The 4% Universe
Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality
(Thomas Allen, 2011)
If there is a main storyline to the physics of the last one hundred years, it is the gradual revelation that the universe is a stranger place than we had once thought. In the late nineteenth century many physicists believed that they pretty much had the bag sewn up; the cosmos was more or less a giant game of billiards. Oh sure, there were a few problems here and there: the frequency spectrum radiated by a hot object was surprisingly hard to calculate, and the equations describing electromagnetic waves seemed to imply the existence of a medium filling all space and time that had impossible properties — but, these few problems apart, things were quite neat and tidy. As we now know, those odd problems were destined to overthrow the foundations of the old physics, giving us quantum mechanics and relativity, and ushering in a world in which, as Richard Feynman once quipped, physicists have to believe six impossible things before breakfast.
This book is about a number of recent discoveries and developments in astronomy, astrophysics, and cosmology that seem to indicate that the world is, once again, turning out to be strange and wonderful in ways we did not expect. In a certain sense this is nothing new: the same disciplines have given us black holes, neutron stars, supernovae, the Big Bang, an expanding universe, billions of galaxies, and an unimaginably vast cosmos. But these new developments are sneaking up, as it were, from behind, and seem to be telling us not just that the universe is strange out there, not just that it is bigger and bigger, but that the whole thing, even here and now, is quite different than it has seemed.
Briefly stated, the picture of the cosmos that is emerging from a number of converging lines of evidence is that the stuff we see around us, both here on earth and through our telescopes in the night sky, accounts for just a small fraction of what is actually present. There seems to be quite a lot of something, which we can detect indirectly through its gravitational influence on things we can see, but that we do not see directly. This “dark matter” accounts, rather surprisingly, for about six times more mass than does the matter that we can see and study directly. What is the dark matter? Originally it was thought that it might be dead stars, but there is too much of it for that to be an adequate explanation. It might be heavy particles left over from the Big Bang that do not interact, or interact only rarely, with the matter that makes up our familiar world; there are numerous experiments around the world trying to detect these particles, but to my knowledge they’ve had only null results so far. It is safe to say, at this point, that no-one knows what dark matter is.
But the picture is stranger still: in addition to dark matter, physicists now talk about “dark energy”, an energy density that fills space and is inferable, again, from its gravitational influence. (In general relativity, all energy sources have a gravitational effect.) It is accessible to study principally from its effect on the expansion rate of the universe. One of the most surprising claims made by physicists working on these topics is that the expansion rate of the universe is actually accelerating; this was not expected. Amazingly, the dark energy seems to be, on the largest cosmic scales, the single largest component of the universe’s overall energy density, accounting for over 70% of the total.
The present accounting is therefore something like this: the universe consists of over 70% dark energy, almost 25% dark matter, and only about 4% ordinary (or, in technical lingo, ‘baryonic’) matter — that is, the matter that makes up stars, airplanes, bodies, and harmonicas.
Richard Panek’s book tells the story of how this picture of the world has developed in the last fifty years or so, and especially in the last few decades. He introduces us to many of the people involved, on both the experimental and theoretical sides. As in any healthy science, these developments have been driven by experimental observations, and he describes the main lines of evidence for the existence of dark matter and dark energy: galactic rotation curves, galactic cluster dynamics and structure formation, gravitational lensing, and cosmic acceleration, for instance. A big chunk of the book is devoted to describing supernovae searches, which are certainly important for determining the expansion rate of the universe, but which, in the grand scheme of things, probably do not warrant quite as much attention as they are given here.
I was impressed at Panek’s ability to convey the ideas behind the science without resorting to overly technical language. For instance, he clearly explains how the expansion rate of the universe — whether it is decelerating or accelerating — affects how cosmologists calculate the age of the universe. I wish, however, that he would have explained how, if energy gravitates, a cosmic energy density can cause the universe to expand more quickly, rather than (as one might expect) more slowly. The dark energy seems to have an anti-gravitational effect, pushing things apart rather than pulling them together, and that is bound to seem strange to the reader.
If dark matter and dark energy are posited because of a discrepancy between the predictions of gravitational theory and observations of the distribution of matter, it is logically possible that the problem is not with the matter, but with the theory — that is, it is possible that the discrepancy is resolved not by introducing new, unseen (“dark”) sources of gravitational influence, but by introducing a modification to our theory of gravity. Panek does mention this briefly, but I would have liked to have seen it discussed more thoroughly.
In addition to describing the scientific ideas, the book introduces us to many of the physicists who work on these topics. There are a lot of such people, but he does a reasonably good job of structuring the story around certain key figures, so that what emerges is really a story about people, and not just about names. There were times when I thought that too many characters were crowding onto the stage, but that is hardly his fault; he has to give credit where it is due.
I believe that the book would be accessible to any reasonably intelligent and interested reader. Panek has a style that is sometimes breezy (chapter headings like “The Tooth Fairy Twice” and “The Thing” give an idea of the tone), but this is not too distracting, and might even be appealing to some. I learned quite a lot from it, and that is probably the highest praise that can be given to a book of popular science.
Note: I received a complimentary copy of this book from the publisher.
I cannot resist adding that a good friend (who occasionally comments here) has recently co-authored a paper on dark energy measurements using the cosmic microwave background radiation. That paper, published by the Atacama Cosmology Telescope research group, is available here.