For a quarter century, cosmology has leaned on one framework to explain how the universe expands. Known as the \u039bCDM model, it assumes about 70 percent of the cosmos is filled with an unseen force called dark energy, represented by Einstein\u2019s cosmological constant (\u039b). <\/p>\n
The other 30 percent consists of matter\u2014both 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\u2019s expansion may not be governed by something so simple and unchanging.<\/p>\n
Hints That Dark Energy Changes Over Time<\/p>\n
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.<\/p>\n
Josh Frieman, Professor Emeritus of Astronomy and Astrophysics at the University of Chicago<\/a>, put it plainly: \u201cThis 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.\u201d<\/p>\n 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<\/a> 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.<\/p>\n Beyond the Cosmological Constant<\/p>\n 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 \u22121, the model matches the cosmological constant. But if w changes over time, then dark energy must be something else.<\/p>\n Frieman and Anowar Shajib, a NASA Hubble Fellowship Program Einstein Fellow, recently co-authored a paper in Physical Review D<\/a>. 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 \u039bCDM model.<\/p>\n Shajib explained, \u201cAlthough there has been interest in the dynamical nature of dark energy since its discovery in the \u201990s, 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.\u201d<\/p>\n By combining all major datasets, our physics-inspired model of dynamical dark energy rules out the standard cosmological model with 99.6% confidence, suggesting the universe is expanding more slowly than expected. (CREDIT: Physical Review D) The Thawing Scalar-Field Model<\/p>\n One way to picture evolving dark energy is through thawing scalar-field models. In these, dark energy begins \u201cfrozen\u201d early in cosmic history, with w at \u22121. As the universe expands and cools, the field slowly \u201cthaws,\u201d rolling downhill like a ball released on a slope. Its density decreases over time, nudging w upward toward less negative values.<\/p>\n In these models, dark energy behaves like an ultralight particle<\/a>, 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\u207b\u00b3\u00b3 electron volts\u2014about 38 orders of magnitude lighter than an electron.<\/p>\n \u201cThe data suggest the existence of a new particle in nature that\u2019s about 38 orders of magnitude lighter than the electron,\u201d Frieman noted.<\/p>\n Putting Theories to the Test<\/p>\n 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.<\/p>\n Current constraints from DES year-3 3\u00d72pt and Planck CMB (no lensing) on \u03a9m. (CREDIT: Physical Review D) <\/p>\n Supernovae provided the strongest push toward thawing models, with DES data showing a significant departure from \u039bCDM. 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.<\/p>\n