How much light has been emitted by all galaxies since the
cosmos began? After all, almost every photon (particle of light) from
ultraviolet to far infrared wavelengths ever radiated by all galaxies that ever
existed throughout cosmic history is still speeding through the Universe today.
If we could carefully measure the number and energy (wavelength) of all those
photons—not only at the present time, but also back in time—we might learn
important secrets about the nature and evolution of the Universe, including how
similar or different ancient galaxies were compared to the galaxies we see
today.