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Key topics in
Physical cosmology
Universe · Big Bang Age of the universe Timeline of the Big Bang
Early universe Inflation · Nucleosynthesis GWB · Neutrino background Cosmic microwave background
Expanding universe Redshift · Hubble's law Metric expansion of space Friedmann equations FLRW metric
Structure formation Shape of the universe Structure formation Reionization Galaxy formation Large-scale structure Galaxy filament
Future of universe Ultimate fate of the universe Future of an expanding universe
Components Lambda-CDM model Dark energy · Dark matter Dark fluid · Dark flow
History of cosmological theories Timeline of cosmological theories History of the Big Bang theory Discovery of cosmic microwave background radiation
Experiments Observational cosmology 2dF · SDSS COBE · BOOMERanG · WMAP · Planck
Scientists Isaac Newton · Einstein · Hawking · Friedman Lemaître · Hubble · Penzias · Wilson Gamow · Dicke · Zel'dovich · Mather Rubin · Smoot · others
Social impact Religious interpretations
Astronomy Portal Category
v t e

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;^[2] 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 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[link]

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.^[3] At the time, Einstein believed in a static universe, but found that his original formulation of the theory did not permit it.^[4] This is because masses distributed throughout the universe gravitationally attract, and move toward each other over time.^[5] 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.^[6] This so-called Einstein model is, however, unstable to small perturbations—it will eventually start expanding or contracting.^[4] The universe described by the Einstein model is static; 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 by Alexander Friedmann in the early 1920s.^[7] 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 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.

Energy of the cosmos[link]

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Light elements, primarily hydrogen and helium, were created in the Big Bang. These light elements were spread too fast and too thinly in the Big Bang process (see nucleosynthesis) to form the most stable medium-sized atomic nuclei, like iron and nickel. This fact allowed for later energy release, as such intermediate-sized elements are formed in our era. The formation of such atoms powers the steady energy-releasing reactions in stars, and also contributes to sudden energy releases, such as in novae. Gravitational collapse of matter into black holes is also thought to power the most energetic processes, generally seen at the centers of galaxies (see quasars and in general active galaxies).

Cosmologists are still unable to explain all cosmological phenomena purely on the basis of known conventional forms of energy such as those related to the accelerating expansion of the universe. Cosmologists therefore invoke a yet unexplored form of energy called dark energy^[8] to account for certain cosmological observations. One hypothesis is that dark energy is the energy of virtual particles (which mathematically must exist in vacuum due to the uncertainty principle).

There is no unambiguous way to define the total energy of the universe in the current best theory of gravity, general relativity. As a result it remains controversial whether one can meaningfully say that 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. Nevertheless some cosmologists insist that energy is conserved in some sense; this would preserve the law of conservation of energy.^[9]

Thermodynamics of the universe is a field of study to explore which form of energy dominates the cosmos - relativistic particles which are referred to as radiation, or non-relativistic particles which are referred to as matter. The former are particles whose rest mass is zero or negligible compared to their energy, and therefore move at the speed of light or very close to it; the latter are particles whose kinetic energy is much lower than their rest mass and therefore move much slower than the speed of light.

As the universe expands, both matter and radiation in it become diluted. However, the universe also cools down, meaning that the average energy per particle is getting smaller with time. Therefore the radiation becomes weaker, and dilutes faster than matter. Thus with the expansion of the universe, radiation becomes less dominant than matter. In the very early universe, radiation dictated the rate of deceleration of the universe's expansion, and the universe is said to have been 'radiation dominated'. 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[link]

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[link]

The equations of motion governing the universe as a whole are derived from general relativity with a small, positive cosmological constant.^[10] 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[link]

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 Failed to parse (Missing texvc executable; please see math/README to configure.): 1/H

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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[link]

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[link]

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[link]

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[link]

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[link]

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 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[link]

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[link]

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[link]

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^[who?] 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[link]

Cosmologists also study:

• whether primordial black holes were formed in our universe, and what happened to them.
• the 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.

