One of the biggest mysteries in science – dark energy – doesn't actually exist, according to researchers looking to solve the riddle of how the Universe is expanding.
For the past 100 years, physicists have generally assumed that the cosmos is growing equally in all directions. They employed the concept of dark energy as a placeholder to explain unknown physics they couldn't understand, but the contentious theory has always had its problems.
Now a team of physicists and astronomers at the University of Canterbury in Christchurch, New Zealand are challenging the status quo, using improved analysis of supernovae light curves to show that the Universe is expanding in a more varied, "lumpier" way.
The new evidence supports the "timescape" model of cosmic expansion, which doesn't have a need for dark energy because the differences in stretching light aren't the result of an accelerating Universe but instead a consequence of how we calibrate time and distance.
It takes into account that gravity slows time, so an ideal clock in empty space ticks faster than inside a galaxy.
The model suggests that a clock in the Milky Way would be about 35 per cent slower than the same one at an average position in large cosmic voids, meaning billions more years would have passed in voids. This would in turn allow more expansion of space, making it seem like the expansion is getting faster when such vast empty voids grow to dominate the Universe.
Professor David Wiltshire, who led the study, said: "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."
He added: "The research provides compelling evidence that may resolve some of the key questions around the quirks of our expanding cosmos.
"With new data, the Universe's biggest mystery could be settled by the end of the decade."
The new analysis has been published in the journal Monthly Notices of the Royal Astronomical Society Letters .
Dark energy is commonly thought to be a weak anti-gravity force which acts independently of matter and makes up around two thirds of the mass-energy density of the Universe.
The standard Lambda Cold Dark Matter (ΛCDM) model of the Universe requires dark energy to explain the observed acceleration in the rate at which the cosmos is expanding.
Scientists base this conclusion on measurements of the distances to supernova explosions in distant galaxies, which appear to be farther away than they should be if the Universe's expansion were not accelerating.
However, the present expansion rate of the Universe is increasingly being challenged by new observations.
Firstly, evidence from the afterglow of the Big Bang – known as the Cosmic Microwave Background (CMB) – shows the expansion of the early Universe is at odds with current expansion, an anomaly known as the "Hubble tension".
In addition, recent analysis of new high precision data by the Dark Energy Spectroscopic Instrument (DESI) has found that the ΛCDM model does not fit as well as models in which dark energy is "evolving" over time, rather than remaining constant.
Both the Hubble tension and the surprises revealed by DESI are difficult to resolve in models which use a simplified 100-year-old cosmic expansion law – Friedmann's equation.
This assumes that, on average, the Universe expands uniformly – as if all cosmic structures could be put through a blender to make a featureless soup, with no complicating structure. However, the present Universe actually contains a complex cosmic web of galaxy clusters in sheets and filaments that surround and thread vast empty voids.
Professor Wiltshire added: "We now have so much data that in the 21st century we can finally answer the question – how and why does a simple average expansion law emerge from complexity?
"A simple expansion law consistent with Einstein's general relativity does not have to obey Friedmann's equation."
The researchers say that the European Space Agency's Euclid satellite, which was launched in July 2023, has the power to test and distinguish the Friedmann equation from the timescape alternative. However, this will require at least 1,000 independent high quality supernovae observations.
When the proposed timescape model was last tested in 2017 the analysis suggested it was only a slightly better fit than the ΛCDM as an explanation for cosmic expansion, so the Christchurch team worked closely with the Pantheon+ collaboration team who had painstakingly produced a catalogue of 1,535 distinct supernovae.
They say the new data now provides "very strong evidence" for timescape. It may also point to a compelling resolution of the Hubble tension and other anomalies related to the expansion of the Universe.
Further observations from Euclid and the Nancy Grace Roman Space Telescope are needed to bolster support for the timescape model, the researchers say, with the race now on to use this wealth of new data to reveal the true nature of cosmic expansion and dark energy.
