Because of this correspondence, many aspects of the very early universe can be described with models of particle physics. Conversely, we are now beginning to learn things about particle physics through observations that use telescopes and other methods to look far into the universe and deep into its past. The recent abundance of results from such observations has made the subject of particle cosmology blossom. A field , or classical field, is a mathematical entity which can be thought of as a collection of numbers that tells you something about each point in space-time, such as the temperature at each point in a room. A quantum field is a more complicated mathematical entity which, in particular, can describe the creation and destruction of particles.
Field theory is a collection of rules that determine how fields behave and evolve.
Quantum field theory has been exceedingly successful in explaining phenomena in particle physics involving the strong, weak, and electromagnetic forces, which together with gravity constitute the forces of Nature. For example, there is a particle called the muon which is a heavier cousin of the electron. The muon's magnetic dipole moment , i.e. ability to respond like a compass needle to a magnetic field, has been calculated so accurately using quantum field theory that the difference between the calculated value and the value measured in experiments is only a few parts in a million. Field theory is successful not only for tiny particles, but can be applied to the universe as a whole. The best explanation of the observed expansion of our universe is that it originates from a point-like superdense state, the Big Bang.