For over a century, scientists have known that the Universe is rapidly expanding. This phenomenon, named in honor of the astronomers who independently confirmed it (Edwin Hubble and Georges Lemaître), is known as the Hubble-Lemaitre Constant (or the Cosmological Constant). By the 1990s, the Hubble Space Telescope (designed to measure this constant) revealed that the rate at which the Universe was expanding was slower during the early Universe, which was in “tension” with measurements of recent cosmic epochs. This is what led to the “Hubble Tension” in astrophysics and cosmology, and the theory of Dark Energy (DE) as a possible means of explaining the discrepancy.
With the deployment of the James Webb Space Telescope (JWST), scientists hoped to resolve the Hubble Tension once and for all. Unfortunately, Webb’s observations have only deepened the mystery further, indicating that cosmic expansion was briefly faster in the very early Universe. This has led to new theories regarding DE, including Early Dark Energy (EDE) or the possibility that the cosmic force could be weakening over time. The latter is argued in a recent study by researchers from the University of Chicago based on data obtained by the Dark Energy Survey (DES) and Dark Energy Spectroscopic Instrument (DESI).
The study was conducted by Anowar J. Shajib and Joshua A. Frieman, a Kavli Institute for Cosmological Physics (KICP) Fellow and Einstein Fellow and a Professor Emeritus of Astronomy and Astrophysics at the University of Chicago (respectively). Shajib is also the co-convener of the strong lensing topical team within the Rubin Observatory LSST’s Dark Energy Science Collaboration (DESC) and co-Principal Investigator I of the STRIDES collaboration, while Frieman is a researcher with the SLAC National Accelerator Laboratory. The paper detailing their study was published in the journal Physical Review D.
Combining constraints from all major datasets into a physics-inspired model of dynamical DE shows that the Universe will expand more slowly over time. Credit: Shajib & Freiman (2025)
Last year, DES and DESI released findings that first hinted at the possibility that Dark Energy might evolve, triggering great excitement in the astrophysical community. In their new paper, Frieman and Shajib combined current data from many observatories and found that this data is more consistent with dynamical models of DE rather than being constant. To be fair, scientists have theorized since the 1990s that DE could be dynamic in nature to resolve observational discrepancies. This was based on observations by Hubble and missions that have provided increasingly sensitive measurements of the Cosmic Microwave Background (CMB).
However, it was only recently that most major and robust datasets were inconsistent with non-evolving DE models that stated that the energy density remains constant over time while space is expanding. This interpretation of the Cosmological Constant is foundational to the Standard Model of Cosmology, also known as the Lambda Cold Dark Matter (LCDM) model. Nevertheless, interest in this alternative theory of DE has grown due to a combination of data regarding supernovae, baryonic acoustic oscillations (BAOs), and missions like the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck space telescope.
These missions hinted that the Cosmological Constant, introduced by Einstein over 100 years ago as a force to “counter gravity,” might not be so constant. Instead, the new data suggest that DE may be a dynamical phenomenon that loses density over time. Shajib and Freiman also found that over the last several billion years, the density of DE has decreased by about 10%, much less than the densities of other matter and energy, but still significant. As they explained in an interview with UChicago Physical Science:
The goal of this study is to compare the predictions of a physical model for evolving dark energy with the latest data sets and to infer the physical properties of dark energy from this comparison. The evolving dark energy ‘model’ used in most previous data analyses is just a mathematical formula that isn’t constrained to behave as physical models do.
In our paper, we directly compare physics-based models for evolving dark energy to the data and find that these models describe the current data better than the standard, non-evolving dark energy model. We also show that near-future surveys such as DESI and the Vera Rubin Observatory Legacy Survey of Space and Time (LSST) will be able to definitively tell us whether these models are correct or if, instead, dark energy really is constant.
The Vera C. Rubin Observatory pictured atop Cerro Pachón in Chile. Credit: NSF/DOE
Their models are based on particle physics theories regarding a type of hypothetical particle called axions, which were first predicted by physicists in the 1970s. In recent years, axions have been proposed as a possible candidate for the elusive Dark Matter, and many detectors worldwide are actively searching for them. In the models Shajib and Freiman developed, an ultra-light version of axions would act as DE instead, which was constant for the first several billion years of cosmic history, then started to evolve as their density slowly decreased.
If DE is the reason for the Universe’s accelerated expansion and its density decreases over time, that acceleration will decrease with time. Depending on the nature of DE, the fate of the Universe could come down to two extreme outcomes. On the one hand, there’s the Big Rip scenario, where cosmic expansion continues to accelerate, eventually ripping the very fabric apart (aka. the Big Rip). On the other hand, there’s the Big Crunch, where cosmic expansion will eventually slow and reverse, causing all matter to recollapse on a single point in a “reverse Big Bang.”
Ultimately, their models suggest that the universe will avoid both of these extremes and undergo accelerated expansion for many billions of years instead. This will eventually result in a cold, dark universe—aka. the Big Freeze. Alas, there are still many questions that are waiting to be resolved. Said Freiman:
We now know precisely how much Dark Energy there is in the Universe, but we have no physical understanding of what it is. The simplest hypothesis is that it is the energy of empty space itself, in which case it would be unchanging in time, a notion that goes back to Einstein, Lemaitre, de Sitter, and others in the early part of the last century. It’s a bit embarrassing that we have little to no clue what 70% of the universe is. And whatever it is, it will determine the future evolution of the Universe.
Near-future surveys such as the Dark Energy Spectroscopic Instrument and the Vera Rubin Observatory Legacy Survey of Space and Time (LSST) are expected to clarify the history and nature of cosmic expansion. What these surveys reveal will help determine which model of cosmology is correct—the Standard LCDM model or the dynamical DE model.
Further Reading: UChicago