5.5. Concluding Remarks
We have emphasized three themes. First, a robust estimate of the present baryon budget is of central importance in serving to clarify and sharpen issues of research in present-day cosmology. Second, there is no guarantee that physical processes in the formation and evolution of structure will have conspired to place a significant fraction of the baryons in forms we can hope to detect by their emission or absorption of radiation or otherwise infer in a convincing way. Third, the developing evidence is that Nature in fact has been kind: we may actually be able to arrive at a close to complete budget of the baryons in the observable universe.
The value for baryon in equation (38) suggests or is consistent with three lines of ideas. First, it suggests baryonic MACHOs make an insignificant contribution to the global budget. Perhaps the MACHOs are non-baryonic entities formed in the early universe, or perhaps like the stellar components of the Milky Way galaxy they are more concentrated to the inner halo than is nonbaryonic dark matter. The second idea is that there are relatively few baryons in the voids defined by the galaxies, likely because the baryons were swept out of the voids by gravity. Gravity would gather low pressure dark matter with the baryons and galaxies, implying that the mass density in matter than can cluster is well below the Einstein-de Sitter value. This is consistent with our estimate of the total density parameter in equation (24). The third idea is that the bulk of the forest baryons at z = 3 have ended up now in warm plasma around the galaxies outside the great clusters. The mass in neutral hydrogen in the DLAs is about half that needed to account for the present-day mass in stars. We know some mass already is in stars at z = 3; formation of the rest of the star mass at z < 3 would be a modest drain on the mass in the forest.
An interesting feature of the budget is that all stars and their remnants, together with cold phases of collapsed gas, comprise a small fraction even of the small fraction of the matter which is baryonic (Gnedin & Ostriker 1992; Persic & Salucci 1992). Just the 0.3% of the baryons in irregular galaxies shine brightly enough at z = 0.4 to 1 to make an important contribution to the rapid increase of B band number counts of galaxies with increasing apparent magnitude. It would not be difficult in these circumstances to imagine that the distribution of the starlight has little to do with the mass. But the consistency of the estimates of dynamical estimates of the total density parameter, as in equations (24), (25), (27) suggests starlight nevertheless is a useful tracer of mass on the scale of the distance between galaxies.
The baryon budget still is quite uncertain: the sums in line 8 in Table 3 for the maximum and minimum low redshift budget differ by a factor of six. We suspect this is largely a result of an underestimate in line (7a) for plasma in groups; improved information on soft X-ray emission would be of considerable help. Perhaps improvements in the observational constraints will confirm the tentative concordance in equation (38); perhaps better observations will reveal an inconsistency that shows we have to reconsider some aspect of the standard concepts of astronomy and cosmology.
We are grateful to J. Bahcall for detailed discussions of the
local star luminosity function and to R. Carlberg,
J. Stadel, C. Stubbs, and E. Vishniac for useful
comments. This work was supported in part by
a Grant-in-Aid of the Ministry of Education of Japan (09640336) and the
, by , and by
the . M.F. acknowledges support from
the Fuji Xerox Corporation at Princeton.