Our view of the large-scale distribution of luminous objects in the Universe has changed dramatically during the last 25 years: from the simple pre-1975 picture of a distribution of ``field'' and ``cluster'' galaxies, to the discovery of the first single superstructures and voids, to the most recent results showing an almost regular web-like network of interconnected clusters, filaments and walls, separating huge nearly-empty volumes. The increased efficiency of redshift surveys, made possible by the development of fast spectrographs and - especially in the last decade - by an enormous increase in their multiplexing gain (i.e. the ability to collect spectra of several galaxies at once), has allowed us not only to do cartography of the nearby Universe, but also to statistically characterise some of its properties. At the same time, parallel advances in the theoretical modeling of the development of structure, with large high-resolution gravitational simulations coupled to a deeper - yet limited - understanding of how to form galaxies within dark-matter halos, have provided a more realistic connection of the models to the observable quantities. Despite the large uncertainties that still exist, this has transformed the study of cosmology and large-scale structure into a truly quantitative science, where theory and observations can progress side by side.
I have been asked by the organizers of the 19th Texas Symposium to review this progress, and this paper is the result of this effort. It is clearly impossible, and actually beyond the scope of a review this size, to provide a thorough and complete summary of all the work done in the field of large-scale structure during this intense period of growth. There are a number of excellent reviews that appeared in the literature in recent years, from which the interested reader can build his/her own personal and more comprehensive view of the historical development of this relatively young branch of cosmology. If I were to suggest a pedagogical tour on this subject, I would personally start with Rood [1], who provides an enthusiastic first-hand description of the early pioneering years. Within this paper, the reader can find all the relevant references to almost anything done before its publication. I would then continue with Geller & Huchra [2], and Giovanelli & Haynes [3], who give a summary of the important work done during the eighties, which saw the completion of the CfA1 survey by Davis and collaborators, its extension (CfA2), as well as the Perseus-Pisces [3] survey. More recently, Strauss & Willick [4] provide a tutorial about the study of both the distribution and motions of galaxies, while in Borgani [5] one can find a thorough introduction to several statistics applied to the distribution of galaxies. Further updates, including early descriptions of projects which are also discussed here in a more advanced stage of development, are given by Guzzo [6], Strauss [7], and more recently by Chincarini & Guzzo [8] and da Costa [9].
Here, I will try and elaborate on a few selected highlights, in the attempt of clarifying - or at least giving a hint of - what we know and what we do not seem to understand yet concerning the properties and the origin of large-scale structure. Emphasis will be on the ideas, and I hope my colleagues will forgive me if the discussion is not always as rigorous as it might formally be. Even if this cannot be a comprehensive review paper, I have done my best to be as complete as possible in terms of at least mentioning the most relevant work done or in progress, with a proper link to a corresponding paper (or web page). Conversely, I have also tried, whenever possible, to recall the basic concepts required to make the discussion as self-contained as possible. Obviously, the topic selection reflects my personal taste, interests, and ignorance, so I apologise in advance to all colleagues whose work I might have overlooked (1) .
Unless differently specified, throughout the paper the Hubble constant
will be parameterised as H0 = 100 / h km
s-1 Mpc-1, and a model with
q0 = 0.5 and
= 0 will be adopted.
1 Although the core of this review reflects the talk given at the 19th Texas Symposium in December 1998, I found it appropriate to include also a few references to works that appeared till March 1999. Back.