Next steps

This final chapter consists of a brief discussion of where the reader can go from here: active research topics in general relativity and gravitation, open questions, and ideas for further study. Topics include exact and approximate solutions of the field equations, including numerical methods and perturbation theory; problems in mathematical relativity, including global geometric methods, singularity theorems, cosmic censorship, and asymptotic conditions; alternative models such as scalar-tensor models; approaches to quantum gravity; and experimental gravity. These topics are not discussed in any depth; rather, the chapter is meant as a “teaser” to encourage readers to look further.

The Einstein field equations

The Einstein field equations are the fundamental equations of general relativity. After a brief qualitative discussion of geodesic deviation and Newtonian gravity, this chapter derives the field equations from the Einstein-Hilbert action. The chapter contains a derivation of Noether’s theorem and the consequent conservation laws, and a brief discussion of generalizations of the Einstein-Hilbert action.

Gravity as geometry

General relativity describes gravity as a byproduct of the geometry of spacetime. This chapter explains why this notion makes sense, introducing the principle of equivalence and the universality of gravitational interactions.

Cosmology

Starting with the assumptions of homogeneity and isotropy, the cosmological solutions of the Einstein field equations—the Friedmann-Lemaitre-Robertson-Walker metrics—are derived. After a discussion of constant curvature metrics and the topology of the Universe, the chapter moves on to discuss observational implications: expansion of the Universe, cosmological red shift, primordial nucleosynthesis, the cosmic microwave background, and primordial perturbations. The chapter includes a brief discussion of de Sitter and anti-de Sitter space and an introduction to inflation.

The weak field approximation

The Einstein field equations are a complicated set of coupled partial differential equations, which are usually too complicated to find exact solutions. This chapter introduces a simple approximation for weak fields. It discusses the lowest order solution, which gives back Newtonian gravity, and the next order, which includes “gravitomagnetic” or “frame-dragging” effects. The chapter briefly discusses higher order approximations, expansions around a curved background, and the evidence that gravitational energy itself gravitates. It concludes with a brief description of an alternative derivation of the Einstein field equations, starting from flat spacetime and “bootstrapping” the gravitational self-interaction.

Black holes

Chapter 3 used the Schwarzschild metric to obtain predictions for the Solar System. In this chapter, that metric is derived as the unique static, spherically symmetric solution of the vacuum Einstein field equations. For the Solar System, this vacuum solution must be joined to an “interior solution” describing the interior of the Sun. Such solutions are discussed briefly. If, on the other hand, one assumes “vacuum all the way down,” the solution describes a black hole. The chapter analyzes the geometry and physics of the nonrotating black hole: the event horizon, the Kruskal-Szekeres extension, the horizon as a trapped surface and as a Killing horizon. Penrose diagrams are introduced, and a short discussion is given of the four laws of black hole mechanics.

Manifolds and tensors

The mathematical basis of general relativity is differential geometry. This chapter establishes the starting point of differential geometry: manifolds, tangent vectors, cotangent vectors, tensors, and differential forms. The metric tensor is introduced, and its symmetries (isometries) are described. The importance of diffeomorphism invariance (or “general covariance”) is stressed.

Geodesics

The generalization of a “straight line” in Euclidean geometry is a geodesic, the shortest distance between two points in a (possibly curved) space or spacetime. This chapter introduces geodesics, starting with examples and leading up to the general geodesic equation. Along the way, the metric and the notion of causal structure are explained, and it is shown that the geodesic equation can reproduce the equations for motion of an object in Newtonian gravity.

The Hamiltonian formalism

So far, general relativity has been viewed from the four-dimensional Lagrangian perspective. This chapter introduces the (3+1)-dimensional Hamiltonian formalism, starting with the ADM form of the metric and extrinsic curvature. The Hamiltonian form of the action is served, and the nature of the constraints—and, more generally, of constraints and gauge invariance in Hamiltonian systems—is discussed. The formalism is used to count the physical degrees of freedom of the gravitational field. The chapter ends with a discussion of boundary terms and the ADM energy.

Gravitational waves

In the weak field approximation, the Einstein field equations can be solved, and lead to the prediction of gravitational waves. After showing that gravitational radiation depends on changing quadrupole moments, this chapter describes the production, propagation, and detection of gravitational waves. It includes discussions of the speed of gravity, detectors, the “chirp” waveform for a compact binary system, and the nature of astrophysical sources.