3.4. There's More Out There Than Just Light

The previous discussion has all been concerned with structure as it manifests itself via light emitting objects (e.g., galaxies). In chapter 4 we will explicitly consider the evidence for large amounts of non-luminous material in the Universe. However, it makes some sense to introduce that topic here before moving on to a discussion of redshift surveys. In particular, if there is a difference between the distribution of mass and the distribution of light (see linear biasing below) then what we measure in redshift surveys as structure, may not identically correspond to the underlying large scale mass distribution. In addition, even the light we measure may not be representative of the distribution of all baryonic material.

In a compelling paper, Persic and Salucci (1992) compare the baryonic mass from light emitting objects from that expected from primordial nucleosynthesis (see Chapters 4 and 6). In brief, the observed abundance of light elements in the Universe (e.g., Walker et al. 1991) suggests that the baryonic density (_b) is

where h is defined as h = H₀ / 100. The visible contribution from baryons to _b is given by the sum

where the latter two terms are meant to incorporate intracluster gas that is present in groups and clusters (and which usually emits X-rays). The inventory of Persic and Salucci yields a total of _b = 2.2 ± 0.6 x 10^-3 h^-3/2 which for a reasonable range of h means that most of the baryons which exist are dark (this subject is considered in detail in Chapter 6).

In contrast with this, is the work of White et al. (1993) who, from X-ray observations of rich clusters of galaxies (e.g., the Coma cluster) infer that _b / 0.1-0.3 and most of the baryonic material is in the ICM. This observation is in apparent conflict with the nucleosynthesis constraint if = 1.0 (see also Carr 1994). This conflict is explored in greater detail in Chapter 4 but in evaluating the results from redshift surveys, the reader should bear in mind that if = 1.0 then luminous baryons contribute 1% to the total mass density of the Universe. In that scenario, one might very well expect that the light and mass distributions are fundamentally different. For the case of 0.1, it is likely that most of the mass in baryonic and that the dark baryons should be located in the same regions as luminous galaxies. Most of them, in fact, would be in the halos of those galaxies.