What does it mean if The universe was opaque to radiation at one time?

[land_of_ice]

What does it mean if "The universe was opaque to radiation" at one time?

The time period to which this is referring is during the epoch of decoupling, if you know what that is. (During the formation of atoms in our universe)

What do astronomers mean by "The universe was opaque to radiation" and, also, by saying that,
at early times matter was ionized? And that, the universe was filled with free electrons? And that matter was ionized? By the way what does it mean for matter to be ionized? Does this just mean that, the matter's configuration was in the standard model (protons, nucleus, and electrons) and that the matter had too many electrons, which made it ionized? or that it was a cation and an anion?

 

[DaveC426913]

It was too hot for electrons and protons to make atoms so they raced about freely. Since they're not canceling each other out, they are ions i.e. positive ions and negative ions.

In this state, photons interact strongly with them. This means that any given emitted photon would travel a very short distance before being absorbed again by another ion.

Photons traveling very short distances mean an opaque universe. Think of what you'd see right now in your living room if none of the photons that hit your retina were emitted more than an inch away from your face. No chairs, no walls, no ceiling lights visible. Nothng but a seething fog one inch from your face.

 

[land_of_ice]

It was too hot for electrons and protons to make atoms so they raced about freely.

Thanks <3

for that part and the whole thing, that makes more sense now , the book that this chapter is about wasn't very clear, but yeah thanks :)