A Dark Day for Dinosaurs

By Pete Edwards

On average, 91 people are killed by asteroids each year. This number – taken from a 2010 report published by the US National Academy of Science – might seem alarmingly high, especially since we know of only a few such deaths in recorded history. But in fact it is simply a consequence of statistics: the average value takes into account deaths from enormous collisions that happen only rarely, but could potentially cause massive destruction. For example, it is generally thought that, roughly 66 million years ago, the impact of an object at least 10 km in diameter produced effects that led to the extinction of around three-quarters of life on Earth, including the majority of the world’s dinosaurs.

Many questions remain unanswered about the sequence of events that produced this cataclysm. In her book Dark Matter and the Dinosaurs, theoretical physicist Lisa Randall focuses on a novel one: how did the dinosaur-killing asteroid end up on its collision course with Earth in the first place? It has been suggested that the impactor was a comet nudged from an innocuous orbit in the Oort Cloud at the outermost reaches of the solar system. But if that is the case, Randall asks, what did the nudging?

Initial inspections of the fossil record (which Randall describes in detail) reveal some evidence to suggest that large impacts tend to occur in groups separated by around 35 million years. Although this periodicity has not been clearly established, could it provide a clue to the nature of the dislodging force?

After ruling out the possibility that the gravitational influence of an invisible companion star of our Sun might be responsible, Randall considers the motion of our solar system through the Milky Way galaxy. As it orbits the galactic centre, the solar system also oscillates through the plane that contains most of the galaxy’s stars. If there was a thin, dense region of matter contained within this stellar plane, then our orbit would periodically take us through regions of space with a density greater than the average. In that case, the additional gravitational interactions that take place during that period could be responsible for nudging Oort Cloud objects towards the Sun.

At first sight, this idea is hard to justify, as we have no observational evidence for such dense patches of space. But if these regions were made of dark matter – the invisible substance that makes up around 85% of the mass in the universe – then an absence of observational evidence is exactly what we would expect. This is an intriguing idea, because it is now widely accepted that dark matter has helped to create the cosmic scaffold on which the large-scale structure of the universe was built.

The story is that these dark-matter particles were created in the Big Bang and, through gravitational interactions, began to concentrate in the very first moments of the life of the universe. The resulting clumps acted as seeds that attracted (again via gravity) the stable atoms that began to form once the temperature of the universe had fallen sufficiently. As the universe expanded, gravity magnified these irregularities in the matter distribution, finally producing the pattern of galaxies we see today.

Unfortunately for Randall’s dinosaur-killing story, the cold and essentially collisionless dark matter described above does not behave in the same way as conventional matter. In particular, it could not be distributed in the galactic plane as a thin, dense disc capable of triggering a comet’s fatal trajectory through gravitational attraction, because galaxy-formation models fail to predict the existence of an overdense region within the plane of the galaxy.

This leads Randall to introduce a more hypothetical variety of dinosaur-killing dark matter, via the radical concept of a whole new “dark sector” in which partially interacting dark matter interacts with itself through the new force of “dark electromagnetism” to which ordinary matter is oblivious. Such self-interactions might allow this new class of dark matter to lose energy and slow down enough to form a disc within the galaxy.

There is little doubt that a mas- sive impact 66 million years ago was, at least in part, responsible for the demise of the dinosaurs. The nature of the impactor remains unknown, but if it was indeed a comet dislodged from the Oort Cloud, then Randall’s book provides an entertain- ing and radical explanation of the events leading up to their ultimate extinction. However, the partially interacting dark-matter model that spawned this explanation is, as Randall concedes, “speculative”, with little experimental evidence to support it. Many of the observations that provided the original motivation for creating the model have now been questioned, while some reported discrepancies between observations and the results of dark-matter simulations have since been resolved. And as Randall herself states, “If there is a conventional explanation for an observation, it is almost always the right one. Radical departures should be accepted only when they explain phenomena that older ideas fail to accommodate. In only very rare instances are new ideas truly necessary to explain observations.”

Perhaps I am more comfortable with established ideas, aligning myself with the majority of scientists who are (in Randall’s opinion) a “conservative lot”. But you may wish to read this entertaining book before making up your own mind. After all, to misquote Robert Frost, “We dance round in a ring and suppose, but nature sits in the middle and knows”.

Forthcoming data from the Large Hadron Collider at CERN and the European Space Agency’s Gaia mission – which is designed to produce a 3D map of the Milky Way with unprecedented accuracy – may well cast some light on the properties of dark matter in our galaxy and the universe at large. In doing so, there is a chance that we could come closer to discovering what was ultimately responsible for the demise of the dinosaurs.

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