An Important Force of the Universe Appears to Be Changing, Scientists Find

Dark energy, the mysterious force that makes up 70 percent of everything in existence, was hypothesized to explain why the expansion of the universe was accelerating. Ever since, it's been thought of as a constant, immutable presence.

Now, the latest observations from the Dark Energy Spectroscopic Instrument (DESI) indicate that dark energy has actually changed over time — a development that could potentially upend the prevailing cosmological model, and perhaps hint at a new understanding of physics. The findings, detailed in a series of papers currently awaiting peer review, have implications not only for how the universe has evolved, but what its eventual fate might be as well.

"It sounds like it will be a paradigm shift, something that will change our understanding and the way we are putting all the pieces together," Mustapha Ishak-Boushaki, a cosmologist at the University of Texas and DESI team member, told Quanta Magazine of the findings.

The DESI telescope, located in Kitt Peak, Arizona, searches and measures galaxies to tease out the effects of dark energy. It's now surveyed a staggering 15 million of these realms as far as 11 billion light years away, providing the most comprehensive portrait to date of the galaxies as they shifted and clustered together over the eons — movements thought to betray the presence of dark energy.

Following up on preliminary findings shared a year ago, the latest DESI results strongly suggest that the acceleration of the universe's expansion started sooner than once thought, peaked early on, and is currently weakening.

This is a big deal. Dark energy, as it's currently theorized, stems from the idea of a cosmological constant. Proposed by Albert Einstein, it assumes that there's some unseen background force powerful enough to explain why the universe, with all its mass, doesn't collapse under its own gravity. 

Einstein later called the cosmological constant his "biggest blunder," but it found a second life decades later with the idea of dark energy, along with dark matter, in the late 1990s. Dark energy, envisioned as this constant, is now a cornerstone of the lambda-CDM model, the standard model of cosmology.

In this model, dark energy pushes against the literal weight of existence to make sure it all doesn't come crashing down, accelerating the universe's expansion at a fixed rate. Meanwhile, invisible dark matter, thought to make up roughly 25 percent of the universe compared to a measly five percent of the regular matter we're made out of, is thought to govern the formation of galaxies from the shadows with the pull of its gravity.

Though it may be the standard theory, lambda-CDM has always been contentious, not least of all because it doesn't explain what dark energy actually is (Einstein thought it was a force intrinsic to the vacuum of space itself.)

It's too early to say that the dominant model has been trumped, but it's on the ropes. The DESI results, combined with extensive observations of the cosmic microwave background — the leftover light from the Big Bang — and of thousands of supernovas, indicate a discrepancy of 4.2 sigma, a measurement of uncertainty indicating, in this case, that there's only a one in 30,000 chance that the lambda-CDM model is correct, per Quanta Mag. Five sigma, however, is the standard needed to be considered a bona fide discovery. Though it hasn't quite made the cut yet, the latest work yields a higher sigma level than reported a year ago — and there's still two more years of DESI data to parse through.

Comfortingly, one possible implication of a waning dark energy is that the universe won't relentlessly expand until it rips itself apart, as one theory holds. On the other hand, if dark energy's powers are diminishing, it's possible its effects could not halt at zero but go on to reverse course, dooming the cosmos to implode on itself. Then again, the mere fact that dark energy can change at all could mean that everything's up in the air.

"As far as theoretical models, Pandora's box just opened," Ishak-Boushaki told New Scientist. "We were stuck with a cosmological constant. We are not stuck anymore."

More on cosmology: Physicists Find That the Universe Could "Collapse Like a House of Cards"

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