See also[link]

Physics portal

• String cosmology
• Physical ontology
• List of cosmologists
• Hubble's law
• Photon

References[link]

Further reading[link]

Popular[link]

• Brian Greene (2005). The Fabric of the Cosmos. Penguin Books Ltd. ISBN 0-14-101111-4. 
• 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. 
• Simon Singh (2005). Big Bang: the origins of the universe. Fourth Estate. ISBN 0-00-716221-9. 
• Steven Weinberg (1993; 1978). The First Three Minutes. Basic Books. ISBN 0-465-02437-8. 

Textbooks[link]

• 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 0-19-852682-2. 
• Benjamin Gal-Or, “Cosmology, Physics and Philosophy”, Springer Verlag, 1981, 1983, 1987, ISBN 0-387-90581-2, ISBN 0-387-96526-2.

External links[link]

From groups[link]

• Cambridge Cosmology- from Cambridge University (public home page)
• Cosmology 101 - from the NASA WMAP group
• Center for Cosmological Physics. University of Chicago, Chicago, Illinois.
• Origins, Nova Online - Provided by PBS.

From individuals[link]

• 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?" 

Stephen Hawking
Stephen Hawking at NASA, 1980s
Born	Stephen William Hawking (1942-01-08) 8 January 1942 (age 70) Oxford, England, United Kingdom
Residence	United Kingdom
Nationality	British
Fields	Mathematics Theoretical physics
Institutions	Cambridge University California Institute of Technology Perimeter Institute for Theoretical Physics
Alma mater	Oxford University Cambridge University
Doctoral advisor	Dennis Sciama
Other academic advisors	Robert Berman
Known for	Black holes Theoretical cosmology Quantum gravity Hawking radiation
Influences	Dikran Tahta
Notable awards	Albert Einstein Award (1978) Wolf Prize (1988) Prince of Asturias Award (1989) Copley Medal (2006) Presidential Medal of Freedom (2009)
Spouse	Jane Hawking (m. 1965–1991, divorced) Elaine Mason (m. 1995–2006, divorced)

Stephen William Hawking, CH, CBE, FRS, FRSA (born 8 January 1942) is a British theoretical physicist, cosmologist, and author. His key scientific works to date have included providing, with Roger Penrose, theorems regarding gravitational singularities in the framework of general relativity, and the theoretical prediction that black holes should emit radiation, which is today known as Hawking radiation (or sometimes as Bekenstein–Hawking radiation).

He is an Honorary Fellow of the Royal Society of Arts, a lifetime member of the Pontifical Academy of Sciences, and in 2009 was awarded the Presidential Medal of Freedom, the highest civilian award in the United States. Hawking was the Lucasian Professor of Mathematics at the University of Cambridge between 1979 and 2009. Subsequently, he became research director at the university's Centre for Theoretical Cosmology.

Hawking has a motor neurone disease related to amyotrophic lateral sclerosis, a condition that has progressed over the years. He is now almost completely paralysed and communicates through a speech generating device. He has been married twice and has three children. Hawking has achieved success with works of popular science in which he discusses his own theories and cosmology in general; these include A Brief History of Time, which stayed on the British Sunday Times best-sellers list for a record-breaking 237 weeks.

Early life and education[link]

Stephen Hawking was born on 8 January 1942 to Frank Hawking, a research biologist, and Isobel Hawking.^[1] He has two younger sisters, Philippa and Mary, and an adopted brother, Edward.^[2] Although Hawking's parents were living in North London, he was born in Oxford as his parents felt it was safer to stay in Oxford for the later stages of the pregnancy. (London was under attack at the time by the Luftwaffe).^[3]

When his father became head of the division of parasitology at the National Institute for Medical Research^[1] in 1950, Hawking and his family moved to St Albans, Hertfordshire.^[3] Hawking attended St Albans High School for Girls from 1950 to 1953 (At that time, boys could attend the girls' school until the age of 10).^[2] and then from the age of 11, he attended St Albans School, where he was a good, but not exceptional, student.^[3]

Inspired by his mathematics teacher, Hawking originally wanted to study the subject at university. However, Hawking's father wanted him to apply to University College, Oxford, where his father had attended. As University College did not have a mathematics fellow at that time, applications were not accepted from students who wished to study that discipline. Hawking therefore applied to read natural sciences with an emphasis in Physics. He was accepted and gained a scholarship.^[3] While at Oxford, he coxed a rowing team, which, he stated, helped relieve his immense boredom at the university.^[4] His physics tutor, Robert Berman, later said "It was only necessary for him to know that something could be done, and he could do it without looking to see how other people did it. ... his mind was completely different from all of his contemporaries".^[3]

