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New measurement of Hubble constant adds to cosmic mystery
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.
Journal Reference:
Geoff C-F Chen, et. al. A SHARP view of H0LiCOW: H0 from three time-delay gravitational lens systems with adaptive optics imaging. Monthly Notices of the Royal Astronomical Society, 2019; 490 (2): 1743 DOI: 10.1093/mnras/stz2547
(Score: 3, Interesting) by FatPhil on Saturday October 26 2019, @12:26PM
|$H_{0}=82.8^{+9.4}_{-8.3}~\rm km\, s^{-1}\, Mpc^{-1}$| for PG 1115+080,
|$H_{0}=70.1^{+5.3}_{-4.5}~\rm km\, s^{-1}\, Mpc^{-1}$| for HE 0435−1223, and
|$H_{0}=77.0^{+4.0}_{-4.6}~\rm km\, s^{-1}\, Mpc^{-1}$| for RXJ 1131−1231.
The joint AO-only result for the three lenses is
|$H_{0}=75.6^{+3.2}_{-3.3}~\rm km\, s^{-1}\, Mpc^{-1}$|.
The joint result of the AO + HST analysis for the three lenses is
|$H_{0}=76.8^{+2.6}_{-2.6}~\rm km\, s^{-1}\, Mpc^{-1}$|.
So it's not that precise, it's best described as "highish seventies". When I was younger, I remember it was "lowish seventies", so I'm not sure the progress towards actual "precision" is that tangible.
And even when they're ramping up precision, it still says nothing about accuracy, as all that's presented with the rider: "All of these results assume a flat Λ cold dark matter cosmology with a uniform prior on Ωm in [0.05, 0.5] ...". And conclusions are only as good as the assumptions you base them on.
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