Astronomy Buff

Why is the Sky Dark at Night?

by Tony on February 7th, 2007

This is my third post in the Just Science Week Challenge.

There is enough luminous matter in the universe to completely light up the sky brighter than the surface of the Sun.

If you add up all the photons spewing out of all the stars and galaxies, it turns out that there is enough to light up the universe, yet that is not the case. WTF?

People have been thinking about this for a long time, Kepler wondered about how this could be, but it wasn’t until a guy named Olber made it a paradox that it became widely discussed.

Simply stated, Olber’s paradox is: if the universe is infinite and static, then the sky ought to be bright because there would be a star everywhere you looked, in every piece of sky.

Well, the clincher here is the ‘if the universe is static and infinite’ part. We’ve already established that the universe is neither of those things. It has a finite age and is an extremely dynamic place. The universe isn’t just sitting around letting us watch YouTube videos in it all day, it is also expanding and evolving.

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Still, this whole idea lends support to what we have been talking about so far: the Hubble expansion and the finite age of the universe. The fact that the sky is dark is one more piece of evidence that the Big Bang occurred some finite time ago and that the universe is indeed getting bigger all the time.

Here is what we know:

• The number of photons flying around the universe is a conserved quantity, meaning that the number of them don’t change as they fly around.
• What does change is their wavelength, due to the doppler shift.
• The energy of a photon is proportional to its frequency and inversely proportional to its wavelength. So, as wavelength increases, it’s energy decreases.
• Stars (sources of photons) are not evenly distributed in the universe, they are clumped up in galaxies and galaxies are all bunched up in groups. The universe is a very gloppy place.
• The universe is not infinitely old, it is 13.5 billion years old.

So where are all the photons?

The sky is not lit up like a car dealership parking lot because of several factors:

• The universe is relatively young. The light from the most distant sources hasn’t reached us yet.
• Most of the light is redshifted out of our perception due to the expansion of the universe.
• The universe has a finite number of stars.
• The distribution of luminous sources is not even. Some places are brighter than others and some light is hidden behind other.

Of the above list, the first and second are the most responsible for the dark sky. The finite age of the universe, coupled with its expansion keep the number of photons low enough for a dark sky. As the universe gets bigger, photons get redshifted away and since everything has only been around for 14 billion years or so, photons do not accumulate.

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POSTED IN: cosmology