Large-scale Structure In The Universe

Type of physical science: Astronomy; Astrophysics

Field of study: Cosmology

There is evidence of connective patterns of galaxy clusters across dimensions at least as large as many hundreds of millions of light-years. Explaining the observed features of this large-scale structure is a challenge to cosmologists that severely limits acceptable models of the origin and evolution of the universe.

Overview

The hundred thousand light-years which span the diameter of the Milky Way galaxy enclose complex organizations of matter and energy, including stars emitting visible light, dense molecular clouds radiating at infrared wavelengths, and dark matter known only through its gravitational influence. To the contemporary cosmologist, all of these are small-scale structures when viewed from the perspective of attempting to construct a comprehensive model of the principal features of the composition and evolution of the entire physically observable universe.

In this context, a large-scale structure means a feature still distinguishable when comparing the average contents of volumes each at least several million light-years across. One such volume encloses the Local Group of galaxies, whose dynamical behavior is determined by its gravitational interaction with other such volumes.

Understanding the structure and evolution of the universe is thus dependent upon a theory of gravitation and evidence about the distribution of matter and energy on the largest scales. As soon as Albert Einstein's theory of space-time and gravitation was completed, in 1916, it was immediately recognized that this theory would have a profound impact on cosmological models. The most extensively studied cosmological solutions of the general theory of relativity are those that also satisfy the cosmological principle. This is the assumption that at each moment in the history of the universe, an observer at any place would find the large-scale structure of surrounding matter the same as an observer at any other place (homogeneity) and appearing the same in all directions (isotropy).

In 1922, Aleksandr Aleksandrovich Friedmann derived the only three possible spatial geometries that can be solutions of Einstein's gravitational field equations under these restrictions. One of these is a space of uniform positive curvature and finite volume though without boundary. In two dimensions, the surface (not including the interior volume) of a sphere is such a space: Motion on it is never blocked by a barrier (perimeter line), but its area is finite.

Another, and particularly simple, possibility is a flat space satisfying all the assumptions (and thus displaying all the derived features) of Euclidean geometry. In two dimensions, the surface of a plane without a boundary must be infinite. The only other homogeneous and isotropic space possible in Einstein's theory is a negatively curved one, also of infinite extent. In two dimensions, such a surface is termed "hyperboloid": A saddle is an example of a part of such a surface, which has unlimited area, if lacking a boundary curve. The common feature of these models is that their scale factors (distance between representative points, such as clusters of galaxies) are required to change with time: The universe must be either expanding or contracting.

Before 1930, Edwin Powell Hubble published the first evidence that the universe is indeed expanding. If the scale factor of the universe was smaller in the past, its local densities of matter and radiation were higher. Since the mass density of matter varies inversely as the scale factor cubed, while the energy density of radiation varies inversely as the scale factor to the fourth power, at sufficiently early times, the pressure driving the expansion of the universe came from radiation. This is the basis of the "hot big bang" concept of the origin of the universe.

In modern years, there has been increasing interest in the study of inhomogeneous and anisotropic cosmological models. Initially, the motivation came primarily from the desire to understand better the implications of Einstein's theory of gravity. Besides the fundamental intellectual challenge of discovering and analyzing new solutions of the gravitational field equations, this work has been driven by the need to understand which features of the standard models are robust--that is, not sensitively dependent on the assumptions of exact homogeneity and isotropy. After all, this is a universe which has different small features in various places and presents slightly different views as one gazes outward in various directions.

Concurrently, observational and theoretical evidence has been accumulating, which challenges the old assumption that large-scale structure is not important in formulating a model that explains the general features of the universe. Pioneering work by George Abell and others, beginning after World War II, suggested that the distribution of rich clusters (each containing at least a thousand members) of galaxies included many of irregular shape and diverse contents.

Yet, since the late 1970's, it was believed that the largest inhomogeneities in the distribution of matter in the universe were less than 300 million light-years across. Since then, new data and new analysis of older data have disclosed substantially larger structures. For example, a three-dimensional map of the tens of thousands of galaxies within a sphere more than 1 billion light-years in diameter, centered on the Milky Way, has been constructed by John Huchra and Margaret Geller. Based on more than five years of data gathered by a telescope, on the surface of the earth, of a size comparable to the Hubble Space Telescope, this map shows voids and filamentary structures on scales greater than 400 million light-years. Understanding the origin of structure in the universe on such large scales challenges the creativity of cosmologists.

