New measurements of the rate of expansion of the universe, led by
astronomers at the University of California, Davis, add to a growing mystery:
Estimates of a fundamental constant made with different methods keep giving
different results.
“There’s a lot of excitement, a lot of mystification and from my point of
view it’s a lot of fun,” said Chris Fassnacht, professor of physics at UC Davis
and a member of the international SHARP/H0LICOW collaboration, which made the
measurement using the W.M. Keck telescopes in Hawaii.
A paper about the work is published by the Monthly Notices of the
Royal Astronomical Society.
The Hubble constant describes the expansion of the universe, expressed in
kilometers per second per megaparsec. It allows astronomers to figure out the
size and age of the universe and the distances between objects.
Graduate student Geoff Chen, Fassnacht and colleagues looked at light from
extremely distant galaxies that is distorted and split into multiple images by
the lensing effect of galaxies (and their associated dark matter) between the
source and Earth. By measuring the time delay for light to make its way by
different routes through the foreground lens, the team could estimate the
Hubble constant.
Using adaptive optics technology on the W.M. Keck telescopes in Hawaii,
they arrived at an estimate of 76.8 kilometers per second per megaparsec. As a
parsec is a bit over 30 trillion kilometers and a megaparsec is a million
parsecs, that is an excruciatingly precise measurement. In 2017, the H0LICOW
team published an estimate of 71.9, using the same method and data from the
Hubble Space Telescope.
Hints of new physics
The new SHARP/H0LICOW estimates are comparable to that by a team led by
Adam Reiss of Johns Hopkins University, 74.03, using measurements of a set of
variable stars called the Cepheids. But it’s quite a lot different from
estimates of the Hubble constant from an entirely different technique based on
the cosmic microwave background. That method, based on the afterglow of the Big
Bang, gives a Hubble constant of 67.4, assuming the standard cosmological model
of the universe is correct.
An estimate by Wendy Freedman and colleagues at the University of Chicago
comes close to bridging the gap, with a Hubble constant of 69.8 based on the
luminosity of distant red giant stars and supernovae.
A difference of 5 or 6 kilometers per second over a distance of over 30 million
trillion kilometers might not seem like a lot, but it’s posing a challenge to
astronomers. It might provide a hint to a possible new physics beyond the
current understanding of our universe.
On the other hand, the discrepancy could be due to some unknown bias in the
methods. Some scientists had expected that the differences would disappear as
estimates got better, but the difference between the Hubble constant measured
from distant objects and that derived from the cosmic microwave background
seems to be getting more and more robust.
“More and more scientists believe there’s a real tension here,” Chen said.
“If we try to come up with a theory, it has to explain everything at once.”
Journal article: https://academic.oup.com/mnras/article/490/2/1743/5568378
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