Chapter 3 Particle Cosmology So far, we have been concentrating on the ΛCDM model of the Universe. We are now going to discuss Particle Cosmology , which is the field concerned with a particle physics description of the (early) Universe. The temperature and energy scales that were dominant during the very early Universe were such that Quantum Field Theory is required for a complete description of that period. As already discussed, the Big Bang Theory is hugely successful in explain- ing many of the properties and features that we observe in our Universe. However, it will become clear that it is not a sufficient theory on its own to explain everything that we observe. There are five main problems with the Big Bang and each of these will be discussed now. We will then see in Section 3.2 how the theory of Inflation can solve these problems. 3.1 Problems of Big Bang Cosmology 24