On Breaking Cosmic Degeneracy

It has been argued that the power spectrum of the anisotropies in the Cosmic Microwave Background (CMB) may be effectively degenerate, namely that the observable spectrum does not determine a unique set of cosmological parameters. We describe the physical origin of this degeneracy and show that at small angular scales it is broken by gravitational lensing: effectively degenerate spectra become distinguishable at ℓ ∼ 3000 because lensing causes their damping tails to fall at different rates with increasing ℓ. This effect also helps in distinguishing nearly degenerate power spectra such as those of mixed dark matter models. Forthcoming interferometer experiments should provide the means of measuring otherwise degenerate parameters at the 5 − 25 % level. It has recently been pointed out 21 that in a parameter space including open models the contours of the estimated likelihood function for planned experiments are highly elongated in certain directions. This indicates that there is a near degeneracy between cosmological parameters which will limit the accuracy with which the individual parameters can be measured using only CMB observations. Note that with sufficiently good data as expected from