Book Reviewed: The Story of Collapsing Stars: Black Holes, Naked Singularities, and the Cosmic Play of Quantum Gravity, by Pankaj S. Joshi

Cosmic Censorship of Naked Singularity

Conventional wisdom in cosmology and theoretical physics suggest that a large star ends its life by collapsing itself into a black hole. But some theoretical studies suggest it could become a spacetime singularity, i.e. a black hole but without an event horizon. In fact, physical processes leading to star collapse suggests formation of naked singularities, but they have not been detected in the cosmos. If they are discovered, it would revolutionize our understanding of spacetime and quantum gravity. They could provide laboratory conditions to study quantum-gravitational effects and help support or contradict string and loop quantum gravity theories.

In this book Professor Pankaj Joshi argues that cosmic censorship is not necessarily universal; naked singularities (black holes without event horizon) are formed under certain circumstances and may be observed. The spacetime ripples (gravitational waves) are known to be formed when two black hole or two neutron stars collide, but naked singularities may also generate gravitational waves. Gravitational force dominates at the end of a massive star’s life, and its fate is determined by Einstein’s theory of gravitation, which predicts that the star’s core-collapse results in a singularity. Physicist Roger Penrose conjectured that visible (or “naked”) singularities are forbidden in nature, hence they reside within an event horizon of a black hole.

A brief description of this book is as follows:
Just before the star’s death, nearly a quarter of the mass of the star is ejected within the final fraction of a microsecond. Just before this, a faraway observer would have seen a sudden dip in the intensity of radiation from the collapsing star, which would be caused by quantum gravity. In the early universe, the conditions were extreme and quantum-gravitational effects dominated; and big bang became a unique event. If singularities are detected, they would allow astronomers to observe the equivalent of a big bang every time a massive star collapse to form a spacetime singularity. This event associated with the emission of high-energy gamma rays, cosmic rays, high energy particles and neutrinos. The energy spectrum and its physical characteristics would provide a basis for evaluating theories of quantum gravity.

Stellar life cycle consists of birth from gigantic clouds of dust and galactic material in deep space. They evolve and shine for billions of years, and finally descend into its final phase and eventual death. Stars shine by burning their nuclear fuel; initially hydrogen atoms which fuse it into helium, and later into heavier elements. Each star attains a balance between the force of gravity, which pulls matter toward the center, and the outward pressures generated by fusion. This balance keeps the star active and stable. When all the fuel is converted to iron; nuclear fusion stops. During the final phase of star’s life, the force of gravity dominates and the star contracts and collapse on its core to a size of the earth. At this stage, it will be supported by the force exerted by fast-moving electrons, called electron degeneracy pressure, and the resulting cosmic body is called a white dwarf. Smaller stars lead to red and brown dwarfs. If the star is three to five times the mass of the Sun, it will become a neutron star; here the gravity is so strong that nuclei dissociate into neutrons. The core of this dead star is supported by neutron pressure, and its size may be as little as six miles in dimeter. For more massive stars gravitational force dominate and its final fate is determined by Einstein’s theory of gravitation, which predicts the star’s core-collapse into a singularity.

This book describes theoretical arguments for the plausibility of naked singularities, and ways to detect them through experimentally verifiable predictions. The book reads well, but there is not much of “take-home-message” in this book beyond what is already discussed in his 2009 article in “Scientific American.”