Dark Galaxies

We see stars all around, so why don't their combined lights add up to make our night sky-and surrounding space, for that matter-bright? German physicist Heinrich Wilhelm Olbers put the same puzzle this way in 1823: If the universe is infinite in size, and stars (or galaxies) are distributed throughout this infinite universe, then we are certain to eventually see a star in any direction we look. As a result, the night sky should be aglow. Why isn't it? In fact, the answer is far more profound than it appears. There have been many attempts at explaining this puzzle, dubbed Olbers' Paradox, over the years. One version implicated dust between stars and perhaps between galaxies. The idea was that the dust would block the light from faraway objects, making the sky dark. In reality, however, the light falling on the dust would eventually heat it up so that it would glow as brightly as the original source of the light. Another proposed answer to the paradox held that the tremendous red shift of distant galaxies-the lengthening of the wavelength of light they emit due to the expansion of the universe-would move light out of the visible range into the invisible infrared. But if this explanation were true, shorter, wavelength ultraviolet light would also be shifted into the visible range-which doesn't happen.