The Universe by Fraknoi Andrew & Asimov Isaac & Pohl Frederick & Anderson Poul & Bradbury Ray

Author:Fraknoi, Andrew & Asimov, Isaac & Pohl, Frederick & Anderson, Poul & Bradbury, Ray [Fraknoi, Andrew]
Language: eng
Format: mobi
Publisher: Byron Preiss Visual Publications
Published: 2010-08-24T16:00:00+00:00

So far, we have examined galaxies and clusters for what they have to tell us about themselves and their contents. But now I would like to take a brief look at one of the most exciting of astronomical endeavors: using our observations of remote galaxies and clusters to pin down the details of the expansion of the universe.

Allan Sandage will discuss the expansion of the cosmos in more detail in his chapter. Here let us just remind ourselves that during the 1920’s and 1930’s, Edwin Hubble and his colleagues realized that the universe was expanding when they measured the distances and speeds of many galaxies independently and found that there was a correlation between the two quantities. The farther away a galaxy is, the faster it is receding from us.

The technology of Hubble’s day allowed us to measure only the distances of relatively close-by galaxies, but today, with large modern telescopes and light-detecting instruments, we can make distance measurements much farther out. We discussed earlier in this chapter how astronomers look for celestial objects that act like standard light bulbs to help them in this quest. At great distances, we can no longer measure individual stars, or even clusters of stars or bright clouds of gas, and thus we cannot use them as our standard bulbs. But it turns out that the brightest E galaxy in every rich cluster has roughly the same intrinsic brightness—the same general “wattage,” we might say. (Another standard bulb we sometimes use is the brightest radio elliptical—an elliptical galaxy that emits not only light but copious amounts of cosmic radio waves. You can read more on such “active galaxies,” as they are called, in Harding Smith’s chapter.)

In general, then, what this endeavor involves is finding a standard-bulb E galaxy, measuring its apparent brightness, and comparing it with the usual (or standard) brightness such galaxies have. The difference between how bright the galaxy appears, and how bright it is “supposed to be” (up close) tells us how much dimming has taken place and thus how far away it is. Then we measure the galaxy’s red shift, which tells us the speed at which it is moving away from us. Comparing the two will show us how well the galaxy in question is following the pattern of the expanding universe we have found in nearby galaxies.

At this point, you may ask why we bother to make these often excruciatingly difficult observations. Why not accept that the universe is expanding smoothly, just as our observations of nearby galaxies imply? To begin with, there is the simple reason that in science we take nothing for granted. But more importantly, our theories tell us that the universe may not always have been expanding at the same rate. The expansion may be slowing down, although the precise deceleration depends on what sort of universe we live in. Clearly, it would be very exciting to be able to test these ideas by comparing the way galaxies were expanding in the remote past—say, 7 to 10 billion years ago—to the way they are receding from one another today.

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