The rate at which the Universe is expanding, H0, and its evolution across time, H(z), are fundamental for our understanding of the cosmos and its evolution. Yet, precision observations have revealed a ~5 sigma tension between empirically measured H0 values based on today's Universe and the value of H0 inferred from early Universe observations assuming the otherwise highly successful ΛCDM model. Could this imply a crisis for cosmology? Is there a need to revise even basic ingredients of cosmology to explain this difference? Conversely, what is needed to make such a case even more convincingly, and what may we have overlooked?
This workshop will bring together experts on subjects relevant for studying present-day cosmic expansion, Baryon Acoustic Oscillations, and the early Universe to answer these and more questions. Together, we will discuss: 1) “classical” distance ladders and their uncertainties (Cepheids, the TRGB, Supernovae, etc. with a focus on systematic uncertainties & biases); 2) other, possibly new, methods for independent H0 measurements (including strong lensing, gravitational wave events, megamasers, surface brightness fluctuations, expanding photospheres of type-IIP supernovae, fast radio bursts, among others); 3) large scale flows and voids; 4) Baryon Acoustic Oscillations that connect to the early Universe’s sound horizon and allow the cosmic expansion to be traced as a function of time; 5) early-Universe probes such as the cosmic microwave background; 6) theoretical implications of tensions and guidance for further observational tests. In so doing, this workshop will connect cosmic expansion from end to end, from recombination to the Hubble constant.
By July 2023 we expect to be able to review first results based on James Webb Space Telescope observations, the upcoming Gaia Data Release 3 (June 2022), the Dark Energy Spectroscopic Instrument, and several other large surveys, while considering opportunities arising by upcoming surveys such as the ESA Euclid mission, the Vera Rubin Observatory, among others. We particularly wish to foster an environment where new collaborations can flourish and tackle difficult obstacles using creative novel approaches.