HOW did our universe begin? This is among the most profound questions of all, and you would be forgiven for thinking it is impossible to answer. But says she has cracked it. A听cosmologist at the University of North Carolina at Chapel Hill, she was born and raised under communist dictatorship in Albania, where her father was considered ideologically opposed to the regime and exiled. She later won a Fulbright scholarship to study in the US,听forging a career in cosmology in which she听has tackled the origins of the universe听鈥� and made an extraordinary proposal.
Mersini-Houghton鈥檚 big idea is that the universe in its earliest moments can be understood as a quantum wave function 鈥� a mathematical description of a haze of possibilities 鈥� that gave rise to many diverse universes as well as our own. She has also made predictions about how other universes would leave an imprint upon our own. Those ideas have been controversial, with some physicists arguing that her predictions are invalid. But Mersini-Houghton argues that they have been confirmed by observations of the radiation left over from the big bang, known as the cosmic microwave background.
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Here, she tells New Scientist about her ideas and her life, which she has described in her new book Before the Big Bang: The origins of听our universe in the multiverse.
Rowan Hooper: Let鈥檚 start with your own story of听growing up in Albania. To what extent has that听shaped your thinking?
Laura Mersini-Houghton: It鈥檚 contributed a听lot.听I was lucky because I had the kind of听parents who spotted early on that I was interested in natural sciences and math and听then nurtured my interest. Another factor听that helped a lot was that my mom worked at a non-profit organisation in Albania听called the League of Writers and Artists. I got to spend a lot of time with composers, writers and artists, and that opened my horizons to spot early on that there听is so much joy in听creativity.
You talk in your book about how you think of your life in quantum terms; if you hadn鈥檛 made a听particular decision then you would be living a听different life. Can you elaborate?
In a quantum universe, everything is based on probability: until you make a measurement on a subatomic particle, all you have are probabilities regarding all possible outcomes. All certainty about the world is gone. Sometimes, during late-night contemplation, I can see parallels between my life in our 鈥渃lassical鈥� world and the quantum world. I can see how my life resembles a quantum world interwoven with uncertainties and unlikely events. If you think of communist Albania, there was a kind of quantum ambiguity where many bad things could happen at any time for no reason at all, certainly not because of your own doing. There is an analogy with the quantum universe, where any outrageous possibility has a non-zero chance to occur.
Getting back to physics, can you explain why our听universe is so unlikely and how that led you听to think about the multiverse?
It starts with the second law of thermodynamics, which states that the entropy of any system 鈥� which roughly means its level of disorder 鈥� always increases with time. Therefore, the entropy of our universe at its first moment of existence must have been incredibly small. But the probability of finding a universe that arises spontaneously is directly proportional to its entropy. Since our universe started with a very small entropy, that gives it an exponentially small chance of existing.
We know from observations and theory that听the universe started as a very small, smooth patch full of energy, and the Oxford mathematician Roger Penrose found that the chance of starting with the universe like that is 1 chance in 10 to the power 10 to the power 123. So there鈥檚 almost no chance to spontaneously start with a universe like ours. And that number intrigued me because, well, here we are 鈥� we can observe the universe around us. This is our home. We know it exists for sure.
Yet I can鈥檛 ask the question 鈥渨hy do we have听this universe?鈥� if all I am allowed to start听with is this universe: I need a pool of possible universes from which I can choose from. So听that led me to think that we really need an initial quantum multiverse. That would allow me to meaningfully ask the question of why we got this universe rather than something else.
What do you mean by 鈥渁n initial quantum听multiverse鈥�?
I mean that in the very first moment, before the universe emerged in space-time, you can think of the universe as a wave function in an abstract space of energies. I began thinking about all this in the early 2000s, around the time that string theory was听the leading candidate for a 鈥渢heory of everything鈥� that unifies gravity with the other听three quantum forces to explain our universe.
String theory is the idea that nature at a fundamental level is 11 dimensional and particles are actually just the bit we can see of tiny loops of vibrating strings. With string theory, after curling up the extra spatial dimensions to make them sufficiently small to be invisible, you end up with a whole landscape of possible initial energy states, or potential big bang energies, that could start a whole family of different universes.
