Dark energy just got even weirder and why the Universe may end in a 'Big Crunch'

There is growing controversy over recent evidence suggesting that a mysterious force known as dark energy might be changing in a way that challenges our current understanding of time and space.

An analysis by a South Korean team has hinted that, rather than the Universe continuing to expand, galaxies could be pulled back together by gravity, ending in what astronomers call a "Big Crunch".

The scientists involved believe that they may be on the verge of one of the biggest discoveries in astronomy for a generation.

Other astronomers have questioned these findings, but these critics have not been able to completely dismiss the South Korean team's assertions.

What is dark energy?

Astronomers previously thought the Universe's expansion, which began with the Big Bang about 13.8 billion years ago, should gradually slow due to gravity.

Then evidence for dark energy was discovered in 1998 as a force accelerating the expansion of the Universe. Studies of very bright exploding stars called supernovas showed that rather than slowing down, distant galaxies were actually accelerating away from each other.

Some theories suggested that this ever‑accelerating Universe could first spread the stars so far apart that almost nothing would be visible in the night sky and, more shockingly, might eventually tear even atoms themselves apart in a "Big Rip".

The controversy began in March with unexpected results from an instrument on a telescope in the Arizona desert called the Dark Energy Spectroscopic Instrument (Desi).

Desi was built to discover more about dark energy. It tracked the acceleration of millions of galaxies in exquisite detail, but astronomers were not expecting the results it came up with.

The data hinted that acceleration of the galaxies had changed over time, something not in line with the standard picture, according to Prof Ofer Lahav of University College London, who is involved with the Desi project.

"Now with this changing dark energy going up and then down, again, we need a new mechanism. And this could be a shake up for the whole of physics," he says.

Then in November the Royal Astronomical Society (RAS) published research from a South Korean team that seems to back the view that the weirdness of dark energy is weirder still.

Prof Young Wook Lee of Yonsei University in Seoul and his team went back to the kind of supernova data that first revealed dark energy 27 years ago. Instead of treating these stellar explosions as having one standard brightness, they adjusted for the ages of the galaxies they came from and worked out how bright the supernovas really were.

This adjustment showed that not only had dark energy changed over time, but, shockingly, that the acceleration was slowing down.

"The fate of the Universe will change," Prof Lee tells BBC News starkly.

"If dark energy is not constant and it's getting weakened, this will change the whole paradigm of modern cosmology."

If, as Prof Lee's results suggest, the force that is pushing galaxies away from each other - dark energy - is weakening, then one possibility is that it becomes so weak that gravity begins to pull the galaxies back together.

"Rather than ending with a Big Rip, a Big Crunch is now a possibility.

"Which outcome wins, depends on the true nature of dark energy, for which we still do not know the answer," says Prof Lee.

Prof Lee's work has been checked by fellow experts and published in a respected journal of the RAS. But his claims have not gone down at all well with many senior astronomers working in the field, such as Prof George Efstathiou of the Institute of Astronomy at Cambridge University.

"I think that this is just reflecting the messy details of supernovas," he says. "The correlation with age is not very tight, so I think it is dangerous to apply a 'correction'. It looks weak to me."

The mainstream view is that the Universe is still accelerating with almost unchanging dark energy.

But Prof Lee pushes back strongly on such criticisms.

"Our data is based on 300 galaxies. The statistical significance is roughly one-in-a-trillion chance of being a fluke. So, I strongly feel that already our research is very, very significant."

Since the South Korean results, two teams have reassessed the brightness of some of the supernovas. They looked at an earlier study that fed into the Desi results from March which began the whole hoo-hah. They did so to double-check the March results because the claim that dark energy was changing was so controversial.

Both teams have pulled back slightly from the original hints, but the hints have not gone away, even after extensive scrutiny.

Consequently there will continue to be passionate, sometimes contentious, debate over whether the cosmos is gently whispering to us about its true nature, or whether astronomers are chasing celestial ghosts.

Hundreds of scientific papers have been published on the subject and astronomers are split on what they think is the best explanation. This is no bad thing, according to Prof Robert Massey, who is RAS's Deputy Director.

"Who doesn't want to understand how the universe is going to end and how it began?" he says. "Human beings have always been interested in that, whether you take it from a religious perspective or a scientific one.

"Being able to think, okay, this is how things will end in many, many billions of years, wouldn't that be extraordinary?"