Olbers, meet SETI

Recall Olbers’ Paradox: why is the night sky dark? If the universe were infinite and static, then a look in any direction in the sky should eventually intercept a distant star; so the sky should be a solid sheet of sunfire. Why is it not?

The answer now given is that the universe is expanding; so it is finite in age, so only a finite amount of light has reached us; and also that distant light sources are receding, and so their light is doppler-shifted to the red. So, for instance, the 3 degree background radiation is the redshifted light of the Big Bang fireball. So in a sense the night sky is a solid sheet of sunfire!

Now recall Fermi’s Paradox: where are they? Meaning, if there are extraterrestrial civilizations, why do we not see them? A special case of Fermi’s Paradox is the Silent Universe. Try as the SETI people might, they have yet to detect radio transmissions from alien civilizations. Why not? After all, in an infinite static universe, no matter which way we point our radio telescopes, we should find in its field of view a radio-transmitting civilization. Therefore the night sky should be a solid sheet of intelligent radio sources. But it isn’t. Why not?

No doubt you see the similarity of the Silent Universe Paradox with Olbers’ Paradox. The latter wonders why the sky doesn’t glow in light; the former wonders why the sky doesn’t babble in radio. Therefore they have the same resolution; the expansion of the universe, which gives the universe a finite age and hence only finite time to beam light or radio; and which also redshifts distant light or radio sources down to invisibility.

So riddle me this: why is night dark and quiet?

Answer: because the universe is expanding.