Hawking's unimpressive study habits resulted in a final examination score on the borderline between first and second class honours, making an "oral examination" necessary.^[3] Berman said of the oral examination: "And of course the examiners then were intelligent enough to realize they were talking to someone far more clever than most of themselves".^[3] After receiving his B.A. degree at Oxford in 1962, he left for graduate work at Trinity Hall, Cambridge.^[3]

Career[link]

1962-1975[link]

Hawking in Cambridge

Hawking with string theorists David Gross and Edward Witten

Although Hawking started developing symptoms of amyotrophic lateral sclerosis as soon as he arrived at Cambridge and did not distinguish himself in his first two years at Cambridge, he returned to working on his PhD after the disease had stabilised and with the help of his doctoral tutor, Dennis William Sciama.^[5]

When Hawking began his graduate studies In the 1960s, there was much debate in the physics community about the opposing theories of the creation of the universe: big bang, and steady state.^[5] Hawking and his Cambridge friend and colleague, Roger Penrose, showed in 1970 that if the universe obeys general relativity and if the universe fits any of the Friedmann models, then the universe must have begun as a singularity.^[6] This work showed that, far from being mathematical curiosities which appear only in special cases, singularities are a fairly generic feature of general relativity.^[7]

Hawking's work with Brandon Carter, Werner Israel and D. Robinson, strongly supported John Wheeler's no-hair theorem – that any black hole is fully described by the three properties of mass, angular momentum, and electric charge.^[8] With Bardeen and Carter, he proposed the four laws of black hole mechanics, drawing an analogy with thermodynamics.^[9] In 1974, he calculated that black holes should thermally create and emit subatomic particles, known today as Bekenstein-Hawking radiation, until they exhaust their energy and evaporate.^[6]

Hawking was elected one of the youngest Fellows of the Royal Society in 1974,^[10] and in the same year he accepted the Sherman Fairchild Distinguished Scholar visiting professorship at the California Institute of Technology (Caltech) to work with his friend, Kip Thorne, who was a faculty member there.^[11] He continues to have ties with Caltech, spending a month each year there since 1992.^[12]

1975-present[link]

The mid to late 1970s were a period of growing fame and success for Hawking, his work was now much talked about, he was appearing in popular television documentaries and in 1979 he was made the Lucasian Professor of Mathematics at the University of Cambridge, a post he held for 30 years until his retirement in 2009.^[13][14] Hawking's inaugural lecture as Lucasian Professor of Mathematics had the title "Is the end in sight for Theoretical Physics" and promoted the idea that supergravity would help resolve many of the outstanding problems physicists were studying. ^[14]

In collaboration with Jim Hartle, Hawking developed a model in which the universe had no boundary in space-time, replacing the initial singularity of the classical Big Bang models with a region akin to the North Pole: one cannot travel north of the North Pole, as there is no boundary.^[15] While originally the no-boundary proposal predicted a closed universe, discussions with Neil Turok led to the realisation that the no-boundary proposal is also consistent with a universe which is not closed.^[16]

Along with Thomas Hertog at CERN, in 2006 Hawking proposed a theory of "top-down cosmology", which says that the universe had no unique initial state, and therefore it is inappropriate for physicists to attempt to formulate a theory that predicts the universe's current configuration from one particular initial state.^[17] Top-down cosmology posits that in some sense, the present "selects" the past from a superposition of many possible histories. In doing so, the theory suggests a possible resolution of the fine-tuning question.^[18]

Thorne–Hawking–Preskill bet[link]

U.S. President Barack Obama talks with Stephen Hawking in the Blue Room of the White House before a ceremony presenting him and 15 others the Presidential Medal of Freedom on 12 August 2009. The Medal of Freedom is the United States' highest civilian honour.

