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)
~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