Book review: “Dark Matter and the Dinosaurs” by Lisa Randall

On the one hand, we have the elusive dark matter particles, dispersed throughout the universe across billions of light-years; on the other, we have the sorely missed dinosaurs, who lived in our own proverbial backyard but were driven extinct by a mysterious impactor 66 million years ago. What if these fascinating yet disparate phenomena, separated by so much space and time, were somehow related?

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That, in essence, is the premise of Lisa Randall’s book, “Dark Matter and the Dinosaurs.” Maybe the “vanilla” cold dark matter model we have isn’t the only possible explanation of observations of the expanding universe and the cosmic web of millions of surveyed galaxies, she argues. It’s more fun to consider other more exotic models, even if they turn out to be wrong.

Dark matter particles don’t interact with each other the way our familiar atoms do. In fact, they hardly interact at all. They mostly just move apart with the growing universe and then clump together as they feel the effects of gravity over time. As a result, we end up with nearly spherical dark matter clumps throughout the universe, and we and the rest of the Milky Way are living inside one of those clumps. But if some dark matter interacts like normal matter, it could form a dense and thin disk—even thinner than the disk of our own galaxy. (Picture a compact disk hidden inside a bagel. Here’s a good composite image of our galaxy, on edge, which would be the bagel.)

If that’s the case, then as our solar system moves up and down through the disk, we’ll experience an extra little gravitational nudge each time we go through. This could periodically dislodge comets traveling in tenuous orbits in the Oort cloud in the distant realms of our solar system, flinging one comet away forever and sending another in an unfortunate Earthbound direction, where the consequences of its destructive impact in the Yucatan kills off the dinosaurs some 66 million years ago, thus finally linking dinosaurs to dark matter.

It’s worth taking each step of this argument with a grain of salt, and the result would be a pretty salty concoction you might not want to taste. In addition to the dark matter assumptions, she’s also assuming that it was a comet, not an asteroid, that caused the extinction, and she’s assuming that Earth suffered these impacts periodically with a period of around 30 million years. I don’t know how much Randall believes all this herself, but what it does do is give her an excellent excuse to take the reader on an entertaining tour of cosmology, astrophysics, planetary physics, the origins of life, and paleontology.

Randall explains complex processes and technical subjects well, though her writing seems more “textbooky” when she’s not writing about cosmology and particle physics, which are her expertise and had been the subject of her previous three books. She has a nerdy sense of humor, which I like, and I think many of you will, too. Occasionally her jokes fall flat and her analogies to pop culture don’t work though.

Recently I reviewed Sean Carroll’s new book, “The Big Picture.” I think Randall writes a bit more dryly and with less poetry, if you will, than Carroll. But I appreciate that she also provides more nuance and writes a little more modestly or self-deprecatingly. (I liked Carroll’s book too but I thought he made too many sweeping, oversimplified arguments in it.) Randall also goes into more detail about the mass extinction at the end of the Cretaceous period than did Elizabeth Kolbert in her book, Sixth Extinction (which I also briefly wrote about in a recent post).

Randall’s model is based on a shaky edifice, however, with a lot of “ifs” piled onto each other. If you’re curious about her dark matter research, you can read about the “double-disk dark matter” model she and her colleagues developed and published. Following that, she published her key paper, “Dark Matter as a Trigger for Periodic Comet Impacts,” in Physical Review Letters. I noticed that it’s been cited only 11 times in the scientific literature—in line with my own least popular (or most underappreciated?) papers—while a Google or Bing search generates tens of thousands of hits, especially in the popular media (including Nature, where I work).

It’s an interesting story though. Maybe more importantly, she describes well the convoluted process of actually doing science. This includes her deciding whether to pursue some research, interacting with colleagues and collaborating on studies, arguing with peers at conferences, and investigating ideas, many of which don’t fare well, while a few others occasionally turn out to be promising. She also mentions mentions how feedback from students as well as chance encounters with nonscientists (including a taxi driver) gives her ideas, too.

Finally, Randall has an interesting discussion of Ockam’s razor, and I wish she’d gone into more detail, as this is an important issue when it comes to prioritizing theories, models, or lines of scientific research. It’s the idea that, all things being equal, the simplest theory is the best one (though not necessarily the right or true one). But are things ever equal? And how do you judge simplicity? If a theory makes fewer assumptions than another, then it could be trading one shortcoming for another. But if its competitor is overly complex to the point of being hard to test or assess, then that’s not an improvement. And when do you decide that a theory’s assumptions are too egregious or exotic?

I haven’t decided where I stand when it comes to Randall’s double-disk partially interacting dark matter idea, but I’m glad she proposed it. I’m glad she explored its implications too, as this dark matter matters to us and our poor dinosaur friends.

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