CMB-BBN complementarity

I will review the measurement of eta, the photon-to-baryon ratio, coming from observations of the cosmic microwave background. Eta is also obtained independently from the measurements of light element abundances. These two independent determinations of eta amount to a consistency check on (and hence a test of) the standard model of cosmology. If our standard model of cosmology passes this test we obtain constraints on non-standard features, such as decaying particles, or new particle interactions. If we are willing to assume the standard model, the measurement of eta from the cosmic microwave background can be taken as a starting point for standard BBN, which would give definite predictions for the primordial abundances of light elements, a key input for other areas of cosmology. I will discuss how this results in an additional consistency check due to the sensitivity of the CMB anisotropies to the Helium abundance.