Quarks and the cosmos

Cosmology is in the midst of a period of revolutionary discovery, propelled by bold ideas from particle physics and by technological advances from gigapixel charge-coupled device cameras to peta-scale computing. The basic features of the universe have now been determined: It is 13.7 billion years old, spatially flat, and expanding at an accelerating rate; it is composed of atoms (4%), exotic dark matter (20%), and dark energy (76%); and there is evidence that galaxies and other structures were seeded by quantum fluctuations. Although we know much about the universe, we understand far less. Poised to dramatically advance our understanding of both the universe and the laws that govern it, cosmology is on the verge of a golden age.     

Origins

The farthest of the galaxies that can be seen through the large ground-based telescopes of modern astronomy, such as those on La Palma in the Canary Islands, are so far away that they appear as they did close to the time of the origin of the universe, perhaps some 10 billion years ago. Much has been learned, and much has still to be learned, about the young universe from optical and radio telescopes, but these instruments cannot be used to look directly at the universe in its first few hundred thousand years. Instead, they are used to search the relatively recent past for relics of much earlier times. Together with experiments planned for the next generation of elementary particle accelerators, astronomical observations should continue to extend what is known about the universe backward in time to the Big Bang and may eventually help to reveal the origins of the physical laws that govern the universe.     

Size and age of the universe

The age of the universe based on abundances of isotopes is in the range 10 billion to 15 billion years. This is consistent with the age range 12 billion to 20 billion years calculated from the evolution of the oldest galactic stars. A third estimate of the age of the universe is based on the Hubble relation between the velocities of galaxies and their distances from us, where the inverse of the Hubble parameter H is a measure of the age of a uniformly expanding universe. Evidence that has been accumulating over the past few years indicates that the expansion of the universe may exhibit a rather large local perturbation due to the gravitational attraction of the Virgo supercluster. Different types of observations still produce conflicting evidence about the velocity with which the Local Group of galaxies (of which our Milky Way system is a member) is falling into the Virgo cluster. The results to date indicate that this velocity lies somewhere in the range 0 to 500 kilometers per second. The resulting ambiguity in the flow pattern for relatively nearby galaxies makes values of H derived from galaxies with radial velocities less than 2000 kilometers per second particularly uncertain, and this restricts determinations of H to distant galaxies, for which distances are particularly uncertain. The best that can be said at present is that H(-1) yields a maximum time scale in the range 10 billion to 20 billion years.     

Measuring spacetime: from the big bang to black holes

Space is not a boring static stage on which events unfold over time, but a dynamic entity with curvature, fluctuations, and a rich life of its own. Spectacular measurements of the cosmic microwave background, gravitational lensing, type Ia supernovae, large-scale structure, spectra of the Lyman alpha forest, stellar dynamics, and x-ray binaries are probing the properties of spacetime over 22 orders of magnitude in scale. Current measurements are consistent with an infinite flat everlasting universe containing about 30% cold dark matter, 65% dark energy, and at least two distinct populations of black holes.     

Cosmological element production

Two recent observations appear to have provided critical information about the past history of the universe. The thermal character of the microwave background radiation suggests that the universe has expanded from a state of high temperature and density, and places constraints on such a big-bang cosmology. The observations of very weak helium lines in the spectra of certain stars in the halo of our galaxy are possibly due to a low primeval abundance of this element. However, the simplest model of a big-bang cosmology leads to much higher helium abundances, such as are observed in the solar system and in many stars. The production of helium can be reduced either by altering the early expansion rate or by introducing degenerate electron neutrinos. Observations of interstellar and intergalactic deuterium and He(4), and possibly even He(3) and Li(7), are needed to test the various models.     

Extracting primordial density fluctuations

The combination of detections of anisotropy in cosmic microwave background radiation and observations of the large-scale distribution of galaxies probes the primordial density fluctuations of the universe on spatial scales varying by three orders of magnitude. These data are found to be inconsistent with the predictions of several popular cosmological models. Agreement between the data and the cold + hot dark matter model, however, suggests that a significant fraction of the matter in the universe may consist of massive neutrinos.     

New light on dark matter

Dark matter, proposed decades ago as a speculative component of the universe, is now known to be the vital ingredient in the cosmos: six times more abundant than ordinary matter, one-quarter of the total energy density, and the component that has controlled the growth of structure in the universe. Its nature remains a mystery, but assuming that it is composed of weakly interacting subatomic particles, is consistent with large-scale cosmic structure. However, recent analyses of structure on galactic and subgalactic scales have suggested discrepancies and stimulated numerous alternative proposals. We discuss how studies of the density, demography, history, and environment of smaller-scale structures may distinguish among these possibilities and shed new light on the nature of dark matter.     

