Paranormal
Why is the night sky dark?
It's a very simple question, but one that holds profound consequences regarding our understanding of the Universe. In the Netwtonian Universe, there are an infinite number of stars, the Universe is infinitely big, and infinitely old. The simple explanation of this paradox is, everywhere you look in this infinite Universe, there are stars. So this should add up, and the night sky should be as bright as a star.
The slightly wordy explanation of this paradox is: It would be safe to say, these stars can move, but there is no overall contraction or expansion of the Universe. Also, the stars, being infinite in number, would be distributed randomly throughout space. This would logically imply that every direction we look, our line of sight would fall upon a star's surface. Stars that are farther away, would appear individually dimmer, but there are more of them. Nearby clusters of stars would contain several bright stars, and more distant clusters would have dimmer stars, and these would cancel each other out. Hence, the amount of light reaching us, on average, is the same in every direction. Although the light from these would be feeble compared to, say, a light bulb, but considering the Universe is infinite, all these would add up, and the sky would be as bright as the surface of a star. The very fact that this isn't what we observe gives us some insight into how the Universe is built.
This is Olbers' paradox. It can be traced as far back as Kepler in 1610. It was rediscussed by Halley and Cheseaux in the eighteen century, but was not popularized as a paradox until Olbers took up the issue in the nineteenth century.
In 1826 Heinrich Wilhelm Mathäus Olbers, the discoverer of the minor planets Pallas and Vesta, reformulated the paradox; "Why is the sky dark at night? The intensity of light reduces with the square of the distance from the observer. If the distribution of stars is uniform in space, then the number of stars at a particular distance from the observer should be proportional to the surface area of a sphere whose radius is that distance. At each radius therefore the amount of light should be both proportional to the radius squared and inversely proportional to the radius squared. These two effects will cancel and so every shell should add the same amount of light. In an infinite universe the sky would be infinitely bright."
So de Cheseaux thought the sky would be as bright as the sun, whereas Olbers argued that it would be even brighter, and yet as everyone knows the sky is dark at night. This simple fact firmly roots cosmology in the observable sciences, and takes it out of the realm of pure theory. At least one of the assumptions that the universe is static, uniform and infinite in extent must be wrong. Olbers' solution was that the stars had not all been shining in the past but that something had caused them to switch on. What that was he did not know.
Explanations that Don't Work
de Chesaux and Lord Kelvin (who gave his name to the temperature scale) suggested that there might be dust in between us and many of the stars, blocking out the light that we receive from them. However, absorbing dust would eventually come to equilibrium, and emit as much radiation as it absorbed. Even if it was at a different wavelength, we would still receive the same amount of light as before.
Some astronomers thought that if the universe was expanding (as Hubble showed that it is) light from distant stars could be redshifted by the Doppler effect. While this effect provides a contribution, it doesn't account for enough light to darken the entire sky; the remainder should still be detectable.
Explanations that Do Work
This means that at least one of our initial assumptions was wrong - the Universe is not Euclidean, isotropic, homogeneous and infinite in space and time. Here are a couple of popular explanations that do work, although there are others:
Benoit Mandelbrot suggested that there is fractal distribution of galaxies. This would mean that there was a lot of empty spaces between stars, and galaxies, so the light coming from each direction would not add up to infinity.
Maybe the universe is not infinite in time - because the speed of light is finite, perhaps there hasn't been time for the light from stars further than a certain distance to reach us yet. Surprisingly, according to some scholars, this argument was first put forward in Edgar Allen Poe's prose-poem Eureka.
Currently, the finite age solution of the paradox is preferred because it supports Big Bang theory, which says that the universe started a few billion years ago, and has been expanding ever since. This alone (with a contribution from the expanding universe explanation) is enough to account for the sky being as dark as it is. Additionally, the average lifetime of a star is 1010 years, and the time that the Universe should take to reach thermal equillibrium is 1024 years. The the number of stars in existence at any one time is simply not enough to fill the volume of the Universe with enough light to light the night sky.
