Probing statistical isotropy of cosmological radio sources using SKA

TL;DR

This paper reviews observed anisotropies in cosmological radio sources and CMBR, discussing their potential implications for the cosmological principle and proposing measurements with SKA to test these anomalies.

Contribution

It introduces new observational strategies with SKA to test large-scale anisotropies and explore possible physics beyond the standard cosmological model.

Findings

Evidence of preferred directions in radio source distributions

Hemispherical anisotropy observed in CMBR fluctuations

Proposed measurements to detect dipole and polarization alignments

Abstract

There currently exist many observations which are not consistent with the cosmological principle. We review these observations with a particular emphasis on those relevant for Square Kilometre Array (SKA). In particular, several different data sets indicate a preferred direction pointing approximately towards the Virgo cluster. We also observe a hemispherical anisotropy in the Cosmic Microwave Background Radiation (CMBR) temperature fluctuations. Although these inconsistencies may be attributed to systematic effects, there remains the possibility that they indicate new physics and various theories have been proposed to explain them. One possibility, which we discuss in this review, is the generation of perturbation modes during the early pre-inflationary epoch, when the Universe may not obey the cosmological principle. Better measurements will provide better constraints on these theories. In particular, we propose measurement of the dipole in number counts, sky brightness, polarized flux and polarization orientations of radio sources. We also suggest test of alignment of linear polarizations of sources as a function of their relative separation. Finally we propose measurement of hemispherical anisotropy or equivalently dipole modulation in radio sources.