Is dark energy weakening? DESI’s results are ambiguous | by Ethan Siegel | Starts With A Bang! | Apr, 2025

DESI, by mapping galaxies, has claimed they see evidence for dark energy evolving by getting weaker. But that’s only one interpretation.

There’s an extremely powerful idea in science that we take for granted, but apply all the time. The idea is simply this: that if we know the laws and rules governing a physical system, and we also know what the initial conditions of that system are, then we can apply the known rules to those initial conditions and evolve our system forward in time, making exquisite predictions for that system’s properties at all times. We can even do this for the entire Universe, with initial conditions given by the inflationary hot Big Bang and the types of energy present within our Universe, and then evolve it forward to form atomic nuclei, neutral atoms, stars, galaxies, and the grand cosmic web, all as the Universe expands and cools.

Our standard picture for this scenario, which fell into place in the late 1990s and early 2000s, is simply known as ΛCDM today. The Λ is for dark energy, which is assumed to be Einstein’s cosmological constant (from General Relativity) in its simplest form and makes up 68% of the Universe’s total energy today. The CDM is for cold dark matter, which makes up the majority of the rest of the cosmic energy budget (27%), with the remaining 5% made up of normal atom-based matter, plus a little bit more in the forms of photons and neutrinos.

Although this picture is excellent, it isn’t perfect. The Hubble tension shows us how measuring the Universe in different ways leads to different values for the expansion rate. There’s a tension in how rapidly structure forms on specific scales: the Sigma-8 tension. And now, with the latest data release from the DESI collaboration, we have strong, but not overwhelming, evidence in favor of evolving dark energy. But despite some very intriguing claims, things aren’t necessarily pointing to a cosmic revolution. Here’s the science that everyone should understand.