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

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.

5 pages + references; 1 figure; science white paper submitted to the Astro2020 decadal survey

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• The trouble with $H_0$
• Nonperturbative Dynamics Of Reheating After Inflation: A Review
• The effects of He I 10830 on helium abundance determinations

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• Cosmological Implications of Axion-Matter Couplings
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• Finding eV-scale Light Relics with Cosmological Observables
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• Free-Streaming Neutrinos and Their Phase Shift in Current and Future CMB Power Spectra
• Inferring the covariant $\Theta$-exact noncommutative coupling in the top quark pair production at linear colliders
• Prospects and Limitations for Constraining Light Relics with Primordial Abundance Measurements
• Warm Hawking Relics From Primordial Black Hole Domination
• Neutrinos in N-body simulations
• Dark Matter-Radiation Scattering Enhances CMB Phase Shift through Dark Matter-loading
• SpectrAx: Spectral Search of Axion-Like Particles Using Multi-Band Observations of Galaxy Clusters from SKA, SO, CMB-S4 and eROSITA
• Cosmological Implications of a Neutrino Mass Detection
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