'If I'm wrong, I can retire': Why some physicists are now betting big that dark energy doesn't exist

 If you’ve ever read anything about cosmology, you’ll have come across an astonishing statistic: all the matter around us, everything we see and touch, every star and gas cloud and planet, makes up just five per cent of the energy in the Universe.

Of everything else, around 25 per cent is made up of dark matter, and the rest – a baffling 70 per cent of everything that exists – is dark energy.

Cosmologists say this must be the case because of the way the Universe is expanding. But it isn’t just that the Universe is expanding; it’s that the expansion is accelerating, and something must be driving that. We call that unknown force dark energy.

But there’s a niche area of cosmology research that disputes this explanation. It argues that things appear to be expanding faster due to a mistake in our understanding of gravity. What’s more, it also proposes that, maybe, dark energy doesn’t exist at all.

False assumptions about dark energy

“The assumptions that we make in General Relativity in cosmology, basically, are that the Universe is very close to being the same everywhere and the same in all directions,” says Hayley Macpherson, a General Relativity and cosmology researcher at the University of Chicago.

Or, as cosmologists put it, the assumption is that the Universe is homogeneous and isotropic.

That makes sense when we’re looking at the very early stages of the Universe, when it was small and dense, like a kind of primordial soup.

But as the Universe evolved and expanded, it became more like what we see today – having some regions that are full of stuff, like stars and galaxies, and other regions that are much more empty.

So, do these assumptions of homogeneity and isotropy still hold? Obviously not in a literal sense – the Universe clearly doesn’t look like soup – but are the assumptions good enough for cosmology? That’s the key question affecting calculations.

Some theorists, like Prof David Wiltshire at the University of Canterbury, argue that these assumptions are so far off base that they’re leading us to see the Universe in the wrong way.

He started off working as a theoretical physicist, he says, and when he began looking into cosmology he wasn’t particularly looking to criticise the Standard Model.

“It’s just that the more I got into it and started speaking with people who actually do the observations, the more I realised that we’re making huge simplifications,” he says.

“In looking for some average notion of isotropy and homogeneity, we’re really assuming the answer rather than trying to explain it.”

In his view, if you take Einstein’s work seriously, you need to think of the expansion as something that’s dependent on the ‘clumpiness’ of the Universe.

Gravity slows down time, so time passes differently in regions of the Universe with more matter than in parts with less matter. Different parts of the Universe will expand at different rates based on how much stuff is in them – that is, whether you’re looking at a bunch of galaxies or a void.

And so perhaps we don’t need the concept of dark energy at all. Perhaps the expansion of the Universe only looks like it’s becoming faster, because we’re looking at voids where the expansion appears faster, rather than areas with galaxies in them where the expansion appears slower.

Perhaps it’s all just the result of the counterintuitive effects of gravity on time.

“It really is saying there’s this effect in Einstein’s theory, which we’ve not thought of before,” Wiltshire says.