Answer
a) The cosmic microwave background radiation (CMB) is the electromagnetic radiation emitted at around 400,000 years after the Big Bang.
b) The temperature is now so low because the universe has expanded and cooled significantly since the Big Bang.
Work Step by Step
After almost 400,000 years after the Big Bang, the universe eventually cooled sufficiently that electrons were able to combine with free nuclei to form stable atoms. CMB photons interact very weakly with neutral atoms, so it could travel in straight lines through the universe making the universe transparent. The radiation during this period would have been a blackbody radiation at a temperature of 3000 K. As the universe continued to expand and cool, the wavelengths of this radiation have increased according to Wien's Law. The peak wavelength is now much larger and corresponds to the blackbody radiation at a temperature of 2.7 K.
