Primordial density fluctuations and the microwave background spectrum

We use recent COBE observational limits on departures of the spectrum of the cosmic microwave background (CMB) from a pure blackbody curve to constrain models of galaxy formation. The damping of adiabatic density perturbations at high redshifts |(z~10⁵)| injects energy into the primordial plasma which cannot be thermalized and therefore distorts the CMB spectrum. We find that we can eliminate flat |(Ω₀ = 1)| baryon-dominated models with power-law primordial fluctuations having n > 4 and open |(Ω₀ = 0.1)| baryonic models with n > 1. The results complement other constraints from microwave background temperature anisotropies, nucleosynthesis and galaxy clustering and, taken together, they suggest very strongly that there is either a preponderance of non-baryonic dark matter, a non-zero cosmological constant, a source of isocurvature (rather than adiabatic) fluctuations in the early Universe or some other source of density perturbations, such as non-superconducting cosmic strings.