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
Edwin Hubble discovered that the universe contains a
huge number of external galaxies beyond our own Milky Way; then, work
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
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.
1 Subject history
Energy of the cosmos
3 History of the universe
3.1 Equations of motion
Particle physics in cosmology
3.3 Timeline of the Big Bang
4 Areas of study
4.1 Very early universe
Big Bang Theory
4.2.1 Standard model of
Big Bang cosmology
4.3 Cosmic microwave background
4.4 Formation and evolution of large-scale structure
4.5 Dark matter
4.6 Dark energy
4.7 Gravitational waves
4.8 Other areas of inquiry
5 See also
7 Further reading
8 External links
8.1 From groups
8.2 From individuals
view • discuss • edit
Earliest universe (−13.80)
Omega Centauri forms
Milky Way Galaxy
spiral arms form
Alpha Centauri forms
Earliest Earth (−4.54)
Earliest sexual reproduction
Axis scale: billion years
Human timeline and
Timeline of cosmology
Timeline of cosmology and List of cosmologists
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 (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
Alexander Friedmann in the early 1920s. His equations describe
Friedmann–Lemaître–Robertson–Walker universe, which may
expand or contract, and whose geometry may be open, flat, or closed.
History of the
Universe – gravitational waves are hypothesized to
arise from cosmic inflation, a faster-than-light expansion just after
the Big Bang
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
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
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
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
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. One
hypothesis is that dark energy is the energy of virtual particles,
which are believed to exist in a vacuum due to the uncertainty
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
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
See also: Timeline of the Big Bang
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
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 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
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
Main article: 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
it will continue expanding forever.
Areas of study
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Below, some of the most active areas of inquiry in cosmology are
described, in roughly chronological order. This does not include all
Big Bang cosmology, which is presented in Timeline of the Big
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.[vague] 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
Main article: 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
Standard model of
Big Bang cosmology
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
Λ), associated with dark energy, and cold dark matter (abbreviated
CDM). It is frequently referred to as the standard model of Big Bang
Cosmic microwave background
Main article: Cosmic microwave background
Evidence of gravitational waves in the infant universe may have been
uncovered by the microscopic examination of the focal plane of the
BICEP2 radio telescope.
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 recent
measurements made by WMAP, for example, have placed limits on the
Newer experiments, such as
QUIET and the Atacama
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
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
Planck data was published and the
European Space Agency
European Space Agency announced that
the signal can be entirely attributed to interstellar dust in the
Formation and evolution of large-scale structure
Main articles: Large-scale structure of the cosmos, Structure
Galaxy formation and evolution
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
Sloan Digital Sky Survey
Sloan Digital Sky Survey and the 2dF
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.
Main article: Dark matter
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.
Main article: 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
Apart from its density and its clustering properties, nothing is known
about dark energy.
Quantum field theory
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
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.
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.
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.
List of cosmologists
Universal Rotation Curve
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Brian Greene (2005). The Fabric of the Cosmos. Penguin Books Ltd.
Alan Guth (1997). The Inflationary Universe: The Quest for a New
Theory of Cosmic Origins. Random House. ISBN 0-224-04448-6.
Hawking, Stephen W. (1988). A Brief History of Time: From the Big Bang
to Black Holes. Bantam Books, Inc. ISBN 0-553-38016-8.
Hawking, Stephen W. (2001). The
Universe in a Nutshell. Bantam Books,
Inc. ISBN 0-553-80202-X.
Ostriker, Jeremiah P.; Mitton, Simon (2013). Heart of Darkness:
Unraveling the mysteries of the invisible Universe. Princeton, NJ:
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Simon Singh (2005). Big Bang: The Origin of the Universe. Fourth
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Cheng, Ta-Pei (2005). 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 (2003). Modern Cosmology. Academic Press.
ISBN 0-12-219141-2. An introductory text, released slightly
Grøn, Øyvind; Hervik, Sigbjørn (2007). Einstein's General Theory of
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Harrison, Edward (2000). Cosmology: the science of the universe.
Cambridge University Press. ISBN 0-521-66148-X. For
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Kutner, Marc (2003). Astronomy: A Physical Perspective. Cambridge
University Press. ISBN 0-521-52927-1. An introductory
Kolb, Edward; Michael Turner (1988). The Early Universe.
Addison-Wesley. ISBN 0-201-11604-9. The classic reference
Liddle, Andrew (2003). An Introduction to Modern Cosmology. John
Wiley. ISBN 0-470-84835-9.
Cosmology without general
Liddle, Andrew; David Lyth (2000). Cosmological
Large-Scale Structure. Cambridge. ISBN 0-521-57598-2. An
introduction to cosmology with a thorough discussion of inflation.
Mukhanov, Viatcheslav (2005). Physical Foundations of Cosmology.
Cambridge University Press. ISBN 0-521-56398-4.
Padmanabhan, T. (1993).
Structure formation in the universe. Cambridge
University Press. ISBN 0-521-42486-0. Discusses the
formation of large-scale structures in detail.
Peacock, John (1998). 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. (1993). Principles of Physical Cosmology. Princeton
University Press. ISBN 0-691-01933-9. Strong historical
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Princeton University Press. ISBN 0-691-08240-5. The classic
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Scientific American (print). pp. 52–59.
(subtitle) Could cosmic inflation be a sign that our universe is
embedded in a far vaster realm?
Galactic / Extragalactic
Extremely large telescope
Gran Telescopio Canarias
Hubble Space Telescope
Large Binocular Telescope
Southern African Large Telescope
Very Large Telescope
Astrology and astronomy
List of astronomers
Age of the universe
Chronology of the universe
Discovery of cosmic microwave background
History of the
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Timeline of cosmological theories
Cosmic microwave background
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Future of an expanding universe
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Large quasar group
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Major subfields of astronomy and astrophysics
Themes and subjects
Chronology of the universe
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