10 ASTRONOMY • SEPTEMBER 2019
QUANTUM GRAVITY
Scientists know the universe is
expanding. But there’s a conun-
drum: Studies of the early universe to
derive the expansion rate, called the
Hubble constant or H 0 , don’t mesh
with measurements of the universe
today. Instead, observations suggest it is
expanding about 9 percent faster than
the theoretical value obtained from the
early universe.
There are two ways to determine how
fast the universe is expanding. One is
to use measurements of conditions inthe very early universe to calculate the
expansion rate, based on cosmological
models. The other is to measure directly
how quickly objects are receding from
our position. Based on previous mea-
surements, astronomers believed there
was a 1 in 3,000 chance that the cal-
culated and measured expansion rates
don’t actually disagree. (That is, there
was a 1 in 3,000 chance the universe is
actually expanding at the rate predicted
by cosmological models.) But in a paper
published April 25 in The AstrophysicalJournal Letters, researchers led by Nobel
laureate Adam Riess at Johns Hopkins
University reduced those chances to
1 in 100,000. Their conclusion: The
universe is definitely expanding faster
than expected, although the cause
remains unknown.MEASURING UP
Astronomers measure the expansion
of the modern-day universe based on
the distances and speeds of faraway
galaxies. But measuring distance can
be quite tricky. Since 2005, Riess and
the Supernovae H 0 for the Equation of
State of dark energy (SH0ES) team have
been using the Hubble Space Telescope
(HST) to measure distance, and thus
the expansion rate, more accurately.
Their recent work used HST to look
at 70 variable stars, called Cepheid
variables, in the Large Magellanic
Cloud (LMC), a nearby satellite galaxy
of the Milky Way. Cepheids cycle in
brightness with a relationship related
to their intrinsic brightness. So,
measuring how long a Cepheid takes to
change brightness allows astronomers
to determine how bright it actually is.
Then they compare that number to how
bright it appears; the difference gives
the star’s distance.
Cepheids are one rung on astrono-
mers’ cosmic distance ladder. Because
these stars are only visible in nearby
galaxies, they are used to measure
distances in the local universe. OnceHUBBLE CONFIRMS UNIVERSE’S
FAST EXPANSION RATE
The cosmos is expanding more quickly than expected,
prompting astronomers to ask why.
For a deeper dive into this
topic, check out “Tension at the
heart of cosmology,” on page 20
of our June issue.BONUS
FLICKERING LIGHTS. Astronomers measured
the distance to 70 variable stars in the Large
Magellanic Cloud to reduce errors in the
distance measurements to farther galaxies. The
results show the universe is expanding faster
than theory predicts. CAFUEGO/FLICKR