Why the Night Sky Is Dark

Stand outside on a clear night. The sky between the stars is black. That blackness was, for two centuries, a problem nobody could solve.

The argument runs like this. If the universe is infinitely old and infinitely large and roughly evenly populated with stars, then every line of sight from your eye, extended far enough, should eventually land on the surface of a star. Distant stars look fainter, but there are also vastly more of them in any given shell of space. The two effects cancel exactly. The whole sky should glow as bright as the surface of the Sun.

It doesn't. Heinrich Wilhelm Olbers wrote about the contradiction in 1823, though Kepler had noticed something similar in 1610 and Edmund Halley in 1721. For most of the 19th century, the standard guess was interstellar dust soaking up the starlight. That fix doesn't work — heated dust would re-radiate the energy and glow back.

The real answer needed cosmology to grow up. The universe is not infinitely old. Light from sufficiently distant stars has not had time to reach us, because the sources only began shining roughly 13.8 billion years ago. On top of that, expansion stretches the light from the farthest objects out of the visible spectrum into the infrared and microwave bands.

So there is a glow filling the whole sky, exactly where Olbers' logic said it should be. We just can't see it. We call it the cosmic microwave background, and it was found by accident in 1964 by two engineers trying to clean pigeon droppings off a horn antenna in New Jersey.