According to the Wilkinson Microwave Ansotropy Probe mission (WMAP), the time most likely for a given CMB photon to last scatter during the period of decoupling of photons and baryons is 372 000 years. This figure corresponds to a maximum probability. However, still following WMAP, the period of decoupling lasted at least 115 000 more years after this maximum until the probability curve of last scattering of a CMB photon reached a value of 1/2 the maximum. This brings us to 487 000 years. What about further reaching 1/10 or 1/20 of the maximum ? Could it bring us to 600 000 or 700 000 years ? Does it make sense to consider that full decoupling of photons and baryons is not actually realized before such times ?
Photons of the CMB will still very occassionally scatter off the IGM but the number of photons affected is so low now that it is of no overall significance. I believe the mean time and FWHM are generally sufficient to characterise decoupling.
Thank you for your answers. I fully agree with both of you. I anyway have the feeling that, instead of the classical sentence "Decoupling occurred 380,000 years after the Big Bang. The universe became transparent", as heard and read a bit everywhere, one could alternatively say something more encompassing like : "Decoupling occurred a few 100,000 years after the Big Bang with a maximum after 372,000 years. It lasted an additional 115,000 years and another 95,000 (?) years to reach respectively 50 % and 10 % of the maximum, but these periods are of less significance compared to the flash around 380,000 years where the universe got most of its transparency." This makes it more complete for non experts in the field and, even expressed that way, one is amazed of the accuracy cosmological results can reach nowadays !
Thanks for your answer, Dmitri ! My reaction was before I read it.
I usually say it happened "after 380,000 years, over a period of about 140,000 years". For the non-scientists I talk to, that's usually more than they want to know. If you want to go to percentages, then you need a graph of the ionization fraction versus time, IIRC, it's not symmetrical about the 50% time. Note also most of these numbers are not measured, they are calculated from theory based on the time of the mean so "accuracy of results" is somewhat misleading.
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