skyandtelescope.com • JUNE 2019 29BOSS
(baryon acoustic
oscillations)
Dark Energy Survey
(cosmic structure +
lensing + baryon density)
Planck
(CMB)Expansion rate (km/s/Mpc)60 65 70 75 80SH0ES
(Type 1a + Cepheids)
Type IaLensed quasarsGravitational wavesCepheidsHubble Constant Measurements
Primordial imprints PhenomenaClarity Will Come
In the closing session of the symposium, Schmidt stoically
observed that “clearly, we have not solved things today.” The
hope is that new and better observations will shed light on
the Hubble constant controversy. Astronomers look forward
to more precise parallax data from the European astromet-
ric satellite Gaia, to detailed supernova observations by the
James Webb Space Telescope, to high-precision measurements
of the CMB by the future Simons Observatory in northern
Chile, and to planned surveys of the large-scale structure of
the universe at various look-back times in cosmic history.
By far the best, however, would be a direct measurement of
the expansion of the universe. “It’s hard, but worth it,” says
Rachel Webster (University of Melbourne, Australia). The
idea would be to precisely measure the redshift (and, thus,
the recession velocity) of a distant quasar, and then to repeatthe measurement ten years later, to determine how much it
has increased. The expected change in redshift would be on
the order of one part in a billion, but with future facilities
like the European Extremely Large Telescope (ELT) or the
Square Kilometre Array (SKA), this is “potentially doable,”
according to Webster.
Colless, like most of his colleagues, remains optimistic.
“The nice thing about this fi eld is that many outstanding
questions will be answered in due time, unlike the situation
with cosmic infl ation or string theory,” he says. “Five years
from now, we’ll have a much clearer view.”¢S&T Contributing Editor GOVERT SCHILLING writes about
astronomy and space science from his hometown of Amers-
foort, The Netherlands. Since he was born, the universe has
expanded by about 0.00000044%. 0he i
000 4 %%“That would be
unwarranted by common
sense,” says Loeb. “It would
be like killing a fl y with an
atomic bomb.”
Clustering and weak lensing Combined with baryon acoustic
oscillations and the density of baryons, observations of the
clustering properties of galaxies over time and measurements
of so-called weak lensing yield an independent, pretty precise
value of the Hubble constant. This approach has been fol-
lowed by the Dark Energy Survey — a comprehensive survey
of the positions, redshifts, and shapes of a few hundred million
galaxies, carried out with the 520-megapixel Dark Energy
Camera on the 4-meter Blanco Telescope at Cerro Tololo,
Chile. Preliminary results, published in November 2018, arrive
at a value for H 0 of 67.4 km/s/Mpc — again, in good agree-
ment with the “cosmological” estimate.Gravitational waves In August 2017, astronomers and physicists
detected a burst of gravitational waves — minute ripples in
spacetime — from a pair of colliding neutron stars (S&T: Feb.
2018, p. 32). From the detected wave patterns, it was possible
to deduce the masses of the colliding stars and the correspond-
ing energy that was emitted in the form of gravitational waves.
Because the waves’ amplitude shrank as they traveled, measur-
ing the amplitude in the detectors here on Earth then provided
an independent distance estimate for the host galaxy of the
merger (NGC 4993): 143 million light-years, with an error margin
of 15%. Combining this with the known redshift of the galaxy
yields a (rather imprecise) value for the Hubble constant of 70
km/s/Mpc, smack in the middle of the “local” and “cosmologi-
cal” values. However, according to Bernard Schutz (Cardiff Uni-
versity, UK), future gravitational-wave detectors like the Einstein
Telescope and the Cosmic Explorer will be able to “see” every
binary neutron star merger in the observable universe, and
eventually, the precision of this method may improve to 0.1%.