The Cosmic Microwave Background Dipole as a Cosmological Effect

A conventional explanation of the dipole anisotropy of the cosmic microwave background (CMB) radiation is in terms of the Doppler effect: our galaxy is moving with respect to CMB frame with ∼ 600 km s. However, as the deep redshift surveys fail to reveal a convergence of the large scale flow to zero at distances as large as d ∼ H15, 000 km s (Lauer & Postman, 1994), the uniqueness of the conventional interpretation has to be investigated. A possible alternative might be a cosmological entropy gradient, as suggested by Paczyński & Piran (1990). We find that contrary to that suggestion a quadrupole anisotropy is generically of the same order of magnitude as the dipole anisotropy (or larger) not only for adiabatic but also for iso-curvature initial perturbations. Hence, the observed dipole cannot be explained with a very large scale perturbation which was initially iso-curvature. Subject headings: cosmic background radiation cosmology gravitation