Why is the cosmic background radiation so weak today?

Question

a
The Big Bang wasn't as powerful as originally suspected
b
The energy from cosmic background radiation is absorbed by galaxies
c
Cosmic background radiation cannot be measured by current science equipment
d
It started as a fixed amount that has been spread out wide across the entirety of the expanding Universe

Answer 1

The correct answer is:

d. It started as a fixed amount that has been spread out wide across the entirety of the expanding Universe.

The cosmic background radiation (CMB) was released when the Universe cooled enough for electrons and protons to combine into neutral hydrogen atoms, allowing photons to travel freely. As the Universe has continued to expand, the wavelengths of these photons have stretched (redshifted), leading to a decrease in energy and making the radiation appear weaker in intensity today.