DARK-MATTER AND COSMOLOGY - CDM WITH A COSMOLOGICAL CONSTANT (ACDM) VS CDM WITH HOT DARK-MATTER (CHDM)

The fact that the simplest modern cosmological theory, standard Cold D ark Matter (sCDM), almost fits all available data has encouraged the s earch for variants of CDM that can do better. Here we discuss what are perhaps the two most popular variants of CDM that might agree with th e data: Lambda CDM and CHDM. While the predictions of COBE-normalized Lambda CDM and CHDM both agree well with the available data on scales of similar to 10 to 100 h(-1) Mpc, each has potential virtues and defe cts. Lambda CDM with Omega(0) similar to 0.3 has the possible virtue o f allowing a higher expansion rate H-0 for a given cosmic age t(0), bu t the defect of predicting too much fluctuation power on small scales. CHDM has less power on small scales, so its predictions appear to be in good agreement with data on the galaxy distribution, but it remains to be seen whether it predicts early enough galaxy formation to be co mpatible with the latest high-redshift data. Also, several sorts of da ta suggest that neutrinos have nonzero mass. And two very recent obser vational results favor high cosmic density, and thus favor Omega = 1 m odels such as CHDM over Lambda CDM - (1) the positive deceleration par ameter q(0) > 0 measured using high-redshift Type 1a supernovae, and ( 2) the low primordial deuterium/hydrogen ratio measured in two differe nt quasar absorption spectra. If confirmed, (1) rules out a cosmologic al constant large enough to help significantly with the He-to problem; while (2) suggests that the baryonic cosmological density is at the u pper end of the range allowed by Big Bang Nucleosynthesis, perhaps hig h enough to resolve the ''cluster baryon crisis'' for Omega = I models . We try to identify ''best'' variants of both Lambda CDM and CHDM, an d discuss critical observational tests for both models.