Why is the Sky Dark at Night?

Singer Johnny Cash explained in his song "Farmer's Almanac" that God gave us the darkness so we could see the stars. One doesn't have to be religious to gaze upward in awe at the incredible lamp of stars, and, in fact, thoughts about the night sky and stars have led many scientists over the centuries to ponder the deep question, "Why is the sky dark at night?" In 1823, the German astronomer Heinrich Wilhelm Olbers presented a paper that discussed this question, and the problem subsequently became known as Olbers' Paradox.  Here is the puzzle. If the universe is infinite, as you follow a line of sight in any direction, that line must eventually intercept a star. This characteristic appears to imply that night sky should be dazzlingly bright with starlight. Your first thought might be that the stars are far away and that their light dissipates as it travels such great distances. Star light does dim as it travels, but by the square of the distance from the observer. However, the volume of the universe and hence the total number of stars would grow as the cube of the distance. Thus, even though the stars become dimmer the further away they are, this dimming is compensated by the increased number of stars. If we lived in an infinite visible universe, the night sky should indeed be very bright.  

Here's the solution to Olbers' Paradox. We do not live in an infinite and static visible universe. Our visible universe has a finite age and is expanding. According to the Big Bang theory, our universe evolved from an extremely dense and hot state, and space has been expanding ever since. In particular, the Big Bang occurred 13.7 billion years ago, and today, most galaxies are still flying apart from one another. Because only 13.7 billion years have elapsed since the Big Bang, we can only observe stars out to a finite distance. This means that the number of stars that we can observe is finite. Because of the speed of light, there are portions of the universe we never see, and light from very distant stars has not had time to reach the Earth. If this is difficult to visualize, imagine standing in Kansas, while, only an hour ago, two locomotives have started out from California and New York, racing toward you. Obviously you will not see them. The locomotives are metaphors for the unseen light beams racing to us today from far-away stars. Interestingly, one of the first people to suggest this kind of resolution to Olbers' Paradox was the writer Edgar Allan Poe.

Another factor to consider is that the expansion of the universe also acts to darken the night sky because starlight expands into an ever vaster space. Also, the Doppler Effect causes a redshift in the wavelengths of light emitted from the rapidly receding stars. This effect, named after Austrian physicist Christian Doppler, refers to the change in frequency of a wave for an observer as the source of the wave moves. For example, if a car is moving while its horn is sounding, the frequency of the sound you hear is higher (compared to the actual emitted frequency) as the car approaches you and is lower as it moves away. Although we often think of the Doppler Effect with respect to sound, it applies to all waves, including light.

Life as we know it would not have evolved without the Olbers' effect because the night sky would otherwise have been extremely bright and hot. Olbers was not the first to pose this problem, and physicist Lord Kelvin provided a satisfactory resolution of the paradox in 1901. Note also that although the sky might be very bright in an infinite visible universe, it would not be infinitely bright. Beyond a certain distance, the surface areas of the stars would appear to touch and make a "shield." As a final part of our explanation, note that the stars themselves have a finite lifetime, which should be considered in our explanation and resolution of Olbers' Paradox. The next time we gaze up at the night sky, we can be thankful that we are not blinded by the light and live in a universe in which darkness competes with the stars.