Reviewed by Peter Hobson
This new book by world famous theoretical physicist Lisa Randall, subtitled “The astounding interconnectedness of the Universe”, gives the reader an excellent insight into how physicists think and how far you can reasonably push an idea that might seem either preposterous or at least highly improbable. The “dark matter” of the title is one of the current mysteries of our universe as this strange stuff appears to make up the majority of the universe compared to the “normal” matter that we can “see”. I use see here in the sense of detect rather than visibly emitting light. Dark matter, on the other hand, does not interact with light nor emit it however hot it might be. The only interaction it has with what you and I regard as “normal” matter is via the force of gravity, though as Randall hypothesises dark matter may have more complex interactions with itself.
Randall discusses in terms accessible to any interested reader what dark matter (and indeed dark energy) is and what the observational evidence for it might be. To date this comes from a number of astronomical observations which are, I must say, very convincing. To quote from a recent (and fairly readable) review  in Annalen der Physik.
After many decades of increasingly precise astrophysical observations, we have unequivocal evidence that the majority of the material that forms galaxies, clusters of galaxies and the largest observed structures in the cosmos is non-luminous, or dark.
So far everything Randall discusses is essentially standard and uncontroversial. What makes this book interesting is her and her colleagues` proposal that the now well understood and accepted mass extinction of the dinosaurs about 65 million years ago has not only a meteoroid explanation (the relatively recent discovery of the impact crater at Chicxulub in Mexico was the final evidence) but that the oscillation of our solar system through an as yet undetected dark matter disk in our galactic plane was responsible for deviating the orbits of bodies in the Oort cloud, from where many of our comets arise. This oscillation coupled with the gravitational interaction with the dark matter, otherwise undetectable, could generate periodically a few objects large enough to impact the earth and cause extreme environmental damage.
This short review is not the place to go further into her arguments but I would like to recommend this book as an excellent example of how radical ideas, commensurate with what is both known and understood and also what is not actually excluded, can be generated by clever people with open minds. Randall writes well and entertainingly and provides a real insight into the processes by which exotic hypotheses are generated and defended. I greatly appreciated the way that she visibly assigns credit to much more junior colleagues at every point in her collaborative journey. Her most recent paper which summarises (in a condensed form for the physicist reader) the hypothesis and potential evidence to support it, is readily available to anyone .
 Baudis L, “Dark Matter Searches”, Annalen der Physik, 2015 [http://arxiv.org/abs/1509.00869]
 Randall L, Reece M, “Dark Matter as a Trigger for Periodic Comet Impacts”, Physical Review Letters, 2014 [http://arxiv.org/abs/1403.0576]
Peter Hobson is a particle physicist who was never as fascinated by dinosaurs as many other people appear to be. He posts from time to time on his weblog Morgana’s Cat, where very occasionally other extinct forms of life may be seen.
Lisa Randall, Dark Matter and the Dinosaurs (Bodley Head, 2015). 9781847923066, 412 pp., hardback.
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