Is the universe expanding or is our understanding of what dark energy is – or isn’t – only making us think it is?
Despite centuries of looking up into the sky at night, and the advancement of machine learning and astronomy, there is still much about the universe we have yet to figure out.
One such mystery is dark energy, a placeholder for a form of energy that might make up 70pc of the universe. While we know almost nothing about it, it could now be challenging the firmly held belief that the universe is expanding.
Current models of the universe use dark energy to explain the observed acceleration in the rate at which it is expanding, based on the measurements of the distances to supernova explosions in distant galaxies.
If the universe’s expansion was not accelerating, researchers argue, the supernovae wouldn’t appear to be farther away than they should be, based on a 100-year old cosmic expansion law called Friedmann’s equation.
This assumes an expansion with no complicating structure, yet astronomers observe a complex cosmic web of galaxy clusters in sheets and filaments that surround and thread vast empty voids.
A team of researchers from the University of Canterbury in New Zealand, led by Prof David Wiltshire, has suggested that the accelerating expansion of the universe may not be real – it might just seem like it is.
Missing an essential point
The team came to this conclusion after it calculated that the fit of a type Ia supernovae to a model universe with no dark energy was slightly better than the fit to the standard dark energy model.
“The past debate missed an essential point: if dark energy does not exist, then a likely alternative is that the average expansion law does not follow Friedmann’s equation,” Wiltshire said.
For more than a year now, debate has pitted two arguments against one another: the Lambda cold dark energy theory and the empty universe theory.
But still, both sides follow Friedmann’s equation, so Wiltshire and his team introduced a third model known as ‘timescape cosmology’, which features a slight variation on the ‘no dark energy’ theory.
However, the statistical evidence is not yet strong enough to rule definitively in favour of one model or the other, but future missions – such as the European Space Agency’s Euclid satellite – will have the power to distinguish between the standard cosmology and other models, and help scientists to decide whether dark energy is real or not.
The team’s research has been published in the journal Monthly Notices of the Royal Astronomical Society.
Updated, 1.19pm, 14 September 2017: This article has been updated to clarify the subject as dark energy, not dark matter.