Physics.stackexchange

Никчемные чмошники не захотели поддержать инициативу по продвижению High energy physics stackexchange. Вчера/сегодня открылся общий раздел под названием Physics. Я там зарегистрировался и задал для порядку простой вопрос:

I have a question about cosmology. At popular level people explain the time of decoupling of matter and radiation as the moment when temperature falls enough for nuclei and electrons to recombine into atoms. People say that the Universe became “transparent”. The photon cross section by an electro-neutral system is smaller than Thomson's one, i.e., technically it means that the mean free path became very large or even infinite.

However, there is another mechanism for the mean free path becomes infinite. It is because during its expansion the universe becomes less dense. Let me explain it by example. Imagine that you are in the forest where the diameter of tree is a, and mean distance between trees is b. What is the mean diameter of the observed area? It is proportional to b²/a, I suppose. Now let's imagine that our forest is in the expanding universe, i.e., b grows (linearly, for example) with time while a remains constant. Then at some moment “the mean diameter of the observed area” starts to grow faster than the speed of light, i.e., becomes infinite.

It implies that the recombination is not necessary for the decoupling. Did the recombination start earlier than the moment I described above? Or both mechanisms (recombination and desity fall) are equally important for the decoupling.

Внятного ответа пока ещё не получил. Посмотрим как будут развиваться события.