The equations of general relativity, which determined how the universee volves in time, are too complicated to solve in detail. So what
Friedmann did, instead, was to make two very simple assumptions about the universe: that the universe looks identical in whichever direction we look, and that
this would also be true if we were observing the universe from anywhere else.
On the basis of general relativity and these two assumptions, Friedmann
showed that we should not expect the universe to be static. In fact, in 1922,
several years before Edwin Hubble's discovery, Friedmann predicted exactly what Hubble found.
The assumption that the universe looks the same in every direction is clearly not
true in reality.
For example, 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 distant galaxies, there seems to be more or less the same number of them in each direction.
So the universe does seem to be roughly the same in every direction, provided
one views it on a large scale compared to the distance between galaxies.
For a long time this was sufficient justification for Friedmann’s assumption—
as a rough approximation to the real universe. But more recently a lucky accident uncovered the fact that Friedmann’s assumption is in fact a remarkably
accurate description of our universe. In 1965, two American physicists, Arno
Penzias and Robert Wilson, were working at the Bell Labs in New Jersey on
the design of a very sensitive microwave detector for communicating with
orbiting satellites. They were worried when they found that their detector was
picking up more noise than it ought to, and that the noise did not appear to
be coming from any particular direction.
First they looked for bird droppings
on 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 is not pointing straight up than when it is, because
the atmosphere appears thicker when looking at an angle to the vertical.
The extra noise was the same whichever direction the detector pointed, so it
must have come from outside the atmosphere.
It was also the same day and
night 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 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. Since it appears to be the same in different directions, the universe must also be the same in every direction, at least on a large
scale. We now know that whichever direction we look in, 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.
At roughly the same time, 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 this glowing, because light from very distant parts
of the early universe 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 looking 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.
Friedmann did, instead, was to make two very simple assumptions about the universe: that the universe looks identical in whichever direction we look, and that
this would also be true if we were observing the universe from anywhere else.
On the basis of general relativity and these two assumptions, Friedmann
showed that we should not expect the universe to be static. In fact, in 1922,
several years before Edwin Hubble's discovery, Friedmann predicted exactly what Hubble found.
The assumption that the universe looks the same in every direction is clearly not
true in reality.
For example, 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 distant galaxies, there seems to be more or less the same number of them in each direction.
So the universe does seem to be roughly the same in every direction, provided
one views it on a large scale compared to the distance between galaxies.
For a long time this was sufficient justification for Friedmann’s assumption—
as a rough approximation to the real universe. But more recently a lucky accident uncovered the fact that Friedmann’s assumption is in fact a remarkably
accurate description of our universe. In 1965, two American physicists, Arno
Penzias and Robert Wilson, were working at the Bell Labs in New Jersey on
the design of a very sensitive microwave detector for communicating with
orbiting satellites. They were worried when they found that their detector was
picking up more noise than it ought to, and that the noise did not appear to
be coming from any particular direction.
First they looked for bird droppings
on 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 is not pointing straight up than when it is, because
the atmosphere appears thicker when looking at an angle to the vertical.
The extra noise was the same whichever direction the detector pointed, so it
must have come from outside the atmosphere.
It was also the same day and
night 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 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. Since it appears to be the same in different directions, the universe must also be the same in every direction, at least on a large
scale. We now know that whichever direction we look in, 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.
At roughly the same time, 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 this glowing, because light from very distant parts
of the early universe 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 looking 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.
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