Olber's Paradox

This chapter gives you the Big Picture called cosmology. Cosmology is the study of the nature, origin, and evolution of the universe as a whole. The observational aspect of cosmology deals with finding distances to galaxies which is necessary for determining the geometry of the universe. This was covered in the last chapter.

Observations and Some Implications

At first you might think that in order to understand the structure of something as large as the universe, which by definition contains everything there is, you would need some very powerful telescope to see to the farthest reaches of space and a complex theoretical model. Actually, there are some powerful conclusions you can draw from observations with the naked eye. We will explore that first and then move on to conclusions we can draw from extending our eyesight. We will explore the basic questions that human beings have been asking themselves ever since we have walked the Earth: where did we come from and where are we going?

Universe Contains Mass---Why has the Universe Not Collapsed?

The universe is not empty. There is matter with mass, so the attraction of gravity is present. Newton knew that if the universe has existed forever and is static, that is, it has no net pattern of motion, then there must be enough time for gravity to collapse the universe, but this has clearly not happened! He knew of three ways to resolve this paradox. Either the universe is infinite in volume and mass or it is expanding fast enough to overcome the gravitational attraction or the universe has a beginning and/or an end. The last two ways violate the assumptions of an eternal and static universe, of course. So Newton chose the infinite universe option. Notice that we are able to arrive at the conclusion of an infinite universe from just one observation: the universe is not empty. No telescopes are needed, just the ability to follow a train of logical thought to its conclusion.

Olbers' Paradox and the Dark Night Sky

If you ever look outside, you will notice that the night sky is dark. Why? If there are stars in space everywhere we look, then should not the night sky be lit by countless myriad stars, making the night sky light? If the night sky were to be completely lit by various star's light, the glare would be overwhelming. Why then is the night sky dark?

Additionally, the light from so many stars would cause the temperature of the Earth to rise above 2000 degrees. This would make life on Earth impossible. Therefore, why too is the night sky cold?

The question of why the night sky is not dark if stars are everywhere was considered by Viennese physician Heinrich Olders in 1826. He was not the first to come up with this question. Thomas Digges wrote about it in 1576, Kepler thought about it in 1610, and Edmund Halley and Jean Philippe de Cheseaux talked about it in the 1720's, but Olbers stated it very clearly, so he was given credit for it. Known today as Olbers' Paradox, the conflict between observation and theory has been difficult to resolve. If the universe is infinite and uniformly filled with stars, then our line of sight in any direction we peer ought to fall upon the surface of a star.

Imagine being in a dense forest. No matter where you look, you see trunks of trees. In the astronomical example, everywhere we look we ought to see stars. The two images below are of the Sagittarius constellation region of the night sky, showing just how densely pack stars are at naked eye levels on your left, and optical telescope levels on your right. Even with such a dense mat of stars, there is plenty of darkness in there.

You might be thinking, the more distant stars are fainter than nearby stars because of the inverse square law. But the farther you look into space, the larger the volume you will see, and thus the two effects cancel out. Every spot on the sky must be occupied by a star, and the night sky should be dark. Now we know that stars are grouped into galaxies, but the paradox remains: our line of sight will eventually intersect a galaxy.

The brightnesses of stars does decrease with greater distance (remember the inverse square law) BUT there are more stars further out. The number of stars within a spherical shell around us will increase by the same amount as their brightness decreases. Therefore, each shell of stars will have the same overall luminosity and because there are a lot of ever bigger shells in an infinite universe, there is going to be a lot of light!

Olbers concluded that the night sky was dark because intervening clouds of dust and gas absorbed the starlight from distant sources. However, these same clouds would heat up from the collection of light energy and glow brightly. Any intervening material absorbing the starlight would eventually heat up and radiate as much energy as it absorbed, so the problem remains even if you try these 'shields'. Of course, stars are not points. They do have a definite size, so they can block light from other stars. The total brightness of the universe will not be infinite, but only as bright as the surface of a star (!). You can substitute 'galaxy' for 'star' in the preceding paragraphs if you want to update Olbers' Paradox for modern times.

The way to resolve a paradox like this is to look at the assumptions that are used (the 'if' statements) and determine whether or not they are valid.

Today, we lean toward a view of the universe different from that of Olbers. Heinrich believed the universe to be infinite. If the universe is finite, then it cannot go out forever nor can it be infinitely old. As astronomers probe the depths of the night sky, the see objects whose light is arriving on our planets after incredible journeys of time. We are seeing galaxies whose light left their locations over 13 billion years ago. Beyond these objects, astronomers do not see stars or galaxies. It is not that the light from those most distant stars has not reached us yes, but that there are no stars that far out because we are looking at the universe as a depth in space before stars were formed. This concept is called "look back time" and means that the farther out in space that you look, the farther back in time you look.

With these thoughts in mind, we venture into a study of the origins of the universe. How did it begin? When did it begin? What is happening to it today? What will happen to it in the future?

I would like you to move on now to Red Shifts. You could check out a great Cosmology website, or go to the Syllabus .

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