In 1997 Hawking made a public scientific wager with Thorne and John Preskill of Caltech concerning the black hole information paradox.^[19] Thorne and Hawking argued that since general relativity made it impossible for black holes to radiate, and lose information, the mass-energy and information carried by Hawking Radiation must be "new", and must not originate from inside the black hole event horizon. Since this contradicted the idea under quantum mechanics of microcausality, quantum mechanics would need to be rewritten. Preskill argued the opposite, that since quantum mechanics suggests that the information emitted by a black hole relates to information that fell in at an earlier time, the view of black holes given by general relativity must be modified in some way.^[20] The winner of the bet was to receive an encyclopedia of the loser's choice, from which information may be accessed.^[19]

In 2004, Hawking announced that he was conceding the bet, and that he now believed that black hole horizons should fluctuate and leak information, in doing so providing Preskill with a copy of Total Baseball.^[19] Comparing the useless information obtainable from a black hole to "burning an encyclopedia", Hawking commented, "I gave John an encyclopedia of baseball, but maybe I should just have given him the ashes".^[19]

Recognition[link]

Acclaim[link]

On 19 December 2007, a statue of Hawking by artist Ian Walters was unveiled at the Centre for Theoretical Cosmology, University of Cambridge.^[21] The Stephen W. Hawking Science Museum in San Salvador, El Salvador, is named in honour of Stephen Hawking, citing his scientific distinction and perseverance in dealing with adversity.^[22] The Stephen Hawking Building in Cambridge opened on 17 April 2007. The building belongs to Gonville and Caius College and is used as an undergraduate accommodation and conference facility.^[23]

Awards and honours[link]

Personal life[link]

Stephen Hawking being presented by his daughter Lucy Hawking at the lecture he gave for NASA's 50th anniversary

Hawking has stated that, having been diagnosed with ALS during an early stage of his graduate work, he did not see much point in obtaining a doctorate if he were to die soon after. Hawking later said that the real turning point was his 1965 marriage to Jane Wilde, a language student.^[29] Jane cared for him until 1990 when the couple separated.^[1] They had three children: Robert, Lucy, and Timothy.^[1] Hawking married his personal care assistant, Elaine Mason, in 1995; ^[1] the couple divorced In October 2006^[30] amid claims by former nurses that she had abused him.^[31] In 1999, Jane Hawking published a memoir, Music to Move the Stars, detailing the marriage and its breakdown; in 2010 she published a revised version, Travelling to Infinity, My Life with Stephen.^[32]

Hawking supports the children's charity SOS Children's Villages UK^[33] and has stated that his view on how to live life is to "seek the greatest value of our action".^[34] He strongly opposed the Iraq War, calling it "a war crime" and "based on two lies" at a demonstration in Trafalgar Square, where he participated in a public reading of the names of Iraqi war victims.^[35][36]

Hawking has named his secondary school mathematics teacher Dikran Tahta as an inspiration.^[37] He maintains his connection with St Albans School, giving his name to one of the four houses and to an extracurricular science lecture series.^[38]

Illness[link]

Hawking on 5 May 2006, during the press conference at the Bibliothèque nationale de France to inaugurate the Laboratory of Astronomy and Particles in Paris and the French release of his work God Created the Integers

Hawking has a motor neurone disease that is related to amyotrophic lateral sclerosis (ALS), a condition that has progressed over the years. He is now almost completely paralysed and communicates through a speech generating device. Hawking's illness has progressed more slowly than typical cases of ALS: survival for more than 10 years after diagnosis is uncommon.^[39][40]

Symptoms of the disorder first appeared while he was enrolled at University of Cambridge; he lost his balance and fell down a flight of stairs, hitting his head.^[4] The diagnosis of motor neurone disease came when Hawking was 21, shortly before his first marriage, and doctors said he would not survive more than two or three years. By 1974, he was unable to feed himself or get out of bed. His speech became slurred so that he could be understood only by people who knew him well. During a visit to CERN in Geneva in 1985, Hawking contracted pneumonia, which in his condition was life-threatening as it further restricted his already limited respiratory capacity. He had an emergency tracheotomy, and as a result lost what remained of his ability to speak.^[41] A speech generating device was built in Cambridge, using software from an American company, that enabled Hawking to write onto a computer with small movements of his body, and then have a voice synthesiser speak what he typed.^[42]

