Large‐Scale Structure in the Universe

The 1980s are likely to be thought, in retrospect, a turning point in the study of large-scale structure of the universe. This is partly due to a change in motivation for the research. We have come to believe that the fundamental laws of high-energy physics have carved their signature into the distribution of matter and galaxies and that, indeed, the Big Bang may have provided the only available experiment to test models of elementary particle physics. Thus, the emphasis in large-scale structure research has shifted away from the sometimes obsessive search for two numbers, H, and qo, which offer only the most rudimentary description of the universe in which we live. Instead, observational cosmologists are focusing on the frothy distribution of galaxies and the coherent motions of these galaxies in order to elucidate the nature and distribution of the dark matter and the processes that drafted the surprisingly complex structure of the galaxy distribution. We know that tools and ideas have gone hand in hand throughout history. As an observational astronomer, I recognize advances in technology that matured in the 1980s and played a major role in advancing our knowledge of large-scale structure. The ambitious redshift surveys that are charting the intricate distribution of galaxies were intractable a decade ago. Likewise, we have only recently developed adequate techniques of measuring galaxy distances independent of redshift, which allow us to map departures from a smooth expansion field caused by a nonuniform distribution of the dark matter. This wonderful symbiosis of new ideas and previously unobtainable data has led to a rapid growth of the field, which I will briefly review here. As summarized by Gunn,’ only two ideas about the formation of structure on the large scale are seriously entertained at present. One idea presumes that structure grows from gravitational clustering of inhomogeneities that were imprinted at a very early epoch. The popular versions of this idea further imagine that these fluctuations arose in a dark matter component of weakly interacting particles, which provide the critical density for closure. In the “cold-dark-matter” (CDM) paradigm, this structure is dominated by small-scale fluctuations that grow hierarchically; in contrast, “hotdark-matter’’ (HDM) universes, for example, those dominated by massive neutrinos, build from large scales downward because the shorter wavelength perturbations are erased by free streaming. Topological defects, for example, cosmic strings, could also couple with either CDM or HDM to provide a very different spectrum of initial fluctuations whose growth is more complex than these simple “bottom-up” or “top-down” pictures. The N-body simulations of Davis, Efstathiou, White, and Frenk have provided much of what we know about the behavior of CDM universes.**’ They are fond of pointing out that the CDM model is exceedingly good a t predicting many properties of