Messengers from the Early Universe: Cosmic Neutrinos and Other Light Relics

TL;DR

This paper discusses how future cosmological observations, especially detailed CMB measurements, can detect or constrain light relic particles from the early universe, revealing new physics beyond current experiments.

Contribution

It highlights the potential of upcoming CMB observations to probe the universe's thermal history and detect new light particles like axions or sterile neutrinos.

Findings

Future small-scale CMB maps will greatly enhance sensitivity to early universe relics.

Detection of excess light relics would indicate new physics beyond the standard model.

Cosmological data can access epochs inaccessible to terrestrial experiments.

Abstract

The hot dense environment of the early universe is known to have produced large numbers of baryons, photons, and neutrinos. These extreme conditions may have also produced other long-lived species, including new light particles (such as axions or sterile neutrinos) or gravitational waves. The gravitational effects of any such light relics can be observed through their unique imprint in the cosmic microwave background (CMB), the large-scale structure, and the primordial light element abundances, and are important in determining the initial conditions of the universe. We argue that future cosmological observations, in particular improved maps of the CMB on small angular scales, can be orders of magnitude more sensitive for probing the thermal history of the early universe than current experiments. These observations offer a unique and broad discovery space for new physics in the dark sector and beyond, even when its effects would not be visible in terrestrial experiments or in astrophysical environments. A detection of an excess light relic abundance would be a clear indication of new physics and would provide the first direct information about the universe between the times of reheating and neutrino decoupling one second later.