The photons that enter microwave detectors today (and that astronomers interpret as the cosmic microwave background radiation) had their last interaction with matter
A) at the instant of the Big Bang.
B) during the inflationary epoch.
C) during the era of recombination.
D) at the time of symmetry breaking.

Question

Quizplus · Accepted Answer

The cosmic microwave background radiation photons had their last interaction with matter during the era of recombination, approximately 380,000 years after the Big Bang, when the universe cooled enough for electrons to combine with protons, forming neutral hydrogen atoms and allowing photons to travel freely.

The Photons That Enter Microwave Detectors Today (And That Astronomers