Assuming the standard cosmological model, different data sets lead to inconsistent values of the Hubble constant. In this talk I will reformulate this discrepancy in terms of the sound horizon - the comoving distance traveled by sound waves from the beginning until recombination of the primordial plasma. The sound horizon can be inferred in a relatively model-independent manner using classic distance ladder techniques with BAO observations at the highest rung. It can also be inferred by assumption of the standard cosmological model and use of a variety of data sets, most notably CMB power spectra. I will argue that a model change that can reconcile these somewhat discrepant inferences has to be important in the 1 to 1.5 decades of scale factor growth immediately preceding recombination. I will bring up a number of model adjustments and the challenges they face, and conclude that among the listed options, an additional dark component appears to be the most viable.
Speaker: Lloyd Knox, UC Davis
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