Why is the night sky dark?
This is a classic question, often known as Olbers’ paradox. The brightness we see is the brightness of whatever object emitted the light entering our eye along that line of sight. To get a dark patch of sky one needs there to be either no objects in that particular direction, or have only dim ones. Stars are very bright so if there was a star along that ray we would expect a bright patch unless there was some way to make it dimmer. Distance alone does not dim an object, it just spreads the energy out more. However, the redshift due to expansion does reduce the brightness somewhat. Thus there are two possible solutions to the puzzle – the night sky is dark either because on average there are no stars along the line of sight or because the expansion dims their brightness. In our universe it turns out that both effects enter, in different areas. For objects like stars, there are too few within our horizon to give good odds for a particular ray to hit a star. Since the horizon depends on t
