Thread: When did the cosmic background radiation come into the universe?

A. When proton and neutrons were first formed.
B. When gas in the universe is thin enough for the photons to travel more less freely.
C. When the big band first began to expand.
D. The inflationary period.
E. When gamma rays had enough energy to destroy nuclei.

C. When the Big Band played God Save the Queen

None of the above. Whoever asked the question tried to get it right and messed up completely. Write "my teacher is clueless about physics and astronomy" on the sheet and let the chips fall.

The cosmic microwave background radiation and the cosmological red shift are together regarded as the best available evidence for the Big Bang (BB) theory. The discovery of the CMB in the mid-1960s curtailed interest in alternatives such as the steady state theory. According to standard cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 400,000 years after the Big Bang ? this time period is generally known as the "time of last scattering" or the period of recombination or decoupling ? the temperature of the Universe was about 3,000 K. This corresponds to an energy of about 0.25 eV, which is much less than the 13.6 eV ionization energy of hydrogen. Since then, the temperature of the radiation has dropped by a factor of roughly 1100 due to the expansion of the Universe. As the universe expands, the CMB photons are redshifted, making the radiation's temperature inversely proportional to the Universe's scale length.

B. Until about 350,000 years *after* the Big Bang, the density and temperature of the universe was such that it was totally opaque. At about T+350,000 years the density and temperature had decreased enough (..down to about 3,000 degrees K..) that light first shined in our universe.

The cosmic back ground radiation began close to 100 million years after the big bang,when the first stars began to light up.