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Cosmological Probes of Light Relics



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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]





Baumann, Daniel


astro-ph.CO, astro-ph.CO, hep-ph, hep-th


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
The author acknowledges support by a Cambridge European Scholarship of the Cambridge Trust, by the Department of Applied Mathematics and Theoretical Physics, by a Research Studentship Award of the Cambridge Philosophical Society, from a Starting Grant of the European Research Council (ERC STG Grant 279617), by an STFC Studentship, by Trinity Hall and by a Visiting PhD Fellowship of the Delta-ITP consortium, a program of the Netherlands Organisation for Scientific Research (NWO) that is funded by the Dutch Ministry of Education, Culture and Science (OCW). This work uses observations obtained by the Planck satellite (, an ESA science mission with instruments and contributions directly funded by ESA Member States, NASA and Canada. This research is also partly based on observations obtained by the Sloan Digital Sky Survey III (SDSS-III, Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation and the U.S. Department of Energy Office of Science. Parts of this work were undertaken on the COSMOS Shared Memory System at DAMTP (University of Cambridge), operated on behalf of the STFC DiRAC HPC Facility. This equipment is funded by BIS National E-Infrastructure Capital Grant ST/J005673/1 and STFC Grants ST/H008586/1, ST/K00333X/1. Some analyses also used resources of the HPC cluster Atócatl at IA-UNAM, Mexico, and of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.