For over a century, physicists assumed the Universe expanded uniformly, relying on dark energy to address gaps in understanding. However, the new research supports the "timescape" model of cosmic expansion, which attributes variations in expansion to gravitational effects on time calibration rather than a mysterious anti-gravity force.
Professor David Wiltshire, who led the study, explained: "Our findings show that we do not need dark energy to explain why the Universe appears to expand at an accelerating rate. Dark energy is a misidentification of variations in the kinetic energy of expansion, which is not uniform in a Universe as lumpy as the one we actually live in."
The timescape model considers the impact of gravitational time dilation, where clocks in low-gravity voids tick faster than those in denser galactic regions. This effect would make cosmic voids appear to expand more, creating an illusion of acceleration without invoking dark energy.
The findings, published in the Monthly Notices of the Royal Astronomical Society Letters, challenge the standard Lambda Cold Dark Matter (CDM) model, which relies on dark energy to explain supernovae measurements and the apparent accelerating expansion of the Universe.
Recent data has increasingly questioned the CDM model. For example, discrepancies between the early Universe's expansion rate (inferred from the Cosmic Microwave Background) and its current rate, known as "Hubble tension," remain unresolved. Furthermore, the Dark Energy Spectroscopic Instrument (DESI) has provided evidence suggesting that dark energy may evolve over time, contradicting CDM's assumption of constancy.
"The present Universe is a complex cosmic web of galaxy clusters, sheets, and filaments surrounding vast voids," added Professor Wiltshire. "A simple expansion law consistent with Einstein's general relativity does not have to obey Friedmann's equation."
Testing the timescape model further will require high-precision observations. The European Space Agency's Euclid satellite and NASA's Nancy Grace Roman Space Telescope are expected to gather critical data, with Euclid alone needing over 1,000 independent high-quality supernovae observations to distinguish timescape from CDM.
The research builds on a 2017 analysis where the timescape model performed marginally better than CDM. By collaborating with the Pantheon+ team to study 1,535 supernovae, the Christchurch researchers now claim "very strong evidence" for timescape, which could resolve anomalies like Hubble tension.
"With new data, the Universe's biggest mystery could be settled by the end of the decade," Professor Wiltshire said.
Research Report:Cosmological foundations revisited with Pantheon+
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University of Canterbury
Understanding Time and Space
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