The particular voice synthesiser hardware he uses, which has an American English accent, is no longer being produced. Asked why he has still kept the same voice after so many years, Hawking stated that he has not heard a voice he likes better and that he identifies with it even though the synthesiser is both large and fragile by current standards. Although a mid-2009 corporate press release said that he had chosen NeoSpeech's VoiceText speech synthesiser as his new voice,^[43] a 30 December 2011 interview with Hawking's technician indicates that Hawking is still using an older synthesiser containing a card "which dates back to the 1980s" and that any upgrade would have to be the same voice, otherwise "it wouldn't be Stephen's voice any more".^[44]

For lectures and media appearances, Hawking appears to speak fluently through his synthesiser; however when preparing answers his system produces words at a rate of about one per minute.^[44] Hawking's setup uses a predictive text entry system, which requires only the first few characters in order to auto-complete the word, but as he is only able to use his cheek for data entry, constructing complete sentences takes time.^[44] During a TED Conference talk, it took him seven minutes to provide a brief answer to a question.^[45]

He describes himself as lucky, despite his disease. Its slow progression has allowed him time to make influential discoveries and has not hindered him from having, in his own words, "a very attractive family".^[42]

In popular culture[link]

Hawking in 2007, experiencing weightlessness

Hawking has played himself on numerous television shows and has been portrayed in many more. He has played himself on a Red Dwarf anniversary special, played a hologram of himself on the episode "Descent" of Star Trek: The Next Generation,^[46] and appeared on the Discovery Channel special Alien Planet.^[47] He has also played himself in several episodes of The Simpsons and Futurama, and has had an action figure made of his Simpsons likeness.^[48] The 2004 BBC4 TV film Hawking dealt with his early life and the onset of his illness. He was portrayed by Benedict Cumberbatch. In 2008, Hawking was the subject of and featured in the documentary series Stephen Hawking, Master of the Universe for Channel 4. In September 2008, Hawking presided over the unveiling of the 'Chronophage' (time-eating) Corpus Clock at Corpus Christi College Cambridge.^[49] His actual synthesiser voice was used on parts of the Pink Floyd song "Keep Talking" from the 1994 album The Division Bell, as well as on Turbonegro's "Intro: The Party Zone" on their 2005 album Party Animals, Wolfsheim's "Kein Zurück (Oliver Pinelli Mix)". On 5 April 2012 he appeared as a guest star in an episode of The Big Bang Theory.^[50]

Space and spaceflight[link]

At the celebration of his 65th birthday on 8 January 2007, Hawking announced his plan to take a zero-gravity flight that year, with an intention to later take a sub-orbital spaceflight. Billionaire Richard Branson pledged to pay all expenses for a future flight on Virgin Galactic's space service, costing an estimated £100,000.^[51] Stephen Hawking's zero-gravity flight in a "Vomit Comet" of Zero Gravity Corporation, during which he experienced weightlessness eight times, took place on 26 April 2007.^[52] He became the first quadriplegic to float in zero gravity. The fee is normally US$3,750 for 10 to 15 plunges, but Hawking was not required to pay. Hawking was quoted before the flight saying:

Many people have asked me why I am taking this flight. I am doing it for many reasons. First of all, I believe that life on Earth is at an ever-increasing risk of being wiped out by a disaster such as sudden nuclear war, a genetically engineered virus, or other dangers. I think the human race has no future if it doesn't go into space. I therefore want to encourage public interest in space.^[53]

In an interview with The Daily Telegraph, he suggested that space was the Earth's long term hope.^[54] He continued this theme at a 2008 Charlie Rose interview.^[55]

Hawking has indicated that he is almost certain that alien life exists in other parts of the universe, "To my mathematical brain, the numbers alone make thinking about aliens perfectly rational. The real challenge is to work out what aliens might actually be like".^[56] He believes alien life not only certainly exists on planets but perhaps even in other places, like within stars or even floating in outer space. He has also warned that a few of these species might be intelligent and threaten Earth.^[57] "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said.^[56] He has advocated that, rather than try to establish contact, humans should try to avoid contact with alien life forms.^[56] At a George Washington University lecture in honour of NASA's fiftieth anniversary, Hawking discussed the existence of extraterrestrial life, believing that "primitive life is very common and intelligent life is fairly rare".^[58]

