Tension in the Hubble constant – Triton Station
https://tritonstation.wordpress.com/2017/02/27/tension-in-the-hubble-constant/
There has been some hand-wringing of late about the tension between the value of the expansion rate of the universe – the famous Hubble constant,
H0, measured directly from observed redshifts and distances, and that obtained by multi-parameter fits to the cosmic microwave background. Direct
determinations consistently give values in the low to mid-70s, like Riess et al. (2016): H0 = 73.24 ± 1.74 km/s/Mpc while the latest CMB fit from
Planck gives H0 = 67.8 ± 0.9 km/s/Mpc. These are formally discrepant at a modest level: enough to be annoying, but not enough to be conclusive.
The widespread presumption is that there is a subtle systematic error somewhere. Who is to blame depends on what you work on. People who work on
the CMB and appreciate its phenomenal sensitivity to cosmic geometry generally presume the problem is with galaxy measurements. To people who work
on local galaxies, the CMB value is a non-starter.
This subject has a long and sordid history which entire books have been written about. Many systematic errors have plagued the cosmic distance
ladder. Hubble’s earliest (c. 1930) estimate of H0 = 500 km/s/Mpc was an order of magnitude off, and made the universe impossibly young by what
was known to geologists at the time. Recalibration of the distance scale brought the number steadily down. There followed a long (1960s – 1990s)
stand-off between H0 = 50 as advocated by Sandage and 100 as advocated by de Vaucouleurs. Obviously, there were some pernicious systematic errors
lurking about. Given this history, it is easy to imagine that even today there persists some subtle systematic error in local galaxy distance
measurements.