Red teaming the H0 Tension

Despite the spectacular success of the standard cosmological model over the past two decades, recent observations from large sky surveys and distance measurements with a broad range of cosmological probes suggest cracks to the accepted paradigm: differences now appear in measurements of quantities the current cosmological model predicts as equal. These are called “tensions”. The most important tension concerns the Hubble parameter, H0, which quantifies the expansion rate of the Universe now, some 13 billion years after Big-Bang. The Hubble tension is the discrepancy between the value of H0, when inferred as a global parameter of the standard cosmological model relying heavily on early-Universe physics, and when measured directly in the late-time Universe from the redshift-to-distance relation. While the tension is well-established, there is no consensus on its actual significance nor on what it means for the standard cosmological model, even broadly speaking. Comparing the values of H0 from the early and the late Universe is like “Threading a needle from the other side of the Universe”; in a very unique way it enables vital end-to-end tests of cosmology to be performed. RedH0T proposes to determine, in a self-consistent manner and using the synergies between all available cosmological probes, whether the tension arises from observational issues or from limitations in the current cosmological model. In the former case, we will impose strong limits of possible deviations from the standard cosmological. But if the latter is confirmed, it will represent one of the most significant discoveries of the 21st century, with profound implications not only for cosmology, but also for physics in general. The team gathers all required expertise to tackle the problem from all view-angles, addressing in a self-critical and constructive way all theoretical and observational aspects simultaneously.