Why is the Sky Dark at Night?

5/14/06 – 5/20/06
by C. Zaitz

Technically, the sky it isn’t all that dark, especially in the Metro Detroit area. But let’s think about that glorious silky black sky we remember from our summers up North or some other dark place we’ve been. These are rare places where city lights are so scarce that you can see the hazy arms of our Milky Way Galaxy. The haze represents hundreds of millions of stars, but they are so distant and numerous they appear to cut a luminescent swath across the summer sky.

If you tear your gaze away from that beauty, you will find other parts of the sky that can look quite dark and devoid of stars. How can this be? You might wonder how there could be any dark places in the night sky if there are an infinite number of stars in an infinite universe. Since the time of Kepler people have wondered about this seeming paradox; if there are an infinite number of stars in the sky, then no matter where you look, even out to infinity, eventually you should see the surface of a star. It’s called Olbers’ Paradox.

As stated, the paradox is true. If there are an infinite number of stars, then the entire sky should be lit up as bright as the surface of the sun by their combined light. But perhaps they are not distributed evenly throughout the universe. We know they collect into spirals and blobs known as galaxies. But even so, the infinite number of galaxies should fill up every degree of sky we can see. So why is the night sky dark?

The answer has to do with the fact that universe is expanding, and that it isn’t infinitely old. That really means that it’s not infinite after all. Astronomers have figured out that the observable universe is about 13.7 billion years old. We can’t see anything that might lie beyond a radius of 13.7 billion light years away because the light just hasn’t had time to reach us yet.

The universe is also expanding. The light we get from very distant galaxies is dimmer than light from closer galaxies due to something called their redshift. Their light has traveled so far in something that is expanding on the way (space) that by the time it reaches us it’s pretty tired and weak (shifted toward the red end of the spectrum). The ultimate evidence of this is the Cosmic Background Radiation we find in everywhere in the universe. It is the radiation created in the Big Bang, but it is so weak and redshifted after traveling through expanding space that it is nearly invisible to us in any wavelength.

These are very short (incomplete) explanations to a question which is actually quite deep and somewhat interesting, so if you’d like to know more of the science behind Olbers’ Paradox, you can check out this website: http://en.wikipedia.org/wiki/Olbers'_paradox or Google “Olbers’ Paradox.” Meanwhile you can always enjoy the glorious rays of the planets shining down from east to west as the night progresses. Mars and Saturn chase the sun down to the northwestern horizon at sunset, and beautiful, radiant Venus entices the sun up in the northeast at sunrise, but steady, royal Jupiter, King of the Planets, stays out all night long.

Until next week, my friends, enjoy the view!