In our expanding universe, galaxies rush away from one another like a dispersing mob. Any two galaxies recede at a speed proportional to the distance between them: a pair 500 million light-years apart separates twice as fast as one 250 million light-years apart. Therefore, all the galaxies we see must have started from the same place at the same time¿the big bang. The conclusion holds even though cosmic expansion has gone through periods of acceleration and deceleration; in spacetime diagrams (right), galaxies follow sinuous paths that take them in and out of the observable region of space (yellow wedge). The situation became uncertain, however, at the precise moment when the galaxies (or their ancestors) began their outward motion.
This article was originally published with the title The Myth of the Beginning of Time.