At the time, string theorists thought this was听really bad because they were looking to听end up with only one universe听鈥� one that looked like ours 鈥� described by one theory, and听they were ending up with a nearly infinite number of universes. But to me it was great news because I needed a fundamental theory to provide that pool of energies that would allow me to ask the听question, 鈥渨hy did I start with this one rather than something else?鈥�.
You had your breakthrough in a coffee shop, and you wrote 鈥淨M on the landscape鈥� on a napkin听鈥� quantum mechanics on the landscape of string theory. Tell us what you meant.
I had realised something that seems obvious in hindsight. We know for sure that our universe was very small in its first moments of existence. Therefore, it obeys the laws of quantum physics. What dawned on me specifically was that, based on the wave-particle duality of quantum mechanics, I could think of the universe as a wave function instead of as an object. The wave function is the mathematical entity that encodes quantum probabilities. But you can imagine it as a tree made up of many branches, each of which can produce a universe, and it spreads through the energy valleys of the string theory landscape, from where it takes its big bang energy.
Is this where quantum entanglement comes in, the idea that there can be this subtle quantum connection, between the branches?
You get these branches, these many worlds, but you need to decouple them from each other 鈥� you need to break that quantum entanglement. Think about when we separate gold from ore. We put the ore mixture into a bath of a compound called borax, and since borax interacts differently with different minerals, they start separating from each other. In my hypothesis, the string theory landscape was our borax 鈥� it broke the entanglement and separated out the many worlds.
Somehow, early on, our universe went through a quantum-to-classical transition. It became a classical object where each event is determined with certainty. This could not have been the case unless the branches of the wave function of the universe completely decoupled. All the branches decouple as they are going through cosmic inflation. This is the phase, shortly after the big bang, when the universe went through a period of exponential expansion in size. My proposal was that, if this decoupling did happen, we would be able to see the remnants of it in the cosmic microwave background [or CMB], the radiation left over from those first moments of our infant universe. The idea was that, as the branches decoupled, traces of the entanglement would have been left behind.
So you made a testable prediction: that we should be able to see signs of our universe鈥檚 primordial entanglement with other universes.
I made with Richard Holman and Tomo Takahashi in 2005 and 2006. We said we would be able to see signatures of this early entanglement. Our present universe is just a rescaled version of听its听infant self, with all its 鈥渂irthmarks鈥� still听there. If you think of all these quantum universes as tiny quantum particles, they were听all interacting with each other听鈥� gravitationally they were pulling on each other, and that left scars in our sky.
One prediction was the existence of a giant听void or cold spot in the CMB. And such a void [about 900 million light years wide] was found in the observations of the Wilkinson Microwave Anisotropy Probe, a space-based observatory. It was confirmed by the Planck satellite, which also observed the CMB. We were the first to show how you can actually test听the multiverse and that you don鈥檛 need to听go beyond the universe鈥檚 observable horizon听鈥� you can just see it in our sky.
As you probably expected, your ideas proved controversial. For example, there was an that didn鈥檛 support your conclusions. How have you reacted听to the criticism?
I did a new analysis to check the status of my theory using the most recent data from the Planck satellite experiment with cosmologist Eleonora Di Valentino. The series of predictions we made in 2005 for anomalies in the CMB supports the origin of the universe from a quantum landscape multiverse.
There has been a long prejudice against 迟丑别听尘耻濒迟颈惫别谤蝉别. It is an idea that goes back to ancient Hindu and Greek beliefs. But it took a听long time to push the idea into mainstream physics. One thing that helped is my proposal to use quantum entanglement as a tool for testing the existence of the multiverse right here, thereby circumventing the speed of light听limit constraints. That provided hope that听even if we can鈥檛 see the multiverse directly, we can still indirectly derive evidence and make predictions on where those signatures can be found in our sky.
Max Tegmark at the Massachusetts Institute of Technology has talked of having doppelgangers in other universes when articulating his vision of听the multiverse. Would your version of the idea听have those?
I don鈥檛 think there is another Laura out there in some other universe that is my twin or my sister. Once the branches of the wave function have decoupled, they have their own independent existence from each other. So I don鈥檛 think of having copies of myself in other worlds 鈥� and I hope there aren鈥檛. If you asked my family, they鈥檇 say one eccentric scientist is enough.
Rowan Hooper is podcast editor at New Scientist. His latest book is
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