Origin of pregalactic microwave background

Abstract

THE photon density in the observed thermal (∼2.7 K) microwave background is nγ0=≃400 cm−3. This contrasts with the present mean baryon density in the Universe, nb0, which is in the range 10−5–10−6 cm−3. In the standard ‘hot big bang’ cosmology, the ratio nγ/nb is essentially constant during the expansion, its value being 𝒮=nγ/nb≃3.6 × 107(Ωh2)−1. (1) h denotes the Hubble constant H0 in units of 100 km s−1 Mpc−1 and Ω = 8/3πGρH0−2 is the usual density parameter. 𝒮 is an undetermined free parameter of the ‘primordial fireball’: the standard ‘hot big bang’ theory as yet gives no reason that 𝒮 should be ∼108 rather than (say) ∼104 or ∼1012. Its observed value could be more readily understood in a different theory that attributes the radiative energy and entropy production to calculable processes in a specified epoch. There have been various proposals along these lines1–9, but none has found as much favour as the standard ‘hot big bang’ cosmology. This is because production of the radiation at a redshift z requires a mass–energy conversion efficiency of the order ργ/ρb≃2.5 × 10−5(1+z)(Ωh2)−1 (2) This is implausibly high if z is large enough (»103) to permit thermalisation by H and He plasma alone7–9; on the other hand, if z corresponds to the epoch of galaxy formation (or later), adequate thermalisation at centimetre wavelengths requires implausibly efficient opacity3–5. Here a possible non-primordial origin of the microwave background is outlined that seems less contrived than other such schemes.