COSMOLOGICAL inflation is the idea that, in its first moments, the universe underwent a sudden, extreme expansion. This is widely accepted because it explains why space-time is almost perfectly flat and why matter in the cosmos appears so smoothly distributed on the largest scales. Or does it? The trouble is that there are many versions of inflation, most of which wouldn’t lead to the universe we observe – and the need for such “fine-tuning” of the theory to match observations makes some physicists nervous.
Among them is , former director of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and now at the University of Edinburgh, UK. Turok, alongside Latham Boyle at the Perimeter Institute, has proposed an alternative to inflation that can explain the evolution of the early universe without fine-tuning. In 2018, by taking seriously one of the deepest symmetries of nature, they arrived at a mind-boggling hypothesis: a mirror universe stretching backwards in time from the big bang.
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An unobservable anti-cosmos is hard to swallow. It didn’t help that the observations of strange particles by the ANITA telescope in Antarctica, initially invoked as potential evidence for the idea, turned out to be a false alarm. But Turok and Boyle have developed their thinking. Now, following a flurry of papers, they argue that the mirror universe explains all the stuff that inflation can, but also several other mysteries, including that of dark matter and dark energy. They have even made testable predictions in an attempt to win over sceptics.
Thomas Lewton: Can you first explain how the idea of inflation became dogma?
Neil Turok: Inflation was based on an “aha!” moment around 1980. People were building grand unified models of particle physics, which encompass all the known fundamental forces and particles, except for gravity. To make the models work, they had to introduce fields with potential energy. When you couple these fields to gravity, their potential energy behaves a bit like an explosive: it causes the universe to blow up in size. The explosive expansion can turn a small, lumpy universe into a huge, smooth, flat universe like the one we see around us.
The second insight, which persuaded many people, is that inflationary expansion isn’t perfectly smooth and uniform. The field that drives inflation fluctuates quantum mechanically, so that inflation lasts longer in some places than in others. As a result, the early universe becomes slightly lumpy. Much later, the denser regions collapse to form galaxies while the less dense regions expand to form the voids between galaxies.
It seems to explain a lot about the universe we see today. What is the problem?
What makes some of us uneasy about inflation is that it is contrived. You must assume that inflationary potential energy was dominant in the early universe and strong enough to start the explosion. You must adjust the initial conditions and the potential energy to keep inflation going for a sufficiently long time. Then, you must adjust the parameters in the model to get the right level of lumpiness.
For example, the temperature of the radiation from the hot big bang, known as the cosmic microwave background, varies across the sky by only a few parts in a hundred thousand. Inflationary models don’t explain that small number: they are just adjusted to fit it. Because there are so few independent ways available to test inflation, and so much freedom to build and adjust inflationary models, inflation can seem more like a “just so” story than a compelling explanation.
The great theories of physics are quite different. James Clerk Maxwell’s theory of electromagnetism has very few adjustable parameters in its equations and makes a vast array of testable predictions. Likewise, Albert Einstein’s theory of gravity has essentially only one adjustable number, telling you how strongly gravity couples to matter. Yet it predicts a great diversity of phenomena, from black holes to gravitational waves, each of which has been verified by experiments.
How did those suspicions lead you to the idea of a mirror universe?
Our first step was a surprisingly trivial observation. We know that the early universe was dominated by hot radiation. 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.
Our universe and its mirror image are related by a symmetry of nature called CPT, or “charge-parity-time reversal”, symmetry. CPT symmetry is based on deep principles of quantum theory and general relativity that have been confirmed in many experiments. The “charge” bit means you take every particle in our universe and exchange it with an anti-particle. The “parity” bit means that you take a right-spinning particle in our universe and replace it with its left-spinning version. And the “time reversal” aspect means that you run time backwards in the mirror universe.
To be clear, our universe by itself does not seem to respect CPT symmetry. Time only runs forward, and there is more matter than antimatter. But the combination of our universe and the mirror universe no longer violates CPT symmetry. This was the driver behind .
If you were a resident on the other side, in the mirror universe, how would you know?
You wouldn’t. It’s impossible to determine by any local measurement which “side” you are on. We’re not postulating a pre-big bang universe, somehow different from our universe. Instead, the pre-bang and post-bang universes are mirror images of each other.
Are there mirror image copies of the sun, Earth and even us in the mirror universe?
Classically, the mirror universe is the exact mirror image of ours. Quantum mechanically, things are more subtle, because you have to take quantum uncertainty into account. When a quantum state is observed, there are many possible outcomes, each with its own probability of being measured. In the mirror universe picture, there are strong correlations between what would be observed on the two sides of the big bang, but the exact pattern of variations would not be identical. So, most likely there isn’t another Thomas Lewton or a żěè¶ĚĘÓƵ magazine on the other side of the big bang. Nor can we communicate with the other side because the time an observer would perceive only progresses forward, away from the big bang and we cannot alter our past.
What makes you think this mirror universe explains our universe better than inflation?
It wasn’t until last year that the mirror universe picture began to fall into place. We understood how the smoothness and flatness of the universe, on large scales, could be explained without any need for inflation.
