Directional dependence of LCDM cosmological parameters

TL;DR

This paper investigates hemispherical power asymmetry in WMAP 9-year data, finding a statistically significant anisotropy that affects key cosmological parameters like n_s, suggesting potential deviations from isotropy in the universe.

Contribution

It provides the first detailed analysis of directional dependence of cosmological parameters using WMAP data, identifying a preferred asymmetry axis and its impact on parameter estimates.

Findings

Significant hemispherical power asymmetry at 3.4sigma confidence level.

Consistent asymmetry direction across multiple multipole ranges.

Variation in spectral index n_s between opposite hemispheres.

Abstract

We study hemispherical power asymmetry (Eriksen et. al. 2004, Hansen et. al. 2004, Hansen et. al. 2009) in the WMAP 9-year data. We analyse the combined V- and W-band sky maps, after application of the KQ85 mask, and find that the asymmetry is statistically significant at the 3.4sigma confidence level for ell=2-600, where the data is signal dominated, with a preferred asymmetry direction (l,b)=(227,-27). Individual asymmetry axes estimated from six independent multipole ranges are all consistent with this direction. Subsequently, we estimate cosmological parameters on different parts of the sky and show that the parameters A_s, n_s and Omega_b are the most sensitive to this power asymmetry. In particular, for the two opposite hemispheres aligned with the preferred asymmetry axis, we find n_s = 0.959 \pm 0.022 and n_s = 0.989 \pm 0.024, respectively.