Why Is the Night Sky Dark?

Olbers' paradox: a dark sky is evidence of the entropy gradient

Static, Infinite Universe

In an infinite, static universe every line of sight eventually reaches a star. The sky should be uniformly bright. The night sky is dark.

Young, Expanding Universe

In a young, expanding universe a finite age and finite stellar lifetimes leave most sightlines empty. Expansion adds a secondary effect: the most distant light is redshifted beyond visibility.

Step the young-universe sky from the static prediction to the sky we observe

The darkness between the stars is not emptiness: it is evidence. The night sky is dark because the universe is young.