Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe, by Lisa Randall

Book of the week: An invisible force causes extinctions – science or sci-fi? Nature will decide, says Marcus Chown


On Earth, death rains down from the sky with metronomic regularity. At least, that was the contention of palaeontologists Jack Sepkoski and David Raup. In 1984, they claimed that mass extinctions of life on Earth occur every 30-odd million years.

The obvious culprit is comet impacts. A fragment of comet-ice the size of a terrace of houses disintegrated above Siberia’s Tunguska River in 1908. The blast wave, equivalent to 1,000 Hiroshimas, flattened more than 2,000 sq km of forest. But why should comet impacts occur regularly?

The solar system is embedded in an enormous bee-swarm of comets, so evidently something must stir up this “Oort Cloud” (named for the Dutch astronomer Jan Oort) at regular intervals, sending a flurry of icy bodies sunward, some on a collision course with the Earth. Also in 1984, scientists Michael Rampino and Richard Stothers suggested that the 30-million-year periodicity is related to the solar system’s motion around our galaxy. Think of our Milky Way as a compact disc. The Sun – which is about two-thirds of the way out to the edge – orbits the centre every 240 million years and, as it does so, it bobs up and down through the plane of the CD. Huge stellar nurseries are concentrated in this plane, and it is the gravity of these “giant molecular clouds”, claimed Rampino and Stothers, that stirs up the Oort Cloud every 30 million years.

Lisa Randall does not agree. The eminent theoretical physicist and best-selling science writer contends that giant molecular clouds are not concentrated enough in the galactic plane to create such a well-defined “extinction signal” (and even Rampino and Stothers eventually came to this conclusion). She invokes something else to impart the necessary sharp kick to the Oort Cloud: “dark matter” – the mysterious, invisible stuff that, although you are utterly unaware of it, is flying through you at this very moment.

In fact, you are only reading these words because of dark matter. As the fireball of the Big Bang expanded, the galaxies, including the Milky Way, congealed out of the cooling debris. This happened because regions of the fireball that were slightly denser than average had slightly stronger gravity, enabling them to pull in more matter, which made their gravity even stronger, and so on. But this process – akin to the rich getting richer and the poor poorer – could not possibly have built up a galaxy as big as our Milky Way in the mere 13.82 billion years that have elapsed since the Big Bang. Not, that is, without the help of the gravity of a lot of extra matter: dark matter.

Dark matter outweighs the universe’s visible stars and galaxies by a factor of about 6. It gives out no light (technically, it does not experience the “electromagnetic force” and so gives out no “electromagnetic radiation”). And, crucially, because it cannot shed heat – radiant heat being a form of light – its internal heat, pushing against gravity, prevents it being crushed down to the CD shape of our galaxy. Dark matter therefore lingers in a giant spherical cloud, or “halo”, around the CD. At least, that is what everyone believes – apart from Randall and her collaborators.

Most physicists think that dark matter is composed of hitherto undiscovered subatomic particles that do not interact with ordinary matter, except by pulling on it with the force of gravity. (Dark matter could just as well be made of astrophysical objects such as fridge-sized black holes the mass of Jupiter, but particle physicists, whose subject is currently far more data-poor than astrophysics, have more time on their hands.)

But what if a small fraction of the dark matter particles, despite shunning normal matter, interact with each other via a new force – a “dark electromagnetic force”? If so, they could shed their heat as “dark light”, which would be invisible to us. This is Randall’s contention. If such dark matter particles were heavy enough, they would not linger in the spherical halo of the galaxy, but sink to its central CD-plane. And, as the solar system bobs up and down, their gravity would shake up the Oort Cloud at 30 million-year intervals, creating the comet showers that have extinguished all manner of creatures on Earth, including the dinosaurs 66 million years ago. The smoking gun of that particular mass extinction is the Chicxulub crater, some 180km in diameter, submerged beneath the waves just off the coast of Yucatán in Central America.

You would be forgiven for thinking there are a lot of “ifs” here. Randall’s mechanism works ifdark matter is made of subatomic particles; if a small fraction, but not all, are self-interacting; ifat least one of them is heavy enough – at least 100 times the mass of a proton – to settle into the CD of the Milky Way and make a sufficiently dense second CD…And that is not to mention ifthe 30 million-year period of extinctions is actually real. It is notoriously difficult to know from the fossil record how many species existed at any particular time in history and what fraction of them disappeared abruptly.

Randall, however, is a compelling writer who presents a broad range of up-to-date science – everything from particle physics to palaeontology, cell biology to cosmology – in a lively and accessible way. You could do worse than read Dark Matter and the Dinosaurs for this engagingly written overview alone.

Moreover, unlike most scientists, Randall is willing to have fun. Piling speculation on speculation, she explores the way-out science fiction possibilities of her idea. Perhaps, in addition to a dark electromagnetic force, she says, there are dark versions of nature’s two nuclear forces too, and together these glue together dark stars and dark planets, complete with dark life. “Dark objects or dark life could be very close – but if the dark stuff’s net mass isn’t very big, we wouldn’t have any way to know,” says Randall. (For a second, the thought flitted through my mind that there might well be a dark matter family living in my house at this very moment…)

But what of Randall’s central thesis – that a thin disc of dense dark matter periodically roughs up the Oort Cloud and causes mass extinctions? Frankly, I think it is a long shot. The fortunate thing about science, however, is that opinions, including mine, do not matter a jot. Nature is the ultimate arbiter of what is right or wrong. At the end of 2013, Nasa launched its Gaia space experiment, and during its five-year mission it will measure the motions of a billion stars in our galaxy. Since those stars are moving under the gravitational influence of both the visible and invisible stuff, Gaia will enable the creation of a 3D map of the invisible world. Randall’s disc of dark matter will either be there or it will not.

Randall herself is remarkably honest on the book’s opening page. “Though both dark matter and dinosaurs are independently fascinating, you might reasonably suppose that this unseen substance and this popular biological icon are entirely unrelated,” she writes. “And this might well be the case.” But science invariably turns out to be stranger than science fiction. So who knows?


Review URL: https://www.timeshighereducation.com/books/review-dark-matter-and-the-dinosaurs-lisa-randall-bodley-head


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