4.8. Summary

This chapter has contained a lot of information and after reading it one might well be left with lingering suspicions about the validity of the inflationary paradigm as well as the nature of the dark matter. The blunt truth is that the inflationary paradigm is so theoretically attractive that one is loathe to discard it simply on the basis that few, if any, observations support the = 1 prediction. In fact, as we discuss in Chapter 5, inflation makes a very specific prediction about the nature of the perturbation spectrum which produces the large scale structure and this prediction is verified by observation. So there is ample reason to retain the inflationary model but perhaps it requires a non-zero in order for the Universe to be spatially flat. Moreover, discarding of this model in favor of a Hot Big Bang baryonic model then forces very special initial conditions that are somehow imprinted on the epoch when observations are being made.

To solidify the discussion we conclude with the following statements that are consistent with the current data:

The best evidence for the existence of dark matter as the dominant mass in some potential is provided by the flat rotation curves of galaxies as measured by their gas content out to scales 5r_h. It is unclear if these specific results can be generalized to all spiral galaxies.

Elliptical galaxies need to have a large scale halo of dark matter if they are to retain their interstellar gas which has been heated and ejected by supernovae. The observations of X-ray halos around elliptical galaxies support this statement.

Full consideration of substructure effects in clusters argues that the use of cluster dynamics does not yield reliable measures of M / L on the 1-2 Mpc scale. It remains unclear if cluster potentials have 5 or 50 times as much dark matter as individual galaxy potential. Recent results from gravitational lensing studies (Squires et al. 1995), X-ray spectroscopy (Markevitch 1996), and profile fitting of X-ray clusters (Evrard et al. 1996) generally indicate M / L to be in the range 300-500.

Determination of the cosmological parameter ₀ has not yet been done convincingly by any group. The unknown biasing between light and mass as a function of scale provides an additional complication. Values of ₀ = 0.2 - 1.0 are consistent with some one's data set, although most dynamical determinations using large scale flows suggest that ₀ lies in the range 0.1 - 0.3 (see Strauss and Willick 1995). Possible systematic large scale variations in individual galaxy M / L may give rise to false peculiar velocity signals thus reducing the credibility of these dynamical determinations. The crucial issue of biasing between the galaxy distribution and the mass distribution has not been adequately resolved.

There is encouraging evidence from the particle physics side of cosmology that the neutrino has a non-zero rest mass. On going searches for CDM particles remain negative.

If ₀ = 0.1 then it is very likely that most of the dark matter is baryonic and in the form of stellar remnants and brown dwarfs. The suggestion that the halo potential of our galaxy is dominated by Jupiter mass objects has been ruled out by the microlensing experiment.

If ₀ = 1.0 then the Universe is clearly dominated by an unknown particle that does not have a distribution like the clustered distribution of light. The only observed candidate for this particle is the neutrino.

As in the reconciliation of the ages of globular clusters with H₀^-1, the introduction of non-zero can reconcile many of the above issues. If we let the neutrino have a small mass (1-3 eV), and allow to be the dominant term in producing a spatially flat Universe, then we can accommodate both inflation and large scale structure. Strangely, there is even data that supports this view (see Chapter 5).