Two theoretical physicists in the US have made a surprising connection between dinosaur extinction and dark matter. Lisa Randall and Matthew Reece of Harvard University believe that some of this mysterious invisible matter – which makes up 85% of all matter in the universe – could exist in a special form that affects the rate at which comets strike our planet. A comet crashing into Earth about 66 million years ago is one possible reason why these giant creatures died off.

Comets have smashed into Earth throughout its history, creating huge craters and possibly causing mass extinctions, such as that which befell the dinosaurs. Many of these comets come from the Oort cloud, which is a huge halo of small icy objects that surrounds the Sun, out to a distance of about one light year. But rather than being entirely random, there is some evidence that the frequency of comet impacts oscillates on a timescale of about 35 million years.

Although this oscillation is not certain, if it is true, there could be something on that timescale that affects the rate at which comets from the Oort cloud are sent towards Earth. Two possible explanations have been proposed so far. One – dubbed the "nemesis hypothesis" – involves the gravitational pull of an as-yet-undiscovered distant companion star to the Sun. The other involves the oscillating pull of the dense galactic disc as the solar system crosses and re-crosses the plane of the Milky Way.

Wrong kind of dark matter

In their new study, Randall and Reece have focused on this second hypothesis, and set about rectifying some of its known flaws. One shortcoming is that the density gradient of normal matter in the galactic disc is too small to have much of an effect on the Oort cloud. As dark matter accounts for about 85% of all matter in the universe, one might think a disc packed with lots of dark matter could resolve the problem. Unfortunately, both theory and observations suggest that dark matter forms a near-spherical halo around galaxies like the Milky Way, rather than being concentrated in the disc.

The problem for anyone suggesting that dark matter affects the rate at which Oort-cloud comets hit the Earth is that the most likely candidates for dark matter – known as weakly interacting massive particles (or WIMPs) – only interact via gravity and the weak force. These interactions are not strong enough for a disc to form, which normally requires strong electromagnetic interactions between atoms, molecules, dust and other conventional types of matter.

However, last year Randall and Reece – along with JiJi Fan and Andrey Katz – proposed a different type of dark matter called partially interacting dark matter (PIDM). Such dark-matter particles interact via an electromagnetic-like interaction, perhaps involving the emission of "dark photons". The four physicists argued that a small fraction of dark matter could be PIDM without much affecting the known dark-matter distribution of galaxies. Furthermore, they argued that the interactions between PIDM particles could be strong enough to form a dark galactic disc that shadows the visible disc (see "Do dark-matter discs envelop galaxies?").

Ample interacting dark matter

By calculating the effects that such a dark disc could have on the shapes of galaxies and how galaxies interact with each other, Randall and colleagues reckon that 5% or less of all dark matter in a galaxy could be PIDM – roughly on a par with the amount of conventional matter in our galaxy. This finding has now prompted Randall and Reece to calculate the effects of this dark disc on the Oort cloud. They conclude that their dark-disc scenario for an oscillating comet cratering rate is about three times more likely that a simple constant cratering rate, which they describe as a "mild preference". The study suggests that the surface density of the Milky Way's dark disc is about 10 solar masses per square parsec and has a thickness of about 10 parsecs. By comparison, astronomers believe that the density of normal matter in the disc of the Milky Way is about seven solar masses per square parsec.

Randall and Reece point out that their dark disc is large enough that it should be detectable by the European Space Agency's Gaia space mission, which is currently studying the Milky Way in unprecedented detail. The research is described in a preprint on arXiv.

  • In the video below, Luke Davies of the University of Bristol explains why physicists believe that the universe is full of dark matter.