Dark energy—the term used to describe whatever is causing the universe to expand at an increasing rate—is one of the universe's greatest mysteries. The most widely accepted theory currently suggests that dark energy is constant, and the energy of empty space drives cosmic acceleration.

However, last year, findings from the Dark Energy Survey and Dark Energy Spectroscopic Instrument sparked excitement within the cosmology community by hinting that dark energy may actually be evolving.

"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, University of Chicago Professor Emeritus of Astronomy and Astrophysics.

In a new paper published in Physical Review D, Frieman and Anowar Shajib, a NASA Hubble Fellowship Program Einstein Fellow in Astronomy and Astrophysics at UChicago, combine current data from a multitude of probes and find that dynamical models of evolving dark energy can better explain the data than the cosmological constant. If so, their models find, there may be an undiscovered particle out there which is many orders of magnitude smaller than an electron.

University of Chicago spoke with Shajib and Frieman about the new models described in their paper, the implications of these results, and what's next.