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Derivation of recombination epoch It is possible to find a rough estimate of the redshift of the recombination epoch , starting by considering that during the era preceding recombination , the photons were primarily coupled to matter through the reaction This reaction requires that the photon ( γ ) have an energy of at least 13.6 electron volt s. As long as photons are coupled to matter , this reaction will be in statistical equilibrium , and so the Saha equation can be applied to determine the equilibrium values of the constituents .
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Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: location, person, organization 
Text: {0} 
 Entities:  
Answer:
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