Firstly, the Universe looks Identical in whichever direction we look.
Next and the other, this would also be true if we were observing the Universe from Anywhere Else.
From these two ideas alone Friedmann showed that we should not expect the Universe Static. In fact, in 1922, several years before Edwin Hubble's discovery, Friedmann predicted what Edwin exactly found! The Assumption of that the Universe looks the same in every direction is clearly not true in reality. For example, as we have seen, the other stars in our galaxy form a distinct band of light across the Night Sky, called the Milky Way. But if we look at distinct galaxies, thus seems to be more or less the same number of them. So the Universe does seem to be the roughly same in every direction, provided one views it on a large scale compared to the distance b/w galaxies and ignores the differences on small scale!
For, a Long Time, this was sufficient justification for Friedmann's assumption - as a rough approximation in the real Universe. But a lucky accident uncovered the fact that Friedmann's assumption is in fact remarkably accurate description of the Universe. In 1965, two American physicist at the Bell Telephone Laboratory in New Jersey, Aro Penzias and Robery Wilson, were testing a very sensitive microwave detector. They were worried when they found that the detector was picking up more noise than it ought to. The noise did not appear to be coming from a particular direction. First they discovered bird droppings in their detector and checked all the other possible malfunction and sought them out. They knew that any noise from within the atmosphere would be stronger when the detector was pointing straight up than when it was, because light rays travel through much more atmosphere when received from near the horizon than when received from directly overhead.
The extra noise was the same whichever direction the detector was pointed, so it must come from outside the atmosphere. It was also the same day and night and the throughout the year, even though the earth was rotating on its axis and orbiting around the sun. This showed that the radiation must come from beyond the Solar System, and even from beyond the galaxy, as otherwise it would vary as the movement of Earth pointed the detector in different directions. In fact, we know that the radiation must have travelled to us across most of the observable universe, and since it appears to be the same in different directions, the universe must also be the same in every direction, if only on a large scale. We now know that whichever direction we look, this noise never varies by more than one part in ten thousand - so Penzias and Wilson had unwittingly stumbled across a remarkably accurate confirmation of Friedmann's first assumption.
Now at first sight, all this evidence that the universe looks the same whichever direction we look in might seem to suggest that there is something special about our place in the universe. In particular, it might seem that if we observe all other galaxies to be moving away from us, then we must be at the centre of the universe. There is, however, an alternative explanation: the universe might look the same in every direction as seen from any other galaxy, too. This as we have seen, was Friedmann's second assumption. We have no scientific evidence for, or against, this assumption. We believe it only on grounds of modesty: it would be most remarkable if the universe looked the same in every direction around us, but not around other points in the universe!
In Friedmann's model, all the galaxies are moving directly away from each other. The situation is rather like a balloon with a number of spots painted on it being steadily blown up. As the balloon expands, the distance between any two spots increases, but there is no spot that can be said to be the centre of the expansion. Moreover, the farther apart the spots are, the faster they will be moving apart. Similarly, in Friedmann's model the speed at which any two galaxies are moving apart is proportional to the distance between them. So it predicted that the red shift of a galaxy should be directly proportional to its distance from us, exactly as Hubble found.
0 Comments