GoKawiilKatie Mack, Hawking Chair in Cosmology and Science Communication at Perimeter Institute.
The seminar announcement called it “the week that broke cosmology.”
In March of 2025, a paper from the Dark Energy Spectroscopic Instrument (DESI) rocked the cosmology world by presenting tantalizing evidence for a deep, fundamental change to our understanding of the universe.
Since the late 1990s, cosmologists have known that the universe is not only expanding, but speeding up. The discovery of expansion was first made way back in the 1920s, as astronomers saw that more distant galaxies seemed to be moving away from us more quickly – the exact behavior you would expect if the cosmos were getting bigger in a uniform way everywhere. The assumption at the time was that the big bang set off the expansion, and now gravity should be causing that expansion to slow down. When two groups of observers showed in 1998 that the expansion was, in fact, accelerating, it created something of a revolution in our understanding of the cosmos. Instead of a universe filled with just the usual matter and energy, both of which would slow expansion via gravity, there must be some extra component actively stretching the universe out. The name given to that new feature was dark energy, and despite it being completely unexpected, astronomers had a pretty good hypothesis for what it might be – one that could be traced all the way back to Albert Einstein.
Before astronomers had good enough observations to know for sure that other galaxies existed, it appeared that the universe must be eternal and unchanging. But that came with a problem: why hadn’t gravity pulled everything together into one big clump? Einstein, working out his equations of general relativity that would describe gravity’s influence on matter and space (and vice versa), proposed an explanation for what could be out there holding the stars apart. He added a new term to balance out the inward pull of gravity that he called the cosmological constant. This term described an inherent property of spacetime that imbued it with just a bit of a tendency to stretch, so that the space in between clumps of matter could resist their gravitational attraction. When 1920s-era observers eventually discovered that the universe was actually expanding, rather than remaining static, Einstein realized the term was unnecessary. The cosmos wasn’t collapsing on itself because it was still in the process of expanding from the big bang. He threw the term out as a mistake.
Flashing forward again to the discovery of acceleration in 1998, the cosmological constant was widely accepted as an obvious candidate for the mysterious dark energy. All the observations seemed to be consistent with the idea that the expansion started accelerating in the relatively recent cosmic past, because that’s when matter got diffuse enough that the cosmological constant started to win the gravitational tug-of-war. As the overall density of matter reduced with the increasing size of space, the cosmological constant continued to be a feature of all of space, so its density stayed the same. That meant it could keep stretching space while matter’s gravity became less and less important. Now, instead of just keeping galaxies from falling together, it was actively making the expansion speed up.
For nearly three decades, cosmologists have been testing this hypothesis from every angle. If dark energy is a cosmological constant, it shouldn’t get more or less powerful over time – its influence on the evolution of the cosmos should be determined completely by the overall density of matter and energy as the cosmos expands, via the Friedmann equations. In the face of the data, the cosmological constant hypothesis worked so well that it became a cornerstone of the concordance model of cosmology, commonly referred to in the field as ΛCDM. Here, Λ is the Greek letter (Lambda) used to label the cosmological constant in Einstein’s equations, and CDM stands for cold dark matter – the invisible matter that makes up the scaffolding upon which galaxies and the large-scale structure of the universe appear to be built.
In recent years, however, apparent disagreements between ΛCDM and new cosmological data have become impossible to ignore.
First came the Hubble tension. Different measurements of the current expansion rate of the universe – a parameter we call H0 – have been giving different results, for reasons that no one has yet been able to satisfactorily explain. Using data from the cosmic microwave background (CMB), the background light coming to us from the final stages of the big bang, combined with our cosmological equations, we can infer a higher present-day expansion rate than what we get looking at data from surveys of supernovae in the nearby universe. The jury is still out on how to resolve this discrepancy, but if it’s not due to some kind of previously unknown bias in the data, it’s a major challenge to the robustness of ΛCDM itself.
Then, there’s the neutrino mass problem. Ghostly neutrinos, which are tiny particles produced in stellar fusion and other high-energy astrophysical processes, contribute a minuscule amount to the total matter in the universe, but have measurable effects on the structure and distribution of galaxies across the cosmos. Yet as soon as we developed galaxy surveys sensitive enough to measure these effects – which should have been able to tell us something about neutrino masses – we started getting results that appeared completely nonsensical. Including neutrinos in the data analysis seemed to have the opposite effect that it should, causing cosmologists to refer to the result as “negative neutrino masses.” To be clear, as far as I’m aware, no one actually thinks the mass of a neutrino can be negative. Instead, the shorthand demonstrates that the analysis isn’t working, for one reason or another. Maybe there’s some new physics hiding in the data that mimics neutrinos misbehaving. One culprit could be a change in dark energy.
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