We used a mathematical tool called gravitational entropy – originally devised by Stephen Hawking and others to count the number of ways a black hole could be made from quantum units of space-time. You can use the idea of entropy, a measure of disorder, to explain the most likely state of a physical system. For example, when you carefully count the number of ways that the air molecules in a room can be arranged, in the vast majority of cases the molecules are very evenly distributed around the room. The probability that they will pile up in a corner is tiny.
Using Hawking’s method we were able to calculate the number of possible cosmic histories for a mirror-symmetric universe filled with radiation, matter and dark energy. Dark energy is added to explain the accelerating expansion of the universe. We found that , and that a small amount of dark energy is favoured. Our universe is expected in the mirror universe theory. We no longer need inflation to understand the smoothness and flatness of the universe.
What is different in terms of the birth and evolution of the early universe in this picture compared with the big bang plus inflation?
According to inflation, the early universe explodes into an infinite number of wildly different universes, known as the inflationary multiverse. Observations show no evidence for this. Indeed, as we examine the universe at larger and larger scales, we find it becomes more and more simple.
The mirror universe picture is far more economical, predictable and uniform. 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.
You also say the mirror universe goes further, explaining things inflation can’t.
In 2018, we realised , the mysterious substance that holds galaxies together, in terms of particles that we have not directly seen but already have strong evidence for. 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.
You will release the final paper in your series on the mirror universe later this week, which you expect to cause a bit of a stir. Can you explain why?
Having explained dark matter and the flatness and smoothness of space-time, we faced a final, huge puzzle. Inflation causes tiny variations in the density of the early universe, known as primordial vacuum fluctuations, which become large-scale variations in the density of matter in the universe. These fluctuations lead to galaxy clusters and voids, and they are seen directly in the cosmic microwave background radiation. If inflation never happened, where did they come from?
Last year, we had this idea that it’s to do with strange hypothetical fields that don’t have any particles, called “dimension zero fields”. When you add these fields to the fields in the standard model of particle physics, they create fluctuations in the expansion of the universe of the right form to match the fluctuations that we see in the cosmic microwave background. In the mirror universe, the cosmic microwave background fluctuations are a direct image of the primordial vacuum fluctuations.
So, you don’t have to blow anything up?
Exactly. And here’s the really remarkable result. We can predict the strength of the fluctuations from our mirror universe theory. It turns out to agree with the very precise measurements made by the Planck satellite and other experiments, without the need for fine-tuning. Inflationary models have to fine-tune in order to match the same data.
There is more. Virtual particles pop in and out of existence in the vacuum as quantum theory allows them to borrow energy for a short amount of time. People have worried about this for a long time because if you add up the energy in all these virtual particles, it’s infinite. There are various mathematical tricks for ignoring the infinity, but this is most likely telling us that something is wrong. A surprising property of these dimension zero fields is that they can .
You have said it can solve another, closely related puzzle, too.
Yes. The quantum fields in the standard model of particle physics have important symmetries which we believe are fundamental to their mathematical consistency. But when we study quantum fields in a curved space-time, like a black hole or an expanding universe, some of the symmetries are spoiled by infinities – just like those in the vacuum energy. When we added just the right number of dimension zero fields to the standard model, all of these infinities cancelled. The infinite vacuum energy and the symmetry violations all disappear. This cancellation also requires that there are three and only three generations of elementary particles – including, for example, electrons, muons and taus – just as we see.
It’s hard to convey how much more predictive the mirror universe idea has turned out to be than we ever expected. You have to pinch yourself! It’s taken us by surprise, because with a few small tweaks to known physics we’re essentially rewriting the whole story of cosmology.
If it explains so much, why are many cosmologists hostile to the idea?
We haven’t faced outright hostility. 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.
Do you make solid predictions?
I take observations extremely seriously. Good science has to prove itself. I’ve opted for economical theories, which are predictive and can be ruled out. If they prove successful, rival theories will fall by the wayside. But we’re not yet at the point where our story is compelling and fully supported by observations.
What observations would persuade people?
Number one: show that the lightest neutrino is massless. If dark matter is composed of stable, right-handed neutrinos – as in our mirror universe picture – then this must also be true. Fortunately, within three to five years, large-scale galaxy surveys will make that measurement. If they find that it’s massless, then we’ll really be on a good road.
Number two: when you look at the fluctuations in the cosmic microwave background on the largest scales, those predicted by the mirror universe are slightly different to inflation. There are already hints that the inflationary models don’t quite fit with observations. It may be that our theory does better. But I am not sure when the observations will become accurate enough to allow a decisive comparison.
What are the chances we will actually resolve these questions around the big bang?
Einstein famously said that the Lord is subtle, but not malicious. In cosmology, nature has posed some profound puzzles: the big bang, the origin of matter, energy and time. Nature is subtle, but it’s also been extremely generous to us. From the same laws of physics that we learned in our backyard – on Earth and in the solar system – we predicted black holes and gravitational waves. We have no right to understand such remote phenomena. It’s kind of absurd. In order for us to understand the universe, we must hope that nature’s generosity continues. I work on the assumption that it will.
Thomas Lewton is a science journalist based in London, UK
