Physical cosmology
Physical cosmology, as a branch of
astronomy, is the study of the largest-scale structures and dynamics of the
universe and is concerned with fundamental questions about its formation and evolution.
[1] For most of human history, it was a branch of
metaphysics and
religion. Cosmology as a
science originated with the
Copernican principle, which implies that celestial bodies obey identical
physical laws to those on Earth, and
Newtonian mechanics, which first allowed us to understand those laws.
Physical cosmology, as it is now understood, began with the twentieth century development of
Albert Einstein's
general theory of relativity and better
astronomical observations of extremely distant objects. These advances made it possible to speculate about the
origin of the universe, and allowed scientists to establish the
Big Bang Theory as the leading cosmological model. Some researchers still advocate a handful of
alternative cosmologies; however, cosmologists generally agree that the Big Bang theory best explains observations.
Cosmology draws heavily on the work of many disparate areas of research in
physics. Areas relevant to cosmology include
particle physics experiments and
theory, including
string theory,
astrophysics,
general relativity, and
plasma physics. Thus, cosmology unites the physics of the largest structures in the universe with the physics of the smallest structures in the universe.
History of physical cosmology
Modern cosmology developed along tandem observational and theoretical tracks. In 1915, Albert Einstein developed his theory of
general relativity. At the time, physicists believed in a perfectly static universe without beginning or end. Einstein added a
cosmological constant to his theory to try to force it to allow for a
static universe with matter in it. The so-called
Einstein universe is, however, unstable. It is bound to eventually start
expanding or contracting. The cosmological solutions of general relativity were found by
Alexander Friedmann, whose equations describe the
Friedmann-Lemaître-Robertson-Walker universe, which may expand or contract.
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 directly 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 at the time about different types of Cepheid variables.
Given the
cosmological principle, Hubble's law suggested that the universe was expanding. There were two primary explanations put forth for the expansion of the universe. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other possibility was
Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time.
For a number of years the 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
Stephen Hawking and
Roger Penrose in the 1960s.
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
ΛCDM 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 are cooled down and become diluted. At first, the expansion is slowed down by
gravitation due to the
radiation and matter content of 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 has already happened, billions of years ago.
Particle physics in cosmology
Particle physics is important to the behavior of the early universe, since 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 or comparable to the time scale of the expansion of the universe, which is
1 / H with
H being the
Hubble constant at that time. This is roughly equal to the age of the universe at that time.
Timeline of the Big Bang
Observations suggest that the universe began around 13.7 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.
The very early universe
While the early, hot universe appears to be well explained by the Big Bang from roughly 10
−33 seconds onwards, there are several
problems. One is that there is no compelling reason, using current particle physics, to expect 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, however 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 more particles than
antiparticles. 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. This problem is called the baryon asymmetry, and the theory to describe the resolution is called
baryogenesis. The theory of baryogenesis was worked out by
Andrei Sakharov in 1967, and requires a violation of the particle physics
symmetry, called
CP-symmetry, between matter and antimatter. Particle accelerators, however, measure too small a violation of CP-symmetry to account for the baryon asymmetry. Cosmologists and particle physicists are trying to find additional violations of the CP-symmetry in the early universe that might account for the baryon asymmetry.
Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and
experiment, rather than through observations of the universe.
Big bang nucleosynthesis
Big Bang Nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when the universe was about three minutes old and its
temperature dropped below that at which
nuclear fusion could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced. Starting from
hydrogen ions (
protons), it principally produced
deuterium,
helium-4 and
lithium. Other elements were produced in only trace abundances. The basic theory of nucleosynthesis was developed in 1948 by
George Gamow,
Ralph Asher Alpher and
Robert Herman. It was used for many years as a probe of physics at the time of the Big Bang, as the theory of Big Bang nucleosynthesis connects the abundances of primordial light elements with the features of the early universe. Specifically, it can be used to test the
equivalence principle, to probe
dark matter, and test
neutrino physics. Some cosmologists have proposed that Big Bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.
