Cosmology

views updated

COSMOLOGY

Although astronomers and natural philosophers have always been interested in cosmology, it is only in the twentieth century that a science of the universe has become a reality. In particular, from about 1950 cosmological theory began to interact ever more strongly with nuclear and particle physics. This development has accelerated in the later part of the 20th century with the result that parts of cosmology (especially the early big bang theory) have become thoroughly integrated with new theories of elementary particles and fundamental interactions. Not only have theories of microphysics greatly advanced cosmological knowledge, but the early universe has also proved a valuable testing ground for fundamental particle physics.

Early Cosmology

Modern cosmology dates essentially from 1917, when Albert Einstein suggested a cosmological model based on his new theory of gravitation, the general theory of relativity. Einstein's universe was static, spatially finite, and filled with dilute matter. The same year, 1917, Willem de Sitter showed that the theory of relativity allowed another cosmological solution, corresponding to an empty universe. Both models made use of the cosmological constant that Einstein had introduced in his field equations as a parameter of a hypothetical antigravity force.

Nonstatic or evolutionary models were first investigated in 1922, by the Russian physicist Alexander Friedmann, but his work was ignored. Five years later, in 1927, the Belgian Georges Lemaître rediscovered the expanding universe and suggested a model in which the universe expands steadily from an Einstein world. The theories of Friedmann and Lemaître only became known in 1930, after Edwin Hubble had found observational evidence for the expanding universe. From that time onward, most physicists and astronomers accepted that the universe is in a state of expansion. At the same time, the cosmological constant fell in discredit and was often seen as an unnecessary complication.

A further step was taken by Lemaître in 1931, when he suggested for the first time what would later be called a Big Bang model, that is, a universe of finite age that has originated from an ultradense state. Lemaître called this state the primeval atom and suggested that its explosion was a quantum process, like in radioactive decay. The Einstein–de Sitter model of 1932, an early Big Bang model, assumed that there is just enough matter to keep the universe spatially flat. This matter density, which depends on the value of the Hubble constant, is called the critical density.

When Lemaître suggested his primeval atom hypothesis, the proton and the electron were the only known (massive) elementary particles. With the emergence of nuclear physics in 1932, new possibilities were offered for astrophysics and a more physical approach to cosmology. The entry of nuclear and particle physics into cosmology was largely a result of astrophysicists' attempts to understand the energy production of stars and the formation of heavier elements out of protons and neutrons. In a 1938 paper, Carl Friedrich von Weizsäcker sought to infer the state of the early universe by assuming that the chemical elements had been formed under conditions that would reproduce their present individual abundance. Von Weizsäcker was led to a kind of Big Bang universe, starting in an extremely hot and dense state, but he did not develop his ideas further. Such development was left to George Gamow, the Russian-American nuclear physicist and pioneer of cosmology.

Gamow's Physical Cosmology

Von Weizsäcker's methodology—to use the cosmic distribution of chemical elements as evidence for the physical state of the early universe—is sometimes known as nuclear archaeology. It was this method that guided Gamow's development of physical Big Bang cosmology in the years between 1946 and 1956. Together with his collaborators Ralph Alpher and Robert Herman, he was led to consider the primordial universe as a hot, dense gas of neutronic matter. Right after the Big Bang, neutrons would decay into protons and electrons, and some of the protons would combine with neutrons to form deuterons. The essential process in the building-up of higher nuclei was believed to be the capture of neutrons. Between 1948 and 1950 it was realized that this picture was too primitive and that the assumption of a matter-dominated early universe was un-tenable. Gamow and Alpher argued that during the first phase of the expansion, the energy content of the universe would be governed almost entirely by electromagnetic radiation (that is, photons). Only at a later stage, when the universe had become colder, would matter begin to dominate over radiation.

Gamow's program of Big Bang cosmology culminated in a 1953 paper, which can be considered the first example of particle cosmology. Written by Ralph Alpher, Robert Herman and James Follin, the paper provided a detailed analysis of the early universe, now thought to consist mainly of photons, neutrinos, and electrons (including positrons), but also with small amounts of nucleons and muons. The Alpher-Herman-Follin theory made full use of the most recent progress in particle physics, including the theory of weak interactions as applied to beta decay and processes involving electrons, neutrinos, and muons. The three physicists found the present abundance of helium to be about 30 percent and also calculated the neutron-to-proton ratio at the time nucleosynthesis started. However, the wider aim of the Gamow program—to account for the formation of all chemical elements—failed. Gamow and his collaborators were unable to build up elements heavier than helium, and this was widely considered a failure of the Big Bang theory itself.

