Causality, randomness, and the microwave background

Fluctuations in the cosmic microwave background (CMB) temperature are being studied with ever increasing precision. Two competing types of theories might describe the origins of these fluctuations: inflation'' and defects’'. Here we show how the differences between these two scenarios can give rise to striking signatures in the microwave fluctuations on small scales, assuming a standard recombination history. These should enable high resolution measurements of CMB anisotropies to distinguish between these two broad classes of theories, independent of the precise details of each.