Dark matter is a hypothetical substance postulated by scientists to help explain astronomical observations. When astronomers peer out at distant objects, they find certain clusters and galaxies behaving in a way that we cannot seem to account for by using merely the gravitational attraction of visible matter. So astronomers speculate that a large fraction of the universe's matter is some completely invisible form of matter that they call dark matter.
Despite some astronomers who speak as if dark matter is fact, there is so far no conclusive evidence that dark matter actually exists. This has not stopped physicist Lisa Randall, who has written a highly readable new book entitled Dark Matter and the Dinosaurs. Like many of her colleagues, Randall is a fearless speculator, not afraid to spin a web of speculations that seems to be built of gossamer threads.
In Chapter 16 of the book, readers will learn one of the reasons why dark matter isn't actually on very solid ground: the fact that scientists have not identified any particles of which dark matter is made up of. The triumphant Standard Model of Physics tells us nothing at all about dark matter particles. In Chapter 17 of the book, Randall discusses attempts to observe dark matter, which have come up short, despite a few false alarms and ambiguous hints.
In Chapter 18 of the book, Randall discusses how computer simulations using dark matter have come up short:
Explaining why density profiles look flat or cored and not cuspy, as per dark matter predictions, is an important challenge to the simplest dark matter models. This, along with the missing satellite problem (fewer dwarf galaxies than predicted orbiting around bigger central galaxies) and the too big to fail problem (a related issue in which the predictions for the densest, most massive galaxies do not agree with observations), possibly point to inadequacies of the standard cold dark matter paradigm.
But Randall has a solution that seems to satisfy her: make the model more complicated to try to explain these discrepancies. Much more complicated. While scientists often claim that they will cheerfully give up a theory when it falls short, it seems that usually when observations conflict with a scientist's favored theory, he or she will prefer not to give up the theory but just to make it more complicated to try to explain away the discrepancies.
What Randall proposes is a model she calls “partially interacting dark matter.” Below are some of her speculations (page 320-321):
I'll call the force that is experienced by the interacting dark matter dark light, or more generally I'll call it dark electromagnetism....It would be an entirely different influence acting on particles charged under a distinct additional force that is communicated by an entirely different new type of particle – a dark photon if you will...Two types of electrically charged particles in the same place that don't interact with each other is really not so mysterious.
What Randall proposes is that dark matter has some type of electromagnetism physics similar to the electromagnetism physics of regular matter, but one that only works with dark matter. This is, of course, wild speculation. One would think that the likelihood of such a thing would be extremely small. It would seem far more likely that dark matter might follow its own weird rules rather than duplicating the rules followed by regular matter. The chance of the two sets of rules matching would seem like the chance of marriage customs on some alien planet matching the marriage customs in America.
Another problem is this: the idea of dark matter was introduced so scientists could avoid having to speculate about “dark forces” – undiscovered laws of nature that might supplement gravitation. But why propose a combination of dark matter and dark forces, when dark forces alone could explain any observational discrepancy?
Randall also proposes that there is a “dark disk” of dark matter that occupies our galaxy, occupying much of the space of our galaxy's disk. She proposes that this “dark disk” somehow had some role in causing a comet to stray from the Oort Cloud that surrounds our solar system, and that such a comet may have wiped out the dinosaurs.
The Oort Cloud (Credit: NASA)
This is all speculation run rampant, and there is no good evidence for such a “dark disk” in our galaxy. I would estimate that the likelihood of dark matter existing is only about 50%, that the likelihood of such a “dark disk” existing in our galaxy is only about 10%, and that the likelihood that dark matter had anything to do with the demise of the dinosaurs is no greater than 1%.
One almost wonders whether the idea for Randall's book wasn't hatched by some book publisher looking to boost sales. You can imagine a conversation like this:
Joe: We can't call it All About Dark Matter. No one will buy that.
John: What about the combo approach? We can call it Dark Matter and Jesus.
Joe: No, those markets don't mesh. What about Dark Matter and Lady Gaga?
John: How can you write a book with that title?
Joe: I've got it! Dark Matter and the Dinosaurs.
John: That's it!
Randall's book is well-written and entertaining, but I am puzzled by why the Midtown branch of the New York Public Library has purchased seven copies of this not terribly important book, and placed them right in the physics section, next to textbooks of physics. It would have been more appropriate to have the book placed in the section with equally speculative books about things like ancient astronauts.