Physical cosmology is a branch of cosmology
concerned with the studies of the largest-scale
structures and dynamics of the Universe and
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
