**What drew you to theoretical physics and cosmology in particular?**

Cosmology has all the most fascinating questions that I’d been daydreaming about since I was a child: where did the universe come from and what was there before it existed? As for working in theoretical rather than experimental physics, I’m really not a practical person – if put in a lab, I’d probably accidentally set it on fire.

**You write that the start of this century was a good time to enter the field of cosmology. Why?**

Because the knowledge had advanced so much, and for the first time we could actually ask those big questions that fascinated me as a child. There were two major findings that really propelled that curiosity. In 1998, a group of supernova astronomers discovered that there is dark energy in the universe, and in fact it’s the dominant component; that is the same type of energy as what existed at the time of the big bang.

The other ingredient was the theoretical findings in string theory. Now, string theory was designed to fulfil that lifelong Einsteinian dream of one single universe explained by the theory of everything. But around 2004, string theory ended up with a whole landscape of many potential energies that could start universes like ours.

**You describe a eureka moment you had in a North Carolina coffee shop. What did you realise?**

I was very intrigued by [Nobel prize-winning physicist] Roger Penrose’s estimate that there was nearly zero chance for our universe to come into existence. I kept dissecting his argument, which is based on the second law of thermodynamics, trying to find out if he did something wrong. Then I realised that the problem was not with the actual calculation, it was more with our way of thinking, that we needed a paradigm shift from one universe to many. And that’s where I started borrowing the landscape of string theory to perform the calculation. In the coffee shop I thought, OK, I’ve convinced myself that I need a pool of many possible infant universes from which to choose, but how can I derive the answer, which one is ours? And then I realised, well, of course: quantum mechanics on the landscape of string theory. In other words, think of the universe as a wave, and then quantum equations will tell me what happens to that wave.

**A lot of hard mathematical work followed. You were stumped after your first round of calculations. What had you overlooked?**

I had missed the most important ingredient and that is: the solution to that equation is not just one branch or one universe, it’s a whole family. So these branches that might grow and become universes are all quantum-entangled with each other. In order for each to create their own identity as they grow into classical universes, they have to decouple from each other. This in physics is known as decoherence, or washing out any trace of quantum entanglement that does not have any counterpart in classical physics. I had not taken this into account.

**Once your calculation was ready, how did you go about testing it?**

When the process of separation [of universes] happens, that’s the point when the cosmic microwave background (CMB) is created. So all the inflation fluctuations will leave scarring or dents as a result of this entanglement, and those will be imprinted on our [universe’s] CMB. That was something that we could calculate. So I calculated the strength of entanglement between the different branches and how quickly that entanglement gets washed out. That allowed me to find out how much denting or scarring that entanglement would leave on our sky as it was being created during inflation, and then fast-forward to the present day, to make predictions on how those very large-scale anomalies would look. One of the key predictions of cosmic inflation is that everything is sprinkled uniformly throughout the sky. But now the scarring coming from entanglement with the other universes is modifying or denting that uniformity, violating it at very mild scales. We predicted those, and they were seen by the Planck satellite [in 2013].

**That must have been an amazing moment of validation.**

Yes. And I think that’s when people started paying much more attention to this work. Until then, the belief had been that, to see beyond the horizon of our universe, we would have to break the speed of light, which we can’t do. So if we can’t test the multiverse, then why bother researching it? But Rich Holman, Tomo Takahashi and I showed that you don’t need to get out of this universe, you can actually find all the traces inside your sky. That’s when the whole field suddenly shifted and everybody was doing research on the multiverse.

**And would you say it’s mainstream now?**

Oh, absolutely. All the great minds are working on it. Roger Penrose has got his own multiverse theory. And Stephen Hawking, in the last few years of his life, started working on the multiverse. Wherever I look, suddenly everybody’s got some version of the multiverse.

**The multiverse is a mind-boggling concept. Do you often think about the other universes out there?**

Yes, I do. In one way, it’s the most natural extension of the Copernican principle, because once we thought the Earth was the centre of the universe, and then the solar system and our galaxy, and now we are finding that even our universe is just one tiny grain of dust in a much more intricate and beautiful cosmos. That to me makes much more sense.

**Does it seem likely that other universes could harbour life?**

Absolutely. With Fred Adams, an astrophysicist at [the University of Michigan in] Ann Arbor, I decided to find out if structures would form in universes that had very different conditions to ours. We discovered that you can change Newton’s constant by 10,000 – four orders of magnitude – and you can do the same with Planck’s constant, and still get life in other universes. In fact, our universe seems to be only borderline habitable. We were sitting right at the edge between habitable and non-habitable.