Hemispherical Anomaly from Asymmetric Initial States

TL;DR

This paper explores how asymmetric excited initial states during inflation could produce the observed hemispherical asymmetry in the CMB, predicting specific modulations in the power spectrum and non-Gaussianity that can be tested by future experiments.

Contribution

It demonstrates that asymmetric excited initial states can generate the CMB hemispherical asymmetry and predicts associated non-Gaussianity signatures within observational bounds.

Findings

Hemispherical asymmetry explained by dipole and quadrupole modulation in power spectrum.

Predicted local non-Gaussianity parameter $f_{NL}$ around 4.17, within Planck bounds.

Variation in non-Gaussianity depending on mode configuration, distinguishable from other models.

Abstract

We investigate if the hemispherical asymmetry in the CMB is produced from "asymmetric" excited initial condition. We show that in the limit where the deviations from the Bunch-Davies vacuum is large and the scale of new physics is maximally separated from the inflationary Hubble parameter, the primordial power spectrum is modulated only by position dependent dipole and quadrupole terms. Requiring the dipole contribution in the power spectrum to account for the observed power asymmetry, $A=0.07\pm0.022$, we show that the amount of quadrupole terms is roughly equal to $A^2$. The {\it mean} local bispectrum, which gets enhanced for the excited initial state, is within the $1\sigma$ bound of Planck 2015 results for a large field model, $f_{\rm NL}\simeq 4.17$, but is reachable by future CMB experiments. The amplitude of the local non-gaussianity modulates around this mean value, depending on the angle that the correlated patches on the 2d CMB surface make with the preferred direction. The amount of variation minimizes for the configuration in which the short and long wavelengths modes are around the preferred pole and $|\vec k_3|\approx |\vec k_{l\approx10}|\ll |\vec k_1|\approx |\vec k_2|\approx |\vec k_{l\approx2500}|$ with $f_{\rm NL}^{\rm min}\approx 3.64 $. The maximum occurs when these modes are at the antipode of the preferred pole, $f_{\rm NL}^{\rm max}\approx 4.81$ . The difference of non-gaussianity between these two configurations is as large as $\simeq 1.17$ which can be used to distinguish this scenario from other scenarios that try to explain the observed hemispherical asymmetry.