Signs of Analyticity in Single-Field Inflation

TL;DR

This paper establishes a connection between low-energy inflationary parameters and high-energy physics through a sum rule derived from analyticity and unitarity principles, constraining primordial non-Gaussianity.

Contribution

It derives a new sum rule linking inflationary observables to high-energy theory, providing a consistency condition that constrains non-Gaussianity in single-field inflation models.

Findings

The sum rule relates low-energy parameters to high-energy behavior.

Positivity conditions hold for all known UV completions of inflation.

The bound excludes about half of the currently allowed parameter space.

Abstract

The analyticity of response functions and scattering amplitudes implies powerful relations between low-energy observables and the underlying short-distance dynamics. These 'IR/UV' relations are rooted in basic physical principles, such as causality and unitarity. In this paper, we seek similar connections in inflation, relating cosmological observations to the physics responsible for the accelerated expansion. We assume that the inflationary theory is Lorentz invariant at short distances, but allow for non-relativistic interactions and a non-trivial speed of propagation at low energies. Focusing on forward scattering, we derive a 'sum rule' which equates a combination of low-energy parameters to an integral which is sensitive to the high-energy behavior of the theory. While for relativistic amplitudes unitarity is sufficient to prove positivity of the sum rule, this is not guaranteed in the non-relativistic case. We discuss the conditions under which positivity still applies, and show that they are satisfied by all known UV completions of single-field inflation. In that case, we obtain a consistency condition for primordial non-Gaussianity, which constrains the size and the sign of the equilateral four-point function in terms of the amplitude of the three-point function. The resulting bound rules out about half of the parameter space that is still allowed by current observations. Finding a violation of our consistency condition would point towards less conventional theories of inflation, or violations of basic physical principles.