Origin of pregalactic microwave background

Abstract

THE photon density in the observed thermal (∼2.7 K) microwave background is n_γ0=≃400 cm⁻³. This contrasts with the present mean baryon density in the Universe, n_b0, which is in the range 10⁻⁵–10⁻⁶ cm⁻³. In the standard ‘hot big bang’ cosmology, the ratio n_γ/n_b is essentially constant during the expansion, its value being

𝒮=n_γ/n_b≃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 lines^1–9, 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

ρ_γ/ρ_b≃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 alone^7–9; on the other hand, if z corresponds to the epoch of galaxy formation (or later), adequate thermalisation at centimetre wavelengths requires implausibly efficient opacity^3–5. Here a possible non-primordial origin of the microwave background is outlined that seems less contrived than other such schemes.