Constraining Statistical Isotropy using 21cm Power Spectrum and Bispectrum

TL;DR

This paper investigates how 21cm intensity mapping can constrain deviations from statistical isotropy in the universe, focusing on dipolar and quadrupolar asymmetries using power spectrum and bispectrum analyses.

Contribution

It introduces a Fisher formalism-based forecast for constraining anisotropy parameters with 21cm surveys, highlighting the potential of PUMA to improve constraints over CMB data.

Findings

PUMA can constrain anisotropy parameters to better than 10^{-3} for z > 1.

Bispectrum provides more sensitive constraints than the power spectrum.

Foreground mitigation strategies significantly impact the constraining power.

Abstract

The Cosmological Principle states that the universe is statistically isotropic and homogeneous on large length scales, typically $\gtrsim 70$Mpc. A detection of significant deviation would help us falsify the simplest models of inflation. In this regard, there are potential indications of departures from this principle, e.g., observations from WMAP and Planck show signs of a preferred direction in the temperature fluctuations known as hemispherical asymmetry in CMB. Phenomenologically, this has been studied using a dipole modulation model. In addition to this, a number of possible mechanisms have been proposed in the literature to explain this anomaly. Some of these scenarios generate dipolar asymmetry or predict quadrupolar asymmetry in the primordial power spectrum of curvature perturbations. In this paper, we study both these asymmetries. To fulfill the objective, we employ 21cm intensity mapping technique post during post-reionization era, i.e., $z\lesssim 7$. We apply Fisher formalism to constrain dipolar and quadrupolar anisotropy parameters using both 21cm power and bispectra and give forecasts for three intensity mapping surveys: SKA-Mid, HIRAX and PUMA. Although 21cm intensity mapping is a very promising cosmological probe, the signals are severely affected by foregrounds. To mitigate the foreground effects, we use foreground avoidance approach. For the interferometer mode of operation, we also include the wedge effect. From our analysis, we find that PUMA, on account of its high redshift range is able to constrain both dipolar and quadrupolar parameters to better than $\sim 10^{-3}$ for redshifts $z \gtrsim 1$. This is one order of magnitude better constraints as compared to those provided by the latest CMB surveys. We also find that as compared to power spectrum, the constraining power of bispectrum is more sensitive towards foregrounds.