The Bullet Cluster is made up of two
galaxy clusters that are colliding, one moving through the other, about 3.7
billion light-years away in the constellation Carina. These galaxy clusters act
as gravitational lenses, magnifying the light of background galaxies. This
phenomenon makes the Bullet Cluster a compelling piece of evidence supporting
the existence of dark matter. Credit: CTIO/NOIRLab/DOE/NSF/AURA
The
Dark Energy Survey Collaboration collected information on hundreds of millions
of galaxies across the universe using the U.S. Department of Energy-fabricated
Dark Energy Camera, mounted on the U.S. National Science Foundation Víctor M.
Blanco 4-meter Telescope at CTIO, a program of NSF NOIRLab. Their completed
analysis combines all six years of data for the first time and yields
constraints on the universe's expansion history that are twice as tight as past
analyses.
The Dark Energy Survey (DES) is an
international, collaborative effort to map hundreds of millions of galaxies,
detect thousands of supernovae, and find patterns of cosmic structure that will
help reveal the nature of the mysterious dark energy that is accelerating the
expansion of our universe.
From 2013 to 2019, the DES Collaboration
carried out a deep, wide-area survey of the sky using the 570-megapixel
DOE-fabricated Dark Energy Camera (DECam), mounted on the NSF Víctor M. Blanco
4-meter Telescope at NSF Cerro Tololo Inter-American Observatory (CTIO) in
Chile. For 758 nights over six years, the DES Collaboration recorded
information from 669 million galaxies that are billions of light-years from
Earth, covering an eighth of the sky.
Today, the DES Collaboration is
releasing results that, for the first time, combine all six years of data from
weak lensing and galaxy clustering probes—two techniques for measuring the
universe's expansion history. The collaboration also presents the first results
found by combining all four methods of measuring the expansion history of the
universe—baryon
acoustic oscillations (BAO),
Type-Ia supernovae, galaxy clusters, and weak gravitational lensing—as proposed
at the inception of DES 25 years ago.
The paper, submitted to Physical Review D, represents a summary of 18 supporting papers.
The analysis is currently available on
the arXiv preprint server.
The correlations used by DES scientists
to map the distribution of matter in the Universe. The DES analysis uses shape
measurements of source galaxies, shown in yellow, and the positions of lens
galaxies, shown in red. Credit: Jessie Muir, DES Collaboration
"It
is an incredible feeling to see these results based on all the data, and with
all four probes that DES had planned. This was something I would have only
dared to dream about when DES started collecting data, and now the dream has
come true," says Yuanyuan Zhang, assistant astronomer at NSF NOIRLab and
member of the DES Collaboration.
The analysis yields new, tighter
constraints that narrow down the possible models for how the universe behaves.
These constraints are more than twice as strong as those from past DES analyses
while remaining consistent with previous DES
results.
"These results from the Dark Energy
Survey shine new light on our understanding of the universe and its
expansion," said Regina Rameika, Associate Director for the Office of High
Energy Physics in the DOE's Office of Science (DOE/SC). "They demonstrate
how long-term investment in research and combining multiple types of analysis
can provide insight into some of the universe's biggest mysteries."
The first clue for dark energy was
uncovered about a century ago when astronomers noticed that distant galaxies
appeared to be moving away from us. In fact, the farther away a galaxy is, the
faster it recedes. This provided the first key evidence that the universe is
expanding. But since the universe is permeated by gravity, a force that pulls
matter together, astronomers expected the expansion would slow down over time.
DES footprint in equatorial coordinates.
The ∼
5000 deg2 wide-field survey footprint is shown as a black outline, with
overplotted convergence map using the Wiener filter reconstruction method. The
white circles, scaled to approximately one full DECam field-of-view, show the
supernovae field locations. Other current and planned surveys are shown as
well. Credit: arXiv (2026). DOI: 10.48550/arxiv.2601.14559
Then, in 1998, two independent teams of cosmologists used distant supernovae to discover that the universe's expansion is
accelerating rather than slowing. To explain these observations, they proposed
a new kind of phenomenon that is responsible for driving the universe's
accelerated expansion: dark energy. Astrophysicists now believe dark energy
makes up about 70% of the mass-energy density of the universe. Yet, we still
know very little about it.
In
the following years, scientists began devising experiments to study dark
energy, including DES. Today, DES is an international collaboration of over 400
astrophysicists and scientists from 35 institutions in seven countries led by
DOE's Fermi National Accelerator Laboratory.
For the latest results, DES scientists
greatly advanced methods using weak lensing to robustly reconstruct the distribution of
matter in the universe. Weak lensing is the distortion of light from distant
galaxies due to the gravity of intervening matter, like galaxy clusters. They
did this by measuring the probability of two galaxies being a certain distance
apart and the probability that they are also distorted similarly by weak
lensing. By reconstructing the matter distribution over six billion years of
cosmic history, these measurements of weak lensing and galaxy distribution tell
scientists how much dark energy and dark matter there is at each moment.
In this analysis, DES tested two models of the universe against their data. There is the currently accepted standard model of cosmology—Lambda cold dark matter (ΛCDM)—in which the dark energy density is constant. There is also an extended model, in which the dark energy density evolves over time—wCDM.
DES
found that their data mostly aligned with the standard model of cosmology.
Their data also fit the evolving dark energy model, but no better than they fit
the standard model.
However, one parameter is still off.
Based on measurements of the early universe, both the standard and evolving
dark energy models predict how matter in the universe clusters at later times.
In previous analyses, galaxy clustering was found to be different from what was
predicted. When DES added the most recent data, that gap widened, but not yet
to the point of certainty that the standard model of cosmology is incorrect.
The difference persisted even when DES combined their data with those of other
experiments.
Next, DES will combine this work with
the most recent constraints from other dark energy experiments to investigate
alternative gravity and dark energy models. This analysis is also important
because it paves the way for the new NSF–DOE Vera C. Rubin Observatory to collect complementary data during its
decade-long Legacy Survey of Space and Time (LSST). LSST is a deep and wide
survey that will catalog about 20 billion galaxies across the entire Southern
Hemisphere sky. The data can be combined with those from surveys like DES to
enable high-accuracy measurements of cosmological parameters that will further
refine our understanding of dark energy and the expansion history of the
universe.
"DES has been transformative, and the NSF–DOE Vera C. Rubin Observatory will take us even further," said Chris Davis, NSF Program Director for NOIRLab. "Rubin's unprecedented survey of the southern sky will enable new tests of gravity and shed light on dark energy."
Provided by NSF NOIRLab
Source: Dark energy survey scientists release analysis of all six years of survey data
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