Dark energy ‘doesn’t exist’ so can’t be pushing ‘lumpy’ Universe apart

For the past hundred years, physicists have generally believed that the universe is growing equally in all directions. They used the concept of dark energy as a placeholder to explain unknown physics they couldn’t understand, but this controversial 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 supernova light curves to show that the universe is expanding in a more diverse, “lumpy” way.

The new evidence supports a “timescape” model of the expansion of the universe, which does not require dark energy because the difference in stretching light is not the result of the universe’s acceleration, but the result of our calibration of time and distance.

It takes into account that gravity slows down time, so an ideal clock in a vacuum would run faster than a clock within a galaxy.

The model suggests that clocks in the Milky Way are about 35% slower than clocks at the average location in large cosmic voids, meaning that time in the void would have been billions of years longer. This in turn causes space to expand further, and as such a massive void comes to dominate the universe, it appears to be expanding faster and faster.

Professor David Wiltshire, who led the research, said: “Our results show that we don’t need dark energy to explain why the expansion of the universe appears to be accelerating.

“Dark energy is a misunderstanding of the change in expansion kinetic energy. In the universe we actually live in, dark energy is not evenly distributed.”

He added: “This study provides compelling evidence that may resolve some key questions about the quirks of our expanding universe.

“With new data, the universe’s greatest mysteries may be answered by the end of the century.”

The new analysis has been published in the journal Newsletter of the Royal Astronomical Society monthly notices.

Dark energy is generally thought of as a weak form of antigravity that acts independently of matter and accounts for about 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 expansion rate of the universe.

Scientists reached this conclusion based on measurements of the distances of supernova explosions in distant galaxies, which appear to be further apart than they would be if the expansion of the universe were not accelerating.

However, the current rate of expansion of the universe is increasingly challenged by new observations.

First, evidence from the afterglow of the Big Bang—known as the cosmic microwave background (CMB)—suggests that the expansion of the early universe was inconsistent with the current expansion, an anomaly known as the Hubble tension.

In addition, recent analysis of new high-precision data by the Dark Energy Spectroradiometer (DESI) found that the ΛCDM model is not suitable for models in which dark energy “evolves” over time rather than remaining constant.

Both the Hubble tension and the surprises revealed by DESI are difficult to resolve in models that use a 100-year-old simplified version of the expansion law of the universe (the Friedmann equation).

This assumes that, on average, the universe is expanding uniformly—as if all cosmic structures could be run through a blender into a featureless soup, devoid of complex structures. However, the current universe actually contains a complex cosmic network of galaxy clusters, in sheets and filaments, surrounding vast voids of empty space.

Professor Wiltshire added: “We now have so much data that in the 21st century we can finally answer the question – how and why do simple mean expansion laws emerge from complexity?

“A simple law of expansion consistent with Einstein’s general theory of relativity does not have to obey the Friedmann equations.”

Researchers say the European Space Agency’s Euclid satellite, launching in July 2023, has the ability to test and distinguish the Friedmann equation from timescape alternatives. However, this will require at least 1,000 independent high-quality supernova observations.

When the proposed time-series model was last tested in 2017, analysis showed it was only better than ΛCDM in explaining the expansion of the universe, so the Christchurch team worked closely with the Pantheon+ collaboration, which painstakingly produced a model containing 1,535 different time series directory.

They said the new data now provide “very strong evidence” for the time landscape. It may also indicate that the Hubble tension and other anomalies related to the expansion of the universe have been convincingly resolved.

The researchers say further observations from the Euclid and Nancy Grace Roman Space Telescopes are needed to support the time landscape model, and the race is now on to use this wealth of new data to reveal the true nature of cosmic expansion and dark energy.