Infrared (in)sensitivity of relativistic effects in cosmological observable statistics

TL;DR

This paper demonstrates that cosmological observable statistics are inherently infrared-insensitive when considering full non-linear effects, diffeomorphism invariance, and adiabatic fluctuations, resolving previous concerns about infrared divergences.

Contribution

It proves that the apparent infrared sensitivity in relativistic effects is spurious, showing that observable statistics are actually infrared-insensitive in a fully non-linear framework.

Findings

Infrared sensitivity is spurious due to overlooked cancellations.

Observable statistics are infrared-insensitive under general conditions.

The analysis confirms the consistency with the equivalence principle.

Abstract

The relativistic effects in cosmological observables contain critical information about the initial conditions and gravity on large scales. Compared to the matter density fluctuation, some of these relativistic contributions scale with negative powers of comoving wave number, implying a growing sensitivity to infrared modes. However, this can be inconsistent with the equivalence principle and can also lead to infrared divergences in the observable $N$-point statistics. Recent perturbative calculations have shown that this infrared sensitivity is indeed spurious due to subtle cancellations in the cosmological observables that have been missed in the bulk of the literature. Here we demonstrate that the cosmological observable statistics are infrared-insensitive in a general and fully non-linear way, assuming diffeomorphism invariance and adiabatic fluctuations on large scales.