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This book gives a description of state-sum quantum gravity models which are based on triangulations of a smooth spacetime manifold. It contains detailed descriptions of Regge quantum gravity, spin-foam models and spin-cube models. Some other similar models, like the dynamical triangulations models, are only briefly described, since the sum over the spacetime triangulations is outside the scope of this book.

The book also contains a detailed description of the approach where the piecewise linear (PL) manifold corresponding to a smooth manifold triangulation is considered as the basic structure of the spacetime. Hence the PL structure is not an auxiliary tool used to define the gravitational path integral for a smooth spacetime, but it is taken as a physical property of the spacetime. Consequently, it is straightforward to construct a finite gravitational path integral. Another consequence is that the problems of determination of the classical limit and how to calculate the quantum corrections can be solved by using the effective action method. The smooth manifold limit problem is then replaced by the problem of a smooth manifold approximation for the effective action, which can be obtained by using the standard quantum field theory with a physical cutoff.

Some physical effects of a PL spacetime quantum gravity theory are also described, one of which is that the cosmological constant spectrum contains the observed value.

A short exposition of higher gauge theory is also given, which is a promising way to generalize a gauge symmetry by using the concept of a 2-group. A 2-group is a categorical generalization of a group, and by using this approach one can construct the spin-cube models of quantum gravity.

Contents:

Preface
Introduction
Classical Theories of Gravity on PL Manifolds
State-Sum Models of QG
Effective Actions for State-Sum Models
Applications of PLQG
PLQG and Other QG Models
Appendices:
- 2-Groups
- Proof That a 0
- Regge EA Perturbative Expansion
- Gaussian Sums
- Higher-Loop Matter Contributions to the Cosmological Constant
- Isosceles 4-Simplices
- Fourier Integral of a PL Function
Bibliography

Readership: PhD students in mathematical physics and theoretical physics. Researchers in quantum gravity and related mathematical physics.
Key Features:

Describes a novel approach in the study of quantum gravity (QG) state-sum models, which is based on the application of the effective action method from quantum field theory. Related to that is a study of the effect of a non-trivial path-integral (PI) measure on the PI finiteness, as well as a study on the dependence of the semi-classical expansion of the effective action on the PI measure
Detailed study of the idea that the spacetime at small distances is not a smooth manifold but a piecewise linear (PL) manifold corresponding to a triangulation of a smooth manifold. One consequence of a QG theory on a PL spacetime is that the cosmological constant has a continuous spectrum, and the spectrum contains the observed value of the cosmological constant
A clear explanation of the relationship between a higher gauge theory and general relativity, and how new state-sum models of QG are then constructed. These new state-sum models are called spin-cube models, which are categorical generalizations of the spin-foam models, since one labels the edges, triangles and tetrahedra with representations of 2-groups, which are categorical generalizations of groups

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State-Sum Models of Piecewise Linear Quantum Gravity

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