Have we just solved dark energy? Study says the Universe's expansion is actually slowing down

Dark energy is the name given to the mysterious force that seems to be accelerating the expansion of the Universe.

This 'anti-gravity' force has been largely accepted among the astronomical community for about three decades, as research has shown the Universe is expanding at an ever-quickening rate.

The discovery of dark energy, using exploding stars to accurately measure distances to faraway galaxies, won the Nobel Prize in Physics back in 2011.

But now a team of astronomers say their measurements show the expansion of the Universe is actually slowing down, not speeding up.

Have they finally solved the mystery of dark energy?

Measuring the expansion of the Universe

To measure distances in the Universe, astronomers use a type of exploding star known as a Type Ia supernova.

These stellar explosions always occur at the same luminosity, so astronomers can compare how bright they appear, with how bright they really are, and work out how far away they are.

So-called 'standard candles', Type Ia supernovae have been key to measuring the expansion of the Universe.

It's widely accepted that the expansion of the Universe is speeding up, but a new study suggests the rate of expansion is actually slowing down.

Solving a longheld tension

If the study's results are confirmed, it could revolutionise our understanding of dark energy, and help resolve the Hubble tension, which is the term used to describe how the rate of the expansion of the Universe changes, depending on how you measure it.

Lead researcher of the study, Professor Young-Wook Lee of Yonsei University in South Korea, says: "Our study shows that the Universe has already entered a phase of decelerated expansion at the present epoch and that dark energy evolves with time much more rapidly than previously thought.

"If these results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago."

Have we got standard candles all wrong?

The team of astronomers at Yonsei University put forward evidence that type Ia supernovae are strongly affected by the age of their progenitor stars.

They say supernovae from younger stellar populations appear fainter, while older ones appear brighter. Perhaps, they suggest, these standard candles are not as standard as once thought.

Using a sample of 300 galaxies, the team say their study has confirmed this at '99.999% confidence'.

That suggests the dimming of distant supernovae is caused not only by cosmological effects but also by stellar astrophysics effects.

Taking this into account, the supernova data no longer matches the so-called ΛCDM standard cosmological model with a cosmological constant, they say.

Instead, it aligns better with a newer model derived from baryonic acoustic oscillations – soundwaves from the early Universe – and the cosmic microwave background – heat left over from the Big Bang.

'Correcting' the data and using only results from the new model shows, they say, that dark energy weakens and evolves over time.

The team also say their data shows the standard ΛCDM model was 'ruled out with overwhelming significance'.

And they argue their results show the expansion of the Universe is not accelerating as once thought, but has actually begun to slow down.

"In the DESI project, the key results were obtained by combining uncorrected supernova data with baryonic acoustic oscillations measurements, leading to the conclusion that while the Universe will decelerate in the future, it is still accelerating at present," says Professor Lee.

"By contrast, our analysis – which applies the age-bias correction – shows that the Universe has already entered a decelerating phase today."

What's next?

The team say they're now carrying out more tests on supernovae in young galaxies of the same age at different distances across the Universe.

"Within the next five years, with the Vera C. Rubin Observatory discovering more than 20,000 new supernova host galaxies, precise age measurements will allow for a far more robust and definitive test of supernova cosmology," says research professor Chul Chung, a co-lead on the study along with PhD candidate Junhyuk Son.

Read the full paper via the Monthly Notices of the Royal Astronomical Society