Dark energy is behind the universe's accelerated expansion
20/3/2025 6:03
New data involving
millions of galaxies and luminous galactic cores is providing
fresh evidence that the enigmatic and invisible cosmic force
called dark energy - responsible for the universe's accelerated
expansion - has weakened over time rather than remaining
constant, as long hypothesized.
The findings announced on Wednesday are part of a years-long
study of the history of the cosmos, focusing upon dark energy.
The researchers analyzed three years of observations by the Dark
Energy Spectroscopic Instrument, or DESI, at Kitt Peak National
Observatory in Arizona.
"The DESI results tantalizingly hint at an evolving dark
energy," said Arjun Dey, an astrophysicist at the U.S. National
Science Foundation's NOIRLab and the NOIRLab project scientist
for DESI.
The new analysis used data from DESI's first three years of
observations of almost 15 million galaxies and quasars, which
are extremely bright galactic cores where a supermassive black
hole hungrily consumes surrounding material.
This analysis, combined with other astrophysical data,
offers mounting evidence that the impact of dark energy may be
weakening over time and that the standard model of how the
universe works may need to be revised, the researchers said.
Those other measurements include the light left over from
the dawn of the universe, exploding stars called supernovae and
the manner in which light from distant galaxies is warped by
gravity.
"The new findings, both from DESI and from a number of other
experiments, now suggest that whatever is causing the universal
expansion may be decaying - that is, decreasing in strength,"
Dey said. "This once again changes our fundamental understanding
of nature, and in particular our understanding of the future of
our universe. Will the expansion continue forever, or will the
acceleration slow, stop and turn into a deceleration?"
The Big Bang event roughly 13.8 billion years ago initiated
the universe, and it has been expanding ever since. Scientists
in 1998 disclosed that this expansion was actually accelerating,
with dark energy as the hypothesized reason. The physical nature
of dark energy is presently unknown.
"DESI data tells us about how the size of the universe has
grown over time. We can relate the rate at which it is growing
directly to the strength - or energy density - of dark energy at
a given time, since dark energy is what causes that growth rate
to accelerate," said University of Pittsburgh astrophysicist
Jeff Newman, another of the researchers.
The universe's contents include ordinary matter - stars,
planets, gas, dust and all the familiar stuff on Earth - as well
as dark matter and dark energy.
Ordinary matter represents perhaps 5% of the contents. Dark
matter, which is known through its gravitational influences on
galaxies and stars, may make up about 27%. Dark energy may make
up about 68%.
"Dark energy is definitely one of the most puzzling and
mysterious components of the universe. We don't yet know what it
is, but we can detect its clear effect on the expansion of the
universe," Dey said.
"All the matter in the universe has gravity which should be
slowing down the rate of expansion. Instead, we observe that the
universal expansion is accelerating, and we attribute this
unexpected behavior to dark energy, a component of the universe
which exerts a pressure to push things apart - kind of like a
negative gravity," Dey added.
The new findings were presented at the American Physical
Society's Global Physics Summit in Anaheim, California.
"Our findings in DESI that the dark energy is evolving in
time and is not the cosmological constant is probably the most
important result about cosmic acceleration since its discovery
in 1998 that led to the Nobel Prize in physics in 2011," said
cosmologist Mustapha Ishak of the University of Texas at Dallas
and co-chair of the working group that analyzed the DESI data.
"The new and unexpected result is likely to change the
future of cosmology and our understanding of its standard
model," Ishak added.
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