A buyer's guide to the Hubble Constant
  Since the expansion of the universe was first established by Edwin Hubble and
Georges Lemaitre about a century ago, the Hubble constant H0 which measures its
rate has been of great interest to astronomers. Besides being interesting in
its own right, few properties of the universe can be deduced without it. In the
last decade a significant gap has emerged between different methods of
measuring it, some anchored in the nearby universe, others at cosmological
distances. The SH0ES team has found $H_0 = 73.2 \pm 1.3$ km sec$^{-1}$
Mpc$^{-1}$ locally, whereas the value found for the early universe by the
Planck Collaboration is $H_0 = 67.4 \pm 0.5$ km sec$^{-1}$ Mpc$^{-1}$ from
measurements of the cosmic microwave background. Is this gap a sign that the
well-established $\Lambda$CDM cosmological model is somehow incomplete? Or are
there unknown systematics? And more practically, how should humble astronomers
pick between competing claims if they need to assume a value for a certain
purpose? In this article, we review results and what changes to the
cosmological model could be needed to accommodate them all. For astronomers in
a hurry, we provide a buyer's guide to the results, and make recommendations.
