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Hitting the Gas

Week of March 2, 1998
When I started these Bitesize pages, I wanted to find a topic every week that was small and palatable for easy comprehension, yet chewy enough to make Snackers think. Sometimes, though, I break my own rules (hey, they're my pages after all!). You may have read in the papers or heard on TV some weird astronomy news: the Universe may be accelerating outwards. What I saw didn't explain it well, so I'll use a paragraph or two to put my take on it.

What happened is that a team of people (actually, several teams, but the announcement was made by Bob Kirshner and Adam Riess at Harvard) are studying very distant supernovae (SN). Supernovae are stars that have exploded, and are extremely bright. So bright, in fact, that they can be seen nearly clear across the Universe. There are many reasons to study such objects, but in this case the the idea was to use the properties of the supernovae (how bright they get, how quickly they brighten and then fade, etc.) to get a handle on the shape of the universe.

We know that our Universe is expanding. As we look farther out, to more distant objects, we see that they are rushing away from us. It looks as if we are at the center of a gigantic explosion that made all the galaxies blow outward from us. This is an illusion; any galaxy would see the same thing. They aren't really moving away from us; we are all moving away from each other. We're not really at the center (as a matter of fact, there wasn't any center, but this is complicated enough!). Since we think this started some time ago in a huge explosion, we call the event the "Big Bang" (please feel free to check out the Relativity FAQ from the USENET newsgroup sci.physics.relativity!).

According to our best theories, we live inside a three-dimensional universe that itself is embedded in four dimensions. I know, that may make your head hurt, but don't worry about it just now. The point is, the overall Universe has a geometry, a shape, and we are not sure just what that shape is. Cosmologists (people that study these things) like to say that there are three different possible shapes to the Universe: open, flat or closed. If the Universe is open, it will expand forever. If it is closed, the expansion will eventually come to a halt, and reverse; in some far distant future the Universe will collapse again to a point. If the Universe is flat, it is forever balanced between open and closed; it will expand forever, but always slowing, just balancing gravity. We don't know for sure which of these three geometries describes the Universe best.

Now it turns out that studying SN will help here. Basically, if the Universe is open (will expand forever), closed (will eventually contract again) or flat (expanding just fast enough to balance gravity), you will see differences in the way the SN behave.

What Riess and Kirshner found is that the SN are dimmer than expected for their distance, which means they are farther away than expected. This was quite a surprise! We think we have a pretty good handle on how far away things should be, assuming nothing else has happened since the initial explosion which formed the Universe. But now it looks like something else is going on. The SN must have been pushed outwards by some force that has a negative effect on gravity. This is accelerating them.

Do I believe this? Not yet. Kirshner is a very good scientist, and so is Riess, so I expect they have done a very good job getting and analyzing the data. Is there something they forgot? Maybe, but I doubt it. More likely there is something going on here we don't understand. A repulsive force is the most palatable of the weirdnesses this implies. If this pans out, it has profound implications. I must stress though, that this is not a peer-reviewed article yet, but a preliminary announcement. It's more of a "what have we found here?" type of event. No doubt there will be more about this as time goes on!

©2008 Phil Plait. All Rights Reserved.

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