The photons that enter our microwave detectors today (and that we 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 era of recombination occurred around 380,000 years after the Big Bang when the universe cooled down enough for electrons and protons to combine to form neutral hydrogen. This allowed photons to travel freely through space without interacting with matter. These photons eventually reached us today as the cosmic microwave background radiation, which is what our microwave detectors pick up.

The Photons That Enter Our Microwave Detectors Today (And That