New Light on Dark Matter

By Tom Whipple

The story is as old as the planet we are on. Somewhere, in a nice primordial soup, life develops. It grows in complexity, branches out to fill different niches–and one of the bolder specialisms flops on to land. Over millions of years this brave pioneer becomes flying animals and walking animals, predators and prey, and populates the Earth.

Then bam! From a clear blue sky, like the random vengeance of an Old Testament god, a comet slams into the planet with the force of thousands of nuclear bombs, leaving all but the hardiest animals dead–and life has to start once more.

What, though, if that vengeance is not random at all? What if, instead, our planet’s regular cometary apocalypses are predictable, and periodic? What if–and this is the thesis of Dark Matter and the Dinosaurs– they are caused by the Earth’s galactic passage through a substance so unusual that until recently scientists did not even know it existed?

Lisa Randall, a physicist at Harvard University, believes dark matter might have killed off the dinosaurs. Because she is a physicist, rather than a publicist, she doesn’t go farther than “might”. In fact, she says, it is a “speculative scenario”. But, Dark Matter and the Theory-Based-On-Some-Interesting-Speculation-And-Data-That-Doesn’t-Reach-Statistical-Significance doesn’t quite do it, title-wise.

To understand her theory requires understanding what our universe is made of. Everything you can see–galaxies, planets, trees, and people–is by definition composed by visible matter. In recent decades we have discovered that this visible world constitutes just 15 percent of the mass of the universe. The rest is “dark matter”, a substance that not only does not reflect light, it does not even interact with the rest of the universe at all, except through exerting gravitational force. “Billions of dark matter particles pass through us every second.” Randall says, “yet no one notices that they are there.”

Pinning down the nature of such a slippery substance is the most pressing problem in theoretical physics. Though if it still evades discovery by our most powerful particle accelerators, it could be that its effects have been felt far more tangibly in the past–or more specifically, every 32 million years or so.

Every day, the Earth is hit by rocks from space. Normally, those rocks are small and they burn up far above our heads. Sometimes, they are big enough to leave a crater. Just occasionally, as with the comet that hit 66 million years ago, they are big enough to kill off much of life on Earth. If you examine those craters through the right statistical lens, it is–just–possible to discern a fluctuation that follows a 32-million-year cycle. What could cause it?

An obvious event that happens regularly, over a timeframe that could be that length, is the oscillating passage of the solar system through the plane of our Milky Way galaxy. What if along that same plane there was a flattened disk of dark matter? What if there was enough of it to perturb the paths of the solar system? That would mean that every 32 million years the Earth’s fauna ran the gauntlet of apocalypse, and that 66 million years ago the dinosaurs lost.

It is a fascinating and surprisingly simple theory. So simple that Randall covers it in a few dozen pages near the end of the book. The preceding 300 pages have descriptions of cosmology and geology that, without a clear narrative, at times feels like Wikipedia articles strung together. Sometimes even the clarity of a Wikipedia article would be welcome. There can be no excuse for any professionally published book containing sentences such as: “A crucial piece of information is that in the beginning, atoms–which are electrically neutral bound states of positively charged nuclei and negatively charged electrons–didn’t exist.”

Dark Matter and the Dinosaurs has a great title and a tantalizing premise. Yet if feels as if most of this book, while undoubtedly important, just provides bulk to the interesting bit. A little like dark matter itself.


Published in The Times. January 16, 2016. Page 14.