Taking the Universe's Temperature with Spectral Distortions of the Cosmic Microwave Background

TL;DR

This paper discusses how future spectral distortion measurements of the cosmic microwave background, especially via the tSZ effect, can precisely determine the universe's thermal energy and baryon density, testing cosmological models.

Contribution

It demonstrates that the PIXIE experiment can detect subtle relativistic effects in the sky-averaged tSZ signal, providing new insights into the universe's thermodynamic properties.

Findings

PIXIE can detect the tSZ signal at >1000σ significance.

Relativistic effects in the tSZ signal can be measured at 30σ.

Measurements will constrain the mean baryon density and thermal energy of electrons.

Abstract

The cosmic microwave background (CMB) energy spectrum is a near-perfect blackbody. The standard model of cosmology predicts small spectral distortions to this form, but no such distortion of the sky-averaged CMB spectrum has yet been measured. We calculate the largest expected distortion, which arises from the inverse Compton scattering of CMB photons off hot, free electrons, known as the thermal Sunyaev-Zel'dovich (tSZ) effect. We show that the predicted signal is roughly one order of magnitude below the current bound from the COBE-FIRAS experiment, but can be detected at enormous significance ($\gtrsim 1000\sigma$) by the proposed Primordial Inflation Explorer (PIXIE). Although cosmic variance reduces the effective signal-to-noise to $230\sigma$, this measurement will still yield a sub-percent constraint on the total thermal energy of electrons in the observable universe. Furthermore, we show that PIXIE can detect subtle relativistic effects in the sky-averaged tSZ signal at $30\sigma$, which directly probe moments of the optical depth-weighted intracluster medium electron temperature distribution. These effects break the degeneracy between the electron density and temperature in the mean tSZ signal, allowing a direct inference of the mean baryon density at low redshift. Future spectral distortion probes will thus determine the global thermodynamic properties of ionized gas in the universe with unprecedented precision. These measurements will impose a fundamental "integral constraint" on models of galaxy formation and the injection of feedback energy over cosmic time.