Sky & Telescope - February 2024 - 20

DARK ENERGY EXPERIMENT by Dan Falk
Astronomers are peering into space - and into the past - to map the cosmos.
Along the way, they hope to shed light on the mystery of dark energy.
W
hen it opened on the summit of Arizona's Kitt
Peak in 1973, the Nicholas U. Mayall telescope
was the second-largest optical telescope
in the world. Today, its 4-meter primary mirror no longer
ranks in the top 20. But no matter: It has been given new
life as the home of an ambitious project called the Dark
Energy Spectroscopic Instrument (DESI) survey - a fiveyear
effort to map the universe in three dimensions and
provide our clearest picture yet of cosmic history.
The DESI device at the heart of the survey takes its
name from the mysterious force, discovered only a quarter
century ago, that seems to be causing the universe
to expand at an accelerated rate (see page 26). Scientists
have dubbed this force dark energy as a kind of placeholder;
for now, no one knows exactly what it is or what
role it played over the universe's nearly 14 billion-year
history. " We know the universe is accelerating, " says DESI
collaboration member Stéphanie Juneau (NSF's National
Optical-Infrared Astronomy Research Laboratory, or
NOIRLab). " But how exactly is it accelerating? We need
measurements to really pinpoint the answer. "
DESI, which saw first light in 2019, has already yielded
impressive cosmological measurements. Using data collected
in late 2020 and early 2021 during the instrument's
survey validation phase, astronomers have made the most
precise measurements yet of baryon acoustic oscillations,
an imprint in the distribution of galaxies left by sound
waves that traveled through the early universe before
it had cooled enough to allow the first atoms to form.
Much more is expected by the end of DESI's five-year run,
including data that may illuminate the fate of the universe
itself. That fate will be decided by two competing forces -
gravity, which tries to pull everything together, and dark
energy, which appears to be pushing everything apart.
Dark Energy: A Celestial Surprise
The discovery of dark energy in 1998 came as a shock. In
the Big Bang model of cosmology, the universe is thought
to have come into existence when spacetime began
expanding almost 14 billion years ago. That expansion
carried galaxies along with it, so one can think of those
galaxies as rushing away from one another. But we also
know that gravity causes matter to attract matter, which
ought to slow the outward march of the galaxies. And that
did indeed happen, for the first several billion years.
Then it began to speed up.
Has dark energy's push been constant over all that
time, or might it have varied? Astronomers have a reason20
FEBRUARY 2024 * SKY & TELESCOPE
ably clear picture of how the universe began, and a very
good idea of what it's doing now - but key elements of
the intervening story are missing. " There's 13.8 billion
years in between, which we don't understand well enough
yet, " says DESI director Michael Levi (Lawrence Berkeley
National Laboratory). " So that's a lot of history to fill in. "
Because DESI is a spectroscopic instrument, it reveals
not only where galaxies are in the sky but also their
motion toward or away from us. If a galaxy is receding
from us, its spectrum shifts to longer, redder wavelengths.
The degree of that redshift is proportional to the galaxy's
distance. Thanks to this correlation, DESI astronomers
can map the universe's largest structures at various
epochs in cosmic history.
Along with gravity and dark energy, there's a third
player that complicates the picture: Galaxies receive help
from dark matter, an unknown substance that also exerts
a gravitational tug, so much so that it holds vast clusters
together and warps the light from background galaxies.
To understand cosmic history, then, scientists need to
untangle dark energy's push from gravity's pull, taking
into account that some of that pull is from unseen dark
matter. Data from DESI will help researchers tease apart
these competing influences.
" The nice thing about DESI is that we're going to have
so much data going back so many billions of years that we
can 'time-slice' the data, " says Levi. " So we can ask the
question, what was the universe doing a billion years ago?
What was the universe doing five billion years ago? What
was the universe doing 10 billion years ago? "
The New
Cosm
Sky & Telescope - February 2024

Table of Contents for the Digital Edition of Sky & Telescope - February 2024
