Why is the Sky Dark at Night?

Abstract

BONDI¹ has pointed out that with certain very general assumptions, notably the cosmological principle, it can be inferred from the observed darkness of the night sky that the universe is either young or expanding. Harrison² has discussed the same question from a somewhat different point of view. He considered evolving world models populated by stars of fixed luminosity and finite age and concluded that “the night sky is dark simply because the stars are so widely separated from each other …; … the radiation level is affected only in a comparatively slight way by whether the universe is expanding or contracting …”. Although Harrison's argument is mathematically correct, his models are somewhat unrealistic. The present communication shows that the darkness of the night sky actually has a number of distinct causes, among which the one given by Harrison may not be especially important.