Olber’s Paradox

“The idea of an infinite, Euclidian, homogeneous and static universe was shattered by the discovery of the Hubble expansion…this model…should have been ruled out long before this by a simple argument now known as the Olbers’ Paradox.” (Coles-48-49)  Olber’s paradox is defined as “the fact that the night sky is dark even though in an infinite universe with stars that live forever, the night sky would be as bright as the surface of the a star.  The paradox disappears when it is realized that stars do not live forever.  In the modern big bang model, expansion of the universe also plays a role in making the sky dark at night by red-shifting the cosmic background radiation to a band well below the visible.” (Hawley-482)  Heinrich Wilhelm Matthaus Olbers(1758-1840) made many contributions to the world of astronomy.  Although he is best know for the paradox named after him regarding why the night sky is dark, he also claculate the orbit of the comit of 1779, discovered two minor planets and five comets and invented a method for calculating the velocity of falling stars. 

            Essentially, Olbers’ paradox questions why the night sky appears to be dark, when in theory, it should be as bright as day.  Everyone can see that the sky at night is dark with a few spots of light provided by stars and planets, but not many understand the other aspect of the paradox.  Many people believe the universe to be infinite in size.  If this is true, there are stars that are as old as the universe and are therefore very far away from us.  There are also younger starts that are quite close.  Because there are more of the older, more distant stars, we should see the same amount of light from these as from the fewer, closer stars.  If this were true, the night sky would be incredibly bright, because we would be observing light from all of the stars from all time.  By studying this information, we would observe that there is no night and day and it would be too hot and bright on earth for life to exist.  Because we know that neither of these things are true, we must assume that either the universeis not infinite or it is not homogeneous.  Going back to the old and distant stars , in order for us to observe them would require that they were all “switched on at different times”, implying that we hold a special position in the universe. (Coles-49)  One of these possibilities, that the universe is finite, Newton contested with his law of gravity.  This law states that if this is true, the universe would “contract to a central point.” (Hodge-75)  We know now that this is not true and we can forgive Newton’s mistake because he did not consider the expansion of the universe in his calculations.  Throughout the centuries, many people have tried to explain Olbers’ Paradox and quite a few of these explanations involve the sun holding a special place in the universe. 

However, because we know that we are not the center of the universe (as much as we like to think that sometimes), other explanations must exist.  Some of these explanations are:

            ~The universe is not infinite and only has a finite number of stars.

                        ~There is so much dust in the universe that it does not allow us to see all of the stars and the brightness from them.  This does not hold true because as the dust absorbs the light, it will be brought into “thermodynamic equilibrium” and will give off radiation with the same amount of energy as the light that it absorbed. (Coles-49)  “A dramatic example of this dust can be seen in this image of the constellation of Sagittarius. All of the dark patches and stripes through the image are where dust is blocking out stars behind it. In fact, toward the very center of the Milky Way, the dust is so good at absorbing light that the light coming from stars at the center of the Milky Way is 1 trillion times dimmer than if there was no dust!” (http://www.madsci.org/posts/archives/oct98/905995334.As.r.html)                 

 

Sagittarius

 

~The universe is expanding and therefore, the distant stars are red-shifted to the point where they are unobservable.  This is also not a possibility because as these stars become redshifted into the infrared range and out of view, others are being shifted from the ultraviolet into the visible range. 

                        ~The universe is not infinitely old and therefore the light from the most distant objects has not yet reached us.

Ultimately, however, E.R. Harrison discovered that in order for the night sky to be bright, we would have to be able to see stars that are 1024 years old.  According to Hubble’s law, though, the oldest that stars can be is approximately 1010 years old.  Therefore, the night sky is bright because the stars at the edges of the universe no longer emit light, so we can not therefore observe it. (Pasachoff 604)

 

 

Works Cited:

            Coles, Peter and Francesco Lucchin.  Cosmology:The Origin and Evolution of the Cosmic Structure.  Chichester, John Wiley & Sons, 1995.  pp48-50

 

            Hawley, John F. & Katherine A. Holcomb.  Foundations of Modern Cosmology.  New York, Oxford UP, 1998.  pp319-320

 

            Hodge, Paul W.  Concepts of the Universe.  New York, McGraw-Hill Book Company, 1969.  pp75-76

 

            Pasachoff, Jay M.  Astronomy:From the Earth to the Universe.  Fort Worth, Saunders College Publishing, 1998.  pp603-604

 

            http://pegasus.astro.umass.edu/a100/handouts/olber.html

 

            http://www.weburbia.com/physics/olber.html

 

            http://search.biography.com/print_record.pl?id=7011