Dark energy might not exist, but the mystery remains

Dark energy, long thought to drive the Universe's expansion, may not exist at all, according to new research challenging this cosmic mystery.

The simplest answer to "What is dark energy?" is, "We don’t know yet." For a century, physicists have assumed that the universe expands evenly in all directions, using dark energy to explain unknown forces, though the theory has always been debated.

Recently, a team of physicists and astronomers at the University of Canterbury in New Zealand has challenged the status quo, suggesting that we may not need "dark energy" to explain the Universe's expansion after all.

The Canterbury astronomers used improved analysis of supernova light curves to show that the Universe is expanding in a more varied, "lumpier" way.

“With new data, the Universe's biggest mystery could be settled by the end of the decade," said a study lead Professor David Wiltshire.

What is dark energy?

The universe is constantly expanding, and the rate of expansion is accelerating. This is one of astronomy’s great mysteries that scientists have strived to solve.

Dark energy is the energy driving the accelerating expansion of the universe. It acts like an "anti-gravity" force, stretching spacetime and pushing cosmic objects apart faster and faster, unlike gravity, which pulls them together.

Dark energy is believed to comprise approximately 68% to 72% of the universe's total matter and energy, making it the dominant component over both dark matter and ordinary matter in the cosmic energy budget.

At least, that’s what scientists agreed on. The standard Lambda Cold Dark Matter (ΛCDM) model of the Universe required the concept of dark energy to explain the observed acceleration in the rate at which the cosmos is expanding.

Expansion is not getting faster, time slows

The new evidence supports the "timescape" model, which explains cosmic expansion without dark energy, attributing the differences in light stretching to how we measure time and distance rather than an accelerating Universe.

The model takes into account how gravity slows time, causing clocks in galaxies like the Milky Way to run 35% slower than those in cosmic voids. This time difference results in more space expanding in the voids, which creates the illusion of faster expansion as these vast empty regions come to dominate the Universe.

"Our findings show that we do not need dark energy to explain why the Universe appears to expand at an accelerating rate,” said Wiltshire.

"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.

Scientists have based their 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.

The researchers say the European Space Agency's Euclid satellite, launched in July 2023, can test and compare the Friedmann equation – which models the expansion of the Universe over time – with the timescape model.

Solving the mystery equation

They believe that their findings could resolve the anomaly known as Hubble tension and others related to the expansion of the Universe.

"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?” said Wiltshire.

"A simple expansion law consistent with Einstein's general relativity does not have to obey Friedmann's equation," he concludes.

The study has been published in the journal Monthly Notices of the Royal Astronomical Society Letters.