Page 25 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 3
description of our universe.

In 1965 two American physicists at the Bell Telephone Laboratories in New Jersey, Arno Penzias and Robert
Wilson, were testing a very sensitive microwave detector. (Microwaves are just like light waves, but with a
wavelength of around a centimeter.) Penzias and Wilson were worried when they found that their detector was
picking up more noise than it ought to. The noise did not appear to be coming from any particular direction.
First they discovered bird droppings in their detector and checked for other possible malfunctions, but soon
ruled these out. They knew that any noise from within the atmosphere would be stronger when the detector
was not 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 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 traveled 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 a tiny
fraction: so Penzias and Wilson had unwittingly stumbled across a remarkably accurate confirmation of
Friedmann’s first assumption. However, because the universe is not exactly the same in every direction, but
only on average on a large scale, the microwaves cannot be exactly the same in every direction either. There
have to be slight variations between different directions. These were first detected in 1992 by the Cosmic
Background Explorer satellite, or COBE, at a level of about one part in a hundred thousand. Small though these
variations are, they are very important, as will be explained in Chapter 8.

At roughly the same time as Penzias and Wilson were investigating noise in their detector, two American
physicists at nearby Princeton University, Bob Dicke and Jim Peebles, were also taking an interest in
microwaves. They were working on a suggestion, made by George Gamow (once a student of Alexander
Friedmann), that the early universe should have been very hot and dense, glowing white hot. Dicke and
Peebles argued that we should still be able to see the glow of the early universe, because light from very
distant parts of it would only just be reaching us now. However, the expansion of the universe meant that this
light should be so greatly red-shifted that it would appear to us now as microwave radiation. Dicke and Peebles
were preparing to look for this radiation when Penzias and Wilson heard about their work and realized that they
had already found it. For this, Penzias and Wilson were awarded the Nobel Prize in 1978 (which seems a bit
hard on Dicke and Peebles, not to mention Gamow!).

Now at first sight, all this evidence that the universe looks the same whichever direction we look in might seem
to suggest 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 center of the universe. There is,
however, an alternate 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 center 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. Despite the success of
his model and his prediction of Hubble’s observations, Friedmann’s work remained largely unknown in the West
until similar models were discovered in 1935 by the American physicist Howard Robertson and the British
mathematician Arthur Walker, in response to Hubble’s discovery of the uniform expansion of the universe.

Although Friedmann found only one, there are in fact three different kinds of models that obey Friedmann’s two
fundamental assumptions. In the first kind (which Friedmann found) the universe is expanding sufficiently
slowly that the gravitational attraction between the different galaxies causes the expansion to slow down and
eventually to stop. The galaxies then start to move toward each other and the universe contracts.

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