Constraints on cosmic hemispherical power anomalies from quasars

TL;DR

This paper tests a curvaton-based inflationary model explaining CMB hemispherical power anomalies by analyzing quasar data, finding strong constraints that challenge the simplest versions of the model.

Contribution

It provides observational constraints on a curvaton gradient model explaining CMB anomalies using high-redshift quasar data.

Findings

Rules out the simplest curvaton gradient scenario at 99% confidence.

Sets an upper limit on the gradient of sigma_8 from quasar data.

Provides a novel test of inflationary models with large-scale spatial variations.

Abstract

Recent analyses of the cosmic microwave background (CMB) maps from the WMAP satellite have uncovered evidence for a hemispherical power anomaly, i.e. a dipole modulation of the CMB power spectrum at large angular scales with an amplitude of +/-14 percent. Erickcek et al have put forward an inflationary model to explain this anomaly. Their scenario is a variation on the curvaton scenario in which the curvaton possesses a large-scale spatial gradient that modulates the amplitude of CMB fluctuations. We show that this scenario would also lead to a spatial gradient in the amplitude of perturbations sigma_8, and hence to a dipole asymmetry in any highly biased tracer of the underlying density field. Using the high-redshift quasars from the Sloan Digital Sky Survey, we find an upper limit on such a gradient of |nabla sigma_8|/sigma_8<0.027/r_{lss} (99% posterior probability), where r_{lss} is the comoving distance to the last-scattering surface. This rules out the simplest version of the curvaton spatial gradient scenario.