[Paper Review] Cosmological Probes of Light Relics

Benjamin Wallisch|arXiv (Cornell University)|Jun 8, 2018

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This thesis proposes cosmological probes of light thermal relics—such as cosmic neutrinos and hypothetical weakly coupled particles—using cosmic microwave background (CMB) and large-scale structure (LSS) data. By analyzing baryon acoustic oscillations (BAO) in the CMB and galaxy clustering, the author presents the first direct measurement of neutrino free-streaming via the BAO spectrum, demonstrating sensitivity to relativistic energy density at the sub-percent level and enabling discovery of new light relics or stringent constraints on their couplings beyond the Standard Model.

ABSTRACT

One of the primary targets of current and future cosmological observations are light thermal relics of the hot big bang. Within the Standard Model of particle physics, an important thermal relic are cosmic neutrinos, while interesting extensions predict new light particles which are even more weakly coupled to ordinary matter. These elusive particles may nonetheless be produced efficiently in the early universe and their gravitational influence could be detectable in cosmological observables. In this thesis, we describe how measurements of the cosmic microwave background (CMB) and the large-scale structure (LSS) of the universe can shed new light on the properties of neutrinos and on the possible existence of other light relics. These observations are remarkably sensitive to the amount of radiation in the early universe, partly because free-streaming species such as neutrinos imprint a small phase shift in the baryon acoustic oscillations (BAO) which we study in detail. Building on this analytic understanding, we provide further evidence for the cosmic neutrino background by independently confirming its free-streaming nature in CMB and LSS datasets. In particular, we establish a new analysis of the BAO spectrum resulting in the first measurement of this imprint of neutrinos in the clustering of galaxies. Future cosmological surveys, such as the next generation of CMB experiments (CMB-S4), have the potential to measure the energy density of relativistic species at the sub-percent level and will therefore be capable of probing physics beyond the Standard Model. We demonstrate how this can be achieved and present an observational target which would allow the detection of any light particle that has ever been in thermal equilibrium. Interestingly, even the absence of a detection would result in new insights by providing constraints on the couplings to the Standard Model. [Abridged]

Motivation & Objective

• To investigate the cosmological imprints of light thermal relics, including cosmic neutrinos and hypothetical weakly coupled particles, using CMB and large-scale structure observations.
• To provide independent confirmation of the free-streaming nature of cosmic neutrinos using current cosmological datasets.
• To develop a novel analysis of the baryon acoustic oscillation (BAO) spectrum beyond its standard ruler application for detecting neutrino effects.
• To demonstrate the potential of next-generation CMB experiments (e.g., CMB-S4) to measure relativistic energy density at the sub-percent level.
• To establish observational targets for detecting any particle that was ever in thermal equilibrium in the early universe, including constraints on scalar particles like axions.

Proposed method

• Analyzing the cosmic microwave background (CMB) and large-scale structure (LSS) power spectra to detect phase shifts induced by free-streaming light relics.
• Using the baryon acoustic oscillation (BAO) feature in galaxy clustering as a probe of neutrino free-streaming, going beyond its standard ruler application.
• Applying analytic modeling of the BAO spectrum to extract the imprint of neutrinos on the clustering of galaxies.
• Combining data from Planck, SDSS-III, DESI, and other surveys to constrain the energy density of relativistic species.
• Projecting sensitivity forecasts for future CMB-S4 experiments to detect or constrain additional light relics.
• Using theoretical frameworks to model the effects of weakly coupled particles on cosmological observables, including their impact on the CMB and LSS power spectra.

Experimental results

Research questions

• RQ1 Can the free-streaming nature of cosmic neutrinos be independently confirmed using current CMB and LSS data?
• RQ2 To what extent can the baryon acoustic oscillation (BAO) spectrum be used as a probe of neutrino dynamics beyond its standard ruler function?
• RQ3 What level of sensitivity to relativistic energy density can future CMB experiments like CMB-S4 achieve?
• RQ4 Can cosmological observations detect any new light particle that was ever in thermal equilibrium in the early universe?
• RQ5 How can the absence of a signal in future surveys constrain the couplings of hypothetical particles like axions?

Key findings

• The first direct measurement of the neutrino free-streaming imprint in the clustering of galaxies is achieved through a novel analysis of the BAO spectrum.
• The study confirms the free-streaming nature of cosmic neutrinos using multiple, independent cosmological datasets.
• Future CMB-S4 experiments are projected to measure the energy density of relativistic species at the sub-percent level, enabling discovery of new light relics.
• The absence of a signal in such surveys would constrain couplings of hypothetical scalar particles like axions by orders of magnitude beyond current bounds.
• The method provides a robust, model-independent probe of the early universe's radiation content and the existence of weakly interacting light relics.
• The analysis demonstrates that cosmological observations are sensitive to the phase shift induced by free-streaming species in the baryon acoustic oscillations of the CMB and LSS power spectra.