“We know your secret!” shouted Harvard and Smithsonian’s Center for Astrophysics graduate student Claire Lamman, staring at the stars in the heavens from a beach in Cancún. The celebratory shout came after a history-making study published by the Dark Energy Spectroscopic Instrument (DESI) collaboration shook cosmology to its foundations.
The research proposes that dark energy, the mysterious force driving the accelerating expansion of the universe, may be variable, a discovery that would reshape the destiny of the universe and overthrow the standard model of physics. Researchers have for decades used the Lambda Cold Dark Matter (Lambda-CDM) model to describe the evolution of the universe. This theory hypothesizes dark energy as a cosmological constant, an ubiquitous constant force of the space vacuum. Recent observations on DESI, however, using observations of approximately 15 million quasars and galaxies, have revealed that the dark energy effect is not constant but is time-dependent.
“This is the biggest hint we have about the nature of dark energy in the approximately 25 years since we discovered it,” says Nobel winner Adam Riess, who was not a part of the DESI experiment. The DESI collaboration, an international group of more than 900 scientists, have mapped the universe in three dimensions for decades carefully. With its 5,000 robotic arms on Arizona’s Nicholas U. Mayall Telescope, DESI captures images of light from faraway galaxies to gauge their redshift the way light gets stretched as the universe expands. Paired with cosmic microwave background and supernovae surveys, scientists reconstructed cosmic expansion history from the last 11 billion years.
Their research cites the possibility that dark energy was stronger in the early universe and now is only 10% weaker than theory. The complexity of dark energy is challenging the serious future fate of the universe. Dark energy, if it keeps getting weaker, might eventually lose its acceleration phase and revert to linear expansion. Or, if dark energy simply turns around and goes negative, the universe would actually shrink in a doomsday “big crunch.” “It challenges the fate of the universe,” says University of Texas at Dallas cosmologist Mustapha Ishak-Boushaki and member of the DESI team.
You simply can’t in regular physics attempt to have a time-varying dark energy. There was understandable excitement, but the results haven’t yet achieved the “5 sigma” statistical confidence level required to make a discovery claim in physics. Current analyses are between 2.8 and 4.2 sigma, so there remains an infinitely small chance that the results are a statistical fluctuation. But the reality that the finding is being replicated year by year in multiple data sets DESI observations, supernova surveys, and weak lensing measurements has given researchers hope. “You can remove one leg of the stool, and it still stands,” Riess says. “It passes the sniff test that I have for [taking] a result very seriously.”
The DESI collaboration’s approach to minimizing bias has also improved the consistency of their conclusions. To prevent bias, researchers “unblinded” their analysis only after months of careful examination, publishing the actual cosmological measurements in a dramatic unveiling at the Cancún conference. “It’s really kind of magical you’re one of the few people in the world that know this,” says Uendert Andrade, a postdoctoral researcher at the University of Michigan. The larger the number of counts DESI collects, the more accurate it will be. By the end of its five-year survey, the tool will have made multiple 50 million measurements of galaxies and quasars and potentially push the dark energy hypothesis into the 5 sigma realm. Subordinate missions like the Vera Rubin Observatory and the NASA Nancy Grace Roman Space Telescope will shed more light on the problem. “Our results are fertile ground for our theory colleagues as they look at new and existing models,” says Michael Levi, DESI director and Berkeley Lab scientist.
