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SUNDAY SCIENCE: ‘MORE THAN A HINT’ THAT DARK ENERGY ISN’T WHAT
ASTRONOMERS THOUGHT  
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Katrina Miller, Dennis Overbye
March 19, 2025
New York Times
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_ An international team of astronomers unveiled the most compelling
evidence to date that dark energy — a mysterious phenomenon pushing
our universe to expand ever faster — is not a constant force of
nature but one that ebbs and flows through time. _

Using the Dark Energy Spectroscopic Instrument, or DESI, scientists
have assembled the largest three-dimensional map of the universe to
date. Earth is at the center in this animation.(Screen shot), New York
Times

 

An international team of astronomers on Wednesday unveiled the most
compelling evidence to date that dark energy — a mysterious
phenomenon pushing our universe to expand ever faster — is not a
constant force of nature but one that ebbs and flows through cosmic
time.

Dark energy, the new measurement suggests, may not resign our universe
to a fate of being ripped apart across every scale, from galaxy
clusters down to atomic nuclei. Instead, its expansion could wane,
eventually leaving the universe stable. Or the cosmos could even
reverse course, eventually doomed to a collapse that astronomers refer
to as the Big Crunch.

The latest results bolster a tantalizing hint
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last April that something was awry with the standard model of
cosmology, scientists’ best theory of the history and the structure
of the universe. The measurements, from last year and this month, come
from a collaboration running the Dark Energy Spectroscopic Instrument,
or DESI, on a telescope at Kitt Peak National Observatory in Arizona.

“It’s a bit more than a hint now,” said Michael Levi, a
cosmologist at Lawrence Berkeley National Laboratory and the director
of DESI. “It puts us in conflict with other measurements,” Dr.
Levi added. “Unless dark energy evolves — then, boy, all the ducks
line up in a row.”

The announcement was made at a meeting of the American Physical
Society in Anaheim, Calif., and accompanied by a set of papers
[[link removed]] describing the results, which
are being submitted for peer review and publication in the journal
Physical Review D.

“It’s fair to say that this result, taken at face value, appears
to be the biggest hint we have about the nature of dark energy in the
~25 years since we discovered it,” Adam Riess, an astrophysicist at
Johns Hopkins University and the Space Telescope Science Institute in
Baltimore who was not involved in the work but shared the 2011 Nobel
Prize in Physics
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discovering dark energy, wrote in an email.

DESI imaging the night sky. KPNO/NOIRLab/NSF/AURA/T. Slovinský

But even as the DESI observations challenged the standard model of
cosmology, which predicts that dark energy is constant across time, a
separate result has reinforced it. On Tuesday, the multinational team
that ran the Atacama Cosmology Telescope in Chile released
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most detailed images ever taken of the infant universe, when it was a
mere 380,000 years old. (That telescope was decommissioned in 2022.)

Their report
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not yet peer-reviewed, seems to confirm that the standard model was
operating as expected in the early universe. One element in that
model, the Hubble constant, describes how fast the universe is
expanding, but over the last half-century measurements of the constant
have starkly disagreed, an inconsistency that today has shrunk to
about nine percent. Theorists have mused that perhaps an additional
spurt of dark energy in the very early universe, when conditions were
too hot for atoms to form, could resolve this so-called Hubble
tension.

The latest Atacama results seem to rule out this idea. But they say
nothing about whether the nature of dark energy might have evolved
later in time.

Both reports evoked effusive praise from other cosmologists, who
simultaneously confessed to a cosmic confusion about what it all
meant.

“I don’t think much is left standing as far as good ideas for what
might explain the Hubble tension at this point,” said Wendy
Freedman, a cosmologist at the University of Chicago who has spent her
life measuring the universe and was not involved in either study.

Michael Turner, a theorist at the University of Chicago, who was also
not involved in the studies, said: “The good news is, no cracks in
the cosmic egg. The bad news is, no cracks in the cosmic egg.”

Dr. Turner, who coined the term “dark energy,” added that if there
was a crack, “it has not opened wide enough — yet — for us to
clearly see the next big thing in cosmology.”

Dark expectations

A computerized animation navigating millions of galaxies that were
mapped using coordinate data from DESI.CreditCredit...DESI
collaboration and Fiske Planetarium, CU Boulder

Astronomers often compare galaxies in an expanding universe to raisins
in a baking cake. As the dough rises, the raisins are carried farther
apart. The farther they are from each other, the faster they separate.

In 1998, two groups of astronomers measured the expansion of the
universe by studying the brightness of a certain type of supernova
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or exploding star. Such supernovas generate the same amount of light,
so they appear predictably fainter at farther distances. If the
expansion of the universe were slowing, as scientists believed at the
time, light from faraway explosions should have appeared slightly
brighter than foreseen.

To their surprise, the two groups found that the supernovas were
fainter than expected. Instead of slowing down, the expansion of the
universe was actually speeding up.

No energy known to physicists can drive an accelerating expansion; its
strength should abate as it spreads ever more thinly across a
ballooning universe. Unless that energy comes from space itself.

Bob Stupak, left, an electronics maintenance supervisor, and Matthew
Evatt, a mechanical engineering manager, working inside DESI’s
spectrograph room. Marilyn Sargent/Berkeley Lab

This dark energy bore all the earmarks of a fudge factor that Albert
Einstein inserted into his theory of gravity back in 1917 to explain
why the universe was not collapsing under its own weight. The fudge
factor, known as the cosmological constant, represented a kind of
cosmic repulsion that would balance gravity and stabilize the universe
— or so he thought. In 1929, when it became clear that the universe
was expanding, Einstein abandoned the cosmological constant,
reportedly calling it his biggest blunder.

