© CAMBRIDGE UNIVERSITY PRESS 1999

1.4.8 Conclusions Regarding

The main issue that has been addressed so far is the value of the cosmological density parameter . Arguments can be made for ₀ 0.3 (and models such as CDM; Ch. 4; Ch. 8; Ch. 11) or for = 1 (Section 7; for which the best class of models is probably CHDM), but it is too early to tell which is right.

The evidence would favor a small ₀ 0.3 if: (1) the Hubble parameter actually has the high value H₀ 75 favored by many observers, and the age of the universe t₀ 13 Gyr; or (2) the baryonic fraction f_b = M_b / M_tot in clusters is actually ~ 15%, about 3 times larger than expected for standard Big Bang Nucleosynthesis in an = 1 universe. This assumes that standard BBN is actually right in predicting that the density of ordinary matter _b lies in the range 0.009 _b h² 0.02. High-resolution, high-redshift spectra are now providing important new data on primordial abundances of the light isotopes that should clarify the reliability of the BBN limits on _b. If the systematic errors in the ⁴He data are larger than currently estimated, then it may be wiser to use the deuterium upper limit _b h² 0.03, which is also consistent with the value _b h² 0.024 indicated by the only clear deuterium detection at high redshift, with the same D/H 2.4 x 10^-5 observed in two different low-metallicity quasar absorption systems (Tytler, Fan & Burles 1996); this considerably lessens the discrepancy between f_b and _b. Another important constraint on _b will come from the new data on small angle CMB anisotropies - in particular, the location and height of the first Doppler peak (Dodelson, Gates, & Stebbins 1996; Jungman et al. 1996; Tegmark 1996), with the latest data consistent with low h 0.5-0.6 and high _b h² 0.025. The location of the first Doppler peak at angular wavenumber l 220 indicated by the presently available data (Netterfield et al. 1997, Scott et al. 1996) is evidence in favor of a flat universe; ₀ ~ 0.3 with = 0 is disfavored by this data.

The evidence would favor = 1 if: (1) the POTENT analysis of galaxy peculiar velocity data is right, in particular regarding outflows from voids or the inability to obtain the present-epoch non-Gaussian density distribution from Gaussian initial fluctuations in a low- universe; or (2) the preliminary indication of high ₀ and low from high-redshift Type Ia supernovae (Perlmutter et al. 1996) is confirmed.

The statistics of gravitational lensing of quasars is incompatible with large cosmological constant and low cosmological density ₀. Discrimination between models may improve as additional examples of lensed quasars are searched for in large surveys such as the Sloan Digital Sky Survey. The era of structure formation is another important discriminant between these alternatives, low favoring earlier structure formation, and = 1 favoring later formation with many clusters and larger-scale structures still forming today. A particularly critical test for models like CHDM is the evolution as a function of redshift of _gas in damped Ly systems. Reliable data on all of these issues is becoming available so rapidly today that there is reason to hope that a clear decision between these alternatives will be possible within the next few years.

What if the data ends up supporting what appear to be contradictory possibilities, e.g. large ₀ and large H₀? Exotic initial conditions (e.g., ``designer'' primordial fluctuation spectra, cf. Hodges et al. 1990) or exotic dark matter particles beyond the simple ``cold'' vs. ``hot'' alternatives discussed in the next section (e.g., decaying 1-10 MeV tau neutrinos, Dodelson, Gyuk, & Turner 1994; volatile dark matter, Pierpaoli et al. 1996) could increase the space of possible inflationary theories somewhat. But unless new observations, such as the new stellar parallaxes from the Hipparcos satellite, cause the estimates of H<sub>0</sub> and t<sub>0</sub> to be lowered, it may ultimately be necessary to go outside the framework of inflationary cosmological models and consider models with large scale spatial curvature, with a fairly large (or non-standard sorts of ``matter'' that violate the strong energy condition - cf. Visser 1997) as well as large ₀. This seems particularly unattractive, since in addition to implying that the universe is now entering a final inflationary period, it means that inflation probably did not happen at the beginning of the universe, when it would solve the flatness, horizon, monopole, and structure-generation problems. Moreover, aside from the H₀ - t₀ problem, there is not a shred of reliable evidence in favor of > 0, just increasingly stringent upper limits. Therefore, most cosmologists are rooting for the success of inflation-inspired cosmologies, with ₀ + = 1. With the new upper limits on from gravitational lensing of quasars, number counts of elliptical galaxies, and high-redshift Type Ia supernovae, this means that the cosmological constant is probably too small to lengthen the age of the universe significantly. So one hopes that when the dust finally settles, H₀ and t₀ will both turn out to be low enough to be consistent with General Relativistic cosmology. But of course the universe is under no obligation to live up to our expectations.