Formation of large-scale cosmic structure by the Doppler instability

Hogan suggested that a bulk Doppler instability of homogeneous hydrogen interacting with electromagnetic radiation via atomic resonance scattering could be responsible for the large-scale structure of the universe. We carry out an analysis within the context of the Hogan model to determine the consequences of including the dynamical effects of viscosity, long-range radiation trapping and radiation attenuation forces, heat conduction, radiation loss, and also the effect of matter which does not directly interact with the rear-resonant radiation, but is coupled to the hydrogen by collisions. The Doppler instability survives inclusion of all these effects. We find that (a) the Doppler instability growth rate for large-scale perturbations, proportional to photon density in a spectrally narrow blueshifted feature, is very large (10^-6 yr^-1 even for excess blueshifted photon densities that are 10^-16 of the hydrogen density at z ≈ 1000), (b) it is unlikely that the homogeneous microwave radiation is affected given the small number of photons required to initiate the instability, (c) the long-range radiation trapping and radiation attenuation forces are too weak to affect the growth rate of fluctuations because the atomic density in the cosmos after recombination is small, and (d) the effects of pressure and viscosity limit the fluctuations only for submegaparsec scales. However, the source of spectrally narrow features to the blue of hydrogen (or He) resonance lines is problematic. We propose a mechanism for the production of photons in narrow energy ranges to the blue of Lyman resonance lines of H involving Balmer lines produced in the recombination of He II.