Einstein called it his “biggest blunder.” Now a Berkeley Lab breakthrough is shedding light on the mysteries of dark energy and cosmic expansion

Lawrence Berkeley National Laboratory has released new breakthrough findings on one of science’s biggest mysteries — one that Albert Einstein once called his “biggest blunder.”

In March, Berkeley Lab researchers presented data gathered from 14 million galaxies at the American Physical Society’s Global Physics Summit in Anaheim, which has ignited new theories on dark energy – once thought to be a universal constant like gravity – as an evolving form of matter linked to the accelerating expansion of the universe.

While both dark energy and the better-known dark matter are still theoretical concepts, they are distinctly different. While the latter is thought to be a form of matter that interacts with gravity and serves as a cosmic “glue” that holds galaxies together, dark energy is the mysterious repulsive force hypothesized to counteract gravity that’s pushing the universe apart.

“’Dark energy’ is a label that we put on what is causing this acceleration,” said Andrei Cuceu, a Berkeley Lab researcher and member of the Dark Energy Spectroscopic Instrument (DESI) team that is working to create the largest-ever three-dimensional map of the universe. Managed by the Berkeley Lab, DESI is made up of nearly 1000 researchers from more than 70 institutions around the world. “The goal with this experiment is to try to shine some light in this space to figure out which of these classes of theories are much more likely to be correct.”

The DESI survey is being conducted at the Mayall Telescope on top of Kitt Peak in the Sonoran Desert, about 55 miles south of Tucson, Ariz. Measurements from the device create a three-dimensional map of the universe that researchers can use to track its expansion over the last 12 billion years, which is measured by the distance between galaxies.

If dark energy were a universal constant, then the separation between galaxies would be uniform over time. But the latest findings show a strong indication that dark matter is weakening and causing expansion to slow, according to DESI researchers.

“We expected to get a better measure of that accelerated expansion, and it’s not at all what we found,” said Claire Poppett, a research physicist with UC Berkeley’s Space Science Laboratory working on DESI in Santiago, Chile. “It confirms, even more strongly, that dark energy is changing with time.”

The study of dark energy began in the 1990s, when researchers studying supernovae sought to identify the rate of the universe’s expansion and found it was actually getting faster over time. That breakthrough earned them the Nobel Prize, Poppett said. But it presented new questions for cosmologists on what force could be strong enough to cause the universe to expand more quickly. Lacking an answer, they called this phenomenon dark energy.

Scientists believe dark energy makes up about 68 percent of the energy density of the universe, but the journey to understanding how it works has been one of trial and error.

When the DESI project launched in 2021, Poppet and other researchers wondered if the data they collected would only reduce the margin of error from past theories. By triangulating DESI measurements with the datasets from supernovae and light left over from the “Big Bang” at the beginning of the universe, they found they could not find a simple explanation for the discrepancies they identified.

Poppett said she initially believed they had done something wrong in their analysis, but the team of researchers was actually on the course of discovery. Cuceu’s review of the data in comparison with the best available models showed a changing pace of growth over the past 12 billion years.

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“When we compare this behavior with the behavior that would be predicted if dark energy were a constant, they don’t quite agree. This essentially implies that maybe dark energy is not a constant. It’s evolving somehow,” Cuceu said. “The expansion of the universe is still accelerating, but this acceleration has started to slow down today.”

While the DESI project is not complete — a few million more galaxies are still to be scanned — the team is getting closer to the “gold standard” of discovery in particle physics known as “5 sigma,” an evidence threshold at which scientists declare there is an almost certain likelihood that unexpected data findings are the result of a new phenomenon, and not just a statistical fluctuation.

The current findings are between 2.8 and 4.2 sigma, yet each new round of data brings the DESI team closer to rewriting the understanding of dark energy and sending cosmologists back to their blackboards to invent new theories about the laws of the universe.

“It shows that the more powerful instruments you can build, even when you think you know what the answer is going to be, you actually don’t,” Poppett said. “There’s still so many secrets that we don’t know about.”