Abstract

Local measurements of the Hubble parameter are increasingly in tension with the value inferred from a LCDM fit to the cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) data. Taken at face value, this tension might be pointing towards physics beyond LCDM. We will briefly review how these different measurements are done, highlighting which scales and epochs they are probing. I’ll then argue that the most promising class of solutions to this tension involves increasing the energy density of the Universe close to the epoch of matter-radiation equality. If you ask a particle physicist, they would tell you that the most natural way to do this is to have new relativistic particles at the epoch probed by the CMB.

We describe the appealing and problematic features of this proposed solution, showing that it is in general challenging to resolve the tension between CMB, BAO, and distance ladder measurements. We then ask: What about neutrinos? Could these pesky little particles have anything to do with our collective cosmological headache? In answering this question, we uncover potential new physics in the neutrino sector that is so out-of-this-world that it should be severely ruled out by the latest cosmological data. Or is it? I will discuss this rather strange physical model that apparently refuses to die when confronted with new data, and what it might be telling us about the tensions we are currently facing.