Cosmic microwave background
The cosmic microwave background is radiation left over from
decoupling after the epoch of
recombination when neutral
atoms first formed. At this point, radiation produced in the Big Bang stopped
Thomson scattering from charged ions. The radiation, first observed in 1965 by
Arno Penzias and
Robert Woodrow Wilson, has a perfect thermal
black-body spectrum. It has a temperature of 2.7
kelvins today and is isotropic to one part in 10
5.
Cosmological perturbation theory, which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular
power spectrum of the radiation, and it has been measured by the recent satellite experiments (
COBE and
WMAP) and many ground and balloon-based experiments (such as
Degree Angular Scale Interferometer,
Cosmic Background Imager, and
Boomerang). One of the goals of these efforts is to measure the basic parameters of the
Lambda-CDM model with increasing accuracy, as well as to test the predictions of the Big Bang model and look for new physics. The recent measurements made by WMAP, for example, have placed limits on the neutrino masses.
Newer experiments, such as
QUIET and the
Atacama Cosmology Telescope, are trying to measure the
polarization of the cosmic microwave background. These measurements are expected to provide further confirmation of the theory as well as information about cosmic inflation, and the so-called secondary anisotropies, such as the
Sunyaev-Zel'dovich effect and
Sachs-Wolfe effect, which are caused by interaction between
galaxies and
clusters with the cosmic microwave background.
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
Main article:
Dark matter
Evidence from
Big Bang nucleosynthesis, the
cosmic microwave background and structure formation suggests that about 23% of the mass of the universe consists of non-baryonic dark matter, whereas only 4% consists of visible,
baryonic matter. The gravitational effects of dark matter are well understood, as it behaves like a cold,
non-radiative fluid that forms
haloes around galaxies. Dark matter has never been detected in the laboratory, and the particle physics nature of dark matter remains completely unknown. Without observational constraints, there are a number of candidates, such as a stable
supersymmetric particle, a
weakly interacting massive particle, an
axion, and a
massive compact halo object. Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations (
MOND) or an effect from
brane cosmology.
Dark energy
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 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 (see
string landscape) have used this as evidence for the
anthropic principle, which suggests that the cosmological constant is so small because life (and thus physicists, to make observations) cannot exist in a universe with a large cosmological constant, but many people find this an unsatisfying explanation. 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:
See also
References
Further reading
Popular
Textbooks
- 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 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 0-521-66148-X. For undergraduates; mathematically gentle with a strong historical focus.
- Kutner, Marc (2003). Astronomy: A Physical Perspective. Cambridge University Press. ISBN 0-521-52927-1. An introductory astronomy text.
- Kolb, Edward; Michael Turner (1988). The Early Universe. Addison-Wesley. ISBN 0-201-11604-9. The classic reference for researchers.
- Liddle, Andrew (2003). An Introduction to Modern Cosmology. John Wiley. ISBN 0-470-84835-9. Cosmology without general relativity.
- Liddle, Andrew; David Lyth (2000). Cosmological Inflation and 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 focus.
- Peebles, P. J. E. (1980). 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 (2002). New Perspectives in Astrophysical Cosmology. Cambridge University Press. ISBN 0-521-64544-1.
- Weinberg, Steven (1971). Gravitation and Cosmology. John Wiley. ISBN 0-471-92567-5. A standard reference for the mathematical formalism.
- Weinberg, Steven (2008). Cosmology. Oxford University Press. ISBN 0198526822.
- Benjamin Gal-Or, “Cosmology, Physics and Philosophy”, Springer Verlag, 1981, 1983, 1987, ISBN 0-387-90581-2, ISBN 0387965262.
External links
From groups
From individuals
- Carroll, Sean. "Cosmology Primer". California Institute of Technology.
- 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 (January 2004). "Four Keys to Cosmology". Scientific American (Scientific American). http://www.sciam.com/article.cfm?chanID=sa006&articleID=0005DCFC-253F-1FFB-A53F83414B7F0000. Retrieved 2008-06-27.
- Cliff Burgess; Fernando Quevedo (November 2007). "The Great Cosmic Roller-Coaster Ride" (print). Scientific American (Scientific American): pp. 52–59. "(subtitle) Could cosmic inflation be a sign that our universe is embedded in a far vaster realm?"
