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Harvard University's Lisa Randall is a theoretical particle physicist focusing on extra dimensions of space and dark matter. In her fourth book, Dark Matter and the Dinosaurs(Bodley Head), Randall explores the interconnectedness of the Universe, theorising that dark matter may have caused Earth's major extinction event 66 million years ago.
Researchers suspect that as our solar system orbits the Milky Way's centre, it bobs up and down through the central galactic plane about once every 35 million years, roughly matching the frequency of comet impacts on Earth. Randall's twist on this idea: dark matter could be clumping together to form a dense "dark matter disk" at the centre of the Milky Way's galactic plane. As our bobbing solar system encounters this disk, its gravitational pull yanks on comets, dragging them toward Earth.
WIRED talks to Randall about this new theory, the evidence, and why it's OK if your ideas are overturned.
WIRED: How do you explain dark matter?Lisa Randall: Well, first of all, it is just matter. What do we mean by matter? Something that kind of clumps; it's defined by how it interacts under gravity. So dark matter is like the matter we know about, the stuff we see around us. The big difference is there's no interaction with light that we know of. That makes it different from the point of view of what we see, but not from the point of view of how gravity acts on it. In some ways, I think it would have been better if it had been called transparent matter, because light just passes through it. It's not made up of the stuff we're used to. It's something different; we don't observe directly with our eyes<sup>1</sup>.
What's different about your theory of dark matter?As the solar system goes round the galaxy, it bobs up and down through a plane in the Milky Way. If there is a dark disk, at some point, when it goes through in the mid-plane, there will be an enhanced gravitational force that could conceivably knock comets out of the distant Oort Cloud, where they're very weakly bound. Before our work, people had suggested that as the solar system goes up and down through the Milky Way, comets get dislodged. The problem was that there was nothing that was abrupt enough to explain the spike in comet strikes. It was too continuous and smooth. The dark-matter disk solves these problems.
And the idea of interacting matter is important, because it explains how the dark matter would clump to form a disk?That's right. I think that, if nothing else, this idea that there could be a fraction of the dark matter that interacts will probably survive. Just like ordinary matter comes in many different types, some of which interact in very different ways, maybe there's a small fraction of dark matter that interacts with itself. We realised that if there is an interaction, it could have very important and measurable implications.
The Gaia satellite is out in space, mapping the density of the Milky Way. Could it provide evidence for your theory?One of the really great things was we realised that the Gaia satellite -- which was about to launch in the autumn after we finished the dark-matter research -- was going to do exactly the measurement we wanted, measuring the positions and velocities of stars, and from that, we'd be able to figure out what their galactic potential looks like. That will tell us whether there's a dark disk with the right properties at the very least. There are other conceivable implications, but this one is most directly measuring whether there's a dark disk<sup>2</sup>.
Will the Large Hadron Collider give us further clues about dark matter?There is a possibility that dark matter can be searched for at the Large Hadron Collider, if it has mass that's comparable to that of the Higgs boson. There's a built-in assumption there, that there's a tiny interaction called the Standard Model Interaction. If there is a tiny interaction, there's a possibility that it can be produced at the Hadron Collider and disappear in the form of what we call "missing energy". We'll figure out, was there dark matter present or not? It's going to be hard to determine, even if you found something like this that is indeed dark matter. But you can measure its properties and conclude whether it could play a role.
Do you worry new evidence will overturn your theories?Oh no, I think that's fantastic. It's kind of like going to the doctor if you're sick: if you know something is wrong, you can do something about it. If we find out the idea we were pursuing is wrong, well, we'll look at some other idea. It's good to be able to narrow our focus to the ones that have a promise of being right.
What are you researching next?We're working on a completely different idea with James Unwin and Jakub Scholtz. We're not only thinking about double-disk dark matter, but all of the dark matter. When we have models of dark matter, we often assume that somehow it is related to ordinary matter, despite the absence of any evident interaction. So we're saying, suppose dark matter and ordinary matter start off on unequal footing -- what would we need to accommodate what we see in the universe? Can it help us understand even the structure of small galaxies? We're approaching dark matter from very different angles.
1. In her book, Randall wanted to overturn preconceived notions about dark matter -- for example, that it's equivalent to black holes. In fact, the two are "no more alike than black ink and film noir". 2. Randall makes it clear that her theory about dark matter is speculative -- at least for now. Still, she explains, it gives us an opportunity to explore more about our galaxy and what it's made of.
This article was originally published by WIRED UK
