Mathematicians say dark energy may be unnecessary

Mathematicians Challenge Dark Energy as Cause of Cosmic Expansion

For nearly three decades, dark energy has served as the leading explanation for why the universe’s expansion is speeding up. Now, two independent lines of research are pushing back against that consensus, arguing that the acceleration may be baked into the mathematics of the cosmos itself.

An Instability in the Equations

A team of mathematicians — Christopher Alexander, Blake Temple, and Zeke Vogler — published a paper in Proceedings of the Royal Society A arguing that the standard Lambda-cold dark matter model rests on an unstable foundation. Working with the Einstein-Euler equations reformulated in self-similar variables, they showed that the critical Friedmann spacetime, the idealized uniformly expanding universe at the heart of standard cosmology, behaves as an “unstable saddle rest point”.royalsocietypublishing

Any small perturbation, such as a region where matter density dips slightly below average, pushes the solution away from the Friedmann model and into an accelerating expansion at intermediate times — before eventually decaying back to the same leading-order Friedmann spacetime. The accelerations produced by this instability fall in the same range as those attributed to dark energy.arxiv

“What this shows is that the acceleration of the galaxies could have been predicted from the original theory of General Relativity without invoking the cosmological constant/dark energy at all,” Temple has said of the broader research program, which builds on earlier work he co-authored with the late Joel Smoller.phys

A Cosmic Uncertainty Principle

Separately, theoretical physicist Savvas Koushiappas of Brown University has proposed a different route to the same destination. In a paper posted to the arXiv preprint server, Koushiappas argues that the universe may obey its own version of Heisenberg’s uncertainty principle: its size and its rate of expansion cannot both be specified with perfect precision.universemagazine

When the universe’s scale factor and expansion rate are treated as quantum operators that do not commute, the resulting deformed Friedmann equation naturally produces late-time accelerated expansion without any dark energy term. The model also predicts that the effective dark-energy equation of state should deviate slightly from the value expected for a pure cosmological constant — a deviation that surveys like the Dark Energy Spectroscopic Instrument have already begun to hint at.space

Testing the Alternatives

Both proposals face scrutiny. The Alexander-Temple-Vogler work operates in a simplified, pressureless and radially symmetric setting, leaving open the question of whether the instability survives in a more realistic universe. Koushiappas’s framework, meanwhile, is a single-author theoretical paper that requires observational confirmation.space

The coming years of data from DESI, the Euclid mission, and the Vera C. Rubin Observatory should help determine whether dark energy is a fundamental feature of the cosmos or, as these researchers suggest, a mathematical artifact of assumptions that were never quite right.sciencedaily