Transparency Temperature
At temperatures higher than about 3000 K where the average kinetic energy of particles is about 0.26 electron volts, the formation of stable atoms is hindered. Above that temperature, matter exists in a plasma state of ionized atoms, which strongly absorbs electromagnetic radiation of all wavelengths, i.e., the plasma is opaque.
When the plasma cools below about 3000K, it is cool enough for hydrogen and helium nuclei to collect electrons and become stable atoms. The electrons in stable atoms exist in specific quantum energy states that are characteristic of those atoms. They will absorb only radiation of the specific frequencies (specific photon energies) that will cause an electron to jump to a higher quantum state in that atom, unless the photon energy of the incoming radiation is high enough to take the electron all the way out of the atom (ionize the atom). This means that the cooling gas cloud of the expanding universe has a point at which it forms stable atoms and becomes transparent to almost all wavelengths, at least for photons with quantum energy less than the ionization energy of the atoms.
  Helium At this stage of the universe, almost all the ordinary matter was hydrogen and helium (>99%). | To illustrate the idea of transparency, consider a range of the hydrogen emission spectrum and the helium emission spectrum in the visible wavelengths. The three frequencies of light emitted in the top image represent transitions between the quantum levels of the hydrogen atom. The emitted wavelengths from hot hydrogen gas are also the frequencies that could be absorbed by cool hydrogen, so if you illuminated the hydrogen gas with all of the wavelengths in this range, it would be transparent to most of them - only those three narrow energy bands would be absorbed. So the stable hydrogen gas is transparent to most wavelengths. The helium would have more absorbable frequencies, but for both hydrogen and helium, most wavelengths would pass through without absorption. |
This transparency point is a crucial concept in the modeling of the expanding universe and in the modeling of star formation. Key information about it is provided by the 3 K background radiation.
| Neutrino transparency |
| HyperPhysics***** Astrophysics | R Nave |