Today's universe—exceedingly large (1028 cm) and frigidly cold (∼3K)—has seemingly little to do with the microscopic high-energy world of elementary particle physics. However, because today's universe is expanding, and its temperature is nonzero, the connection between the macroworld of cosmology and the microworld of the elementary particle is unavoidable. The relatively simple equations describing the expanding universe (filled with ubiquitous microwave background radiation) can just as easily be run backwards in time. It is then apparent that the universe evolved from a Big Bang—a hot dense soup consisting of all the particles that inhabit the Standard Model of particle physics. The connection between particle physics and cosmology is so strong that the standard cosmological model (i.e., the Hot Big Bang) assumes the Standard Model of particle physics, along with general relativity, as fundamental components.
There is no evidence for appreciable antimatter anywhere in the universe. For that matter, there are relatively few baryons in the universe: the success of Big Bang nucleosynthesis reveals that today there is roughly one baryon for every 109 photons. That is, the baryon asymmetry of the universe (i.e., the net number of baryons over antibaryons relative to the number of photons) is 10-9. This small number turns out to be a big problem for cosmology. If the universe had started out with equal numbers of baryons and antibaryons, they would have annihilated with great efficiency, and today's baryon-to-photon ratio would be 10-19, much too small. A solution to this baryogenesis problem may be derived from particle physics. Generating an excess of baryons over antibaryons in a universe that starts with an equal number of baryons and antibaryons (i.e., one with no net baryon excess) requires physical processes that violate baryon number. Andrey Sakharov pointed out in the 1960s that out-of-equilibrium(e.g., the decay of a massive particle) baryon number violation (along with the violation of CP [the simultaneous conservation of charge and parity]) is the necessary ingredient in any model that explains the observed baryon excess. These Sakharov conditions, including baryon number violation, are a natural feature of many Grand Unified Theory (GUT) extensions of the Standard Model. Although baryon number is almost perfectly conserved at low energies (protons, no matter how long they are observed, do not seem to decay), GUTs, in their quest to unify the strong, weak, and electro-magnetic interactions, still possess interactions that violate both baryon and lepton number conservation, and these interactions become strong at large energies. As the universe cooled through the temperature associated with these large energies, the baryon violating interactions allow it to dynamically generate a baryon asymmetry of just the right order of magnitude.
GUT baryogenesis is the fair-haired child resulting from the marriage of particle physics and cosmology. Not nearly so pleasant are the cosmological implications of another GUT-cosmology offspring: magnetic monopoles. As the GUT universe symmetry evolves from the unified to the broken (via spontaneous symmetry breaking), it is impossible, in the standard cosmological model, to avoid the production of very massive relics (so-called topological defects). The pointlike variety of these topological relics are called magnetic monopoles, and the prediction of most GUTs is that they would be very massive (1016 GeV) and produced in abundances that would be easily detectable, either by their sheer dynamical mass density or in a variety or other astrophysical environments. It appears that the marriage of GUTs and cosmology is doomed by the monopole problem.
In an effort to save the GUT-cosmology union, particle physicists began to examine the evolution of the early universe during the time that spontaneous symmetry breaking must occur. And there they found not only a solution to the monopole problem but also a significant addition to the standard cosmology: inflation. In the generic theory of inflation, the universe becomes dominated by vacuum energy during one of the GUT symmetry breakings and undergoes a period of exponential growth (in the standard cosmology the universe grows as a fractional power of time), increasing in size by some forty-three orders of magnitude so that a micro-scopic patch can become larger than the visible universe! In this scenario, there remains only one GUT monopole in the universe, and thus the monopole problem vanishes. More important, inflation helps explain several deficiencies of the standard cosmology: (1) the universe, at least at an age of 100,000 years, appears to be very nearly isothermal even though the standard cosmology predicts that it was not causally connected, (2) there is no origin for the density perturbations that eventually give rise to galaxies and clusters of galaxies, and (3) the universe is remarkably close to, if not, flat (i.e., no curvature), which requires a fine-tuning of at least one part in 1060. Inflation solves these problems by inflating a causally connected patch, along with the quantum fluctuations present in the patch, into a nearly zero-curvature region larger than the observable universe. The solution to the monopole problem has led scientists to a significant revision of their thinking about the evolution of the universe that is so compelling that most cosmologists accept an epoch of inflation as a necessary ingredient of the standard Big Bang cosmology.
