On 03/02/2017 23:06, Steve Willner wrote:
In article ,
"Phillip Helbig (undress to reply)" writes:
The real question is whether there is any real tension between Planck
and "local" measurements.

I agree that the most likely reason is some small systematic error.

Indeed. The speed of light in vacuum determination as a function of time
with error bars is a salutary lesson in this regard. I remember the
graph from an introductory relativity textbook in our library.

The gist of the problem was that a very famous experimentalist made an
error in the sign of the correction for imperfect vacuum and everyone
afterwards made exactly the same mistake. It was only when a new method
with even greater precision and radically different systematic errors
came onstream that the problem was uncovered.

BTW Anyone recognise the book title from this description?

it probably makes sense to wait and see if the tension is real
before trying to come up with something which would make the Hubble
constant depend SLIGHTLY on the redshift range in which it is
measured.

While I also agree that coming up with possible explanations isn't
worth a major effort, I think a bit of speculation might not be a bad
idea. It might suggest independent avenues of research to see
whether there might be evidence for some hypothesis or other.

I'm not enough of an expert to have any real idea what might cause
discrepancies between the local and CMB values of the Hubble
constant. I am _guessing_ a "hiccup" in the expansion history might
do it, but I have no idea what might cause such a hiccup nor how one
might test that. Any other ideas?

If the universe is accelerating with time due to dark energy then the
Hubble constant might be expected to vary slightly with increasing Z.
But I suspect by amounts much smaller than the experimental error bars.

--
Regards,
Martin Brown