Galaxies and the Universe: Properties of the universe are revealed by the rotation of galaxies and their distribution in space

A brief review is given of what the study of galaxies has taught us about properties of the universe. It is assumed that the universe started from a general "explosion," and that the general expansion observed today, as well as the 3 degrees K blackbody radiation, are consequences of this explosion. The present average density in the universe is probably close to the critical value of 10(-29) g/cm(3). Only about 3 percent of this is contained in galaxies; the rest consists probably of intergalactic gas at a temperature between 10(5) and 10(6) degrees K. Observations in our own galaxy indicate that this intergalactic gas is still flowing into it.     

The most distant known galaxies

Ever since the proposal of the idea of an expanding universe more than 50 years ago, each generation of investigators has found that some current theory could be (marginally) tested by the properties of the most distant known galaxies. There has consequently been a continuing effort to identify very remote objects, especially to confront theories of the evolution of galaxies (since galaxies are seen as they were at prior epochs) and to confront cosmological theories (which make predictions about the overall dynamics of the expansion of the universe). These theories have yet to be definitively tested, but a new generation of optical telescopes and detectors provides hope for significant progress during this decade.     

The dark night-sky riddle: a "paradox" that resisted solution

The riddle of a dark night sky, now known as "Olbers's paradox," can be traced back to Thomas Digges in 1576. Since the time of Edmund Halley (1721) the riddle of a dark night sky in an infinite universe uniformly populated with stars has been regarded as a paradox. Constant emphasis on the paradoxical aspect of the problem of darkness at night, however, leads to a one-sided interpretation of the riddle. Calling the phenomenon a "paradox" distorts the historical perspective, and consequently we incorrectly attribute the origin of the riddle to Edmund Halley. Also it distorts the cosmological perspective and quite probably has greatly delayed the solution of the riddle.     

Energy Conditions in the Epoch of Galaxy Formation

The energy conditions of Einsteinian gravity (classical general relativity) do not require one to fix a specific equation of state. In a Friedmann-Robertson-Walker universe where the equation of state for the cosmological fluid is uncertain, the energy conditions provide simple, model-independent, and robust bounds on the behavior of the density and look-back time as a function of red shift. Current observations suggest that the "strong energy condition" was violated sometime between the epoch of galaxy formation and the present. This implies that no possible combination of "normal" matter is capable of fitting the observational data.     

A simple description of the 3 k cosmic microwave background

An intuitive model for the expansion of the universe is developed in which special relativity is used to describe events seen by a hypothetical observer in a Lorentz frame of reference. The cosmic microwave background photons he sees are the red-shifted remnants of hot photons emitted from the matter flying rapidly away from him. This special relativistic model, also called the Milne model, represents the extreme case of a Friedmann (general relativistic) universe in the limit of vanishingly small density of matter. The special relativistic model approximates an open universe (one that expands forever) with increasing accuracy as time evolves.     

Particle dark matter in the universe: at the brink of discovery?

The nature of dark matter is one of the central problems of cosmology, particle physics, and gravity. It may be made of still unknown particles produced in the early universe. Much progress has been made in attempts to detect these particles and in the development of the required experimental techniques. Results from direct searches, the Large Hadron Collider, and the Gamma-ray Large Area Space Telescope offer promising opportunities within the next decade to find the missing dark matter.     

Inflationary cosmology: exploring the universe from the smallest to the largest scales

Understanding the behavior of the universe at large depends critically on insights about the smallest units of matter and their fundamental interactions. Inflationary cosmology is a highly successful framework for exploring these interconnections between particle physics and gravitation. Inflation makes several predictions about the present state of the universe-such as its overall shape, large-scale smoothness, and smaller scale structure-which are being tested to unprecedented accuracy by a new generation of astronomical measurements. The agreement between these predictions and the latest observations is extremely promising. Meanwhile, physicists are busy trying to understand inflation's ultimate implications for the nature of matter, energy, and spacetime.     

Age estimates of globular clusters in the Milky Way: constraints on cosmology

Recent observations of stellar globular clusters in the Milky Way Galaxy, combined with revised ranges of parameters in stellar evolution codes and new estimates of the earliest epoch of globular cluster formation, result in a 95% confidence level lower limit on the age of the Universe of 11.2 billion years. This age is inconsistent with the expansion age for a flat Universe for the currently allowed range of the Hubble constant, unless the cosmic equation of state is dominated by a component that violates the strong energy condition. This means that the three fundamental observables in cosmology-the age of the Universe, the distance-redshift relation, and the geometry of the Universe-now independently support the case for a dark energy-dominated Universe.     

