26 ASTRONOMY • MAY 2020
matter, scientists are learning about the
first moments after the Big Bang.
How fast is space
expanding?
In 1929, Edwin Hubble discovered that
galaxies are moving away from us at
speeds proportional to their distances.
This provided the first clear evidence
that our universe is expanding. Ever
since, the current rate of this expansion
— the Hubble constant — has been one
of the key properties of our universe
that cosmologists study.
It’s fair to say that the Hubble constant
has long been difficult to measure.
Hubble’s original determination was
plagued with systematic errors that led
him to overestimate the expansion rate
by a factor of 7. As recently as the 1990s,
textbooks often quoted values ranging
from as low as 50 to as high as 100 kilo-
meters per second for every million
parsecs separating two points in space
— usually written as 50 to 100 km/s/Mpc.
(One megaparsec [Mpc] equals 3.26 mil-
lion light-years.) Although the precision
of these measurements has improved
considerably over the past two decades,
no consensus yet exists regarding the cor-
rect value for this quantity. In fact, as
these measurements have improved, the
results from different methods seem to
disagree with one another even more.One way to determine the Hubble
constant is to directly measure how fast
objects are moving away from us, just as
Hubble did in 1929. For his measurements,
Hubble used a special class of pulsating
stars known as Cepheid variables, whose
intrinsic luminosities track nicely with the
periods over which they brighten and fade.
Modern cosmologists continue to use
Cepheids for this purpose, but they also
employ other classes of objects, including
type Ia supernovae — exploding white
dwarfs that all have the same approximate
luminosity. When researchers combine
the latest data, they find that the universe
is currently expanding at a rate of about
72 to 76 km/s/Mpc.COUNTERCLOCKWISE FROM TOP LEFT: The
IceCube Neutrino Observatory sits under South Pole
ice, hunting for cosmic neutrinos. Some of these
subatomic particles could come from the decay
of weakly interacting massive particles — a prime
candidate for dark matter — though none has been
detected yet. MARTIN WOLF (ICECUBE/NSF)
The Large Underground Xenon experiment (LUX)
attempted to detect interactions between weakly
interacting massive particles and 816 pounds
(370 kg) of liquid xenon inside this tank. The
experiment, which operated from 2013 to 2016 in
an old mine in the Black Hills of South Dakota,
turned up none of these dark matter particles.
A successor, the 7-ton LUX-ZEPLIN, should begin
taking data in 2020. CARLOS FAHAM/WIKIMEDIA COMMONSThe Axion Dark Matter Experiment looks for
hypothetical axions when they decay into
microwaves in the presence of a strong magnetic
field. Here, technicians install the superconducting
magnet in a lab at the University of Washington.
LAMESTLAMER/WIKIMEDIA COMMONSEarthbound experiments on the hunt for dark matter