Annu. Rev. Astron. Astrophys. 1988. 26: 245-294 Copyright © 1988 by Annual Reviews. All rights reserved

3.2.2. ORIGIN OF VOIDS

An excellent introduction to the origin of large-scale structure is provided by Oort (128, pp. 418-25).

It is assumed that the present large-scale structure evolved from fluctuations that occurred in an early era prior to the recombination of hydrogen (28, 76a, 86, 118, 177, 206, 211f, 215c, 218; also cf. Oort (128)]. The primary aim of these theoretical calculations is to generate predictions consistent with all known cosmologically relevant data [e.g. (a) the Hubble relation, V_H = H₀ R_ij; (b) the presence of the 2.75-K cosmic blackbody radiation, its 600 km s^-1 dipole anisotropy, and the nondetection of superimposed irregularities; (c) the correlation scale length of the space distribution of galaxies (10 Mpc) and clusters of galaxies (50 Mpc); (d) the presence of superclusters and voids each with a characteristic length ~ 50 Mpc; and (e) the infall velocity of the Local Group within the Local Supercluster, and other less clearly determined motions]. According to cosmological evolutionary calculations by, for example, Hoffman et al. (91) [see also (29a, 210a)], the density contrasts at recombination required to form the observed voids correspond to predicted irregularities in the cosmic blackbody radiation of characteristic angular size ~ 10 arcmin, which have not been observed (cf. Section 2.2.3). This discrepancy could be understood if the irregularities originated after the recombination era by, for example, the cosmic explosions discussed in Section 3.2.1. Alternatively, the irregularities might have originated in the prerecombination era, but their effective amplitude is smaller than inferred from the distribution of galaxies because of the occurrence of biased galaxy formation. This mechanism was suggested by Kaiser (100) and is being studied extensively (cf. 33a, 118a, 144a, 149; S.D.M. White et al., preprint, 1987; A.L. Melott, preprint, 1987).

The model of biased galaxy formation assumes that at a given epoch, the initial density fluctuations [i.e. the spectrum of (r) - <>, where (r) is the density at a given location and <> is the global average density], can be represented by the superposition of Gaussian functions with a spectrum of amplitudes (A) and wavelengths (). It also assumes that galaxies form whenever a density fluctuation exceeds a threshold value (astrophysically determined, but not necessarily known). Consider, for example, a simple model in which most of the mass of the Universe is contained in a Gaussian wave with ~ (consistent with the smoothness of the sky distribution of the cosmic blackbody radiation), but a small fraction of the mass is in a Gaussian with ~ 50 Mpc (superclusters, voids) and another small fraction is in a Gaussian with ~ 5 Mpc (galaxy clusters). In this model, the peaks above threshold are more clustered than the mass, so that the cluster-cluster correlation scale length is larger than the galaxy-galaxy correlation scale length (cf. Section 2.2.1). In addition, the voids contain matter, so that the predicted cosmic motions are consistent with observational indications (cf. Sections 2.1.4, 2.2.1) and the cosmic blackbody radiation is effectively smooth (cf. Section 2.2.3).

The successes of the biased galaxy model are to be weighed against the ad hoc "save-the-phenomena" nature of its assumptions (a feature that is shared with many cosmological models). Such assumptions are necessitated by a gap in our understanding of fundamentals, which is manifested very noticeably in mass/time-scale problems (cf. Section 4). Cosmological modeling is founded on the postulate that unexplained gravitational effects are caused mainly by dark matter. Efforts to test this hypothesis have led primarily to enigmatic empirical results and logical puzzles that must be resolved before any of the cosmological models developed to explain the observed large-scale structure can be considered to be substantially more than physically speculative mathematical constructs. These models, nevertheless, do provide logical predictions that can be compared with empirical data, and in this way they supply diagnostic probes that may provide clues to the resolution of the mass/time-scale problems reviewed in Section 4.