Only in 1965 did Big Bang cosmology experience a renaissance, this time irreversibly. The discovery of the cosmic microwave background radiation of temperature about 3K provided an important parameter for new and improved calculations of the synthesis of the lightest elements. In 1966, James Peebles calculated from the Big Bang theory a helium abundance between 26 and 28 percent, in excellent agreement with observations. Subsequent calculations of the light elements (including deuterium and lithium) only improved the fit between theory and experimental data. The standard cosmological model accounts satisfactorily for the primordial abundance not only of helium-4, but also of deuterium, helium-3, and lithium-7. (The heavier elements are produced in stars and novae, not cosmologically.) The success of the predictions strongly suggests that the hot Big Bang theory is accurate all the way back to 1 second after the universe came into origin. Moreover, theories of nucleosynthesis give a good estimate of one of cosmology's most important parameters, the density of baryons (essentially protons and neutrons). Calculations imply that the density of baryons is somewhere between 1 and 10 percent of the critical density, a result that has important implications for cosmology.

Antimatter and Baryogenesis

Antiparticles, predicted by Paul Dirac in 1931, have played an important role in the development of cosmology. As early as 1933, Dirac suggested the existence of entire antistars (made up of positrons and antiprotons), and in 1956 Maurice Goldhaber even speculated about an "anticosmos" symmetric to the cosmos in which we live. Neither these nor other speculations about abundant masses of antimatter have received observational support.

On the contrary, observations strongly indicate that there is only very little antimatter in the universe. If the universe were initially symmetric, annihilation between matter and antimatter would have resulted in a present state almost completely dominated by photons and with only trace amounts of baryons and antibaryons. This annihilation catastrophe obviously has not occurred, which means that the early universe must have possessed a slight excess of matter over antimatter. Until about 1970 the only explanation was to postulate a charge asymmetry in the very early universe, a slight predominance of quarks over antiquarks. Why would there be, for every 300 million quarks, just 299 million antiquarks? This question relates critically to the possibility of creation of baryons—baryogenesis—out of processes in which the number of baryons is not conserved.

The earliest attempt to explain how a baryon excess could be generated was published by Andrei Sakharov in a 1967 paper, but his suggestion did not attract much interest at the time. To explain the baryon asymmetry, Sakharov assumed violation of charge conjugation and parity (CP) conservation, a process which in 1964 had been detected in the decay of neutral kaons. More speculatively, he postulated an interaction that violated baryon conservation. With the first versions of Grand Unified Theories (GUTs), the idea of baryogenesis received theoretical support, and Sakharov's speculations were reconsidered. According to the early GUTs, as developed by Howard Georgi, Sheldon Glashow, Steven Weinberg, and others in 1974–1975, transitions between quarks and leptons are possible, that is, the baryon number is not precisely conserved. In 1978, Motohiko Yoshimura used the new GUTs to predict a baryon-antibaryon asymmetry caused by primordial vector bosons, and since then baryogenesis has attracted great interest. Within the GUT framework there is no need to postulate a baryon excess, for the observed baryon number could have been created by baryon number nonconserving processes.

During the years 1978–1980, Yoshimura, Weinberg, Frank Wilczek, and others used GUT to solve another cosmological problem. The number of photons in the observable part of the universe is around 10⁸⁸ and that of baryons is around 10⁷⁹. The photon-to-baryon ratio is considered a fundamental quantity, because theory prescribes it to be constant in time. Why are there one billion times as many photons than baryons? The answer, according to the GUT theorists, is that this has not always been the case but was the result of the slight asymmetry in quark-antiquark annihilation processes in the early universe. Although the GUT-based theory of baryo-genesis has still no direct experimental support, it is considered compelling by many particle physicists who see it as support for an intimate relationship between cosmology and particle physics.

