Tensor Bounds on the Hidden Universe

TL;DR

This paper explores how a large number of hidden fields during inflation can produce observable effects in tensor modes, allowing constraints on hidden sector content through CMB and gravitational wave observations.

Contribution

It demonstrates that hidden fields can generate observable tensor running via large N resummation, enabling bounds on hidden sectors from primordial tensor measurements.

Findings

Large N hidden fields can produce detectable tensor running.

Bounds on hidden field content can be derived from violations of the tensor consistency relation.

Future observations could constrain models with many species addressing naturalness.

Abstract

During single clock inflation, hidden fields (i.e. fields coupled to the inflaton only gravitationally) in their adiabatic vacua can ordinarily only affect observables through virtual effects. After renormalizing background quantities (fixed by observations at some pivot scale), all that remains are logarithmic runnings in correlation functions that are both Planck and slow roll suppressed. In this paper we show how a large number of hidden fields can partially compensate this suppression and generate a potentially observable running in the tensor two point function, consistently inferable courtesy of a large $N$ resummation. We detour to address certain subtleties regarding loop corrections during inflation, extending the analysis of [1]. Our main result is that one can extract bounds on the hidden field content of the universe from bounds on violations of the consistency relation between the tensor spectral index and the tensor to scalar ratio, were primordial tensors ever detected. Such bounds are more competitive than the naive bound inferred from requiring inflation to occur below the strong coupling scale of gravity if deviations from the consistency relation can be bounded to within the sub-percent level. We discuss how one can meaningfully constrain the parameter space of various phenomenological scenarios and constructions that address naturalness with a large number of species (such as `N-naturalness') with CMB observations up to cosmic variance limits, and possibly future 21cm and gravitational wave observations.