Applications

First images, unfortunately of disappointing quality, were returned by the Hubble Space Telescope in May of 1990. Scientists expect that, when its optical problems are corrected, it will produce data of comparable quality to those described above throughout the remainder of its fifteen-year estimated life. It will be looking substantially deeper into space, perhaps ultimately surveying a volume a thousand times greater than heretofore. The motivation for having such data is that there are millions of times as many volumes that could be mapped as have been, and looking out is also looking back in time (because of the finite speed of light) toward the beginning of the universe. In the region already surveyed in detail, the density of galaxies in voids is typically a factor of ten less than average, and the density in the narrow but long filaments is typically a factor of ten thousand greater than average. Scientists are confronted with the major challenge of explaining the origin of such variations in density from an early universe that was amazingly homogeneous. The experimental evidence for this last claim is the extreme uniformity of the cosmic microwave background radiation, as observed by the Cosmic Background Explorer (COBE) satellite, launched in 1990. This radiation, now known to have a temperature of 2.735 Kelvins (absolute degrees) in all directions, is a relic of the primeval fireball, informing scientists of conditions throughout the universe at a time no more than a few hundred million years after its beginning. The difficulty of understanding how a universe that was so smooth at early times became so lumpy on such large scales in later times is called the homogeneity problem.

The solution of the homogeneity problem may involve what has been, somewhat misleadingly, often termed the "missing-mass problem." It is better to call this problem that of the identity of dark matter, since it is light rather than mass that appears to be missing. There are two distinct ways to estimate the total mass of the galaxies in clusters. One is based on assuming that a statistical mechanics result known as the virial theorem, relating kinetic and gravitational potential energies, applies to their observed speeds and separations. Another method is based on applying ratios of mass to light and is derived from knowledge of the composition of the light-producing objects within galaxies. These two methods are in serious conflict: The estimate of mass from light is typically less than that from mechanics by a factor of ten or more. Most astrophysicists consider the arguments for the estimate from mechanics more firmly established, and thus are challenged to explain the nature of the dark matter, which apparently interacts very little (if at all) with electromagnetic radiation. Though this challenge has not been resolved in a manner satisfactory to all the experts in this field, the mere existence of dark matter, which is much more abundant than the familiar matter through its interaction with photons, provides a possible solution to the homogeneity problem. The idea is that the distribution of dark matter may be much more nearly uniform than that of the forms seen. If so, both matter and radiation are quite homogeneous now and were quite homogeneous at early times. It is only where densities of matter are slightly higher than average for the universe that the kinds that can be seen are strongly concentrated. An analogy for this "biased" formation of visible large-scale structure in the universe would be the view of a passenger aboard a large oceangoing vessel who gazes just above the horizon. Such an observer would see only unusually high wave peaks, while all the time being at a great and only slightly fluctuating height above the ocean floor. Unfortunately, it has not yet been possible to gather convincing evidence in support of quantitative tests of biased structure formation scenarios.

Another relevant concept to the understanding of large-scale structure in the universe may be the activity of cosmic strings. These hypothetical concentrations of energy are lines formed during spontaneous symmetry breaking at the end of the era of the grand unification of strong, electromagnetic, and weak interactions. They may stretch across the entire extent of the very early universe or join their ends to form loops. In either case, they could be the "seeds" for the formation of concentrations of matter, like droplets condensing on a wire, which later evolved into filamentary groups of galaxies. The absence of cosmic strings in the later universe (which has been observed) is conveniently explained through the generation of gravitational waves by their accelerations in the early expanding universe. There is hope that gravitational wave astronomy and observations of gravitationally lensed distant quasars may provide direct experimental evidence for the existence and behavior of cosmic strings in the not too distant future.

Finally, going back even closer to the beginning of the space-time and mass-energy of the universe leads one to the era of inflation. There are speculations that the high energy ("elementary particle") quantum processes during these exceedingly brief times after the birth of the universe may have been associated with inhomogeneous inflation. This suggests that the earliest structures, and therefore those now of the largest scales, would be bubbles. If such concepts could be confirmed by any relic evidence surviving from those extraordinarily early times, it could be said that not only the stuff of the cosmos but also its grandest patterns of organization are determined in its earliest moments. The desire to predict what such relics might be and then to find them drives a cutting edge of research in cosmology.

Context

The history of the quest to understand the large-scale structure of the universe has been, at least for the past few centuries, an uneven progression toward the recognition of ever more subtle organization at ever-larger distances.

Perhaps some early observers of the "patterns" that the mind can "recognize" among the stars visible to the unaided human eye believed these constellations to be real large-scale structures in the universe. Nevertheless, in one of the earliest applications of telescopes in astronomy, Galileo's observations from Italy at the start of the seventeenth century revealed many previously unseen stars in the Milky Way. Careful reasoning supported by this new evidence supplanted the reigning paradigm of a sphere of fixed stars centered on the earth with that of a universe of indefinitely large extent, where the earth is at a position of no particular importance. Thus, the constellations came to be seen as merely convenient direction indicators from a vantage point, not necessarily physical associations.

