B-modes and the Nature of Inflation

TL;DR

This paper explores how measurements of B-mode polarization in the cosmic microwave background can constrain the sound speed of scalar fluctuations during inflation, providing insights into the inflationary mechanism beyond slow-roll models.

Contribution

It derives a lower bound on the sound speed from the tensor-to-scalar ratio and discusses implications for inflationary models with non-trivial dynamics and non-Gaussianity.

Findings

Detection of primordial B-modes with r > 0.01 constrains sound speed c_s.

For r ≳ 0.1, the bound on c_s approaches a critical value indicating non-slow-roll behavior.

Order-one equilateral non-Gaussianity is a natural target for probing inflation dynamics.

Abstract

Observations of the cosmic microwave background do not yet determine whether inflation was driven by a slowly-rolling scalar field or involved another physical mechanism. In this paper we discuss the prospects of using the power spectra of scalar and tensor modes to probe the nature of inflation. We focus on the leading modification to the slow-roll dynamics, which entails a sound speed $c_s$ for the scalar fluctuations. We derive analytically a lower bound on $c_s$ in terms of a given tensor-to-scalar ratio $r$, taking into account the difference in the freeze-out times between the scalar and tensor modes. We find that any detection of primordial B-modes with $r > 0.01$ implies a lower bound on $c_s$ that is stronger than the bound derived from the absence of non-Gaussianity in the Planck data. For $r \gtrsim 0.1$, the bound would be tantalizingly close to a critical value for the sound speed, $(c_s)_\star = 0.47$ (corresponding to $(f_{\rm NL}^{\rm equil})_\star = -0.93$), which we show serves as a threshold for non-trivial dynamics beyond slow-roll. We also discuss how an order-one level of equilateral non-Gaussianity is a natural observational target for other extensions of the canonical paradigm.