Chapter 9: Fundamental Physics

Section 3: Irreversibility and the Second Law of Thermodynamics

Alignment of time in the universe

Evidence from astronomy clearly suggests that the direction of irreversible processes is the same throughout the universe. The reason for this is presumably that all parts of the universe are expanding—with the local consequence that radiation is more often emitted than absorbed, as evidenced by the fact that the night sky is dark. Olbers' paradox asks why one does not see a bright star in every direction in the night sky. The answer is that locally stars are clumped, and light from stars further away is progressively red-shifted to lower energy. Focusing a larger and larger distance away, the light one sees was emitted longer and longer ago. And eventually one sees light emitted when the universe was filled with hot opaque gas—now red-shifted to become the 2.7K cosmic microwave background.

From Stephen Wolfram: A New Kind of Science [citation]