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The Universal Expansion Revisited

Week of May 3, 1999

Last year, I wrote a Snack about what was then a new discovery: the finding that the expansion of the Universe was accelerating. I expressed skepticism of the result, as any scientist would, though I speculated on its impact. Later that year, Science Magazine (a reputable journal) called the results the biggest scientific discovery of 1998.

Before I get started here, please go and read that Snack. It'll only take you a minute, and it'll prevent me from having to rewrite it here!

So: having read it, you see that astronomers think the Universe has a shape, and its expansion may be getting faster with time. A few weeks ago, Saul Perlmutter of the University of California at Berkeley, and head of the other team of astronomers mentioned in the article, gave a talk where I work about his findings. I won't go into details, but he was able to sway me that his results are good. I'll mention here that his results and those from the other team also agree very well, even though they were independently arrived at, which is very strong support indeed that they are accurate.

He displayed one plot which had me baffled though. He showed that even though the Universe is accelerating, it is also closed. Now, as I mentioned in the other Snack, textbooks, teachers and most astronomers have always equated the shape of the Universe (open, flat or closed) with its eventual fate (expand forever, glide to a halt, recollapse). How, I asked Dr. Perlmutter at the Q&A session after the main talk, can the Universe be closed yet still accelerate its expansion?

His answer was simple, actually: the shape of the Universe is separate from its fate. What we think of as ``closed'' means the Universe has a finite extent. Think of the surface of a balloon (a flawed analogy, but good enough for my purposes here). It is finite (you can measure the total surface area, say 100 square centimeters). However, if you blow air in the balloon it will expand. Even as it expands, the surface stays finite. You can say it still encloses the entire volume of the balloon. Even if you blow in air faster and faster, the surface remains finite.

The same is true for the Universe. If it is closed, it does not necessarily mean it will eventually recollapse; it just means that it's, well, closed. It encloses a certain volume of space, you might say (even though that's a gross oversimplification). Even as it expands, it remains bound, finite, closed.

After he answered my question, I told Dr. Perlmutter that this means that those of us in the education field have a daunting task: changing our thinking, changing textbooks, changing countless web pages. His answer: a charming smile, and "I know."

The audience laughed. So did I. The auditorium was full of scientists, Good scientists, people who not only aren't afraid to change their mind when better evidence comes along, but also people who delight in having their horizons expanded. And some horizons are indeed infinite.

©2008 Phil Plait. All Rights Reserved.

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