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Wednesday, October 3, 2012

Why is the Sky Dark at Night?

This seemingly childish question, of course because there is no sun shining at night!, is actually a very profound question called Olbers' Paradox. Its answer awaited the discoveries of modern cosmology, including the discovery of the Big Bang and of the Cosmic Microwave Backgroud Radiation (CMBR) by Arno Penzias and Robert Wilson at Bell Labs in 1964.

If the universe is eternal and infinite, when we look out into space at night we should eventually see a star. So, the night sky should be filled with star light, bright as day. But, of course, it is not. Why not? That is the paradox.

The universe had a beginning when it exploded from an infinitely small point, a "singularity", and space, time and matter were created. This singularity did not exist in space or time. Rather space and time were created as it exploded and began expanding. A fraction of a smidge of a tiny bit of a second after this "Big Bang", as it is called, the universe began expanding exponentially faster than the speed of light. This is called the period of Inflation, and no central banker had anything to do with it. When this period of inflation ended, just a smidge of a second after it began, the universe was stretched out and homogeneous in all directions, with little seeds of unevenness that eventually became the stars and galaxies we see today. With the expansion of space-time and all within it, many stars and galaxies have been carried beyond our horizon of visibility, so the light from its earliest era comes from stars too distant to ever see, while much of the rest is shifted into the infrared where it is invisible to our eyes.

So, our universe is neither infinite nor eternal. Some of the stars and galaxies from the earliest era of creation are too far away for us to ever see and others have their light shifted toward infrared wavelengths that are invisible to our eyes. Surrounding and permeating it all is the afterglow of the Big Bang, the Cosmic Microwave Background Radiation, which is the remnant of the Big Bang that started everything 13.72 billion years ago. This CMBR shines at a temperature of just 2.725 degrees Kelvin, invisible to our naked eyes but detected by radio telescopes in the microwave portion of the electromagnetic spectrum. If we had radio telescopes for eyes, the sky would be filled with this "light" of creation. "And G-d said, Let there be light!"

The Cosmic Microwave Background Radiation, how our night sky would look in all directions if we had radio telescopes for eyes.

If you would like a capsule video explanation of Olbers' Paradox here is this from Minute Physics, but it takes four minutes to explain:

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