exception of our nearest neighbors (such as Andromeda), all other galaxies in space emit redshifted light—and are therefore racing away from us, like engines from a firehouse. As Hubble observed, the more distant the galaxy, the faster its recessional speed.

The Milky Way isn’t alone in being shunned by remote galaxies. We occupy no special place in the cosmos and must assume that the behavior of the galaxies in our region is essentially the same as galaxies everywhere. Therefore, all the distant galaxies must be moving away from each other as well, pointing to a grand expansion of space itself.

Note that the universal expansion does not cause Earth itself to grow bigger. Nor does it cause the solar system to enlarge. Rather, it operates solely on the grandest level: the arena of the Milky Way and its many galactic cousins. Their colossal feud has little effect on our planet—except for telltale signs in the light that the distant galaxies produce. As physicist Richard Price of the University of Texas at Brownsville recently said, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe.”

The Hubble expansion includes two effects that bear on Olbers’ paradox and they concern the energy and density of radiation. The redshifting of light causes it to lose energy. Red starlight, for example, is cooler than yellow. Consequently, as the universe grows, its radiation becomes less powerful. Furthermore, the enlargement of space offers ever-increasing room for photons (light particles). As time goes on, each cubic foot contains, on average, fewer and fewer of these particles. Thus, the Hubble expansion has a double-barreled effect: It cools and dilutes the light in the universe. It makes Earth’s night sky darker than it would have been otherwise. Therefore, according to this explanation, the reason we aren’t immersed in light is similar to someone trying to take a hot bath in an ever-expanding bathtub. The growth of the bathtub would continuously cool the water and lower its level. Over time, nothing would remain but cold, isolated droplets. By analogy, Earth’s night sky displays cooled-down, scattered points of light rather than a warm, luminous flood.

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