The general and special theories of relativity discussed in the previous chapters are the tools currently used in the investigation and description of the universe. Most of the objects in the universe are somewhat mundane: stars, planets, rocks and gas clouds. Yet in many respects the universe is far from being a placid and peaceful place. There are stars which explode with the energy of a billion suns, black holes with millions of times the mass of our sun which devour whole planetary systems, generating in one day as much energy as our galaxy puts out in two years. There are enormous dust cluds where shock waves trigger the birth of new stars. There are intense bursts of gamma rays whose origin is still uncertain.
These phenomena are not infrequent, but appear to be so due to the immense distances which separate stars and galaxies; for one of the most impressive properties of the universe is its size. The universe is so large that just measuring it is very difficult, and finding out the distance to various objects we observe can be a very complicated proposition.
In order to extract information about the universe a toolbox of methods has been devised through the years. I will first discuss the most important of these methods, and with these I will describe how measure the universe and discuss its evolution. We need to determine sizes and distances because, as we will see, they provide basic information about the history of the universe.
Most of the data we get from the universe comes in the form of light (by which I mean all sorts of electromagnetic radiation: from radio waves to gamma rays). It is quite remarkable that using only the light we can determine many properties of the objects we observe, such as, for example, their chemical composition and their velocity (with respect to us). In the first two sections below we consider the manner in which we can extract information from the light we receive.
But detecting light is not the only way to obtain information from the universe, we also detect high-energy protons and neutrons (forming the majority of cosmic rays). The information carried by these particles concerns either our local neighborhood, or else is less directly connected with the sources: isolated neutrons are not stable (they live about 10 minutes), so those arriving on Earth come from a relatively close neighborhood (this despite time dilation - Sect. 6.2.3). Protons, on the other hand are very stable (the limit on their lifetime is more than 1032 years!), but they are charged; this means that they are affected by the magnetic fields of the planets and the galaxy, and so we cannot tell where they came from. Nonetheless the more energetic of these particles provide some information about the most violent processes in the universe.
In the future we will use yet other sources of information. Both gravitational wave detectors and neutrino telescopes will be operational within the next few years. Neutrinos are subatomic particles which are copiously produced in many nuclear reactions, hence most stars (including our Sun) are sources of neutrinos. These particles interact very very weakly, and because of this they are very hard to detect. On the other hand, the very fact that they interact so weakly means that they can travel through very hostile regions undisturbed. Neutrinos generated in the vicinity of a black hole horizon can leave their native land unaffected and carry back to Earth information about the environment in which they were born.