Since galaxies are observed over cosmological length and time scales, the description of their formation and evolution must involve cosmology, the study of the properties of space-time on large scales. Modern cosmology is based upon the cosmological principle, the hypothesis that the Universe is spatially homogeneous and isotropic, and Einstein’s theory of general relativity, according to which the structure of space-time is determined by the mass distribution in the Universe., these two assumptions together lead to a cosmology (the standard model) that is completely specified by the curvature of the Universe, K, and the scale factor, a(t), describing the change of the length scale of the Universe with time. One of the basic tasks in cosmology is to determine the value of K and the form of a(t) (hence the space-time geometry of the Universe on large scales), and to show how observables are related to physical quantities in such a universe.
Modern cosmology not only specifies the large-scale geometry of the Universe, but also has the potential to predict its thermal history and matter content. Because the Universe is expanding and filled with microwave photons at the present time, it must have been smaller, denser and hotter at earlier times. The hot and dense medium in the early Universe provides conditions under which various reactions among elementary particles, nuclei and atoms occur. Therefore, the application of particle, nuclear and atomic physics to the thermal history of the Universe in principle allows us to predict the abundances of all species of elementary particles, nuclei and atoms at different epochs. Clearly, this is an important part of the problem to be addressed in this book, because the formation of galaxies depends crucially on the matter/energy content of the Universe.
In currently popular cosmologies we usually consider a universe consisting of three main components. In addition to the ‘baryonic’ matter, the protons, neutrons and electrons1 that make up the visible Universe, astronomers have found various indications for the presence of dark matter and dark energy (see Chapter 2 for a detailed discussion of the observational evidence). Although the nature of both dark matter and dark energy is still unknown, we believe that they are responsible for more than 95 percent of the energy density of the Universe. Different cosmological models differ mainly in (i) the relative contributions of baryonic matter, dark matter, and dark energy, and (ii) the nature of dark matter and dark energy. At the time of writing, the most popular model is the so-called 
model, a flat universe in which ∼ 75 percent of the energy density is due to a cosmological constant, ∼ 21 percent is due to ‘cold’ dark matter (CDM), and the remaining 4 percent is due to the baryonic matter out of which stars and galaxies are made.
