4

Tests of General

Relativity

Before we proceed to construct a theory for a possible description of the Universe we
should take a look in this chapter at several phenomena serving as tests of general
relativity. Recall the statistical meaning of tests, discussed at the end of Section 1.4.
In Section 4.1 we describe the classical tests of general relativity which provided
convincing evidence early on that the theory was valid, if only in the weak field limit.
In Section 4.2 we describe the precision measurements of properties of two binary
pulsars which testify to the correctness of general relativity. A very important gravi-
tational phenomenon is gravitational lensing, encountered already in the early obser-
vations of starlight deflected by the passage near the Sun’s limb. The strong lensing
of distant galaxies and quasars by interposed galaxy clusters, which is discussed in
Section 4.3, has become a tool for studying the internal structure of clusters. and the
distribution of dark matter Weak lensing is a tool for studying the large-scale distri-
bution of matter in the Universe, in particular the dark matter which is not observed
by any radiation.
The existence of gravitational radiation, already demonstrated in the case of the
binary pulsar, is an important prediction of general relativity. However, it remains a
great challenge to observe this radiation directly. How to do this will be described in
Section 4.4.
The extremely important case of black holes as tests of general relativity is left for
the next Chapter, Section 5.4.

4.1 The Classical Tests

The classical testing ground of theories of gravitation, Einstein’s among them, is
celestial mechanics within the Solar System. Ideally one should consider the full

Introduction to Cosmology, Fourth Edition. Matts Roos
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.