Astronomers have found evidence that dark energy may be evolving rather than constant, challenging Einstein's longstanding cosmological model. A new study indicates that time-varying dark energy models fit observational data better, potentially altering our understanding of the universe's fate. Researchers from the University of Chicago analyzed data from major surveys to support this idea.
Dark energy, which drives the universe's accelerating expansion and constitutes about 70 percent of the cosmos, has long been explained as a constant property of empty space, akin to Einstein's cosmological constant proposed over a century ago. However, recent observations are prompting a reevaluation.
Last year, data from the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI) suggested that dark energy might be dynamic. This built on earlier interest from the 1990s, but robust datasets like those from supernovae, baryon acoustic oscillations, and the cosmic microwave background—gathered by DES, DESI, and Planck—showed discrepancies with the constant model. "This would be our first indication that dark energy is not the cosmological constant introduced by Einstein over 100 years ago but a new, dynamical phenomenon," said Josh Frieman, Professor Emeritus of Astronomy and Astrophysics at the University of Chicago.
In a study published in September in Physical Review D, Frieman and NASA Hubble Fellow Anowar Shajib combined data from DES, DESI, Sloan Digital Sky Survey (SDSS), Time-Delay COSMOgraphy, Planck, and the Atacama Cosmology Telescope. Their analysis found that models of evolving dark energy, based on ultra-light axion particles, better match the observations. These axions, predicted in the 1970s for strong interactions and now candidates for dark matter, would behave as dark energy in an ultra-light form. The data indicate that dark energy's density has decreased by about 10 percent over the last several billion years, remaining constant earlier in cosmic history before slowly declining.
The implications are profound. As Shajib explained, evolving dark energy means its density changes with time, potentially slowing the universe's acceleration. Their models predict a "Big Freeze," where expansion continues but leads to a cold, dark universe, avoiding extremes like a Big Rip or Big Crunch.
Future surveys, including DESI and the Vera Rubin Observatory's Legacy Survey of Space and Time (LSST), could confirm whether dark energy evolves. Frieman noted the excitement: after two decades of data supporting constancy, these hints of change could reshape fundamental physics.