Our universe could have a twin where time moves backwards, scientists say

Thanks to Marvel, many of us are now familiar with the concept of multiverse theory – the idea that our universe is just one of countless parallel worlds, all existing simultaneously but impossible for us to ever see or reach.

Well, now we’ve wrapped our heads around that (sort of), things are about to get more confusing. Enter the anti-universe.

That’s right, the anti-universe – a mirror image of our universe running backwards in time from the point of the Big Bang.

That isn’t to say in another world your tomorrow has already happened, today is today and yesterday will be tomorrow.

But the idea that there is another version of our universe on the other side of the Big Bang helps explain a lot of issues thrown up by current thinking about that single moment of creation.

At present, the most widely-accepted theory is known as cosmological inflation.

Yes, inflation, we’ve all had enough of it.

But this was good inflation. It suggests that, a fraction of a second after the Big Bang, the universe expanded exponentially fast. We know the universe is still expanding (what it is expanding into is a question for another day) but at 13.7billion years of age, it’s now moving at a much more sedate pace.

However, this theory does have a few holes in it, including that most mysterious of particles, dark matter – which the anti-universe could explain.

Speaking to New Scientist earlier this year, one of the brains behind the theory, physicist Dr Neil Turok, outlined how an anti-universe would work.

‘We know that the early universe was dominated by hot radiation,’ said Dr Turok. ‘This means that, if you rewind the clock from there, the size of the universe shrinks to zero in a very simple way. Mathematically, you can follow a straight line which cuts through the big bang. This allows us to extrapolate backwards to another “mirror image” copy of our universe on the other side of the big bang.

‘The two sides of the universe grow steadily in opposite directions away from the big bang, governed by the known laws of gravity and particle physics. The extreme simplicity of the large-scale universe, which is very smooth and flat, is a direct result of the simplicity of these laws.’

On the dark matter front, it gets a bit more complicated.

The laws of physics have determined a symmetry in nature known as CPT – short for charge, parity and time. At its simplest, this means if you run a physical interaction in reverse, you get the same result – for instance, if you flip the charges of all particles involved, you’ll get the same interaction.

This means the curious particles known as left-handed neutrinos (so called because they spin left) require a right-handed twin – which the anti-universe theory can provide.

‘We realised our idea could solve the puzzle of dark matter, the mysterious substance that holds galaxies together, in terms of particles that we have not directly seen but already have strong evidence for,’ said Dr Turok. ‘These are called right-handed neutrinos. They have been invoked since the 1970s to explain the tiny masses of left-handed neutrinos, which have been observed. Whereas every other model of dark matter postulates a completely new particle, we don’t have to. 

‘That came as a huge surprise.’

Well, that clears everything up.

Except not quite. The anti-universe theory is far from widely accepted – although Dr Turok said it had been well received, and paper outlining the theory has recently been accepted for publication in the journal Annals of Physics.

‘We haven’t faced outright hostility,’ he said. ‘We’ve been pleasantly surprised. Among the community of people who are more open-minded to alternatives, many are very curious. There is, however, a very large body of scientists who have been focused on building inflationary models and fitting them to the data. They tend to be sceptical about an entirely new framework. Communities who have spent decades working on inflation are naturally reluctant to change.’

Dr Turok and his team will continue working on their theory to explain the beginnings of the universe, as will proponents of cosmological inflation. 

But for now, the idea of a mirror-image universe working in reverse is an exciting one.

‘It’s hard to convey how much more predictive the mirror universe idea has turned out to be than we ever expected,’ said Dr Turok. ‘It’s taken us by surprise, because with a few small tweaks to known physics we’re essentially rewriting the whole story of cosmology.

‘You have to pinch yourself!’ 

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