Early results from the infrared astronomical satellite

For 10 months the Infrared Astronomical Satellite (IRAS) provided astronomers with what might be termed their first view of the infrared sky on a clear, dark night. Without IRAS, atmospheric absorption and the thermal emission from both the atmosphere and Earthbound telescopes make the task of the infrared astronomer comparable to what an optical astronomer would face if required to work only on cloudy afternoons. IRAS observations are serving astronomers in the same manner as the photographic plates of the Palomar Observatory Sky Survey; just as the optical survey has been used by all astronomers for over three decades, as a source of quantitative information about the sky and as a "roadmap" for future observations, the results of IRAS will be studied for years to come. IRAS has demonstrated the power of infrared astronomy from space. Already, from a brief look at a miniscule fraction of the data available, we have learned much about the solar system, about nearby stars, about the Galaxy as a whole and about distant extragalactic systems. Comets are much dustier than previously thought. Solid particles, presumably the remnants of the star-formation process, orbit around Vega and other stars and may provide the raw material for planetary systems. Emission from cool interstellar material has been traced throughout the Galaxy all the way to the galactic poles. Both the clumpiness and breadth of the distribution of this material were previously unsuspected. The far-infrared sky away from the galactic plane has been found to be dominated by spiral galaxies, some of which emit more than 50 percent and as much as 98 percent of their energy in the infrared-an exciting and surprising revelation. The IRAS mission is clearly the pathfinder for future missions that, to a large extent, will be devoted to the discoveries revealed by IRAS.     

A new look at the saturn system: the voyager 2 images

Voyager 2 photography has complemented that of Voyager I in revealing many additional characteristics of Saturn and its satellites and rings. Saturn's atmosphere contains persistent oval cloud features reminiscent of features on Jupiter. Smaller irregular features track out a pattern of zonal winds that is symmetric about Saturn's equator and appears to extend to great depth. Winds are predominantly eastward and reach 500 meters per second at the equator. Titan has several haze layers with significantly varying optical properties and a northern polar "collar" that is dark at short wavelengths. Several satellites have been photographed at substantially improved resolution. Enceladus' surface ranges from old, densely cratered terrain to relatively young, uncratered plains crossed by grooves and faults. Tethys has a crater 400 kilometers in diameter whose floor has domed to match Tethys' surface curvature and a deep trench that extends at least 270 degrees around Tethys' circumference. Hyperion is cratered and irregular in shape. Iapetus' bright, trailing hemisphere includes several dark-floored craters, and Phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours. Within Saturn's rings, the "birth" of a spoke has been observed, and surprising azimuthal and time variability is found in the ringlet structure of the outer B ring. These observations lead to speculations about Saturn's internal structure and about the collisional and thermal history of the rings and satellites.     

A cyclic model of the universe

We propose a cosmological model in which the universe undergoes an endless sequence of cosmic epochs that begin with a "bang" and end in a "crunch." Temperature and density at the transition remain finite. Instead of having an inflationary epoch, each cycle includes a period of slow accelerated expansion (as recently observed) followed by contraction that produces the homogeneity, flatness, and energy needed to begin the next cycle.     

The cosmic web in our own backyard

On the largest scales, matter is strung out on an intricate pattern known as the cosmic web. The tendrils of this web should reach right into our own cosmic backyard, lacing the Galactic halo with lumps of dark matter. The search for these lumps, lit up by stars that formed within them, is a major astronomical endeavor, although it has failed to find the huge expected population. Is this a dark matter crisis, or does it provide clues to the complexities of gas physics in the early universe? New technologies in the coming decade will reveal the answer.     

Entropy in an expanding universe

The question of how the observed evolution of organized structures from initial chaos in the expanding universe can be reconciled with the laws of statistical mechanics is studied, with emphasis on effects of the expansion and gravity. Some major sources of entropy increase are listed. An expanding "causal" region is defined in which the entropy, though increasing, tends to fall further and further behind its maximum possible value, thus allowing for the development of order. The related questions of whether entropy will continue increasing without limit in the future, and whether such increase in the form of Hawking radiation or radiation from positronium might enable life to maintain itself permanently, are considered. Attempts to find a scheme for preserving life based on solid structures fail because events such as quantum tunneling recurrently disorganize matter on a very long but fixed time scale, whereas all energy sources slow down progressively in an expanding universe. However, there remains hope that other modes of life capable of maintaining themselves permanently can be found.