![]() ![]() At this temperature, the Planck spectrum has its peak at microwave frequencies (∼ 1 – 1000 GHz), and its study forms a branch of astronomy called Cosmic Microwave Background astronomy (hereafter CMB astronomy). The radiation from this point in time has been traveling towards us for 15 billion years and has now cooled to a blackbody temperature of 2.7 degrees Kelvin. ![]() The electrons combine with the protons to form atoms, and the cross section for Compton scattering with photons is dramatically reduced. At this stage, the matter does not have sufficient energy to remain ionised. ![]() The Universe then cooled to a temperature of ∼ 4000K through its expansion about 300,000 years after the Big Bang. The very early Universe was opaque due to the constant interchange of energy between matter and radiation. The Universe started from a very hot, dense phase about 15 billion years ago in the Big Bang. ![]()
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