Expanding Universe

The Expanding Universe

In the universe of galaxies the situation is complicated because the galaxies recede from our Galaxy at speeds which increase with increasing distance. The rate of increase is estimated to be about 15 miles a second per million light-years, but is thought gradually to get larger at distances beyond about 1,000 million light-years. 3C-295, one of the most distant galaxies yet photographed, appears to have a velocity of recession of 76,000 miles a second, or about two-fifths the velocity of light. It is therefore unlikely that its distance will be more than 5,000 million light-years, although this estimate is little more than a guess.

If the value 76,000 miles a second seems absurdly large, we must realize that it refers to the speed of a galaxy, not to that of a relatively tiny object like a star. A galaxy comparable in size to our Galaxy, moving at that speed, would still take about 250,000 years to pass through a distance equal to its own diameter. Further, an observer on 3C-295 would regard himself as being stationary while our Galaxy receded from him at a velocity of 76,000 miles a second.

Cluster of Galaxies in Hercules. Source of the photo: NASA, photograph in public domain

In other words, no matter what particular galaxy we found ourselves in, we would regard that galaxy as being at the center of an expanding system of galaxies. Our bun-model now breaks down completely, for the bun has a raisin at or near its center. The universe of galaxies, on the other hand, has no absolute centre. There are as many centres as there are galaxies.

We must not think that astronomers measure the velocities of recession of the galaxies. All they can measure are the relative shifts towards the red of certain lines in the spectra of galaxies. They then interpret the shifts in terms of the wave theory of light, and thereby conclude that the shifts are Doppler effects due to velocities of recession of the sources observed, that is, of the galaxies themselves. This particular interpretation is undoubtedly the best one so far but its is by no means the only one possible.