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Why the Night Sky is Black

Updated on March 27, 2013

The universe is big. Really big. Some even say that it is infinite. It would seem impossible to measure, but as the poet Edgar Allen Poe suggests, ironically, to what is known as Olber's paradox, the universe is probably finite. Olber's paradox asks why the night sky is dark if the universe is infinite? Rather, if the universe went on forever, then the stars should go on forever, thus making the night sky a brilliant white.

As anyone who has looked up to the night sky knows, stars are simply blotted through out the vastness of space. The universe is, therefore, not infinitely old.

It was proposed that dust could obscure the light emitted from other stars. However, this theory is easily shot down by thermodynamics. Consider, the light from the stars would heat the dust clouds and, therefore, cause those dust clouds to emit the same light-energy as the original star.

This conundrum goes further back than Hienrich Winhelm Olber who suggested the paradox. Scholars and writers alike were soliciting their own theories and variables. Edgar Allen Poe even mentions the possibility that the "voids" in the sky could be from a universe so vast that the light has still to reach us. On the other hand, if the universe were infinitely old, then the light that would travel would have had an infinite time to reach our eyes.

What it really comes down to is whether the universe is static or if it actually has a beginning. Enstien struggled with this question as well. The theory of general relativity should make the universe collapse due to gravity pulling everything together, but it of course does not. Enstien then introduced the his constant equation to make the universe static once more.

In 1929, Edwin Hubble changed Enstien's mind. Originally observed by Vesto Silpher, the star light showed wave lengths that suggested it was actually moving away from us. This is measured much in the way that sound waves are measured in the Doppler Effect. When a sound is further away from the person hearing it, the lower the pitch and longer the wave. As it comes closer, the wave becomes shorter and, thus, higher pitched. The most common example is the sound of an engine riding past someone. The person hears the sound grow sharper as the car approaches and taper off as it moves away.

The same can be measured in light, in what scientist call a redshift. As Hubble observed the light from stars, he noticed that the light made a redshift, meaning galaxy were moving away from the Earth. As with the Doppler Effect, light wavelengths change with distance. On one end of the spectrum is blue, the closest, and on the other is red, the furthest.

Among other evidence, the redshift is just part of the evidence in how scientist determine that the universe is expanding. This expansion shows that the universe may have come from a single point of which it derived and determining that the universe is not static. So, when someone looks up to the night sky and see the darkness of space littered with dots of light, he can think of space and time racing outward and appreciate how a simple observation of freckled stars could have so many implications.


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