RUI: Looking beyond LCDM-- observational consequences of models that ease the Hubble tension

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Over the past twenty years, greater precision in cosmological measurements have revealed intriguing tensions that challenge the standard cosmological model. The most pressing of these is a disagreement between two distinct ways scientists estimate the current expansion rate of the universe (known as the Hubble constant). One estimate is based on direct measurement that uses observations of supernovae; the other is based on indirect measurement that uses observations of the afterglow of the big bang. Whereas the data from these two estimates used to agree, advances in measurement precision now yield values for the Hubble constant that are statistically different. This ‘Hubble tension’ may be pointing scientists to new and unexpected physics not included in the standard cosmological model. The proposed research will advance the field of cosmology by refining what this tension may signify about new and unanticipated physical processes and in doing so has the potential to enhance our understanding of the origin and evolution of the universe. As cosmological observations have become more precise, tensions in the data have appeared. The most significant of these is the “Hubble tension”, a 4-6 sigma disagreement between several independent methods to measure the current expansion rate of the universe (known as the Hubble constant). These methods can be grouped based on whether their measurements are indirect (involving the cosmic microwave background or baryon acoustic oscillations) or direct (involving calibrated type Ia supernovae or strong lens time delays). Intriguingly, the indirect methods find a common value that is lower than those found using direct probes. It has been difficult to explain this tension in the context of the standard cosmological model, but recent work has interpreted this tension as evidence for either additional material in the early universe (with a particular set of properties), for non-standard neutrino interactions, or for modifications to the way in which the electrons recombined with protons during the process that formed the cosmic microwave background. The proposed research will explore the essential observational consequences of these models, articulate the basic dynamics required to resolve the Hubble tension, create new tools with which to characterize their predicted features, and articulate their novel and unexpected dynamics. The outcome of this research will advance the field of cosmology by providing the necessary tools to establish if the Hubble tension is pointing us towards a significant revision of our standard cosmological modelThis award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.