Explaining excess dipole in NVSS data using superhorizon perturbation

Abstract

Many observations in recent times have shown evidence against the standard assumption of isotropy in the Big Bang model. Introducing a superhorizon scalar metric perturbation has been able to explain some of these anomalies. In this work, we probe the net velocity arising due to the perturbation. We find that this extra component does not contribute to the CMB dipole amplitude while it does contribute to the dipole in large scale structures. Thus, within this model's framework, our velocity with respect to the large scale structure is not the same as that extracted from the CMB dipole, assuming it to be of purely kinematic origin. Taking this extra velocity component into account, we study the superhorizon mode's implications for the excess dipole observed in the NRAO VLA Sky Survey (NVSS). We find that the mode can consistently explain both the CMB and NVSS observations. We also find that the model leads to small contributions to the local bulk flow and the dipole in Hubble parameter, which are consistent with observations. The model leads to several predictions which can be tested in future surveys. In particular, it implies that the observed dipole in large scale structure should be redshift dependent and should show an increase in amplitude with redshift. We also find that the Hubble parameter should show a dipole anisotropy whose amplitude must increase with redshift in the CMB frame. Similar anisotropic behaviour is expected for the observed redshift as a function of the luminosity distance.