Cosmology and Particle Physics

This is the “odd chapter out” because it does not deal with a particular messenger or its results. Instead you will see what kind of measurements have tranformed cosmology into a quantitative discipline within the last 50 years. The starting point was the detection at short radio wavelengths of the comic background radiation in 1964, which gave strong support to Big Bang cosmology. This was followed by observational work, mostly by the optical astronomers, on the production from hydrogen of the elements heliium, deuterium and lithium in the primordial fireball, which permitted the prediction of the ratio of baryons to photons in the universe as a whole, as well as giving information about the number of types of neutrinos and the mean lifetime of free neutrons. This work showed that the mean density of baryons (protons plus electrons) in the universe is only some 4% of the quantity needed to make it “closed” by its own gravity. This links to the problem of dark matter, needed to explain the rotation curves of galaxies, which for two decades was understood to make up the remaining 96%. Then the optical observers detected that the universe is accelerating, and the dark energy needed to produce this acceleration is estimated at over 70% of the “closure density” leaving 26% as dark matter, and 4% as “ordinary” baryonic matter. This was all tied together by increasingly precise measurements of the variation over the sky of the cosmic background radiation, which shows that the universe is indeed “just closed”. The only problems are that we don’t know what the dark matter or dark energy are. Experiments to detect dark matter particles form the latter part of the chapter.