• DESI data suggests dark energy may be weakening over time, challenging the cosmological constant in the standard LCDM model.
• New studies reveal the universe's matter is less clumpy than predicted, hinting at dark energy influencing cosmic growth more than originally thought.
• The S8 tension persists with different methods of measuring matter density yielding conflicting results, prompting exploration of new physics beyond the standard model.

Recent astronomical findings are stirring debate in the cosmology community regarding the nature of dark energy and the distribution of matter in the universe. The Dark Energy Spectroscopic Instrument (DESI) has presented data suggesting that dark energy, the mysterious force driving the universe's accelerating expansion, might not be a constant as previously assumed. Furthermore, a combined analysis of data from the Atacama Cosmology Telescope (ACT) and DESI indicates that the universe's matter is less 'clumpy' than predicted by existing models. These developments challenge the standard cosmological model (LCDM) and prompt scientists to explore alternative theories involving dynamic dark energy, decaying dark matter, or modifications to our understanding of gravity.

• What: New research suggests that dark energy might not be constant and matter distribution is less clumpy than expected. The 'S8 tension' persists, causing disagreements among cosmologists.
• Who: Key individuals include Joshua Frieman, Daniel Green, Nathalie Palanque-Delabrouille, Paul Steinhardt, Joshua Kim, Mathew Madhavacheril, Surhud S. More. Organizations involved are DESI collaboration, ACT collaboration, University of Chicago, University of California, Lawrence Berkeley National Laboratory, Princeton University, University of Pennsylvania, University of Tokyo, and Inter-University Centre for Astronomy and Astrophysics in Pune.
• When: Findings were released in 2024 and March 2025, with ongoing data collection and analysis.
• Where: Data is collected from observatories like Kitt Peak National Observatory (DESI), Atacama Cosmology Telescope (ACT), and Subaru Telescope in Hawaii.

The emerging picture from these findings is one where the established cosmological model (LCDM) is facing increasing scrutiny. The DESI results, hinting at a dynamic dark energy, challenge the fundamental assumption of a cosmological constant. The ACT/DESI combined analysis reinforces this by suggesting a less clumpy universe than predicted. These findings necessitate a re-evaluation of the models used to describe the universe and its evolution. The 'S8 tension,' where different measurement methods yield conflicting results for matter density, adds another layer of complexity. This suggests that there might be unknown systematics or that the standard model requires modifications to account for the observed discrepancies.

We tend to stick with the simplest theory that works—until it doesn’t.

The tendency should be to say, ‘Hey, why don’t we explore all the possible interpretations?’ DESI didn’t do that many analyses.

Our work cross-correlated two types of datasets from complementary, but very distinct, surveys, and what we found was that for the most part, the story of structure formation is remarkably consistent with the predictions from Einstein’s gravity. We did see a hint of a small discrepancy in the amount of expected clumpiness in recent epochs, around four billion years ago, which could be interesting to pursue.

One of the main difficulties in using deep surveys such as Subaru HSC is our lack of understanding of how fast the galaxies in these surveys are actually receding from us, quantified by the redshift [increase in wavelength] of certain lines in their spectrum. As the millions of galaxies used in these analyses are faint, one cannot analyse the spectrum of light of these galaxies to determine this redshift. This constitutes one of the major uncertainties that still remains unresolved before we start entirely doubting the standard theory of cosmology.

The latest findings from DESI and ACT/DESI collaborations are prompting a re-evaluation of the cosmological constant within the standard cosmological model (ΛCDM). While ΛCDM has been a useful framework, emerging tensions, particularly the Hubble tension and the S8 tension, alongside anomalies at both large and small scales, suggest a more complex picture. DESI's precise measurements of baryon acoustic oscillations, combined with CMB data from Planck and ACT, now indicate a more than 3-sigma conflict with ΛCDM, favoring evolving dark energy. These results align with the possibility that the influence of dark energy may be weakening over time. Future surveys, including those by the Simons Observatory, which will probe the CMB with unprecedented precision to study the primordial universe, neutrino physics, and dark energy, and the Rubin Observatory's LSST, designed to make high-accuracy measurements of cosmological parameters, hold the potential to further illuminate the nature of dark energy and the universe's expansion history, possibly requiring a paradigm shift in our understanding of late-time cosmology and potentially revealing the need to go beyond the standard effective field theory treatment.