The Price of Shifting the Hubble Constant
  An anisotropic measurement of the baryon acoustic oscillation (BAO) feature
fixes the product of the Hubble constant and the acoustic scale $H_0 r_d$.
Therefore, regardless of the dark energy dynamics, to accommodate a higher
value of $H_0$ one needs a lower $r_d$ and so necessarily a modification of
early time cosmology. One must either reduce the age of the Universe at the
drag epoch or else the speed of sound in the primordial plasma. The first can
be achieved, for example, with dark radiation or very early dark energy,
automatically preserving the angular size of the acoustic scale in the Cosmic
Microwave Background (CMB) with no modifications to post-recombination dark
energy. However it is known that the simplest such modifications fall afoul of
CMB constraints at higher multipoles. As an example, we combine anisotropic BAO
with geometric measurements from strong lensing time delays from H0LiCOW and
megamasers from the Megamaser Cosmology Project to measure $r_d$, with and
without the local distance ladder measurement of $H_0$. We find that the best
fit value of $r_d$ is indeed quite insensitive to the dark energy model, and is
also hardly affected by the inclusion of the local distance ladder data.
