Sky and Telescope - May 2018 - 19

er
gy

Exploring the Darkness

Expansion history of the universe

with it in any number of ways, you ruin the theory," he says.
"It's very hard to modify gravity and get a sensible theory."

Today

Final judgment on any of these ideas will depend on what
from
atter
ark m
ation
d
r
e
l
m
e
o
current and future telescopes see. Cosmologists are after two
fr
Acc
ation
eler
main things. The ﬁrst is a more accurate measurement of the
Dec
current expansion rate, which is somewhat contested (see page
20). Astronomers can achieve that with better observations of
things such as supernovae and neutron star collisions.
But they're also measuring primordial sound waves called
baryon acoustic oscillations (S&T: Apr. 2016, p. 22). These
Past
Future
Big
waves were born not long after the Big Bang, when the
Bang
universe was a hot plasma soup. Fluctuations in the density
of the plasma generated sound waves that rang throughout
p INCONSTANT UNIVERSE As astronomers have pushed their distance
the young universe. But when the cosmos cooled and atoms
ladder farther back in time, they've discovered that the expansion rate
formed, the sound waves were silenced, their periodic patdid slow - just not permanently. Matter's gravity decelerated the expanterns frozen in the cosmic microwave background. As the
sion during the first half or so of cosmic history, then dark energy took
over. This behavior would make sense if dark energy were the cosmouniverse continued to expand, those patterns grew, imprintlogical constant: Since expanding space produces more vacuum energy,
ing themselves in the distribution of galaxies.
the cosmic balance would ultimately hit a tipping point, after which the
Like supernovae, these wave patterns act as a cosmic yardvacuum energy's antigravity effect overpowers matter's gravity.
stick. By comparing the apparent size of the patterns with how
big theory predicts they should be, cosmologists can determine
how far away they really are. Researchers can then use these
herculean challenge of solving dark energy. They established
distances to piece together the universe's expansion history.
that a combination of at least two of the four primary techThe second thing cosmologists want to know is how cosniques - supernovae, baryon acoustic oscillations, galaxy
mic structure - the distribution of matter in the universe -
clustering, and weak lensing - is necessary to tease out the
evolved over time. "The pattern of large-scale structure tells
properties of dark energy. They set goals of homing in on a
us about this kind of cosmic tug-of-war
value for w. They laid out what types of
between gravity, which is trying to pull galtelescopes and surveys would be needed.
"The pattern of largeaxies together, and dark energy, which has
And they calculated a so-called ﬁgure of
scale structure tells
been pushing them apart," Frieman says.
merit to quantify how much a progresus about this kind
One way to study cosmic structure is to
sive sequence of experiments would have
of cosmic tug-of-war
analyze how galaxy clusters are scattered
to improve in precision. "We made these
across the sky. Another method involves the
recommendations, and they're pretty
between gravity,
nearly imperceptible distortions of galactic
much all coming to pass," says Albrecht,
which is trying
images caused by dark matter, an effect
who was on the original team along with
to pull galaxies
called weak gravitational lensing. By measurFreedman.
together, and dark
ing how dark matter's gravity bends the
Numerous telescopes are now scanning
light from galaxies, researchers can map the
the
skies for clues about dark energy. The
energy, which has
dark matter - the main component of cosDark Energy Survey, which began in 2013,
been pushing them
mic structure - throughout cosmic history.
is now in its last year, but it has collected
apart,"
Frieman says. so much data that last summer's results
Having these two strategies of measuring cosmic expansion and structure growth
only pulled from the ﬁ rst year of observais vital, as the dual approaches offer independent tests of
tions. Those initial results included lensing and clustermodiﬁed gravity. "Gravity affects the expansion rate of the
ing data. The ﬁ rst big data release, made in January 2018,
universe and the growth of cosmic structure differently," says included three years of observations of several hundred milcosmologist Rachel Mandelbaum (Carnegie Mellon Univerlion galaxies spread over one-eighth of the sky. The collabosity). Comparing measurements of expansion and structure
ration is now analyzing those data to measure large-scale
growth will reveal whether dark energy is real, or if general
structure and lensing effects.
relativity needs to be changed.
The task force's ﬁ nal recommendations included additional large-scale surveys to probe dark energy with unprecedented precision. Those, too, may come to pass. In the
A Wide Open Field
early 2020s, the European Space Agency plans to launch the
In 2005, a team of researchers formed the Dark Energy Task
Euclid space telescope to analyze patterns in the distribution
Force to outline how, as a ﬁeld, researchers would tackle the
s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 8

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Sky and Telescope - May 2018

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