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

Recent cosmic microwave background (CMB) results from BOOMERANG, MAXIMA, and DASI provide cosmological constraints on new physics that can be competitive with those derived from big bang nucleosynthesis (BBN). In particular, both CMB and BBN can be used to place limits on models involving neutrino degeneracy and additional relativistic degrees of freedom. However, for the case of the CMB, these constraints are, in general, sensitive to the assumed priors. We examine the CMB and BBN constraints on such models and study the sensitivity of “new physics” to the assumed priors. If we add a constraint on the age of the universe ${(t}_0\gtrsim 11\hspace{0.5em}\mathrm{Gyr}),$ then for models with a cosmological constant, the range of baryon densities and neutrino degeneracy parameters allowed by the CMB and BBN is fairly robust: ${\eta }_{10}=6.0\mathrm{}\pm 0.6,$ $\Delta N_{\nu }\lesssim 6,$ ${\xi }_e\lesssim \mathrm{0.3.}$ In the absence of new physics, models without a cosmological constant are only marginally compatible with recent CMB observations (excluded at the 93% confidence level).