Dark energy—often cited as one of the greatest mysteries in science—might not exist after all, say researchers investigating the expansion of the universe.
Their work, published in the journal Monthly Notices of the Royal Astronomical Society Letters, challenges the long-held belief that the universe expands uniformly in all directions. Historically, physicists have used the concept of dark energy to explain unknown forces accelerating cosmic expansion, but the theory has faced skepticism.
Now, a team of physicists and astronomers from the University of Canterbury in Christchurch, New Zealand, has presented evidence suggesting the universe’s expansion is more irregular or “lumpy” than previously thought. By analyzing supernovae light curves with refined techniques, they propose a new explanation—one that doesn’t require dark energy. Their findings support the “timescape” model of cosmic expansion, which attributes the perceived acceleration to differences in how time and distance are calibrated.
The model incorporates the effect of gravity on time. It suggests that time moves slower in galaxies due to gravity, meaning clocks in the Milky Way tick about 35% slower than those in vast cosmic voids. Over billions of years, this variation could result in these voids expanding more, creating the illusion of an accelerating universe when viewed from areas like our galaxy.
Lead researcher Professor David Wiltshire explains, “Our findings show that dark energy isn’t necessary to explain why the universe seems to expand at an accelerating rate. What we’ve identified as dark energy is actually variations in the kinetic energy of expansion, which isn’t uniform in a universe as lumpy as ours.”
The timescape model may also help address anomalies like the “Hubble tension”—a discrepancy between the universe’s current observed expansion rate and predictions based on the Big Bang’s afterglow, known as the Cosmic Microwave Background (CMB). Recent data from tools like the Dark Energy Spectroscopic Instrument (DESI) has also highlighted issues with the standard Lambda Cold Dark Matter (ΛCDM) model, which assumes dark energy remains constant over time.
The ΛCDM model relies on Friedmann’s equation, a 100-year-old theory assuming uniform cosmic expansion. However, today’s universe contains a complex cosmic web of clustered galaxies threading large voids, contradicting assumptions of homogeneity. The Christchurch team argues that a simpler expansion law can emerge from Einstein’s general relativity without needing Friedmann’s equation.
Testing the timescape model further will require substantial additional data. The researchers suggest that the European Space Agency’s Euclid satellite and NASA’s Nancy Grace Roman Space Telescope could provide the clarity needed. Euclid, launched in 2023, is equipped to distinguish the Friedmann equation from the timescape alternative by analyzing over 1,000 high-quality supernova observations.
While initial testing of the timescape model in 2017 showed only slightly better results than the ΛCDM model, the Christchurch team’s collaboration with the Pantheon+ team (who compiled a catalog of 1,535 supernovae) now offers much stronger evidence. This may pave the way to resolving key questions about cosmic expansion and anomalies like the Hubble tension.
Professor Wiltshire concludes, “With advancements in data analysis and tools like Euclid, we’re closer than ever to understanding the true nature of the universe’s expansion. By the end of the decade, we may finally solve one of the greatest mysteries in cosmology.”
Further research accompanied by increasingly precise observations will determine the validity of the timescape model and the role, if any, of dark energy in the grand story of cosmic evolution.
Reference: Antonia Seifert et al., Supernovae evidence for foundational change to cosmological models, Monthly Notices of the Royal Astronomical Society Letters (2024). DOI: 10.1093/mnrasl/slae112