Something is pushing the universe apart faster and faster — and it may be changing

The problem

In 1998, two teams measuring distant supernovae found the universe's expansion is speeding up, driven by an unknown 'dark energy' making up roughly 70% of the cosmos. The simplest explanation — energy of empty space itself — fails spectacularly: quantum theory predicts a vacuum energy up to ~120 orders of magnitude larger than observed, often called the worst theoretical prediction in the history of physics. Now the plot has thickened: in 2025 the DESI survey, using nearly 15 million galaxies and quasars, reported that dark energy may be weakening over time, with a 2.8–4.2 sigma preference for evolution when combined with other datasets. That's below the 5-sigma discovery bar, and independent reanalyses dispute its robustness, making this one of the most contested live questions in cosmology.

Why it matters

Dark energy controls the fate of the universe — eternal expansion, a slowdown, or something stranger. If it truly evolves, the cosmological constant of Einstein's equations is wrong and a new field or new gravity is at work.

Progress so far

• 1967Yakov Zel'dovich shows naive vacuum-energy estimates overshoot cosmological observations by tens of orders of magnitude
• 1998two supernova teams independently discover the accelerating expansion
• 2011Nobel Prize in Physics to Perlmutter, Schmidt, and Riess for the discovery
• 2025DESI's three-year data strengthens hints of evolving dark energy: 2.8–4.2 sigma depending on dataset combination
• 2026independent reanalyses contest the robustness of the evolving-dark-energy signal; the question remains unsettled

References

1. Berkeley Lab — official DESI DR2 announcement with dataset size and significance range
2. Nobel Prize — 2011 prize for the 1998 discovery of accelerating expansion
3. Wikipedia — the vacuum-energy discrepancy and Zel'dovich's 1960s calculations