Is Dark Energy Getting Stronger?

Dark energy may not be constant—and a new quasar-based distance test hints it could be getting stronger over cosmic time. That possibility matters because the standard Lambda-CDM model, which treats dark energy as a fixed “cosmological constant” (Lambda) alongside cold dark matter, underpins how astronomers predict the universe’s expansion from the earliest moments to today. If dark energy instead evolves, the future fate of the cosmos could look radically different, potentially even involving a “Big Rip,” where accelerating expansion tears apart structures down to subatomic scales.

The tension starts with a mismatch between two pillars of observational cosmology. Measurements of the cosmic microwave background (CMB) provide starting conditions—how much dark energy and dark matter the universe had when the CMB formed. Using Lambda-CDM, those conditions imply a specific expansion rate for later epochs. But distance measurements based on Type Ia supernovae, which were used to discover accelerating expansion in the late 1990s, suggest the universe is expanding faster than the CMB-calibrated model predicts. The discrepancy could come from systematics in supernova distances, uncertainties in CMB-based calculations, or a failure of Lambda-CDM itself.

Risaliti and Lusso’s new study in Nature Astronomy takes aim at a key limitation of supernova cosmology: supernovae are not bright enough to map the earliest slice of cosmic history. The earliest ~25% of the universe’s expansion timeline is largely missed, and the first half of cosmic time has too few well-measured supernovae. To probe that missing era, the researchers turn to quasars—extremely luminous objects powered by matter accreting onto supermassive black holes. Quasars are visible across nearly the entire age of the universe, but they are messy “standard candles” because their brightness varies widely.

The workaround uses a physical correlation between ultraviolet (UV) emission from the accretion disk and X-rays produced in a hot “corona” above it. UV photons gain energy through Compton up-scattering when they interact with energetic electrons, producing X-rays. Crucially, X-ray output does not scale perfectly one-to-one with UV brightness; there is a diminishing return. That non-linear relationship means the UV-to-X-ray ratio can act as a distance indicator: measure the ratio, infer the quasar’s intrinsic UV brightness, compare it to the observed UV brightness, and derive distance.

Using roughly 1,600 quasars with UV and X-ray data (including observations from major surveys and additional XMM-Newton measurements), the team constructs a Hubble diagram: inferred distance versus redshift. For large redshifts, the quasars sit systematically below the Lambda-CDM expectation line, implying their light is more redshifted—more stretched by expansion—than constant-dark-energy cosmology predicts. A model in which dark energy strengthens with time fits the trend.

Even so, the result is not a verdict on the end of the universe. A Big Rip, if it occurs, would still be tens of billions of years away, and there are alternative explanations that could reconcile the data without requiring steadily increasing dark energy. The leading possibilities remain observational or modeling systematics, limited numbers of high-quality distant X-ray measurements, and statistical scatter in the UV-to-X-ray relation. More X-ray observations and further tests of the method are expected to determine whether this is a genuine crack in Lambda-CDM—or a clue that the measurements need tightening.