In their analysis, published in the Journal of Cosmology and Astroparticle Physics<\/a> and on arXiv<\/a>, researchers found the best-fitting model to the data involves a negative cosmological constant \u2014 a fundamental shift from what has long been considered an unchanging feature of our universe.<\/p>\n This constant, denoted by the Greek letter \u039b, was first introduced by Albert Einstein<\/a>. For over a century, \u039b was assumed to be positive, a sign of continuous growth. But the new analysis shows that \u039b may be negative, implying that gravity will eventually win and reverse expansion, forcing the universe to collapse in on itself.<\/p>\n \u201cIt\u2019s not just about how the universe began,\u201d says Dr. Yucheng Qiu, co-author of the study. \u201cIt\u2019s about when \u2014 and how \u2014 it ends.\u201d<\/p>\n Ultralight Particles Could Trigger the End<\/p>\n Central to this reversal is a theoretical framework involving ultralight axions \u2014 elusive particles proposed as candidates for both dark matter and dark energy. In this model, axions evolve slowly over time and interact subtly with the cosmological constant. The result is a dark energy force that does not remain constant, but gradually shifts toward a state that no longer supports expansion.<\/p>\n When that shift reaches a threshold \u2014 projected to occur in about 10 billion years \u2014 the expansion of the universe will cease. At that point, the gravitational pull of all matter will slowly reverse the cosmic stretch. The contraction will accelerate over the next 9 billion years, ending in a singularity: a final collapse known as the Big Crunch.<\/p>\n This contradicts the widely held Big Freeze theory<\/a>, which assumes expansion will continue indefinitely until the universe becomes cold and empty. Instead, in the model presented by Tye and colleagues, everything ends not in silence, but in compression.<\/p>\n![\"DESI](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2025\/08\/TADtLqKE7u8wsYgAXx6toh.jpg\"\/)
DESI maps distant objects to study dark energy. The instrument is installed on the Mayall Telescope in Arizona, shown here beneath star trails.\u00a0Credits: KPNO\/NOIRLab\/NSF\/AURA\/B. Tafreshi<\/p>\n
![\"ASYBC0121_03\"](\"https:\/\/www.astronomy.com\/wp-content\/uploads\/2023\/02\/ASYBC0121_03.jpg\"\/)
<\/a>If the universe continues to expand indefinitely, as expected, we\u2019ll face a Big Freeze. But if dark energy pushes the expansion rate to near infinity, we\u2019ll have a Big Rip that tears everything, even atoms, apart.\u00a0Credit: Astronomy: Roen Kelly<\/p>\n