Cosmological Parameters and Hyper-Parameters: The Hubble Constant from Boomerang and Maxima

We generalise the procedure for joint estimation of cosmological parameters to allow freedom in the relative weights of various probes. This is done by including in the joint Likelihood function a set of ‘Hyper-Parameters’, which are dealt with using Bayesian considerations. The resulting algorithm is simple to implement. We illustrate the method by estimating the Hubble constant H0 from the recent Cosmic Microwave Background experiments Boomerang and Maxima. For an assumed flat Λ-CDM model with fixed parameters (n = 1,Ωm = 1− λ = 0.3,Ωbh 2 = 0.03, Qrms = 18μK) we solve for a single parameter, H0 = 79± 4 km/sec/Mpc (95 % CL, random errors only), slightly higher but still consistent with recent results from Cepheids. We discuss how the ‘Hyper-Parameters’ approach can be generalised for a combination of cosmic probes, and for other priors on the Hyper-Parameters.