Nucleosynthesis and baryogenesis are examples of particle physics applied to cosmology. Conversely, cosmology has also been used as a probe of fundamental physics, to gain knowledge of physics at very high energies. For example, in 1977 Gary Steigman, David Schramm, and James Gunn showed that the number of neutrino types could not be larger than four if the hot Big Bang theory were correct. Further refinements led to a lower limit of three types or families of neutrinos. At that time, only two neutrino types were known (the electron and muon neutrinos), but in 1993 evidence for a third (tau) neutrino was produced in accelerator experiments, which was seen as a brilliant confirmation of the cosmological prediction. Moreover, calculations showed that the primordial nucleosynthesis required the mass of the tau neutrino to be less than 0.5 MeV. This constraint agreed with, but was finer than, the one obtained experimentally and thus afforded another test of the big bang scenario. No wonder that Schramm concluded that "the marriage between particle physics and cosmology had indeed been consummated" (Schramm 1996, p. xvii).

Inflation Models

Grand unified theories also led to the first inflation models, introduced in the early 1980s by Alan Guth, Andrei Linde, and others. According to the inflation model, the very early universe underwent an extreme phase transition and approached a state known as a false vacuum. In Guth's original model, the energy density of the false vacuum was attributed to a mechanism derived from GUT, spontaneous symmetry breaking. The mechanism of inflation can be imagined as the vacuum energy effectively acting as a cosmological constant that boosts the expansion of the universe exponentially until the vacuum energy is converted into heat and the universe enters its epoch of slow expansion. However, most later versions of the inflation model do not depend critically on GUT or relate to the spontaneous symmetry breaking of particle physics. The essential feature is the false vacuum. Guth's paper of 1981 inspired a massive influx of particle physicists into cosmology and a burst of theoretical activity. By 1997, more than 3,000 papers had been published on inflation theory, most by particle physicists.

Among the many attractive features of the inflation model is that it offers an explanation of how the energy of the universe came into existence, namely, when the huge energy stored in the inflated false vacuum was released at the end of the brief inflation era. Moreover, it avoids the problem of the magnetic monopoles, particles that have never been detected. Most GUTs predict an abundance of primordial monopoles, many of which should still exist, but inflation takes care of the problem.

The confidence that many particle physicists and cosmologists have in the inflation model (in one of its several versions) is related to one of cosmology's most exciting problems, the problem of dark matter. According to the inflation model, the mass (or energy) density of the universe should be critical. However, observations show that there is far from enough ordinary (baryonic) matter to produce a critical density. It is known that most of the matter in the universe must be in an exotic, dark form. Particle physics suggests a number of dark matter candidates, from massive neutrinos to hypothetical particles predicted by certain fundamental theories of the GUT type. However, none of these particles has been detected, and the problem of dark matter is thus still unsolved. Nonetheless, there is a growing consensus that dark matter particles are "cold," that is, slowly moving. Particles called axions and neutralinos are examples of such cold dark matter, but, even if they exist, they may not be the most abundant form of energy in the universe.

An Accelerating Universe

Observations of supernovae in 1998 indicated that the expansion rate of the universe is much greater than hitherto assumed. Subsequent observations have substantiated the result, and it is now generally accepted that the universe is in a state of acceleration rather than deceleration. According to the standard big bang theory, an accelerated universe contains less matter than given by the critical density, but things look different if a form of energy with negative pressure is admitted. Such strange forms of energy were studied prior to the inflation model, and in 1974 Linde argued that a vacuum energy with negative pressure acts as an effective cosmological constant. (The same insight can be found as early as 1934, in a paper by Lemaître.) However, Linde did not realize that this effective cosmological constant may greatly influence the initial stage of the evolution of the universe. This was an insight of the inflation theory in which the cosmological constant has a natural interpretation.

Today, it is widely assumed that the vacuum energy of the accelerated universe must be attributed to a positive cosmological constant which is responsible for as much as two-thirds of the total energy content of the universe. The symbol for the cosmological constant is ?, and cold dark matter is abbreviated CDM. For this reason, physicists sometimes speak of the ?CDM scenario or, because of the connection to inflation theory, the inflation +CDM scenario. Now, in the beginning of the twenty-first century, many cosmologists explore both this and alternative theories of the early universe.