbjørn (2007).
Einstein's General Theory of Relativity with
Modern Applications in Cosmology. New York:
Springer. ISBN 978-0-387-69199-2.
Harrison, Edward (2000). Cosmology: the science
of the universe. Cambridge University Press.
ISBN 978-0-521-66148-5. For undergraduates;
mathematically gentle with a strong historical
focus.
Kutner, Marc (2003). Astronomy: A Physical
Perspective. Cambridge University Press. ISBN
978-0-521-52927-3. An introductory astronomy
text.
Kolb, Edward; Michael Turner (1988). The Early
Universe. Addison-Wesley. ISBN 978-0-201-11604-5.
The classic reference for researchers.
Liddle, Andrew (2003). An Introduction to
Modern Cosmology. John Wiley. ISBN 978-0-470-84835-7.
Cosmology without general relativity.
Liddle, Andrew; David Lyth (2000). Cosmological
Inflation and Large-Scale Structure. Cambridge.
ISBN 978-0-521-57598-0. An introduction to
cosmology with a thorough discussion of inflation.
Mukhanov, Viatcheslav (2005). Physical Foundations
of Cosmology. Cambridge University Press.
ISBN 978-0-521-56398-7.
Padmanabhan, T. (1993). Structure formation
in the universe. Cambridge University Press.
ISBN 978-0-521-42486-8. Discusses the formation
of large-scale structures in detail.
Peacock, John (1998). Cosmological Physics.
Cambridge University Press. ISBN 978-0-521-42270-3.
An introduction including more on general
relativity and quantum field theory than most.
Peebles, P. J. E. (1993). Principles of Physical
Cosmology. Princeton University Press. ISBN
978-0-691-01933-8. Strong historical focus.
Peebles, P. J. E. (1980). The Large-Scale
Structure of the Universe. Princeton University
Press. ISBN 978-0-691-08240-0. The classic
work on large-scale structure and correlation
functions.
Rees, Martin (2002). New Perspectives in Astrophysical
Cosmology. Cambridge University Press. ISBN
978-0-521-64544-7.
Weinberg, Steven (1971). Gravitation and Cosmology.
John Wiley. ISBN 978-0-471-92567-5. A standard
reference for the mathematical formalism.
Weinberg, Steven (2008). Cosmology. Oxford
University Press. ISBN 978-0-19-852682-7.
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
(public home page)
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 (ed.)
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 (LHEA)
NASA Goddard Space Flight Center.
Wright, Ned. "Cosmology tutorial and FAQ".
Division of Astronomy & Astrophysics, UCLA.
George Musser (February 2004). "Four Keys
to Cosmology". Scientific American. Scientific
American. Retrieved 22 March 2015.
Cliff Burgess; Fernando Quevedo (November
2007). "The Great Cosmic Roller-Coaster Ride".
Scientific American (print). pp. 52&ndash,
59. (subtitle) Could cosmic inflation be a
sign that our universe is embedded in a far
vaster realm?
