Posted by: Alexandre Borovik | January 31, 2009

Stretching yardstick II: Photometric Paradox

This is a continuation of my previous post.

If we assume that the Universe is infinite, stationary and evenly filled with stars, then the number of stars at distance about $R$ from Earth is proportional to $R^2$, while intensity of light from them declines as $1/R^2$. Hence contribution of stars at distance $R$ to the brightness of our skies does not depend on $R$ and positive, hence the brightness of the sky has to be infinite. This is the famous Dark Sky Paradox, or Photometric Paradox — its history can be traced, apparently, back to Kepler.

Revisiting the snail on the rubber tape of the previous post — what will happen with the brightness of our skies if the universe was infinite, uniformly filled with stars, but non-stationary and, instead, uniformly expanding?