Direction dependence of cosmological parameters due to cosmic hemispherical asymmetry

TL;DR

This paper investigates how cosmic hemispherical asymmetry in the CMB affects the variation of cosmological parameters across different sky directions, highlighting the susceptibility of certain parameters to anisotropy.

Contribution

It introduces a simulation-based analysis of anisotropic effects on cosmological parameters, emphasizing the impact of scale-dependent and independent modulations.

Findings

Maximum variation in parameters occurs with scale-independent modulation.

Scale-dependent modulation leads to about 1.25σ deviation in A_s.

Parameters n_s and A_s are most affected by anisotropy.

Abstract

Persistent evidence for a cosmic hemispherical asymmetry in the temperature field of cosmic microwave background (CMB) as observed by both WMAP as well as PLANCK increases the possibility of its cosmological origin. Presence of this signal may lead to different values for the standard model cosmological parameters in different directions, and that can have significant implications for other studies where they are used. We investigate the effect of this cosmic hemispherical asymmetry on cosmological parameters using non-isotropic Gaussian random simulations injected with both scale dependent and scale independent modulation strengths. Our analysis shows that $A_s$ and $n_s$ are the most susceptible parameters to acquire position dependence across the sky for the kind of isotropy breaking phenomena under study. As expected, we find maximum variation arises for the case of scale independent modulation of CMB anisotropies. We find that scale dependent modulation profile as seen in PLANCK data could lead to only $1.25\sigma$ deviation in $A_s$ in comparison to its estimate from isotropic CMB sky.