Principal Component Analysis of the Cosmic Microwave Background Anisotropies: Revealing The Tensor Degeneracy

A principal component analysis of cosmic microwave background (CMB) anisotropy measurements is used to investigate degeneracies among cosmological parameters. The results show that a degeneracy with tensor modes – the `tensor degeneracy’ – dominates uncertainties in estimates of the baryon and cold dark matter densities, omega_b = Omega_b h2, omega_c = \Omega_c h^2, from an analysis of CMB anisotropies alone. The principal component analysis agrees well with a maximum likelihood analysis of the observations, identifying the main degeneracy directions and providing an impression of the effective dimensionality of the parameter space.