Sky & Telescope - May 2024 - 8

NEWS NOTES
COSMOLOGY
New Supernova Catalog Used to Measure
Strength of Dark Energy
AT THE TURN OF the century, astronomers
discovered that the universe's
expansion has been speeding up since
the Big Bang. Now, in a study posted
on the arXiv preprint server, a team of
astronomers suggest that dark energy,
the force behind this phenomenon,
might be weaker than we first thought.
Dark energy is an unidentified quantity
that exerts a repulsive pressure.
Evidence for its existence first came
from the study of a few dozen Type Ia
supernovae, exploding white dwarf stars
(S&T: Feb. 2024, p. 26). These supernovae
can be calibrated as standard candles,
meaning they explode with a known
luminosity. Astronomers can then use
them to measure the expansion rate of
the universe.
Until now, supernova studies have
led astronomers to think dark energy
exerts the same force everywhere,
everywhen. In this simple scenario, the
repulsive pressure is inherent to empty
space itself and is labeled the cosmologi8
MAY 2024 * SKY & TELESCOPE
pThe inset shows a Type Ia supernova (dot of
light to upper right of galaxy's bulge) detected
during the course of the Dark Energy Survey.
This supernova is nearby; its light took only 0.6
billion years to travel to Earth.
cal constant. But a study of thousands of
Type Ia supernovae in the Dark Energy
Survey (DES) suggests that dark energy
might not be constant after all.
During a five-year survey, astronomers
used the Dark Energy Camera,
mounted on the Víctor M. Blanco
4-meter Telescope at Cerro Tololo
Inter-American Observatory in Chile,
to discover 1,635 Type Ia supernovae.
The light from these supernovae has
come from a huge range of distances,
traveling between 1 billion and 8 billion
years before arriving at Earth. Using
the supernovae as standard candles, the
team calculated the universe's expansion
rate and established new constraints
on dark energy.
To describe dark energy, physicists
use the equation of state, labeled w,
which is defined as the ratio of pressure
to density. The value of w determines
the nature of dark energy. In
the simplest scenario, dark energy is
the cosmological constant and w = −1.
However, the DES supernovae point to a
value between −0.66 and −0.95; this less
negative number might indicate that the
repulsive force weakens with time.
The result doesn't completely rule
out a cosmological constant, though:
Random fluctuations in the data could
reproduce the findings about 5% of the
time. (In technical terms, dark energy
is still consistent with the cosmological
constant to within two sigma.)
" It's tantalizing, " says study lead
Tamara Davis (University of Queensland,
Australia). " Maybe the universe
isn't quite as simple as we had thought. "
Mickael Rigault (French National
Centre for Scientific Research), who
was not a part of the collaboration, was
impressed with the group's techniques.
However, he's less sure what this means
for dark energy: " It's an interesting fluctuation, "
he notes. " I think we can still
consider dark energy to be a cosmological
constant. But we shall see. "
¢ARWEN RIMMER
DES COLLABORATION / NOIRLAB / NSF / AURA / M. ZAMANI
Sky & Telescope - May 2024

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