Microwave Background Anisotropies and Large Scale Structure in the Universe

Abstract

Recent observations show that the Universe contains more structure on scales ≳ 10h^-1Mpc^* than expected in the standard cold dark matter (CDM) theory of structure formation. This may indicate that the initial fluctuations were not scale-invariant. I investigate how this would affect microwave background anisotropics if the Universe is spatially flat with Ω = 1, as seems most natural if the Universe went through an inflationary phase. If we are allowed arbitrary freedom in choosing the shape of the initial fluctuation spectrum, then it is possible to construct models which can account for observed structure in the galaxy distribution but which result in small (ΔT/T ≲ 10^-6) anisotropics in the microwave background radiation on angular scales ≳ 5°. However, it is very difficult to avoid producing temperature fluctuations of order 10^-5 on angular scales of ~ 1°, even if the Universe were reionised. If the extra large-scale power detected in galaxy surveys really reflects clustering in the mass distribution, then experiments at θ ~ 1° should soon see fluctuations in the microwave background.