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Recent cosmic microwave background (CMB) results from BOOMERANG, MAXIMA, andDASI provide cosmological constraints on new physics that can be competitivewith those derived from Big Bang Nucleosynthesis (BBN). In particular, both CMBand BBN can be used to place limits on models involving neutrino degeneracy andadditional relativistic degrees of freedom. However, for the case of the CMB,these constraints are, in general, sensitive to the assumed priors. We examinethe CMB and BBN constraints on such models and study the sensitivity of ``newphysics" to the assumed priors. If we add a constraint on the age of theuniverse ($t_0 \ga 11$ Gyr), then for models with a cosmological constant, therange of baryon densities and neutrino degeneracy parameters allowed by the CMBand BBN is fairly robust: $\eta_{10} = 6.0 \pm 0.6$, \deln $\la 6$, $\xi_e \la0.3$. In the absence of new physics, models without a cosmological constant areonly marginally compatible with recent CMB observations (excluded at the 93%confidence level).Comment: 6 pages, 5 figures; version to appear in Phys. Rev.

How does the cosmic microwave background plus big bang nucleosynthesis constrain new physics?