What does it mean when they say the universe is expanding?

    When scientists talk about the expanding universe, they mean that it has been growing ever since its beginning with the Big Bang.

The galaxies outside of our own are moving away from us, and the ones that are farthest away are moving the fastest. This means that no matter what galaxy you happen to be in, all the other galaxies are moving away from you.

However, the galaxies are not moving through space, they are moving in space, because space is also moving. In other words, the universe has no center; everything is moving away from everything else. If you imagine a grid of space with a galaxy every million light years or so, after enough time passes this grid will stretch out so that the galaxies are spread to every two million light years, and so on, possibly into infinity.

The universe encompasses everything in existence, from the smallest atom to the largest galaxy; since forming some 13.7 billion years ago in the Big Bang, it has been expanding and may be infinite in its scope. The part of the universe of which we have knowledge is called the observable universe, the region around Earth from which light has had time to reach us.

One famous analogy to explain the expanding universe is imagining the universe like a loaf of raisin bread dough. As the bread rises and expands, the raisins move farther away from each other, but they are still stuck in the dough. In the case of the universe, there may be raisins out there that we can’t see any more because they have moved away so fast that their light has never reached Earth. Fortunately, gravity is in control of things at the local level and keeps our raisins together.

Who Figured This Out?

The American astronomer Edwin Hubble made the observations in 1925 and was the first to prove that the universe is expanding. He proved that there is a direct relationship between the speeds of distant galaxies and their distances from Earth. This is now known as Hubble’s Law. The Hubble Space Telescope was named after him, and the single number that describes the rate of the cosmic expansion, relating the apparent recession velocities of external galaxies to their distance, is called the Hubble Constant.

So, is the Universe Infinite?

It might be easier to explain about the beginning of the universe and the Big Bang Theory, than to talk about how it will end. It is possible that the universe will last forever, or it may be crushed out of existence in a reverse of the Big Bang scenario, but that would be so far in the future that it might as well be infinite. Until recently, cosmologists (the scientists who study the universe) assumed that the rate of the universe’s expansion was slowing because of the effects of gravity. However, current research indicates that the universe may expand to eternity. But research continues and new studies of supernovae in remote galaxies and a force called dark energy may modify the possible fates of the universe.

Related Web Sites

Classics of the Scientific Literature: Hubble’s Guide to the Expanding Universe Proceedings of the National Academy of Sciences (PNAS) - An introduction and guide to landmark papers about Hubble’s expanding universe. Curious About Astronomy: Cosmology and the Big Bang - Questions and Answers about Cosmology and the Big Bang are presented, as well as links and references to related information. Selected Internet Resources in Astronomy - A list of Astronomy Web links selected by the Library of Congress Science Reference Service. LC Science Tracer Bullet: Astronomy and Astrophysics - A reference guide that helps one locate information about astronomy and astrophysics. Science Reference Guide. Astronomy for Schools: Selected Teaching Aids - A guide for teaching astronomy.

Further Reading

Dinwiddie, Robert, and others. Universe. New York, DK Pub., 2005. 512 p. Fox, Karen C. The big bang theory: what it is, where it came from, and why it works. New York, Wiley, c2002. 206 p. Harland, David M.. The big bang: a view from the 21st century. London, New York, Springer; Chichester, Eng., In association with Praxis Pub., c2003. 262 p. Schilling, Govert. Evolving cosmos. Cambridge, Eng., New York, Cambridge University Press, 2004. 135 p. Scientific information about the universe and the scientific theories of the evolution of the universe: an anthology of current thought. Edited by Rick Adair. New York, Rosen Pub. Group, 2006. 192 p. Silk, Joseph. The infinite cosmos: questions from the frontiers of cosmology. Oxford, New York, Oxford University Press, 2006. 248 p

For more print resources...
Search on " Cosmology--Popular works," " Astronomy," or "Physics--Popular Works" in the Library of Congress Online Catalog.

Model of expanding raisin bread. Wilkinson Microwave Anisotropy Probe, NASA Web site.

Edwin Hubble, with the 48-inch telescope on Palomar Mountain. Photo: NASA's Astronomy Picture of the Day Web site.

Hubble Space Telescope - the largest orbiting public optical telescope in history. NASA Web site.

The following four images were taken by the Hubble Space Telescope, and can be found at NASA's Hubblesite image gallery.


Saturn's Dynamic Auroras.

Galaxy NGC 1512 in Visible Light

A Storm of Turbulent Gases in the Omega/Swan Nebula (M17).

Kepler's Supernova Remnant In Visible, X-Ray and Infrared Light.

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