Dark Energy, the theoretical form of energy that proliferates throughout most of our universe, may be losing its momentum over time, leading to a slowdown in universal expansion and a potential reevaluation of our current models of the cosmos.
Researchers examining data from Antarctica’s South Pole Telescope report that their observations align with those of previous studies, which have also detected the unexpected behavior. The new data is the most precise ever collected, setting a new standard as scientists grapple with the possibility of rethinking existing models of the universe.
Modeling All That Is
The commonly accepted model of the universe, the Lambda-Cold Dark Matter (Lambda-CDM) model, states that collapsing dark matter created dense regions that drew in normal matter to form galaxies and stars.
A wrinkle in this model is that the combined gravitational pull of all ordinary and dark matter should be enough to slow the rate of universal expansion. Yet, that is not what we observe: 1998 measurements of distant supernovae showed that the expansion rate is actually increasing.
Astronomers didn’t abandon their model but instead introduced dark energy as a possible explanation. Dark energy is a theoretical form of energy that counteracts gravity, repelling objects rather than pulling them together. Although it has never been directly detected, it appears to make up about 70% of the universe and governs the rate of expansion.
Covering the Sky
The South Pole Telescope, a collaboration among more than a dozen institutions, features 16,000 detectors housed within a 10-meter telescope located at Antarctica’s Amundsen-Scott South Pole Station.
For more than two years, an international team has been gathering data with the instrument, covering 1/25th of the sky. The data focused on temperature measurements and polarization patterns created by matter distributed in the cosmic background radiation. These measurements were combined with 3D galaxy distribution data collected by the Dark Energy Spectroscopic Instrument (DESI).
Analysis of DESI’s earlier work, now supported by the South Pole observations, suggests that dark energy is growing weaker over time, and that universal expansion could eventually halt. Additionally, the South Pole data provides tighter constraints on existing models, refining the sound horizon scale to approximately 13.8 billion years ago. This is the first significant improvement in cosmic microwave background radiation measurements since the Planck satellite dataset set the standard a decade ago.
Taken together, the DESI and South Pole data reduce confidence in the idea of a cosmological constant, suggesting that dark energy evolves over time. Various supernovae observations further support this possibility.
Remodeling the Cosmos for Dark Energy
When Einstein first developed his theory of relativity in the early 1900s, a static universe was assumed. To uphold this assumption in the face of gravitational collapse, Einstein proposed a cosmological constant, though he later retracted the idea when Edwin Hubble discovered the universe was expanding in 1929. Decades later, the cosmological constant returned as an explanation for the accelerating expansion. That idea held until these recent observations.
While more data is needed for a firm conclusion, evidence of weakening dark energy would represent a major shift in our understanding of the universe. Discarding the cosmological constant leaves two paths: reconciling the current model to time-evolving dark energy, or rethinking the general theory of relativity itself.
Researchers face a high bar to make such a radical claim: they would need to achieve a confidence level with less than a 1 in 3.5 million chance of being false—the “gold standard” for scientific acceptance. Scientists are working toward that goal, with DESI-2 already in the planning stages, possibly to be followed by an even larger spectroscopy project, Spec-S5. The South Pole Telescope itself is scheduled for a receiver upgrade in 2028.
Meanwhile, the Simons Observatory will begin surveying later this year, and the CMB-S4 experiment is targeting a launch in the 2030s. Together, these projects aim to provide the data needed to determine whether the universe is indeed beginning to slow down, and what that means more broadly.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.