The spectrum of cosmological density fluctuations and nature of dark matter

Abstract

COSMIC structure is thought to have arisen by the gravitational amplification of small density fluctuations in the early Universe. The evolution of fluctuations with specified magnitude and spectrum is controlled by a few fundamental parameters: the cosmic density Ω, the cosmological constant Λ, and the relative contributions of radiation and of dark and visible matter to the density of the Universe. Maps and statistical descriptions of the large-scale distribution of galaxies, from the QDOT IRAS redshift survey^1–7, along with the COBE measurements of the microwave background fluctuations^8,9 have recently transformed our understanding of large-scale structure, for which the growth of fluctuations is linear and well understood. These two sets of data effectively determine the density fluctuation spectrum in the present Universe on scales from 10 to 1,000 Mpc. Here we examine an array of structure formation models, and show that most are ruled out by the COBE and QDOT observations. We find only one completely satisfactory model, in which the Universe has density Ω = 1, with 69% in the form of cold dark matter, 30% provided by hot dark matter in the form of a stable neutrino with mass 7.5 eV, and 1% baryonic. Certain 'grand unified theories'^10,11 may provide a physical basis for such, hybrid models. A Hubble constant of 50 km s⁻¹ Mpc⁻¹ is preferred to one of 100.