Book contents
- Frontmatter
- Contents
- Preface
- Part 1 Foundations
- 1 The nature of cosmology
- 2 Geometry
- 3 Classical physics and gravity
- Part 2 Relativistic cosmological models
- Part 3 The standard model and extensions
- Part 4 Anisotropic and inhomogeneous models
- Part 5 Broader perspectives
- Appendix: Some useful formulae
- References
- Index
3 - Classical physics and gravity
from Part 1 - Foundations
Published online by Cambridge University Press: 05 April 2012
- Frontmatter
- Contents
- Preface
- Part 1 Foundations
- 1 The nature of cosmology
- 2 Geometry
- 3 Classical physics and gravity
- Part 2 Relativistic cosmological models
- Part 3 The standard model and extensions
- Part 4 Anisotropic and inhomogeneous models
- Part 5 Broader perspectives
- Appendix: Some useful formulae
- References
- Index
Summary
In standard cosmology, gravity is modelled by GR. In this chapter we review how, in GR, gravity is represented by a curved spacetime, with matter moving on timelike geodesics and photons on null geodesics. There is no definition of gravitational force or gravitational energy. Thus although GR has a good Newtonian limit, it has totally different conceptual foundations. It is only in restricted circumstances that gravity will be well represented by Newtonian theory. GR also has its limits: it can only be a good description if quantum gravity effects are negligible. Then it is very good: there are no data requiring us to alter it in such contexts, which include all of cosmology except the very earliest times.
This chapter discusses the Einstein field equations of GR, after a short discussion of physics in a curved spacetime and the energy–momentum tensor.We give a brief introduction to the physical foundations of GR such as the equivalence principle and the motivation of the form adopted for the field equations but do not cover the experimental tests (for which see Will (2006); note that except for the binary pulsar data, these tests are essentially tests of the weak field slow motion regime).
Equivalence principles, gravity and local physics
Using our understanding of spacetime geometry, we now consider how to describe local physics in a curved spacetime. Two principles underlie the way we do this: namely, use of tensor equations, and minimal coupling based on covariant differentiation.
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- Relativistic Cosmology , pp. 56 - 70Publisher: Cambridge University PressPrint publication year: 2012