The Story of Collapsing Stars: Black Holes, Naked Singularities, and the Cosmic Play of Quantum Gravity, Pankaj S. Joshi, Oxford U. Press, 2015. $49.95 (225 pp.). ISBN 978-0-19-968676-6 Buy at Amazon
What happens to a star after it has exhausted its nuclear fuel? If the star’s initial mass is sufficiently small, it collapses gravitationally and becomes a white dwarf or a neutron star. For massive stars, the inevitable final state is a singularity hidden behind an event horizon—that is, a black hole. At least that is the story told by the cosmic censorship conjecture, which does not permit “naked” singularities void of an event horizon as the end state of gravitational collapse. However, the conjecture is still unproven.
In The Story of Collapsing Stars: Black Holes, Naked Singularities, and the Cosmic Play of Quantum Gravity, Pankaj Joshi presents the active field of research on gravitational collapse. He concentrates on possible outcomes of such collapses, probing the conditions under which collapsing stars form black holes, which are not visible to a distant observer, and naked singularities, which can be seen by a distant observer. He also asks how we should formulate the cosmic censorship conjecture so that it can be proved. Joshi outlines his and others’ efforts in the past few decades to address those research questions.
The book starts out with a short, accessible exposition of general relativity. Joshi then describes singularities; in particular, he discusses the singularity theorems, which state the conditions under which singularities form in general relativity. He details the two possible kinds of singularity—black holes and naked singularities. Unfortunately, he neglects to include a short overview of the astrophysical evidence for their existence, as Fulvio Melia did in The Black Hole at the Center of Our Galaxy (Princeton University Press, 2003).
In the subsequent chapters, Joshi presents different models for the gravitational collapse of massive stars. He starts with the idealized collapse of a spherical, symmetric, and homogeneous dust cloud, which unambiguously does lead to a black hole. After that, Joshi relaxes all the assumptions gradually and considers inhomogeneous dust models, different matter models, deviations from spherical symmetry, generalized theories of gravitation, and more. He shows that naked singularities are indeed a viable end state of the collapse of a star whose matter satisfies the standard energy conditions. Unfortunately, the assumptions made beyond the energy conditions—for example, for velocity and density profiles—are not justified astrophysically, microphysically, or otherwise. Thus it might be difficult for readers to evaluate the applicability of the models to realistic collapse scenarios. Additionally, a reference list would have been helpful for expert readers.
Chapter 7, “Cosmic Conundrums,” is a highlight of the book. Therein, Joshi brings together the material of the previous chapters by formulating common questions regarding the cosmic censorship conjecture and additional properties of black holes and naked singularities. He answers those questions directly based on current knowledge and does not shy away from pointing out open issues. For scientists who are interested in studying gravitational collapse, this section will be valuable.
The book concludes with a discussion of the observational signatures of naked singularities and how those objects can be distinguished from black holes. And it discusses possible observations of quantum gravity effects. The extreme conditions near a naked singularity allow processes at such high energies that we might indeed be able to see evidence for a unification of quantum mechanics and general relativity. That would make naked singularities—if they are observed—an exciting laboratory for the discovery of new physics.
Although meant for a popular audience, The Story of Collapsing Stars presents the open research questions so clearly that it can also serve as a valuable starting point for scientists and students considering research in the area of gravitational collapse.
Norman Gürlebeck heads a fundamental physics research group at the Center of Applied Space Technology and Microgravity at the University of Bremen in Bremen, Germany. His research areas include general relativity—in particular, compact objects like neutron stars and black holes—and tests of fundamental physics.
