Cosmos

Though no one can explain why the Big Bang happened, this chapter explains what followed the Big Bang—our expanding universe. Sagan describes the creation of galaxies: the many different forms they take, and the fact that they interrelate and even sometimes collide. Quasars are major deep-space explosions; many differing hypotheses posit how they come into being, but they are still largely mysterious. In the expansion of the universe, quasar explosions seem to proceed at an incredibly fast pace.

The Big Bang and the ongoing movement of galaxies exhibit the Doppler effect. Humans typically understand this phenomenon through the workings of sound waves; for example, the way a blaring car horn goes from a high to a low pitch as the car emitting it speeds by an individual’s stationary position. As the car moves away from the individual, the sound waves caught by the ear are stretched out, lowering the pitch from the individual’s point of view. Similarly, light is also subject to the Doppler effect, called red shift: An object receding from another object appears redder, because its speed affects the frequency of observable light it reflects (blue is a lower frequency light than red).