Scientists could be wrong about dark energy. But new research suggests they’re right about gravity.
Dark energy, a mysterious phenomenon that accelerates the expansion of the universe, is widely believed to have had a constant density throughout the history of the universe. However, dark energy may actually be in decline, researchers from the Dark Energy Spectroscopy Instrument (DESI) announced in a joint report on November 19th. A series of papers posted on the project website and arXiv.org.
This finding reaffirms an April report by the same team that reached similar conclusions (SN: 4/4/24). At the same time, a new analysis (a more thorough look at the same data used in the previous report) shows that the DESI data are consistent with general relativity, Albert Einstein’s theory of gravity, and the alternative It has been confirmed that there is no evidence for the “modified gravity” theory.
DESI creates a three-dimensional map of galaxies throughout the universe. Previous analyzes of the project focused on only one type of information gleaned from its maps. Baryon acoustic oscillations, the sound waves of the early universe that left their mark on the visible universe today (SN: March 4, 2019).
The new analysis adds information about how galaxies and other structures evolved over the history of the universe. “This is the first time we’ve been sensitive to how structures grow over time,” said cosmologist Dragan Hutterer of the University of Michigan in Ann Arbor. “This is important because it is well known that structure growth is very sensitive to dark energy and modified gravity.”
In both analyses, the researchers found signs that dark energy’s equation of state, or the relationship between pressure and density, changes over time. “We show the same conclusion, which is… quite reassuring,” says cosmologist Pauline Zarouk of the CNRS and the Institute for Physics, Nuclear and Advanced Energy in Paris. Because the two analyzes are based on the same data, “even if they do not yield the same results, [conclusion]that would really be a problem. (In both cases, the team combined DESI data with other cosmological data, including data from the cosmic microwave background, the oldest light in the universe.)
Daniel Skolnick, a physicist at Duke University, said DESI researchers were working hard on the initial results. “They’re not backing down from that. It often feels like something big in cosmology is gone a month later.” But in DESI’s case, “their necks… I’m still missing out. I really respect and appreciate that.”
If dark energy fluctuations were confirmed, they would send a shockwave through cosmology, overturning scientists’ standard cosmological model. Although this theory is highly successful in explaining the universe, it includes elements that are not well understood, such as dark energy and an equally unidentified source of mass called dark matter.SN: August 26, 2024).
In an effort to create a more satisfying explanation of the universe, some scientists are tweaking the theory of general relativity, which explains gravity as the result of mass distortions in spacetime. A modified theory of gravity could eliminate the need for dark matter or dark energy (SN: 7/5/24). However, the structure formation observed by DESI was consistent with that predicted by general relativity. And while the theory is not completely ruled out, there is no evidence that gravity has been modified.
A new study unravels a mystery about the mass of neutrinos, lightweight elementary particles that are abundant in the universe. Similar to DESI’s initial analysis, the new findings show that the sum of the masses of the three types of neutrinos is smaller than expected, at least by some explanations (SN: September 20, 2024). That could suggest that cosmologists have misunderstood something about the nature of the universe or about neutrinos themselves.
In 2025, the DESI collaboration plans to publish results based on data from the first three years of the project. That will be a true test of how robust the results are, including whether dark energy actually changes over time.
Skolnick envisions the standard cosmological model as a bonfire. While scientists were enjoying themselves sitting around the warm light, DESI’s results started sparks flying. “This is when we say to everyone, ‘Let’s take a step back from the bonfire to be safe.’ … We’re not going to put a damper on everything, but we’re definitely going to take a step back.”
