Radiation from the Early Universe

Shifting the wavelength to longer and longer values is therefore the equivalent of shifting the characteristic temperature of the radiation to lower and lower tempera- 

In an ionized hot plasma (e.g. inside the Sun and stars) the free electrons strongly interact with radiation, and its progress gets blocked. Therefore, the hot early universe was not transparent. As the universe expanded, the gas cooled. At an age of about 100000 years, when the size of the universe was about 10 of its current size, the temperature dropped to a few 1000 of K. Protons and electrons were able to combine to form hydrogen atoms. This is also called recombination of the universe. The hydrogen atoms interact less effective with radiation, the universe became transparent. Since the time of recombination, radiation was able to travel unimpeded throughout the universe. At the time of recombination, the radiation was peaked around 1pm. Now it is peaked around 1 mm (i.e. a factor of 1000). 

In 1989 the Cosmic Background Explorer, COBE, was launched. It made precise measurements of the CBR at wavelengths from a few micrometers out to 1 cm (Fig. 8.1). COBE unambiguously proved the CBR to follow a Planck curve at 2.73 K so there is no doubt that we are seeing the residual radiation left behind from the primordial Big Bang. Tiny variations in the CBR result from the formation of structures during the early evolution of the universe (see also Fig. 8.2). 

When the first objects in the universe (probably massive population III stars) started to radiate, the universe went from neutral to become ionized again. This reionization occurred between 150 million and 1 billion years after the big bang (corresponds to 6 z 20). 

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