Evidence mounts that universe's dark energy is changing over time
By Will Dunham
WASHINGTON (Reuters) - 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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