Significance of Recombination and Why Reionization is Different

The previous lecture segments describes one of the key events after the Big Bang, how protons acquired matching electrons to form electrically neutral atoms.  Recall that this process is called  "recombination". This is a misnomer as there was no earlier "combination" for there to be a "recombination".  The cosmic microwave backgrond radiation is the energy given off by the electrons as they return to a lower energy state in orbit around protons.

Before recombination, light could not travel far before being absorbed and re-emitted by an electron because the density of matter was very high. The Universe was opaque. After recombination, the Universe is essentially transparent so light can travel long distances.


After recombination, the Universe was filled with neutral hydrogen gas with some helium and traces of a view other light atoms. Eventually the Universe cooled to the point that gravity pulled clumps of gas together and the first stars formed.  The first stars radiated photons that ionized the hydrogen which leads to the term "reionization" since the hydrogen was ionized before recombination.  Light can still travel freely even with the hydrogen ionized because the density is so low.

~400,000 yrs    ---------------------->     ~400,000,000 yrs 


Loeb, 2006, Scientific American


Model atom


Schematic of a helium atom

Spitzer Launch

galaxyform.jpg (8103 bytes)




Computer simulation of galaxy formation in the early Universe; Univ. of Chicago Center for Cosmological Physics,



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