UChicago astrophysicists believe dark energy may be evolving

For a quarter century, cosmology has leaned on one framework to explain how the universe expands. Known as the ΛCDM model, it assumes about 70 percent of the cosmos is filled with an unseen force called dark energy, represented by Einstein’s cosmological constant (Λ).

The other 30 percent consists of matter—both ordinary and dark. This model has worked well in describing many observations, from how galaxies cluster to how the cosmic microwave background looks. But fresh results suggest the universe’s expansion may not be governed by something so simple and unchanging.

Several new surveys have begun to challenge the standard model. Measurements from Type Ia supernovae, baryon acoustic oscillations (BAO), and the cosmic microwave background (CMB) hint that dark energy may evolve instead of staying fixed. These findings point toward a dynamic form of energy, one that shifts slowly with cosmic time.

Josh Frieman, Professor Emeritus of Astronomy and Astrophysics at the University of Chicago, put it plainly: “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.”

Last year, the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI) released data that stirred excitement among scientists. The results suggested that the density of dark energy may have dropped by about 10 percent over the past few billion years. That is not a dramatic change, but enough to make cosmologists reconsider long-standing assumptions.

To explore whether dark energy evolves, researchers use an equation-of-state parameter, w, which compares its pressure to its energy density. If w equals −1, the model matches the cosmological constant. But if w changes over time, then dark energy must be something else.

Frieman and Anowar Shajib, a NASA Hubble Fellowship Program Einstein Fellow, recently co-authored a paper in Physical Review D. They compared mathematical models to physics-based versions of evolving dark energy. Their work showed that models allowing dark energy to shift over time fit current data better than the ΛCDM model.

Shajib explained, “Although there has been interest in the dynamical nature of dark energy since its discovery in the ’90s, until recently most of the robust datasets were consistent with a non-evolving dark energy model. However, interest was vigorously rekindled last year from the combination of supernovae, baryon acoustic oscillation, and cosmic microwave background data.”

One way to picture evolving dark energy is through thawing scalar-field models. In these, dark energy begins “frozen” early in cosmic history, with w at −1. As the universe expands and cools, the field slowly “thaws,” rolling downhill like a ball released on a slope. Its density decreases over time, nudging w upward toward less negative values.

In these models, dark energy behaves like an ultralight particle, possibly a form of axion. Such particles were first suggested in the 1970s and remain strong candidates for explaining cosmic mysteries. In the dark energy context, axions would have masses around 10⁻³³ electron volts—about 38 orders of magnitude lighter than an electron.

“The data suggest the existence of a new particle in nature that’s about 38 orders of magnitude lighter than the electron,” Frieman noted.

The team tested their scalar-field models against multiple datasets, including DES supernovae, BAO results from DESI, CMB data from Planck, and gravitational lensing surveys. Each method highlights a different piece of the cosmic puzzle.

Supernovae provided the strongest push toward thawing models, with DES data showing a significant departure from ΛCDM. BAO results pulled in the opposite direction by favoring a lower Hubble constant, while the CMB offered a middle ground. Combining these datasets produced a more balanced picture.

Statistical tools showed evidence ratios leaning in favor of thawing scalar-field models. Yet one major challenge remains: the Hubble tension. While some local measurements suggest a higher value of the Hubble constant, both ΛCDM and thawing models predict lower numbers. For now, this puzzle continues.

Future surveys promise sharper answers. The extended DESI project will provide more precise BAO maps, while the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) will track weak lensing, galaxy clustering, supernovae, and strong lensing. Together, these observations will tighten the error bars around dark energy’s properties.

Forecasts suggest that combining DESI and LSST data with existing CMB results could reduce uncertainties by a factor of three. If today’s best-fit numbers hold, the deviation from ΛCDM could grow to an undeniable 9.2σ level, essentially proving that dark energy is not constant.

Shajib sees the collective effort as a highlight: “For this paper, we gathered all the major data sets—from the DES, DESI, SDSS, Time-Delay COSMOgraphy, Planck, and Atacama Cosmology Telescope—and combined them to get the most constraining measurement of dark energy to date.”

If dark energy is slowly weakening, what does that mean for the far future? Shajib explained that the universe seems headed toward neither a “Big Rip” nor a “Big Crunch.” Instead, the cosmos will keep expanding at an accelerating pace, but the acceleration will decrease over time. The result will be a “Big Freeze,” a cold and dark universe stretching endlessly into the future.

Frieman added that while the everyday impact may be limited, the technological advances required to study dark energy—such as building powerful telescopes and detectors—could bring more immediate benefits to society.

After decades of viewing dark energy as a constant, the possibility that it is evolving reopens some of cosmology’s most profound questions. As Frieman put it, “We now have the first hint in over 20 years that dark energy might be changing, and if it is evolving, it must be something new, which would change our understanding of fundamental physics.”

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