Back in 2011, three astronomers were awarded the Nobel Prize in Physics for their discovery that the Universe wasn’t just expanding - it was expanding at an accelerating rate.
The discovery led to the widespread acceptance of the idea that our Universe is dominated by a mysterious force called dark energy, and altered the standard model of cosmology forever. But now physicists are questioning the conclusion, and they have a much larger dataset to back them up.
For a bit of background on the 2011 Nobel Prize in Physics, it was shared between cosmologists Saul Perlmutter from the University of California, Berkeley; Adam Riess from Johns Hopkins University; and Brian Schmidt from the Australian National University.
During the 1990s, these three scientists were part of competing teams that were measuring distant Type 1a supernovae - the violent end of a type of star called a white dwarf.
White dwarf stars are made from one of the densest forms of matter in the known Universe - surpassed only by neutron stars and black holes.
While a typical white dwarf will only be slightly larger than Earth, it will have around the same amount of mass as our Sun. To put that into perspective, you could fit roughly 1,300,000 Earths inside the Sun.
Now imagine that incredibly dense, dead star collapsing under the weight of its own gravity. We’re talking about a luminosity level that’s about 5 billion times brighter than the Sun.
Because each Type 1a supernova explodes with roughly the same brightness, the amount of light they give off can be used as an indication of their distance from Earth - and slight shifts in colour can also be used to figure out how fast they’re moving.
When Perlmutter, Riess, and Schmidt measured all the data for known Type 1a supernovae, recorded by the Hubble space telescope and a number of large ground-based telescopes, they found something incredibly strange.
As the Royal Swedish Academy explained on the morning of the Nobel Prize announcement in Stockholm:
"In a Universe which is dominated by matter, one would expect gravity eventually should make the expansion slow down. Imagine then the utter astonishment when two groups of scientists ... discovered that the expansion was not slowing down, it was actually accelerating.
By comparing the brightness of distant, far-away supernovae with the brightness of nearby supernovae, the scientists discovered that the far-away supernovae were about 25 percent too faint. They were too far away. The Universe was accelerating. And so this discovery is fundamental and a milestone for cosmology. And a challenge for generations of scientists to come."
The find was backed up by data collected separately on things like clustering galaxies and the cosmic microwave background - the faint afterglow of the Big Bang.
And earlier this year, NASA and ESA scientists found that the Universe could be expanding around 8 percent faster than originally thought.
By all accounts, the discovery was a solid one (Nobel Prize solid) but it posed a very difficult question - if the collective gravity from all the matter expelled into the Universe by the Big Bang has been slowing everything down, how can it be accelerating?
"There’s something pervading the Universe that physically spreads space apart faster than gravity can pull things together. The effect is small - it’s only noticeable when you look at far-away galaxies - but it’s there. It’s become known as dark energy - 'dark', because no one knows what it is."
Since scientists first proposed dark energy, no one's gotten any closer to figuring out what it could actually be.
But now an international team of physicists have questioned the acceration of the Universe's expansion, and they've got a much bigger database of Type 1a supernovae to back them up.
By applying a different analytical model to the 740 Type Ia supernovae that have been identified so far, the team says they've been able to account for the subtle differences between them like never before.
They say the statistical techniques used by the original team were too simplistic, and were based on a model devised in the 1930s, which can't reliability be applied to the growing supernova dataset.
They also mention that the cosmic microwave background isn't directly affected by dark matter, so only serves as an "indirect" type of evidence.
"We analysed the latest catalogue of 740 Type Ia supernovae - over 10 times bigger than the original samples on which the discovery claim was based - and found that the evidence for accelerated expansion is, at most, what physicists call '3 sigma'," reports lead researcher, Subir Sarkar, from the University of Oxford.
"This is far short of the '5 sigma' standard required to claim a discovery of fundamental significance."
Instead of finding evidence to support the accelerated expansion of the Universe, Sarkar and his team say it looks like the Universe is expanding at a constant rate. If that's truly the case, it means we don't need dark energy to explain it.
"A more sophisticated theoretical framework accounting for the observation that the Universe is not exactly homogeneous, and that its matter content may not behave as an ideal gas - two key assumptions of standard cosmology - may well be able to account for all observations without requiring dark energy," he says.
Now, to be clear, this is just one study, and it's a big, extremely controversial claim that a Nobel Prize-winning discovery is fundamentally wrong. (Because I don't have to tell you that Nobel Prizes aren't given out lightly.)
But replication of results is everything in science, and if we have a larger dataset to go on than we did five years ago, we should use it to support - or correct - previous discoveries.
The question now is whether Sarkar's team applied their new statistical model to the data in a way that best reflects the science, and it will likely spur on a whole lot of physicists to figure out which is right - accelerating Universe, or constant Universe.
"Naturally, a lot of work will be necessary to convince the physics community of this, but our work serves to demonstrate that a key pillar of the standard cosmological model is rather shaky," says Sarkar.
"Hopefully, this will motivate better analyses of cosmological data, as well as inspiring theorists to investigate more nuanced cosmological models."
The research has been published in Scientific Reports.