Religious views[link]

In his early work, Hawking spoke of God in a metaphorical sense, such as in A Brief History of Time: "If we discover a complete theory, it would be the ultimate triumph of human reason – for then we should know the mind of God."^[59] In the same book he suggested the existence of God was unnecessary to explain the origin of the universe.^[60] In the Channel 4 documentary Genius of Britain, in his 2010 book The Grand Design, and in interviews with the Telegraph, Hawking has clarified that he does not believe in a "personal" God.^[61] Hawking writes, "The question is: is the way the universe began chosen by God for reasons we can't understand, or was it determined by a law of science? I believe the second."^[61] He adds, "Because there is a law such as gravity, the Universe can and will create itself from nothing."^[62]

His ex-wife, Jane, has described him as an atheist.^[63] Hawking has stated that he is "not religious in the normal sense" and he believes that "the universe is governed by the laws of science. The laws may have been decreed by God, but God does not intervene to break the laws."^[64] In an interview published in The Guardian newspaper, Hawking regarded the concept of Heaven as a myth, believing that there is "no heaven or afterlife" and that such a notion was a "fairy story for people afraid of the dark."^[59][65] Hawking contrasted religion and science in 2010, saying: "There is a fundamental difference between religion, which is based on authority, [and] science, which is based on observation and reason. Science will win because it works."^[66]

Philosophical views[link]

At Google’s Zeitgeist Conference in 2011, Hawking said that "philosophy is dead." He believes philosophers "have not kept up with modern developments in science" and that scientists "have become the bearers of the torch of discovery in our quest for knowledge.” He said that philosophical problems can be answered by science, particularly new scientific theories which "lead us to a new and very different picture of the universe and our place in it”.^[67]

Publications[link]

Hawking's first popular science book, A Brief History of Time, was published on 1 April 1988. It stayed on the British Sunday Times best-sellers list for a record-breaking 237 weeks.^[68] A Brief History of Time was followed by The Universe in a Nutshell (2001). A collection of essays titled Black Holes and Baby Universes (1993) was also popular. His book, A Briefer History of Time (2005), co-written by Leonard Mlodinow, updated his earlier works to make them accessible to a wider audience. In 2007 Hawking and his daughter, Lucy Hawking, published George's Secret Key to the Universe, a children's book focusing on science that Lucy Hawking described as "a bit like Harry Potter but without the magic."^[69]

Popular[link]

• A Brief History of Time, (1988)
• Black Holes and Baby Universes and Other Essays, (1994)
• The Universe in a Nutshell, (2001)
• On The Shoulders of Giants. The Great Works of Physics and Astronomy, (2002)
• A Briefer History of Time, (2005)
• God Created the Integers: The Mathematical Breakthroughs That Changed History, (2005)^[70]
• The Grand Design, (2010)^[70]

Children's fiction[link]

These are co-written with his daughter Lucy.

• George's Secret Key to the Universe, (2007)
• George's Cosmic Treasure Hunt, (2009)^[70]
• George and the Big Bang, (2011)

Films and series[link]

• A Brief History of Time (1991)
• Stephen Hawking's Universe (1997)
• Horizon: The Hawking Paradox^[71] (2005)
• Masters of Science Fiction (2007)
• Stephen Hawking: Master of the Universe^[72] (2008)
• Into the Universe with Stephen Hawking^[73] (2010)
• Brave New World with Stephen Hawking

See also[link]

• Many-worlds interpretation, or flexiverse
• Gibbons–Hawking ansatz
• Gibbons–Hawking space

References[link]

Bibliography[link]

Further reading[link]

External links[link]

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• Stephen Hawking's web site
• Appearances on C-SPAN
• Stephen Hawking at the Internet Movie Database
• Stephen Hawking at the Mathematics Genealogy Project
• Works by or about Stephen Hawking in libraries (WorldCat catalog)
• Stephen Hawking collected news and commentary at The Guardian
• Stephen Hawking collected news and commentary at The New York Times
• Archival material relating to Stephen Hawking listed at the UK National Register of Archives
• Portraits of Stephen William Hawking at the National Portrait Gallery, London