Robust predictions for an oscillatory bispectrum in Planck 2015 data from transient reductions in the speed of sound of the inflaton

TL;DR

This paper searches for oscillatory features in the Planck 2015 CMB data caused by transient reductions in the inflaton's speed of sound, identifying potential signals in both the power spectrum and bispectrum.

Contribution

It introduces a robust, prior-independent method to detect high-frequency oscillatory features in the CMB power spectrum, extending previous analyses and linking them to bispectrum signals.

Findings

Detected features around multipoles 20-40 in the power spectrum.

Identified higher multipole features (100-1500) consistent with theoretical models.

Assessed the correlation of these features with the CMB bispectrum and secondary effects.

Abstract

We update the search for features in the Cosmic Microwave Background (CMB) power spectrum due to transient reductions in the speed of sound, using Planck 2015 CMB temperature and polarisation data. We enlarge the parameter space to much higher oscillatory frequencies of the feature, and define a robust prior independent of the ansatz for the reduction, guaranteed to reproduce the assumptions of the theoretical model and exhaustive in the regime in which the feature is easily distinguishable from the baseline cosmology. We find a fit to the $\ell\approx20$--$40$ minus/plus structure in Planck TT power spectrum, as well as features spanning along the higher $\ell$'s ($\ell\approx100$--$1500$). For the last ones, we compute the correlated features that we expect to find in the CMB bispectrum, and asses their signal-to-noise and correlation to the ISW-lensing secondary bispectrum. We compare our findings to the shape-agnostic oscillatory template tested in Planck 2015, and we comment on some tantalising coincidences with some of the traits described in Planck's 2015 bispectrum data.