If we are not seeing evidence for rapid expansion near us, could this mean that the universe is no longer expanding?

No, because at the current epoch in the history of the universe some 10 - 15 billion years after the Big Bang, the expansion rate of the universe is not decelerating very rapidly. You would have to look at objects beyond a distance of 5 billion light years or so to begin to see any sign of a change in the expansion rate. This means that for regions about this size, the expansion rate is close to what we measure as the famous Hubble Constant. This gives the RATE at which the separations between objects are increasing AT A SCALE WHERE COSMOLOGICAL INFLUENCES ARE IMPORTANT. At what scale is this? It is determined by how lumpy the universe is. You have to be at a scale where the lumps due to galaxies and clusters of galaxies appear to smooth out. At a scale of 1 million parsecs and a Hubble Constant of 50 kilometers/sec/megaparsecs, the cosmological expansion is only worth 50 kilometers/sec or relative 'motion'. However, individual galaxies and the members of clusters of galaxies are moving in gravitational fields where velocities of 200 - 1000 kilometers/sec are typical. This means that the cosmological expansion effect is undetectable in such 'local' systems. It is only when you reach scales of 10s of megaparsecs or more, that the local 'curvature' effects due to clustered/clumpy matter smooth out and you can see the cosmological effect. In principle, it is present everywhere, but completely swamped by local physics at scales below a megaparsec. At a scale of a kiloparsec, it is only 50 x 0.001 megaparsec = 50 meters per second. At the atomic scale, it is only 50 x 10^-33 mpc = 2 x 10^-26 cm/sec if one wished to stretch the point to its logical absurdity. A


Copyright 1997 Dr. Sten Odenwald
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