Annu. Rev. Astron. Astrophys. 1998. 36: 599-654 Copyright © 1998 by Annual Reviews. All rights reserved

4.3. Other Models

Currently less popular structure formation models include several Gaussian and non-Gaussian models. The oldest one is the hot dark matter (HDM) model, with all the dark matter in the form of massive neutrinos with a free-streaming length (the collisionless analog of the Jeans length) of several megaparsecs. The HDM model was the first physically motivated model studied with simulations (Melott 1982, White et al 1983, 1984). Recent work supports the early conclusion that galaxy formation occurs too late because of the absence of initial small-scale power (e.g. Cen & Ostriker 1992b).

If CDM has too much small-scale power and HDM too little, perhaps a Goldilocks solution exists with warm dark matter. Colombi et al (1996b) investigated warm models with a range of free-streaming lengths (hence varying degrees of suppression of small-scale power). They found that models tuned to match large-scale structure have too much power on small scales.

Peebles (1987a) proposed a low-density model without nonbaryonic dark matter, with _b 0.1 and primeval "isocurvature" fluctuations corresponding to a spatially varying entropy per baryon. Structure formation simulations of this isocurvature baryon model by Suginohara & Suto (1992a), Cen et al (1993b) indicated some difficulties, but the greatest conflict arises with the microwave background (Hu et al 1995).

Several classes of non-Gaussian models have been explored. Early simulations of structure formation with cosmic strings assumed that loops acted as accretion sites (e.g. Scherrer et al 1989), but later work showed that long strings are dominant (e.g. Albrecht & Stebbins 1992a, b). Cosmic textures, another type of hypothetical topological defect, also act as seeds (e.g. Gooding et al 1992), and generic seed models have been explored by Villumsen et al (1991). Recently, Cen (1997b) has shown that models with random seeds cannot account for the strong clustering of rich galaxy clusters.

Other non-Gaussian models include local nonlinear transformations of Gaussian random fields (e.g. Moscardini et al 1991, Weinberg & Cole 1992). While less physically motivated than the other models discussed above, they provide useful foils for assessing the effect of nonlinear gravitational evolution in producing non-Gaussianity from Gaussian initial conditions.

The discussion given above is only a partial summary of nonstandard structure formation models that have been considered. Nonetheless, a general conclusion applies: No model has emerged as a popular alternative to the generalized CDM models discussed in Section 4.2. Their initial conditions are more complicated, and when evolved, they show no compelling advantages to the variants of CDM models. However, nature need not be so kind as to always favor the simplest theories we can conceive. For this reason, it remains valuable to explore nonstandard models with a close eye on observational signatures that can be tested.