Cosmology and physics

By cosmology, I mean the application of the laws of physics to the Universe as a whole. As a result, the validation of theory depends upon observation rather than experiment, placing us at one stage further removed from our ‘apparatus’ than is the case in laboratory physics. Yet, throughout history, astronomical observations have played their role in establishing new physics, which has been rapidly assimilated into the mainstream of established science. In Chapter 2, Tycho Brahe's observations of the motions of the planets, which led to Newton's law of gravitation, were discussed. From observation of the eclipses of the satellites of Jupiter, Ole Rømer showed conclusively that the speed of light is finite and in 1676 estimated it from the time it takes light to travel across the Earth's orbit about the Sun.

To construct self-consistent cosmological models a relativistic theory of gravity is needed, and most of the tests of general relativity involve the use of astronomical objects. The discovery of the binary pulsar PSR 1913 + 16 has proved to be of particular importance for physics. The pulsar is a magnetised, rotating, neutron star, which has mass about 1.4 times the mass of the Sun, radius r ≈ 10 km and general relativistic parameter 2GM/(rc2) ≈ 0.3. Its companion star is another neutron star of similar mass and they orbit their common centre of mass every 7.75 hours.