What is the particle physicist's next cosmological conquest? Most likely, it will occur within the realm of the dark matter problem. When the rotational velocity of hydrogen gas in galaxies is measured, it is found that there is roughly ten to twenty times more mass present than that tied up in stars. When the amount of gravitating material in large clusters of galaxies is measured, there appears to be roughly thirty to forty times more gravitational mass than the mass associated with light mass. Both of these measurements provide irrefutable evidence of the dark matter problem. Galaxies themselves and groups of galaxies contain much more mass than astronomers can see! One finds similar discrepancies in the mass budget if one compares these estimates of the gravitating mass of the universe to the mass associated with baryons (as derived from Big Bang nucleosynthesis). Not only can most of the universe not be seen, but most of the universe is not made of baryons!
Again, just as in baryogenesis, particle physics was quick to provide two candidates to resolve the dark matter problem. They existed as by-products of extensions of the Standard Model designed to solve significant problems in particle physics. One dark matter candidate is known as the weakly interacting massive particle (WIMP), and it can naturally occur in the supersymmetric models designed to solve the hierarchy problem. In these models, there is a symmetry introduced that pairs each existing fermion with a hypothetical boson and likewise for the existing bosons. The lightest new particle in the theory is stable against decay and can easily survive as a Big Bang relic in numbers large enough to contribute significantly to the mass density at the current epoch. Although this type of supersymmetric dark matter has yet to be discovered, experimentalists are hot on its trail. The other popular dark matter candidate is the axion, a particle that results from the breaking of the symmetry introduced to solve the strong CP problem (namely, where nonperturbative effects in quantum chromodynamics [QCD], unless suppressed, would predict an electric dipole moment for the neutron that is ten orders of magnitude too large). The axion that results would be produced in the Big Bang, and it couples to photons with significant strength so that it may be detected via resonant photon production in a large magnetic field. Again, it is likely that experimentalists will soon be able to determine if the axion is a significant component of dark matter.
Over the past several years, a new dark matter problem has appeared—the dark energy problem—and its solution will almost surely come from the world of particle physics. When astronomers looked at distant supernovae, their apparent brightness implied that they were further away than cosmologists would have predicted based on the amount of gravitating material so far surveyed (dark matter). The universe is accelerating! And, the culprit is smoothly distributed throughout the universe, makes up 70 percent of the mass of the universe, and has a negative pressure (thus, the name dark energy rather than dark matter). Albert Einstein first called this newly discovered dark energy the cosmological constant. The cause for acceleration may, in fact, be Einstein's cosmological constant, or equivalently, vacuum energy. The problem with this solution is that the same symmetry breakings that occur in GUT cosmology would contribute to vacuum energy—a rough estimate would be that the vacuum energy should be 100 orders of magnitude larger than the current data suggest. Particle cosmologists have recognized that they do not know what makes up 90 percent of the universe and that two-thirds of that 90 per cent is currently unexplainable by any theory. How ever, many are confident an answer will be found, and it is very likely that such an explanation will come from the world of particle physics.
Peebles, P. J. E. "Making Sense of Modern Cosmology." Scientific American284 , 54–55 (2001).
Terry P. Walker
u·ni·verse / ˈyoōnəˌvərs/ • n. (the universe) all existing matter and space considered as a whole; the cosmos. The universe is believed to be at least 10 billion light years in diameter and contains a vast number of galaxies; it has been expanding since its creation in the big bang about 13 billion years ago. ∎ a particular sphere of activity, interest, or experience: the front parlor was the hub of her universe. ∎ (Logic also u·ni·verse of dis·course) another term for universal set. ORIGIN: late Middle English: from Old French univers or Latin universum, neuter of universus ‘combined into one, whole,’ from uni- ‘one’ + versus ‘turned’ (past participle of vertere).
A. †in u. (L. in universum) universally XIV;
B. the whole of created things XVI; the world XVII. — (O)F. univers or L. ūniversum the whole world, sb. use of n. of ūniversus all taken together, lit. ‘turned into one’, f. UNI- + versus, pp. of vertere turn.