Sky & Telescope - June 2019 - 28

mean they know how to explain the discrepancy. Even very
creative theorists like Harvard's Loeb are stumped. "I tried
to come up with a solution to present at the symposium,"
he says, "but I have nothing new to report. It's not a simple
problem to solve."
One thing's for sure: The suggestion that our Milky Way
Galaxy may sit in a huge local void, which would have a
higher-than-average expansion rate (S&T: Oct. 2017, p. 12),
doesn't work. "That effect is much too small," says Loeb.
"Moreover, it would also leave an imprint on the cosmic
microwave background."
Riess agrees. "If caused by a local void, the difference
between the two estimates of H0 would be less than a percent,
not 9%," he says. Riess is very conﬁdent about the high-H0
results of the SH0ES team, partly because some alternative
methods of determining the Hubble constant arrive at a
similar value (see box below). "Maybe we're just not creative
enough" to solve the riddle, he says.
One far-fetched possibility might be that dark matter has
been destroying itself over time, weakening its ability to slow
the universe's expansion. But coming up with viable models
of decaying dark matter has turned out to be difﬁcult. In
Berlin, theoretical physicist Lisa Randall (Harvard University) presented her very preliminary ideas on what she calls a
"quintessential solution to the Hubble puzzle," in which dark
matter would become less massive over time. "But," she said
at the conference, "if the gap remains as large as 9%, this

can't be the ﬁnal solution. It's a very challenging problem to
address." Since then she's been able to revise the model to
provide "a rather good ﬁt to existing data."
Another potential solution would be a slow change in the
"density" of dark energy over cosmological time. Recent X-ray
observations of 1,600 distant quasars appeared to indicate just
that (S&T: May 2019, p. 8). However, "the jury is deﬁnitely
still out," acknowledges team leader Guido Risaliti (University
of Florence, Italy). "We'll have to look at many more models
in great detail before we can solve this cosmic conundrum."
Both Randall and Loeb are reluctant to consider speculative theories on modiﬁed gravity that might somehow explain
away the problem. "That would be unwarranted by common sense," says Loeb. "It would be like killing a ﬂy with an
atomic bomb."
Still, according to Freedman, an informal vote amongst
scientists during a cosmology conference in Chicago in early
October 2018 revealed that the vast majority thought that
some form of "new physics" will be needed to solve the mystery. "It's what cosmologists really hope," says Huterer. "Who
knows," comments Schmidt, "there might be something
fundamentally wrong with our interpretation of the cosmic
microwave background."
u THE WALL Measurements of the cosmic expansion rate bifurcate,
with studies that use distance scales set in the early universe favoring
lower values than those that use phenomena such as supernovae.

Other Ways to Measure the Hubble Constant
The current expansion rate of the universe can be derived from precise measurements of distances and "recession" velocities of
remote galaxies, or calculated from cosmological models and the observed properties of the cosmic microwave background - both
approaches are discussed in the main story. But there are other ways to measure the Hubble constant, H0 :
Gravitational lensing The light from remote galaxies and quasars
can be split into multiple images by the gravity of a massive
foreground object, such as a huge elliptical galaxy or a galaxy
cluster. Brightness changes in the lensed object (for example,
a supernova explosion in a galaxy, or the temporary flickering
of a quasar core) arrive at Earth at different epochs, because
each light path has its own associated travel time. From the
time differences - and a precise model of the mass distribution of the foreground lens - it is possible to calculate the distances traveled, measuring the Hubble constant to a precision
of some 3%, according to Sherry Suyu (Max Planck Institute
for Astrophysics, Germany). Her team's latest results yield a
value for H0 between 70.2 and 74.6 km/s/Mpc, in fair agreement with the "local" Cepheid/supernova method.

J U N E 2 019 * SK Y & TELESCOPE

Baryon acoustic oscillations (BAOs) Some 370,000 years after
the Big Bang, free electrons combined with atomic nuclei,
and the universe became transparent to radiation. Largescale pressure waves in the primordial gas, produced by the
earlier photon pressure, suddenly froze out. As a result, every
high-density lump of matter was surrounded by a sphere with
a higher-than-average density at a characteristic distance of
something between 400,000 and 500,000 light-years. Consequently, the chance of two galaxies forming at this mutual distance was higher than average. According to Matthew Colless
(Australian National University), it's a small effect, invisible to
the eye, but statistical studies of the current three-dimensional
distribution of 1.5 million galaxies do reveal it, albeit at a much
larger characteristic scale, thanks to the expansion of the
universe (S&T: Apr. 2016, p. 22). The "peak" now appears at a
mutual distance of 147 megaparsecs (480 million light-years),
and from this value, a Hubble constant of 67 km/s/Mpc can be
derived, in accordance with the "cosmological" method.

H 0 ME ASUREMEN TS: G REGG DINDER M A N / S&T; E X PLOSION
G R A PHIC: TATIA N A Z A E TS / ISTOCK / G E T T Y IM AG ES PLUS

Cosmic Expansion Puzzle

Sky & Telescope - June 2019

Table of Contents for the Digital Edition of Sky & Telescope - June 2019