Further progress in understanding the distribution of matter in space was dependent on actual measurements of the distances to the stars. A direct approach through trigonometric parallax (changes in the apparent angular positions of nearby stars in relation to more distant stars as seen from Earth at various points in its orbit around the sun), became possible only when the precision of angular location surpassed the level of 1 second of arc. (Recall that there are 60 seconds in one minute, 60 minutes in one degree, and 90 degrees in the right angle between two perpendicular lines.) The first reports of success, by Friedrich Wilhelm Bessel in Germany, Thomas Henderson in South Africa, and Friedrich Georg Wilhelm von Struve in Russia, came in 1838 and 1839. By 1890, distances were known for nearly one hundred stars in the immediate neighborhood of the sun, but such observations were limited by having to look through the earth's atmosphere at visible light wavelengths. It took slightly more than a century for a variety of indirect distance indicators to be developed and calibrated before astronomers had the data from which to derive a substantially correct view of the size and shape of the Milky Way.

At the start of the twentieth century, most considered the Milky Way the observable universe. By the late 1920's, studies of Cepheid variable stars and observations of spectroscopic redshifts had established that the "spiral nebulas" were structures comparable to the Milky Way, far outside it and on average moving away at speeds proportional to their distances. Thus, the expanding universe once again was perceived to be immensely larger and richer in structure than had been believed only a short time before. Ever since, central goals of observational and theoretical cosmology have been to produce accurate maps of the distribution of these galaxies and explanations of the origin of the features in this distribution.

Principal terms

BUBBLE: the possible structuring of galaxies, galaxy filaments, and galaxy clusters around a void

COSMIC STRING: a hypothetical early concentration of energy that initiated formation of galaxy filaments

DARK MATTER: mass in the universe that does not give off any form of electromagnetic radiation and is thus invisible, but is known by its gravitational influence

HOMOGENEITY PROBLEM: the difficulty of reconciling observations of the extreme uniformity of the cosmic background radiation with the early inhomogeneity required to account for large-scale structure

VIRIAL THEOREM: the result of statistical mechanics used to estimate masses of galaxies in a cluster, assuming their energies of motion and gravitation are balanced

VOID: the region of space hundreds of millions of light-years across, which is relatively free of galaxies

Bibliography

Bahcall, Neta A. "Large Scale Structure in the Universe Indicated by Galaxy Clusters." In ANNUAL REVIEWS OF ASTRONOMY AND ASTROPHYSICS. Vol. 26. Palo Alto, Calif.: Annual Reviews, 1988. An extended, somewhat technical, article with a formidable specialist bibliography, this paper is a masterful survey of evidence up to the late 1980's presented largely through an illuminating collection of maps, diagrams, and graphs.

Cohen, Nathan. GRAVITY'S LENS: VIEWS OF THE NEW COSMOLOGY. New York: John Wiley & Sons, 1988. A book for the general reader by a researcher in general relativity and cosmology, this clear and well-illustrated volume features extensive discussion of the evidence for large-scale structure, and prospects for refined and more extensive observations in the future.

Ferington, Esther, et al. THE COSMOS. Alexandria, Va.: Time-Life Books, 1988. This profusely illustrated large-format book presents a readable and surprisingly comprehensive brief introduction to the field of modern cosmology. The final third concentrates on the connection between high-energy ("elementary particle") physics and the very early universe as the unifying element in understanding the origins of large-scale structure.

Hartwick, F. D. A., and David Schade. "The Space Distribution of Quasars." In ANNUAL REVIEWS OF ASTRONOMY AND ASTROPHYSICS. Vol. 28. Palo Alto, Calif.: Annual Reviews, 1990. A rather technical article with much cited literature summarizing knowledge of the clustering of these most distant discrete objects yet observed. Includes a discussion of their importance in understanding the development of large-scale structure in the universe.

Peebles, P. J. E. THE LARGE-SCALE STRUCTURE OF THE UNIVERSE. Princeton, N.J.: Princeton University Press, 1980. This authoritative text, written at an advanced level, contains many accessible discussions of early evidence for and historical developments in the physical understanding of large-scale structure. Much larger and more complex structures have been found since this volume was published.

Trefil, James. THE DARK SIDE OF THE UNIVERSE. New York: Charles Scribner's Sons, 1988. A lucid account of contemporary cosmology by a physicist for the general reader, this well-written volume concentrates on the evidence for dark matter in the universe, emphasizing its role in explaining large-scale structure.

Hubble Space Telescope

Types of Galaxies and Galactic Clusters

Grand Unification Theories and Supersymmetry