Particle physics in cosmology

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Particle physics, which deals with the interactions of elementary particles at high energies, is an important component of cosmological models of the early universe, when the universe was dominated by radiation and its average energy density was very high. Because of this, pair production, scattering processes and decay of unstable particles are important in cosmology, and the interface between particle physics and cosmology is sometimes referred to as particle cosmology.

As a thumb rule, a scattering or a decay process is cosmologically important in a certain cosmological epoch if its relevant time scale is smaller or even to the time scale of the universe expansion, which is with being the Hubble constant at that time. This is roughly equal to the age of the universe at that time.

For example, the pion has a lifetime of about 26 nanoseconds. This means that particle physics processes involving pions did not take place until roughly that much time passed since the start of the Big Bang.

Cosmological observations of phenomena such as the cosmic microwave background and the cosmic abundance of elements, together with the predictions of the Standard Model of particle physics, place constraints on the conditions of the early universe. The success of the Standard Model at explaining these observations provides a confirmation of its validity outside of laboratory conditions. In addition, phenomena extrapolated from cosmological observations, such as dark matter and CP-violation, suggest a need for physics that goes beyond the Standard Model.

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