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Hansen, Frode Kristian (2002): Data Analysis of the Cosmic Microwave Background. Dissertation, LMU München: Faculty of Physics
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Abstract

The early universe consisted of a dense, hot and ionised gas of electrons, protons, neutrons, some light atomic nuclei and photons. The universe at that time was optically thick as the photons could not travel very far before being scattered on electrons in the dense gas. The temperature was too hot for the electrons to combine with the nuclei and form atoms. But as the universe expanded it cooled. About 300 000 years after the Big Bang, the temperature of the dense gas filling the universe was about 3000 degree Kelvin. This temperature allowed the formation of the first atoms in the universe. The electrons combined with the protons to form atoms. The probability for a photon to scatter on a neutral atom is much less than the probability to scatter on free electrons and protons. For this reason it is said that the universe got transparent when the electrons were bound to the protons. The photons continued travelling in a straight line without being scattered. About 12 billion years later some of these photons hit a detector on the planet called 'the earth'. And it provided scientists with valuable information about the origin of the universe. This radiation which has travelled more or less unchanged from the earliest times until today is called the cosmic microwave background radiation, and is the topic of this Ph.D. thesis.