Hubble Trouble

The Hubble constant, H_0, characterises the current expansion rate of the Universe, as well as setting the timescale for the age of the Universe, along with the extra-Galactic relationship between redshift and distance.  All current measurements of H_0 imply its value is about 70 km/s/Mpc, but in the last decade a significant discrepancy has been revealed between direct local distance ladder measurements (which give a high value of ~73 km/s/Mpc) and the value inferred from the cosmic microwave background radiation (from which Planck data imply ~67 km/s/Mpc).  The explanation for this “Hubble trouble” might be new cosmological physics, but it could stem from misunderstandings about the local or cosmological data-sets.  After reviewing the current situation, I will i) describe some further analysis of the existing local distance ladder data and ii) discuss how observations of gravitational waves from merging binary stars might soon provide a completely independent measurement of H_0 that could arbitrate the current tension.