Olbers’s ParadoxWhy is the sky dark at night?⇒The night sky should be as bright as the surface of a typical star!If the universe is uniform, isotropic, infinite, and eternal,then every line of sight should end on the surface of a star at some point.Since the no. stars per unit area projected onto the celestial sphere increases as r2, and their apparent brightness decreases as 1/r2, then Solution to Olbers’s Paradox:If the universe is not eternal, i.e., it had a beginning, then we can only see light from galaxies that has had time to travel to us since the beginning of the universe.⇒The observable universe is finite!
This preview has intentionally blurred sections.
Sign up to view the full version.
The Cosmological PrincipleConsidering the largest scales in the universe, we make the following fundamental assumptions:1) Homogeneity: On the largest scales, after correcting for look-back time, the local universe has the same physical properties throughout the universe.Every region has the same physical properties (mass density, expansion rate, visible vs. dark matter, etc.)2) Isotropy: On the largest scales, after correcting for look-back time, the local universe has the same physical properties in any direction.You should see similar large-scale structure in any direction.3) Universality: The laws of physics that govern the universe are the same everywhere in the universe, but could have evolved with time.