![]() It is thought these fluctuations were probably laid down only 10 -34 seconds after the big bang, and have their origin in quantum fluctuations of a scalar field. At the epoch of recombination, fluctuations on scales larger than 2° must have been produced by matter perturbations already present at this time. No physical processes will act on scales larger than this. At the last scattering surface the particle horizon corresponds to θ ∼ 2° as seen from Earth today. Any objects further than this point are not, and never have been, in causal contact with the observer. The particle horizon of an observer is the distance to the farthest object that could have affected that observer. If the universe was so smooth, then how did anything form? There must have been some bumps in the early universe that could grow to create the structures we see today.Īs the Universe grows older, the observable Universe gets bigger, due to the finite speed of light. It is seen from all directions in the sky and is very uniform. In 1965, Arno Penzias and Robert Wilson were the first to detect this radiation. ![]() 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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