In article <71609d81-f915-4d7d...@googlegroups.com
> "Is the Universe Younger than We Thought?", is the age of the universe,
> not 13,8 billion years, but 11 billion years old.
> This seems, to me, a rather big shift, specific because it is based on
> gravitational lensing.
All else being equal, the age of the universe is inversely proportional
to the Hubble constant.
The headline doesn't deserve any prizes. There are many measurements of
the Hubble constant, and the field has a history of discrepant
measurement (i.e. measurements which differ by significantly more than
their formal uncertainties). Recently, the debate has shifted from "50
or 100?" to "67 or 73?" but since the formal uncertainties have also
gone down, one could argue that the "tension" is comparable to that in
the old days. There is more than one measurement supporting 67, and
more than one supporting 73. So, ONE additional measurement doesn't
mean "the textbooks will have to be rewritten" or some such nonsense,
but rather is an additional piece of information which must be taken
It should be noted that there are many measurements of the Hubble
constant from gravitational lenses. Not all agree. The biggest source
of uncertainty is probably the fact that the result depends on knowing
the mass distribution of the lens galaxy.
For what it's worth, I am co-author on a paper doing this sort of thing:
Our value back then, almost 20 years ago, was 69+13/-19 at 95%
confidence. The first two authors recently revised this after
re-analysing the data, arriving at 72+/-2.6 at 1 sigma, though this
includes a better (published in 2004) lens model as well. The papers
are arXiv:astro-ph/9811282 and arXiv:1802.10088. Both are published in
MNRAS (links to freely accessible versions are at the arXiv references
above). It's tricky to get right. As Shapley said, "No one trusts a
model except the man who wrote it; everyone trusts an observation except
the man who made it." :-)
The above uses just the gravitational-lens system to measure the Hubble
constant. Such measurements have also been made before for the two lens
systems mentioned in the press release. What one actually measures is
basically the distance to the lens. Since the redshift is known, one
knows the distance for this particular redshift; knowing the redshift
and the distance gives the Hubble constant. In the new work, this was
then used to calibrate supernovae of with known redshifts. (Determining
the Hubble constant from the magnitude-redshift relation for supernovae
is also possible, of course (and higher-order effects allow one to
determine the cosmological constant and the density parameter
(independently of the Hubble constant), for which the 2011 Nobel Prize
was awarded), but one needs to know the absolute luminosity, which has
to be calibrated in some way. Since they measure the distance at two
separate redshifts, the cosmology cancels out (at least within the range
of otherwise reasonable models). Their value is 82+/-8, which is
consistent with the current "high" measurements. There are many reasons
to doubt that the universe is only 11 billion years old, so a value of
73 is probably about right.
The MPA press release is more carefully worded: "While the uncertainty
is still relatively large" and notes that the value that that inferred
from the CMB. However, many would say that the anomaly is that the CMB
(in particular the Planck data) seem to indicate a low value.
> Figure 6 of the /Science/ research article gives a nice comparison
> of some of the recent Hubble-constant measurements, showing that the
> choice of cosmological model (at least within the range of models
> considered by the authors) makes rather little difference.
> -- jt]]
In principle, the cosmological model can make a difference, but these
days we believe that the values of lambda and Omega have been narrowed
down enough that there isn't much room to move; measuring the distance
at two different redshift essentially pins it down.