Expansion Of The Universe

If you look at objects that are really far away (outside our galaxy) you will notice that the further away the object is, the more RedShifted light emitted from those objects appears to be. This effect, first noted by EdwinHubble, is observable no matter in what direction you look. If this universal RedShift is interpreted as being due to the DopplerEffect of receding light sources, the simple conclusion is that the universe is expanding uniformly in all directions.

"Uniformly" means that the speed of recession of the object -- and hence the amount of red shift -- is directly related to the distance of the observer from the object, in all directions. This is thought to be true no matter where the observer is. This remarkable and strange thought is what leads to the idea that the universe itself is what is expanding. One analogy for this idea is that the universe, were it only two-dimensional, might be like the surface of a balloon, where, as the balloon expands, each point on the surface flies away from each other point on the surface. [Isn't "isotropically" the proper term?]

"Thought to be true"? How do we come to think how the universe would appear if viewed from, say, a distant quasar? What good measurement of extreme distance is possible (without making assumptions about red-shift)? Cannot balloon-dwellers imagine the third dimension in which they actually exist? Do we have enough evidence to state the shape of the known universe? The balloon-surface is "closed". Is there any evidence that our space is closed?

Good questions. The actual observation is that *for us* the further away objects are, the more redshifted they are. Expansion of the universe would explain this observation. The only real alternative is that we are at the center of the universe and everything is flying away from us specifically for some (equally strange) reason. As anticipated by your question of extreme measurement, calibrating the distance scale of the expansion is tricky. There are several methods that are excellent for shorter (but still universe-expansion scale) distances. One is to use the apparent parallax of the proper motion of groups of stars. Another is the use of so-called "standard candles". A standard candle is a light source who's absolute brightness can be calculated from first principles or well-established theory. If you know the absolute brightness, and the observed brightness, and assume that brightness falls off as the inverse square of distance, you can calculate distance. Some standard candles are Cepheid variable stars and supernovae explosions. We don't have enough data to state the shape of the known universe, but it is intimately tied in with the current expansion rate. The observation is that the universe is very very close to being perfectly flat and there are theoretical reasons to supposed that it is in fact exactly flat (where flat refers to the geometry of space/time). The cosmic microwave background radiation is the best observational evidence of this. Of course, there are lots of theories... -- AndyPierce

Thanks. In a conventional explosion, an observer need not be located at the center to see every major object is receding. However, (a) an observer at sufficient distance from the center sees nothing when looking away from the center, and (b) given enough observational data, an observer could identify the location of the center. Is there currently enough data to discredit either possibility?

More good questions. It's hard to know what the definition of "enough data" is, but my impression is that all current observations indicate that the "location of the center" is us. Since there is no a priori reason why we should occupy such a privileged position in the universe the expansion of the entire universe is the currently accepted explanation for this observation.

Note: nowadays we think the rate of expansion is changing. Logically it ought to be, because the universe contains mass and mass exerts an attractive force called gravity, so the expansion must be slowing down. However, recent attempts to measure the rate of slow-down have produced a negative answer. The expansion is actually getting faster. This discovery makes no sense, but that's science for you. [Negative from data relating to galaxies at any distance, or just remote ones?]

More info at http://www.bbc.co.uk/horizon/infinity.shtml.

Does "us" mean literally that or merely "where our galaxy is"? Do you have a specific reference for the notion that either is the apparent centre of mass of the universe? I don't recall this being discussed in any television program on the nature of the universe.

"Us" means the center of the visible part of the Universe. So we're at the center by fiat. But understand that the effect, as described, would appear the same no matter where you stood in the Universe, including beyond the horizon of the visible part (as projected forward in time to the present...)

JaneRoberts has said something to the effect that "Modern cosmologists believe in the equivalent of looking at the reflection of a tree in a pond to figure out how deep the pond is."

Your huffy reaction of sarcasm upon hearing this is indeed one of her goals; to challenge you to think outside a very large box.

See:HumanPerfectionInCheckers