# Cherenkov in the Sky: Measuring the Sound Speed of Primordial Scalar Fluctuations

## TL;DR

This paper proposes a novel method to measure the sound speed of primordial scalar fluctuations by analyzing the imprints of relativistic particles produced during inflation, revealing distinctive CMB patterns and non-Gaussian features.

## Contribution

It introduces a new inflationary Cherenkov effect model where relativistic particles produce observable signatures in the CMB, enabling direct measurement of scalar sound speed.

## Key findings

- Distinctive temperature anisotropy patterns predicted
- Unique features on the power spectrum identified
- Significant flattened non-Gaussianity predicted

## Abstract

We consider production of additional relativistic particles coupled to the inflaton. We show that the imprints of these particles on the spectrum of primordial perturbations can be used for the direct measurement of the speed of sound of scalar perturbations, regardless of the mechanism of the production of this species. We study a model where these relativistic localized sources are decay products of heavier particles generated via a resonance mechanism. These particles emit in-phase inflaton particles which interfere constructively on the the so-called sound boom, leading to an "inflationary Cherenkov effect". The resulting shock waves lead to distinctive patterns on the temperature anisotropies of the cosmic microwave background. Moreover, we show that the model predicts unique features on the power spectrum of curvature perturbation and sizeable flattened non-gaussianity for a suitable range of parameters.