another CMB question, from Hawkings book

Ive been reading stephen hawkings "the universe in a nutshell". In one chapter he describes how the light we see from older galaxies show they were much closer together. this makes sense, but he then mentions how by measuring the CMB we find it at a constant temp of 2.7 degrees above absolute zero and this proves the radation came from a region of space opaque to microwaves, and this proves that our light cone ( what we see from our point in space) must pass through a certain amount of matter, enough to curve spacetime. i dont understand how it proves that it comes from a region opaque to radiowaves. where does he get this deduction from

Imagine loads of footballers kicking a loads of balls all over the place in an enclosed stadium.

Every time a ball is kicked it hits another player who kicks it again to another player etc. etc.

This kicking goes on around and around endlessly.The balls never escape because they always hit a player and because the "stadium" is enclosed.

The "balls" are PHOTONS
The "players" are charged ELECTRONS and PROTONS which bounce PHOTONS between them.

This is "thermal equilibrium" and is "opaque" because a given Photon does not get far before it is deflected. (Kicked back)

But the stadium is expanding and the "players" are getting tired (The universe is getting cooler.)

Suddenly, (at temp about 3000 degrees) the Protons and Electrons join together to form neutral atoms and go on strike.

They stop playing ball with the Photons.

The Photons now burst out of the stadium.
(about 300k years after the Big Bang) and fly forever freely across the universe hitting nothing at all.(Eventually some hit our detectors).
The uniform temp and wavelength proves they all did this at the same time...when the universe stopped being "opaque".

Hope that helps.

Peter.

You've been watching too much Champions League me thinks...

excellent explination!! perfect. thanks!

one question, how come this was not gradual? if the universe is expanding at a steady rate, then should some light not have escaped before the rest, and this keep happening until it expanded enough so that all the light escaped? how was it all trapped, then suddenly liberated? was there a big jump all of a sudden in the speed of expansion or something?

found my answer on wikipedia. bless that site:

The Big Bang theory predicted the existence of the cosmic microwave background radiation or CMB which is composed of photons first emitted during baryogenesis. Because the early universe was in thermal equilibrium, the temperature of the radiation and the plasma were equal until the plasma recombined. Before atoms formed, radiation was constantly absorbed and re-emitted in a process called Compton scattering: the early universe was opaque to light. However, cooling due to the expansion of the universe allowed the temperature to eventually fall below 3,000 K at which point electrons and nuclei combined to form atoms and the primordial plasma turned into a neutral gas. This is known as photon decoupling. A universe with only neutral atoms allows radiation to travel largely unimpeded.

thanks for your help all