A discrepancy observed in the dipole anisotropy in the radio sky at 1.4 GHz and that in the CMBR – A threat to the cosmological principle?

Abstract

According to the cosmological principle, the sky brightness at any frequency should appear uniform in all directions to an observer considered to be fixed in the co-moving coordinate system of the expanding universe. However a peculiar motion of the observer introduces a dipole anisotropy in the observed sky brightness, which should be independent of the observing frequency. We have examined the angular distribution in the radio-sky brightness, i.e., an integrated emission from discrete sources per unit solid angle, from the NVSS sample containing 1.8 million discrete radio sources at 1.4 GHz. Our results give a dipole anisotropy which is in the same direction as that of the CMBR from the COBE or WMAP, but the magnitude we find is about 4 times larger at a statistically significant (about 3σ) level. A genuine difference between the two dipoles cannot arise from the observer's motion alone, and it would imply intrinsically anisotropic universe, with anisotropy changing with the epoch. This would violate the cosmological principle where isotropy of the universe is assumed for all epochs, and on which the whole modern cosmology is based upon.