But it was too late. One feature of quantum theory devised in 1955
predicts that empty space is foaming with energy that would produce a
repulsive force just like Einstein’s fudge factor. For the last
quarter-century, this constant has been part of the standard model of
cosmology. The model describes a universe born 13.8 billion years ago,
in a colossal spark known as the Big Bang, and composed of 5 percent
atomic matter, 25 percent dark matter and 70 percent dark energy. But
the model fails to say what dark matter or dark energy actually are.

If dark energy really is Einstein’s constant, the standard model
portends a bleak future: The universe will keep speeding up, forever,
becoming darker and lonelier. Distant galaxies will eventually be too
far away to see. All energy, life and thought will be sucked from the
cosmos.

‘Something to go after’

DESI also captures the light from stars in our Milky Way, as shown in
this video created from the instrument’s first data release.
Generally, the redder the star, the younger its age. Credit DESI
collaboration and Sergey Koposov/University of Edinburgh

Astronomers on the DESI team are trying to characterize dark energy by
surveying galaxies in different eras of cosmic time. Tiny
irregularities in the spread of matter across the primordial universe
have influenced the distances between galaxies today — distances
that have expanded, in a measurable way, along with the universe.

Data used for the latest DESI measurement consisted of a catalog
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other celestial objects. Alone, the data set does not suggest that
anything is awry with the theoretical understanding of dark energy.
But combined with other strategies for measuring the expansion of the
universe — for instance, studying exploding stars and the oldest
light in the universe, emitted some hundred thousand years after the
Big Bang — the data no longer lines up with what the standard model
predicts.

Enrique Paillas, a postdoctoral researcher at the University of
Arizona who announced the DESI measurement publicly on Wednesday,
noted that the data imply that the cosmic acceleration driven by dark
energy began earlier in time, and is currently weaker, than what the
standard model predicts.

The discrepancy between data and theory is at most 4.2 sigma (in the
units of uncertainty preferred by physicists), representing one in
50,000 chances that the results are a fluke. But the mismatch is not
yet at five sigma (equal to one in 3.5 million chances), the stringent
standard set by physicists to claim a discovery.

Still, the disconnect is enticingly suggestive that something in the
cosmological model is not well understood. Scientists might need to
revise how they interpret gravity or make sense of the ancient light
from the Big Bang. DESI astronomers think the problem could be the
nature of dark energy.

“If we introduce a dynamical dark energy, then the pieces of the
puzzle fit together better,” said Mustapha Ishak-Boushaki, a
cosmologist at the University of Texas at Dallas who helped lead the
latest DESI analysis.

Will Percival, a cosmologist at the University of Waterloo in Ontario
and a spokesperson for the DESI collaboration, expressed excitement
about what lies on the horizon. “This is actually a little bit of a
shot in the arm for the field,” he said. “Now we’ve got
something to go after.”

Maps showing different views of the cosmic microwave background from
the Atacama Cosmology Telescope’s sixth data release, based on data
collected between 2017 and 2022. Credit Naess et al., Atacama
Cosmology Telescope

In the 1950s, astronomers claimed that only two numbers were needed
to explain cosmology
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one related to how fast the universe was expanding and another
describing its deceleration, or how much that expansion was slowing
down. Things changed in the 1960s, with the discovery that the
universe was bathed in light from the Big Bang, known as the cosmic
microwave background. Measuring this background radiation allowed
scientists to investigate the physics of the early universe and the
way that galaxies subsequently formed and evolved. As a result, the
standard model of cosmology now requires six parameters, including the
density of both ordinary and dark matter in the universe.

As cosmology has become more precise, additional tensions have arisen
between predicted and measured values of these parameters, leading to
a profusion of theoretical extensions to the standard model. But the
latest results from the Atacama Cosmology Telescope — the clearest
maps to date of the cosmic microwave background — seem to slam the
door on many of these extensions.

DESI will continue collecting data for at least another year. Other
telescopes, on the ground and in space, are charting their own views
of the cosmos; among them are the Zwicky Transient Facility
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space telescope
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NASA’s recently launched SPHEREx mission
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In the future, the Vera C. Rubin Observatory
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begin recording a motion picture of the night sky from Chile this
summer, and NASA’s Roman Space Telescope is set to launch in 2027.

Each will soak up the light from the sky, measuring pieces of the
cosmos from different perspectives and contributing to a broader
understanding of the universe as a whole. All serve as ongoing
reminders of just what a tough egg the universe is to crack.

“Each of these data sets comes with its own strengths,” said
Alexie Leauthaud, a cosmologist at the University of California, Santa
Cruz, and a spokesperson for the DESI collaboration. “The universe
is complicated. And we’re trying to disentangle a lot of different
things.”

_Katrina Miller [[link removed]] is a
science reporter for The Times based in Chicago. She earned a Ph.D. in
physics from the University of Chicago. More about Katrina Miller
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_Dennis Overbye [[link removed]] is the
cosmic affairs correspondent for The Times, covering physics and
astronomy. More about Dennis Overbye
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_Subscribe to the NEW YORK TIMES
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__

‘Global Weirding’: Climate Whiplash Hitting World’s Biggest
Cities, Study Reveals
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Damian Carrington, Environment editor
The Guardian
March 11, 2025

* Science
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* cosmology
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* dark energy
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