Physical cosmology

Physical cosmology is a branch of

Physical cosmology, as it is now understood, began with the development in 1915 of

In the 1910s,

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,[17] 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.^[18]

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.^[19][20][21]

In September 2023, astrophysicists questioned the overall current view of the

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 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.^[28]

Different forms of energy may dominate the cosmos—

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

During the earliest moments of the universe, the average energy density was very high, making knowledge of

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.^{[clarification needed]} The time scale that describes the expansion of the universe is ${\displaystyle 1/H}$ with ${\displaystyle H}$ being the

${\displaystyle 1/H}$

Timeline of the Big Bang

Observations suggest that the universe began around 13.8 billion years ago.^[31] 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

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

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

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.^{[speculation?]}

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

The cosmic microwave background is radiation left over from

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.^[56]

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.^[57]
• The 21-centimeter absorption line of neutral atomic hydrogen also provides a sensitive test of cosmology.^[58]
• gravitational lensing due to dark matter.^[59]

Evidence from

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.^[61]

Apart from its density and its clustering properties, nothing is known about dark energy.

Other possible explanations for dark energy include quintessence^[64] or a modification of gravity on the largest scales.^[65] 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

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.^[67]

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.^[68][69][70] On 15 June 2016, a second detection of gravitational waves from coalescing black holes was announced.^[71] Besides LIGO, many other gravitational-wave observatories (detectors) are under construction.^[72]

Other areas of inquiry

Cosmologists also study:

• Whether primordial black holes were formed in our universe, and what happened to them.^[73]
• Detection of cosmic rays with energies above the
  GZK cutoff,^[74] and whether it signals a failure of special relativity
  at high energies.
• The
  laws of physics are the same everywhere in the universe.^[76]

References

Further reading

Popular

• ISBN 978-0-14-101111-0
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• ISBN 978-0-224-04448-6
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• ISBN 978-0-553-38016-3
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• ISBN 978-0-553-80202-3
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• 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
  .
• ISBN 978-0-00-716221-5
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• 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.
• Baumann, Daniel (2022). Cosmology. Cambridge: Cambridge University Press.
  ISBN 978-0-19-852957-6
  . Modern introduction to cosmology covering the homogeneous and inhomogeneous universe as well as inflation and the CMB.
• Dodelson, Scott (2003). Modern Cosmology. Academic Press.
  ISBN 978-0-12-219141-1. An introductory text, released slightly before the WMAP
  results.
• Gal-Or, Benjamin (1987) [1981]. Cosmology, Physics and Philosophy. Springer Verlag.
  ISBN 0-387-90581-2
  .
• ISBN 978-0-387-69199-2
  .
• 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.
  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
  .

External links

Wikimedia Commons has media related to Physical cosmology.

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.
• 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. Retrieved 22 March 2015.
• Cliff Burgess; Fernando Quevedo (November 2007). "The Great Cosmic Roller-Coaster Ride". Scientific American (print). pp. 52–59. (subtitle) Could cosmic inflation be a sign that our universe is embedded in a far vaster realm?