5 minute read

Cosmology

Trouble In Paradise



In the last quarter of the twentieth century some problems with the standard picture of the big bang emerged. The extreme uniformity of the cosmic background radiation, which seemed initially reasonable, leads to a subtle problem. Consider the age of elements of the cosmic background radiation originating from two widely separated places in the sky. The distance between them is so great that light could not travel between them in an amount of time less than their age. Thus the two regions could never have been in contact during their existence. Why then, should they show the same temperature? How was their current status coordinated? This is known as the horizon problem.



The second problem has to do with the remarkable balance between the energy of expansion of the universe and the energy associated with the gravitational forces of the matter opposing that expansion. By simply counting the amount of matter we see in the universe, we can account for about 1% of the matter required to stop the expansion and close the universe. Because the expansion causes both the expansion energy and the energy opposing the expansion to tend to zero, the ratio of their difference to either one tends to get larger with time. So one can ask how good the agreement between the two was, say, when the cosmic background radiation was formed.

The answer is that the agreement must have been good to about 1 part in a million. If one extends the logic back to the nuclear era where our physical understanding is still quite secure, then the agreement must be good to about thirty digits. The slight departure between these two fundamental properties of the universe necessary to produce what we currently observe is called the "Flatness Problem." There is a strong belief among many cosmologists that agreement to 30 digits suggests perfect agreement and there must be more matter in the Universe than we can see.

This matter is usually lumped under the name dark matter since it escapes direct visible detection. It has become increasingly clear that there is indeed more matter in the universe than is presently visible. Its gravitational effect on the rotation of galaxies and their motion within clusters of galaxies suggests that we see perhaps only a tenth of the matter that is really there. However, while this amount is still compatible with the abundance of deuterium, it is not enough to close the Universe and solve the flatness problem.

Any attempt to run the motion picture further backwards before the nuclear era requires physics which, while less secure, is plausible. This led to a modification of the big bang by Alan Guth called inflation. Inflation describes an era of very rapid expansion where the space containing the matter-energy that would eventually become galaxies spread apart faster than the speed of light for a short period of time.

Inflation solves the horizon problem in that it allowed all matter in the universe to be in contact with all other matter at the beginning of the inflation era. It also requires the exact balance between expansion energy and energy opposed to the expansion, thereby solving the flatness problem. This exact balance requires that there be an additional component to the dark matter that did not take part in the nuclear reactions that determined the initial composition of the Universe. The search for such matter is currently the source of considerable effort.

Finally, one wonders how far back one can reasonably expect to run the movie. In the earliest microseconds of the universe's existence the conditions would have been so extreme that the very forces of nature would have merged together. Physical theories that attempt to describe the merger of the strong nuclear force with the electro-weak force are called Grand Unified Theories, or GUTs for short. There is currently much effort being devoted to testing those theories. At sufficiently early times even the force of gravity should become tied to the other forces of nature. The conditions which lead to the merging of the forces of nature are far beyond anything achievable on Earth so that the physicist must rely on predictions from the early Universe to test these theories.

Ultimately quantum mechanics suggests that there comes a time in the early history of the universe where all theoretical descriptions of the universe must fail. Before a time known as the Planck Time, the very notions of time and space become poorly defined and one should not press the movie further. Beyond this time science becomes ineffective in determining the structure of the universe and one must search elsewhere for its origin.

Resources

Books

Harrison, E.R. Cosmology: The Science of the Universe. Cambridge, England: Cambridge University Press, 1981.

Hawking, Stephen. W. The Illustrated A Brief History of Time. 2nd ed. New York: Bantam Books, 2001.

Kirshner, Robert P. The Extravagant Universe: Exploding Stars, Dark Energy, and the Accelerating Cosmos. Princeton, NJ: Princeton University Press, 2002.

Weinberg, S. The First Three Minutes. New York: Basic Books, 1977.

Periodicals

Glanz, James. "Evidence Points to Black Hole At Center of the Milky Way." New York Times. October 17, 2002.

Guth, A.H., and P.J. Steinhardt. "The Inflationary Universe," Scientific American 250, no. 5 (1984): 116–28.

Other

Cambridge University. "Cambridge Cosmology." <http://www. damtp.cam.ac.uk/user/gr/public/cos_home.html> (cited February 14, 2003).

National Air and Space Administration. "Cosmology: The Study of the Universe." <http://map.gsfc.nasa.gov/m_uni. html> (cited February 5, 2003).


K. Lee Lerner

George W. Collins, II

KEY TERMS

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Grand Unified Theory

—Any theory which brings the description of the forces of electromagnetism, weak and strong nuclear interactions under a single representation.

Hubble constant

—The constant of proportionality in Hubble's Law which relates the recessional velocity and distance of remote objects in the universe whose motion is determined by the general expansion of the universe.

Inflation cosmology

—A modification to the early moments of the big bang Cosmology which solves both the flatness problem and the horizon problem.

Megaparsec

—A unit of distance used in describing the distances to remote objects in the universe. One megaparsec (i.e., a million parsecs) is approximately equal to 3.26 million light years or approximately ten trillion trillion centimeters.

Olbers' paradox

—A statement that the dark night sky suggests that the universe is finite in either space or time.

Planck time

—An extremely short interval of time (i.e., 1043 sec) when the conventional laws of physics no longer apply.

Primeval atom

—The description of the very early expanding universe devised by Abbe Lemaitre.

Spectra

—The representation of the light emitted by an object broken into its constituent colors or wavelengths.

Steady state cosmology

—A popular cosmology of the mid twentieth century which supposed that the universe was unchanging in space and time.

Additional topics

Science EncyclopediaScience & Philosophy: Cosine to Cyano groupCosmology - Evolution Of Cosmological Thought, The Expanding Universe, The Big Bang, Implications Of The Big Bang