Did Dark Matter Kill the Dinosaurs?
Dark matter is one of the most intensely studied subjects in particle physics and cosmology today. It’s certainly curious stuff, behaving in ways unlike anything in our everyday experience. Indeed, dark matter has never actually been isolated or studied in the lab. Its very existence has only been inferred indirectly by its influence on things we can see.
But what is dark matter? When astronomers look at a rotating galaxy, they can work out the mass of the stars and gas clouds within it, and thus the strength of that galaxy’s gravity, as well as how fast it is spinning and so how much gravitational force is required to hold it all together. The problem is that the two figures don’t match. Galaxies like our own apparently contain far more mass than the visible matter of stars and nebulae, or else they would have flung themselves apart in their rapid twirl. The implication is that there must also be something unseen, dark matter, the gravity of which holds it all together. We infer dark matter, as Lisa Randall puts it rather wonderfully in Dark Matter and the Dinosaurs, in the same way we would deduce the invisible presence of a celebrity in the midst of an excited, jostling crowd.
It is now thought that the rapid clumping of dark matter in the primordial universe provided the scaffolding for ordinary matter to gather together gravitationally into the ordered cosmos we see today – it seeded the structures of galaxies. But while dark matter may have made the universe suitable for life on Earth, did this mysterious, invisible stuff also threaten to destroy? Did it kill the dinosaurs?
The answer is: probably not. At least, there’s no evidence yet to support this new suggestion. Randall is careful not to push this particular possibility too hard. Nor is it the most interesting aspect of her book. In exploring the chain of steps in the argument that may indirectly link dark matter in the universe to the huge impact 65 million years ago that wiped out around three-quarters of plant and animals species, including the dinosaurs, Randall has woven a beautiful account of how life on Earth is intimately connected to the cosmos.
The history of life on Earth is punctuated by mass extinctions, including that notorious asteroid or comet collision 65 million years ago. Various studies over recent decades have looked for a periodicity in the impact rate – the possibility that impacts on the Earth come in regular waves – and have hypothesised triggers, such as the elongated orbit of an unseen Planet X or perhaps a companion star to the Sun, dubbed Nemesis. A huge swarm of icy bodies encircle the Sun on the very outer edges of the solar system, known as the Oort cloud; if some of these were to have their orbit disturbed by an external gravitational influence, they could swoop down into the inner solar system as comets, and potentially slam into the Earth. But none of these proposed triggers have stood up to further study.
Could there be a link, Randall posits, between the rate of impacts on the Earth and dark matter in the galaxy? Our galaxy, the Milky Way, is a great rotating spiral of stars, and as our sun orbits the centre, it also bobs up and down through the flat plane of the galaxy: our solar system’s overall motion around the galaxy is like that of a fairground carousel horse. It is this motion, claims Randall, that could cause regular waves of impacts on Earth.
Randall, working with another physicist named Matt Reece, argues that if there is a concentrated invisible flat disc of dark matter in the plane of our galaxy, embedded within the disc of bright stars that we can see, then every time the solar system passes through the midriff of the Milky Way this extra dark matter disturbs the Oort cloud and sends comets plummeting towards the Earth.
This is a satisfyingly elegant idea, linking cosmology and dark matter to the architecture of our solar system, cometary impacts and mass extinction. However, the dark matter we have detected so far exists as diffuse halos around galaxies, whereas Randall’s theory requires at least two different kinds of dark matter, the second able to radiate energy (analogous to emitting “dark light”) in order to collapse into a dense disk in the midplane of the galaxy, disturbing cometary orbits as the solar system passes through it. Even if there were a dark matter disc causing a rhythm in the cratering, it would be hard to prove it was the trigger of any particular impact – and her argument only works for comets, not asteroids.
But, as Randall argues with admirable clarity, this dense dark matter disc in the midplane of our Milky Way is a testable scientific hypothesis. A space telescope called Gaia has already been launched whose observations could support or refute Randall’s predictions. If she is right, there’s also some exciting new physics waiting to be discovered.