Most of the universe is made up of dark energy and dark matter, neither of which is fully understood or explained by current models and theories. In "The 4% Universe," Richard Panek describes the investigations of scientists.
By PETER WOIT
The topic of Richard Panek's "The 4% Universe" refers not to the sliver of the universe that we think we understand but to the other mysterious 96%, made up of what has become known as "dark matter" (23%) and "dark energy" (73%). Here "dark" refers to an invisibility caused by the lack of interaction with light, making the study of these phenomena a difficult challenge. In this wonderful book, Mr. Panek gives a lively account of how astronomers have met the challenge, finding convincing evidence for a relatively simple, but still mystifying, "Standard Model" of cosmology.
.By the mid-1970s, particle physicists had come up with their own Standard Model of fundamental particles and forces, one that has by now passed several decades of stringent tests carried out at higher and higher energies, corresponding to shorter and shorter distances. Meanwhile, astronomers have experienced a golden age of discovery, driven by ever more powerful telescopes and ever more sensitive detectors. Almost all of what they have found agrees with Einstein's general relativity and the particle physicist's Standard Model, with one exception: Some new, unknown sort of matter seems to make up 23% of the universe. This so-called dark matter is observed solely through its gravitational effects, visible for instance in the way stars orbit the centers of galaxies.
Despite a precise determination of how much dark matter there is, its nature remains unknown. One of the major activities of particle theorists in recent years has been the study of possible extensions of their Standard Model that could somehow explain dark matter. Some physicists, for example, posit the existence of new particles, which may be observable in collisions taking place at the Large Hadron Collider (LHC) in Geneva. The fate of such ideas will be determined through data collected over the next few years.
Mr. Panek mostly avoids the somewhat overexposed subject of particle physics and the LHC, instead describing several groups of physicists searching with much smaller experiments for rare laboratory events that could be attributed to dark-matter particles. This painstaking work is punctuated by moments of drama as data that has been collected over months with elaborate protocols is finally "unblinded," potentially revealing a signal that would justify the announcement of a discovery. Nothing has turned up so far; the search continues.
The centerpiece of "The 4% Universe" is a compelling narrative of science at its best, the discovery of dark energy by two vigorously competing groups of scientists—the Supernova Cosmology Project and the High-Z Supernova Search Team—who are gathering observations of supernovae in distant galaxies. A supernova is a catastrophic explosion of a star that causes it to shine briefly with a brightness greater than that of an entire galaxy. These explosions are fairly well-understood by astronomers, making it possible to use them to estimate the distance to the galaxies in which they occur.
Putting this information together with data on the expansion of the universe has made it possible for the first time to map out how the expansion rate changes with distance. The task required overcoming daunting difficulties, not least persuading other astronomers to give up their hard-won time on a large, expensive telescope that happened to be at the right place to make the necessary measurements immediately after the discovery of a supernova. The effort to calculate the expansion rate of the universe is one of the great stories of late 20th-century science, and Mr. Panek does an excellent job of making it accessible.
The two groups racing to gather supernovae observations found their answers at about the same time—and the discovery surprised many: The expansion of the universe is accelerating. Such a possibility was inherent in general relativity, using a term in Einstein's equations that could be interpreted as assigning energy to the vacuum of empty space. Until the supernova-measurement results, nearly everyone had assumed that this vacuum energy was exactly zero, but instead it turned out to be a number equivalent to 73% of the total energy of the universe. The universe was not just expanding; the rate of expansion was increasing.
Physicists have long been trying to unify their Standard Model and general relativity. A vacuum energy of such a large size, however, is hard to make sense of by using current models, thus driving theorists to desperate measures. One popular "solution" is the multiverse, in which all possibilities occur and our universe happens to have the exact vacuum energy that allows our existence.
Mr. Panek's narrative points to an emerging cultural clash between particle physics and astronomy. Astronomers have traditionally worked alone or in small groups, emphasizing the exploration of varied and complicated phenomena. Particle physicists typically strive toward an understanding of the fundamental constituents of nature, with techniques that now require the collaboration of thousands of scientists. The discovery and investigation of dark energy has taken on more of the characteristics of particle physics than astronomers are used to, requiring large groups of researchers and a tight focus on a specific, simple model. Astronomers worry that, by following the path of particle physics, they will be led into some of the same problems that particle physicists have come to face. In the coming years a variety of new experiments should reveal whether such concerns are justified. Until then, "The 4% Universe" will serve handsomely as an illuminating guide to the dark mysteries lying at the heart of the intersection of astronomy and fundamental physics.
Mr. Woit, a senior lecturer in mathematics at Columbia University, is the author of "Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law."