Astronomy – September 2019

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 in

the 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 Astrophysical

Journal 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. Once

HUBBLE 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