The Mystery of Dark Energy Just Got Even Deeper

Dark energy is a dynamic entity, rather than a matter-of-fact constant in the universe’s rapid unfurling, a team of scientists announced this week.

The Dark Energy Spectroscopic Instrument, or DESI, is an international effort involving more than 900 researchers. The project has produced gargantuan sky maps and helped reveal some of the universe’s largest structures, including the source of the universe’s largest known jets. The latest DESI release shakes up the foundational understanding of dark energy and its role in the universe.

As best scientists can tell, the vast majority of the universe is made up of dark matter and dark energy. Dark matter is the catch-all term for about 27% of the universe that is only known to exist because of its gravitational effects on other matter. The stuff is so infinitesimally small or otherwise elusive that it cannot be observed with telescopes, which take in wavelengths of light emitted by objects to better understand those sources.

Observable matter–everything from the coffee cup on your desk, to distant planets, to the largest and most ancient galaxies—makes up just 5% of the cosmos, meaning that dark energy is responsible for a whopping 68% of everything we think exists in the universe.

The notion of dark energy as a constant—which is to say, it manifested the same way 10 billion years ago as it does today, and as it will 10 billion years from now—is “baked into” the predominant model of the universe, Lambda-CDM, according to Rossana Ruggeri, a physicist at the University of Queensland who was involved in the DESI analysis.

“Results from the first batch of data gave a hint that dark energy might not behave like a simple cosmological constant—but it wasn’t strong enough to draw firm conclusions,” Ruggeri said in The Conversation. “Now, the second batch of data has made this evidence stronger.”

Though the data does not yet meet the statistical threshold physicists require to firmly declare a bona fide new discovery, the data strengthen physicists’ resolve that something is going on that will mean the model needs to be revised. “If dark energy is changing over time, it could have profound implications for the ultimate fate of the universe,” Ruggeri wrote.

According to the new DESI numbers, dark energy may be changing over time—either causing the universe’s expansion to accelerate or driving an inward collapse, which would theoretically cause everything we know to collapse into a singularity in a “Big Crunch.”

The team revealed its findings at the American Physical Society’s Global Physics Summit, and the results are hosted in several papers posted to the preprint server arXiv.

At 270 terabytes, DESI’s first data release is just a fraction of what’s to come from the instrument’s five-year survey of the cosmos. It contains information about 18.7 million objects deep in space, including the distances to some extremely remote galaxies, covering 11 billion years of the universe’s history. The data release is free and available for public access; you can also explore some of the instrument’s data via the Legacy Survey Sky Browser.

“The DESI project has maintained the pace of making 3D maps of the universe that are 10 times larger every decade,” said David Schlegel, one of the lead scientists at Berkeley Lab for both DESI and SDSS, in a laboratory release. “That’s our version of Moore’s Law for cosmology surveys. The rapid advance is powered by the clever combination of improved instrument designs, technologies, and analysis of ever-fainter galaxies.”

The instrument works rapidly; with clear observing conditions, it can collect data on more than 100,000 objects in a single night. DESI collects its data from its perch atop the National Science Foundation’s Nicholas U. Mayall 4-meter telescope, an NSF NOIRLab program at Kitt Peak National Observatory in Arizona.

The release contains more than twice as many unique objects outside of the Milky Way than in all previous 3D spectroscopic surveys combined—a testament to the amount of information scientists can garner about the universe at scale, such as the role of dark energy in its expansion.

DESI is currently in its fourth year of a five-year data collection period, and the team intends to record spectra for more than 50 million galaxies and quasars (active galactic cores) before it ends. Thereafter, there will be a lot of data processing and analysis to do. But once it’s done, scientists may have a revised theory of dark energy’s exact machinations in our universe, and how its inexorable role in the universe’s grand scale changes over time.