by Clive Cookson
Sixty-six million years ago, a meteorite almost 15km wide hit Earth with an impact speed of 70,000kph, unleashing a billion times more energy than an atomic bomb and wiping out most life on the planet–including all dinosaurs.
This cataclysmic strike, which cleared an evolutionary path for mammals and eventually humans to dominate the animal kingdom, might have been a random one-off collision. But many scientists, including Harvard physics professor Lisa Randall, prefer to find a pattern in large meteorite strikes, pointing to an underlying cause.
Randall’s splendid book Dark Matter and the Dinosaurs is built around her latest theory. Put very briefly, this links the demise of the dinosaurs to the sun’s passage 66m years ago through a thin disk of “dark matter” in our Milky Way galaxy. The resulting gravitational perturbation dislodged one of the countless icy and rocky objects that orbit within the Oort Cloud in the outer reaches of the solar system and hurled it as a comet towards the sun, until the Earth got in the way.
“I’ll tell you right up front that I don’t know yet if this idea is correct,” the author admits. She takes it seriously enough to be devoting a substantial amount of research time to further investigation, however–and to have written a long book explaining her “double-disk dark matter” or DDDM theory to the general reader, together with much essential background about the development of the universe and the solar system, the nature of dark matter and the history of the Earth’s periodic bombardment by asteroids and comets.
The complexity of Randall’s material makes some passages hard going, despite her skills as a storyteller and scientific populariser, and I felt a little more simplification and pruning would have produced an even better book. In particular, the early sections about different models for the growth of the universe after the Big Bang could have been shortened without losing key information about the nature of dark matter.
As Randall explains, science currently recognises three components of the universe. Dark energy, an expansive and decollated property of space itself, was discovered less than 20 years ago; its nature is a mystery and it plays little part in the DDDM theory. The other two components are types of matter–material that occupies specific regions of space and, through its mass, exerts gravitational pull.
Everything we can observe is ordinary matter. This includes atoms and myriad subatomic particles that make up planets, stars, galaxies and more exotic astronomical objects–even black holes are regarded as ordinary matter in this context.
Dark matter, at the heart of Randall’s story, is known only through its gravitational influence on ordinary matter. No other interaction has been detected and no one knows what it consists of; leading candidates go by names such as axions, wimps (weakly interacting massive particles) and machos (massive compact halo objects). Cosmologists calculate that just 15 percent of matter in the universe is ordinary and 85 percent is dark.
The most innovative aspect of DDDM theory is its proposal that dark matter exists in several forms–in contrast to physicists general assumption that there is just one type. Randall asks why dark matter should not come in different forms interacting with each other in different ways, given the huge variety of ordinary matter.
Her model requires at least two varieties of dark matter. One forms a spherical halo around the pancake-shaped Milky Way, which astronomers have detected indirectly from its gravitational influence. The other, she proposes, is a thin dark disk within the galaxy’s main plane where most of its stars reside.
Our sun moves through this disk every 32m years or so–a period that happens to coincide with the cycle of large meteorites striking Earth, according to Randall’s analysis of impact craters investigated and dated by geologists. Although none of the others was as big as the dino destroyer that hit what is now the Yucatan 66m years ago, she detects periodicity consistent with comets being dislodged by the dark disk’s gravitational disturbance and hurtling toward the inner solar system.
Whether Dark Matter and the Dinosaurs is unwarranted speculation or the basis of a revolution in cosmology may become clearer over the next few years, as the European Space Agency’s Gaia observatory maps the galaxy and its gravitational interactions in unprecedented detail. Its surveys should either confirm or rule out the existence of a dark disk. Every reader of this book will hope that the theory passes the test.