In article , "Richard D.
Saam" writes:

On 11/9/18 1:37 AM, Steve Willner wrote:
MILKY WAY CEPHEID STANDARDS FOR MEASURING COSMIC DISTANCES AND
APPLICATION TO Gaia DR2:
IMPLICATIONS FOR THE HUBBLE CONSTANT
https://arxiv.org/abs/1804.10655
The Planck H0 = 67.4 km/s/Mpc is based on CMB.
The reported H0 = 73.24 km/s/Mpc is based on photometric parallaxes.
What mechanism explains the difference?

The difference is less than 3 sigma so may not be real. If there is
a difference, time-variable dark energy would be one possibility.
The Adam Riess colloquium I posted about earlier
https://www.youtube.com/watch?v=eSPCy-IJaPg
is still relevant and easy to follow.
Referencing a good summary article on the Ho tension:
Measuring cosmic distances with standard sirens Physics Today, Dec 2018

universe expansion using supernovae Ho1 = 73.24 +/-1.74 km s^-1 Mpc^-1
and
Planck satellite's CMB fluctuations Ho2 = 67.74 +/-.46 km s^-1 Mpc^-1

The hope is to resolve Ho tension with standard siren determination.
There is one currently available measurement GW170817
with the hope of many more.

I guess "standard sirens" mean "black-hole mergers detected by
gravitational waves". As the song says, "two men say they're Jesus; one
of them must be wrong". A third determination might disagree with both.
Even if it agrees with one, that doesn't "resolve the tension". I also
doubt whether the standard-siren technique will get the uncertainties
down to a comparable level any time soon.

My guess is that the errors have been overestimated. IIRC, H_0 is one
area where WMAP and Planck don't agree well. So Planck is the odd man
out, considering that most techniques favour a higher value (though with
larger uncertainties).