It’s looking more and more as if dark energy, the mysterious factor that scientists say is behind the accelerating expansion of the universe, isn’t as constant as they once thought.
The latest findings from the Dark Energy Spectroscopic Instrument, or DESI, don’t quite yet come up to the level of a confirmed discovery, but they’re leading scientists to rethink their views on the evolution of the universe — and how it might end.
Readings from DESI’s Data Release 2, published on March 19, suggest dark energy’s influence isn’t as strong as it used to be. Scientists had thought that the universe’s endless expansion would eventually lead to a state of cosmic stasis known as the “Big Chill.” But if dark energy fizzles out, billions or trillions of years from now, the universe may fall back on itself and end up in a reverse Big Bang, or “Big Crunch.”
“What we are seeing is deeply intriguing,” Alexie Leauthaud-Harnett, co-spokesperson for DESI and a professor at the University of California at Santa Cruz, said in a news release. “It is exciting to think that we may be on the cusp of a major discovery about dark energy and the fundamental nature of our universe.”
DESI uses the 4-meter Mayall Telescope at Kitt Peak National Observatory in Arizona to map the universe in 3D, going back 11 billion years in time. The international effort is managed by the U.S. Department of Energy’s Berkeley National Lab. During the first three years of its five-year mission, DESI mapped nearly 15 million galaxies and quasars — and those are the readings that were released this week.
The readings confirm preliminary results that were released last year. Both sets of findings add a new layer of complexity to the debate over dark energy, which is thought to account for about 70 percent of the universe’s cosmic contents.
Dark energy’s influence has been compared to the effect of blowing up a polka-dotted balloon. As the balloon gets bigger, the polka dots spread farther apart. In a similar way, galaxies and galaxy clusters have been expanding farther apart on the fabric of spacetime.
A quarter-century ago, scientists analyzed data about the distance of supernovae during different eras of the universe’s history — and determined that the spreading effect seemed to be accelerating over time. Their findings earned them the Nobel Prize in physics in 2011.
Some scientists noted that dark energy’s effects could be explained most easily by invoking a cosmological constant. Such a constant wouldn’t explain what dark energy is. It would, however, balance out the equations describing the nature of the universe. Theoretical physicist Albert Einstein first came up with the idea in 1917, but he later abandoned the concept and was said to have called it his “biggest blunder.” Decades later, the idea was revived for dark energy.
DESI was designed to bring a higher level of precision to measurements of dark matter’s effects over time. The telescope’s readings were analyzed to focus in on subtle patterns known as baryon acoustic oscillations, which can reveal the distribution of matter at different epochs in the history of the universe. Such readings serve as a “standard ruler” for measuring the strength of dark energy.
The latest findings, released in conjunction with this week’s meeting of the American Physical Society, are based on DESI’s data as well as other types of measurements. Taken together, they suggest that dark matter began making its mark on the universe’s evolution earlier than previously thought, and that those effects are now weaker than what had been predicted.
“It’s not just that the data continue to show a preference for evolving dark energy, but that the evidence is stronger now than it was,” said the University of Portsmouth’s Seshadri Nadathur, co-chair of DESI’s Galaxy and Quasar Clustering working group. “We’ve also performed many additional tests compared to the first year, and they’re making us confident that the results aren’t driven by some unknown effect in the data that we haven’t accounted for.”
To explain what they’re seeing, physicists may need to add new twists to theories relating to the origin and evolution of the universe.
“For a couple of decades, we’ve had this standard model of cosmology that is really impressive,” said Willem Elbers, a postdoctoral researcher at Durham University and co-chair of DESI’s Cosmological Parameter Estimation working group. “As our data are getting more and more precise, we’re finding potential cracks in the model and realizing we may need something new to explain all the results together.”
But don’t resign yourself to the universe’s eventual collapse just yet. The findings haven’t yet reached the level of confidence that’s required for a confirmed discovery just yet. In statistical terms, that level is known as 5 sigma. That means there’s only a 1-in-3.5 million chance that the measurements are the result of a fluke.
In contrast, the findings that rely on DESI’s readings and other observations are in the range of 2.8 to 4.2 sigma — and a 3-sigma event has a 0.3% chance of being a statistical fluke. That’s a small chance, to be sure, but more data will be needed to make sure the DESI team’s claims are rock-solid.
Fresh waves of data are sure to come from DESI, which is still in the midst of its sky survey. Other instruments — including the European Space Agency’s Euclid space telescope, NASA’s newly launched SPHEREx observatory and the Vera C. Rubin Observatory in Chile — are likely to contribute data as well.
In the meantime, theoretical physicists will be putting in long hours.
“Our results are fertile ground for our theory colleagues as they look at new and existing models, and we’re excited to see what they come up with,” said Michael Levi, DESI director and a scientist at Berkeley Lab. “Whatever the nature of dark energy is, it will shape the future of our universe.”
A version of this report was published on Alan Boyle’s Cosmic Log. The view that dark energy can change over time also received a boost from supercomputer simulations that looked at how different theoretical models would be expected to affect the large-scale evolution of the universe. Read about the research.