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Did dinosaurs fall foul of dark matter?

Harvard physicist and author Lisa Randall's dealing with dark matter – and will tend to dark energy too, when she gets around to it. She chats to Andrew Masterson.

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“I mean, it really is just energy,” says Lisa Randall. “So to actually say it’s something is wrong.”

At first blush, this seems like a strangely blasé way for a theoretical physicist to refer to dark energy, the property that comprises just a shade less than 70% of the observable universe and about which we know (to use the technical term) three-fifths of stuff-all.

Then again, Randall, the Frank B Baird Jr Professor of Science at Harvard, is among the handful of people in the world for whom dark energy is less an exotic mystery and more just another workaday problem. For her, dark energy, and all the questions surrounding its density and dynamism, can just take a ticket and wait its turn. Right now she’s concentrating on dark matter.

At least, that’s the case in the parallel non-academic career she enjoys – that of a best-selling pop-science author. Her latest book, which revels in the excellent hot-button title, Dark Matter and the Dinosaurs, was released in late 2015 and promptly went gangbusters.

The book glosses the generally accepted idea that the Cretaceous–Palaeogene mass extinction 66 million years ago – the event that wiped out the dinosaurs and pretty much everything else – was prompted by an extraterrestrial object whacking into Earth. Randall extends the concept, suggesting the object was a meteoroid dislodged from the distant Oort Cloud after a collision with a thin flat disc of dark matter.

The disc, she suggests, is a permanent feature in the Milky Way, the rotation of which brings our own solar system into its proximity roughly every 30 million years. The suggestion potentially explains observations that indicate that while little asteroids and comets tend to bang into planets on a statistically regular basis, really big buggers from the Oort only hit, yes, every 30-odd million years.

The idea, she adds, requires only a proportionately tiny amount of the dark matter assumed to be in the local area to form itself into a disc. The proposition implies that dark matter, which accounts for roughly 27% of the stuff in the universe, is subject at least sometimes to the same forces that affect visible (or “baryonic”) matter. The Milky Way itself, of course, is disc-shaped.

Which is all well and good, except we don’t actually know what dark matter is either.

English physicist, television presenter and author Brian Cox, interviewed recently in Cosmos, suggested that it “must” be some kind of particle. Randall isn’t prepared to go that far.

“It’s more likely than not that it’s a particle,” she says, “but you know, I’m a model builder and I’m a theorist, and what we do is we make suggestions and then we see how to test them. So he [Cox] can imagine whatever he wants. The fact is until we actually find it or measure what it is, we don’t know for sure.”

She notes that research into dark matter involves discovering the types of constraints that can be applied – in essence, identifying what it can’t be, and thus ever limiting the possibilities of what it can. In certain models, she notes, the influence of a black hole of a particular size can’t yet be discounted.

If push comes to shove, though, she’ll put her money on particles.

“I would suspect that it’s even more than one kind of particle,” she says. “We assume it is just one kind of particle, even though our own matter is many different types of particle. So it’s very likely, but I’m not going to tell you it must be true until I know it must be true.”

'I think what dark energy is telling me is that we feel like maybe we’re missing some really big idea.'

In her pop-science writing, Randall has been rightly praised for her ability to render the complexities of cosmology accessible to non-specialist readers – such as the bewildering idea of multidimensional objects dubbed "branes".

Her academic work, in contrast, delves deeply into the multiple potentials of supersymmetry, a field that posits warped branes and creates warped brains with equal vigour.

Her best-known contribution to the field – and one explored in a previous book, Warped Passages (2005) – began in 1999 when, in conjunction with University of Maryland theoretical particle physicist Raman Sundrum, she formulated what became known as the Randall-Sundrum Model.

The model suggests that our world is located in a higher dimensional universe that comprises a five-dimensional negatively curved manifold, in which all elementary particles (except the graviton) are clustered locally in a four-dimensional brane.

There are two basic versions of the model, known as RS1 and RS2, which differ primarily in the distance assumed to exist between two branes – one has them relatively close to each other, the other infinitely far apart. Randall-Sundrum is taken very seriously in the field, with results from CERN's Large Hadron Collider in Switzerland from as recently as August this year being used to continually refine its measurements and predictions.

Given, thus, the depth, complexity and longevity of her research into what exactly makes the universe the universe, it’s hardly surprising that she has little time for portrayals of dark energy as enigmatic.

“Quite seriously, dark energy is just that: it’s just energy permeated through the universe,” she says. “It’s not a matter of saying what it is – it’s really about saying something different: why it carries the amount of energy it does.

“It’s energy. It’s not carried by particles; it’s not carried by matter. We would like to know why there is the amount of it that there is. And that’s been measured. The surprising thing is that there isn’t an enormously bigger amount of energy. That’s really what’s surprising.”

In essence, then, the key question for Randall is not what dark energy is, but what it isn’t. Dark energy has been invoked since the 1990s to explain observations that suggest that not only is the universe expanding, but that it is doing so at an accelerating rate.

The problem is it’s not all that good at doing so. Whether dark energy is a constant or a dynamic force, homogenous or varied, and why it seems much less dense than calculations predict are all open questions.

To Randall, this makes one thing very clear: there’s a lot more science still to be done.

“I think what dark energy is telling me is that we feel like maybe we’re missing some really big idea,” she says.

“Because we would naively expect, not just through the standard model [of particle physics] but also through quantum field theory – that combines together quantum mechanics and special relativity – the amount of energy to be enormously greater.

“The fact that it isn’t tells us there must be something else going on. And that’s the challenge: to understand what that thing is. Is there some underlying principle – that this is the only place where we could end up with the kind of things we see? We really don’t know the answer here.”

Lisa Randall is touring Australia on a speaking tour in November. Click here for tickets and more information.

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Andrew Masterson is news editor of Cosmos.
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