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

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