Hubble, bubble . . .

Edwin Hubble almost became a professional heavyweight boxer.
Instead he chose a career path in astronomy, and now he has the
world’s most famous telescope named after him. What was his
claim to fame? For one thing, he was the first to realize that
other galaxies existed beyond the Milky Way. Until then, it was
thought that the tiny smudges of light that could be seen through
telescopes were clouds of dust, called nebulae, within our own
galaxy. Hubble found that they were too far away to be part
of the Milky Way and therefore had to be galaxies in their own
right. He also discovered that these other galaxies appeared to
be flying away from our own at speeds which were proportional
to how far away they were. The further away a galaxy was, the
faster it appeared to be receding from us. What was remarkable
was that this was happening whichever direction he pointed his
telescope. He had shown experimentally that Friedmann’s model
of the expanding Universe was correct. Einstein was forced to
admit that including the cosmological constant in his equation
had been the biggest mistake of his scientific career.
Hubble argued, correctly, that since the Universe is now
expanding, then in the past it must have been smaller than it
is today. Imagine that the expansion of the Universe could
have been filmed from a vantage point that has to be somehow
‘outside’ the Universe—of course this is impossible since all of
space is, by definition, within the Universe. By running the film
backwards you would see the Universe shrinking. If you went
back far enough into the past you would reach a time when all the
galaxies overlapped each other and things would have been pretty
crowded. Go back even further in time and all the matter will get
more and more squashed up and squeezed closer together until
you reach the moment of the Universe’s birth, the Big Bang2.
Hubble made his discovery by measuring something called
the cosmological redshift of light. To understand what this means
consider a more familiar phenomenon called the Doppler shift which, as you probablyknow, is the change in pitch you hear when,
say, a fast ambulance goes past you. The reason for this effect is the
change in frequency of the sound waves which reach you from the
ambulance when it is in two different situations: moving towards
you and moving away from you. When it approaches, the waves
of sound get squashed up, giving rise to a higher frequency (high
pitch) but when it is receding the waves are stretched out to give
a lower frequency (low pitch).
The same thing happens to light. When an object is moving
away from us—say a distant galaxy—the waves of light that
reach us from it get stretched and the frequency of the light goes
down. Instead of the frequency of the light we more often talk
about its wavelength. You probably remember something about
wavelengths from your school physics. You know, ripple tanks,
long springs that stretched across the class. Whatfun! Anyway, the
wavelength is the distance between two consecutive wave crests.
So a drop in frequency of light is really due to the stretching of the
wavelengths.
Since we are confident that a distant galaxy should contain
stars similar to the ones in our own Galaxy, and since we
know what wavelength the light should have—the nuclear
processes inside all stars cause them to shine with light of specific
wavelengths—then by measuring the change in wavelength of the
light we can work out how fast the galaxy is moving away from
us. Of course, astronomers will be quick to point out that it is not
as simple as this, but the basic principle is correct. I will come back
to some of the subtleties of measuring the rate of expansion later
on.
The effect is called a redshift because the wavelength gets
stretched as the galaxy recedes, and longer wavelengths of visible
light are associated with a redder colour. The term ‘redshift’, while
only really applying to visible light, is nevertheless used for all
parts of the electromagnetic spectrum.
We must first consider whether this reddening of light from
the distant galaxies observed by Hubble could be interpreted in
another way. Astronomers certainly tried to since they did not
initially want to believe that the Universe was really expanding. An obvious way this could happen is if light loses energy on its
way from its source to our telescopes, since a decrease in energy
would also make the wavelength longer. The only way the light
would lose energy would be if it was having to fight its way
through any interstellar dust or gas it encountered while on its
long journey through space. But there was a fatal difficulty with
this explanation. Light loses energy by bouncing off the atoms
of matter in its path. So it would tend to move in a zigzagged
path and this would make the image of the galaxy appear blurred.
Since there was no observed blurring of the images of the galaxies
this explanation had to be ruled out. The only other explanation
was the Doppler shift due to an expanding universe. A few
physicists, including a colleague of mine who taught me relativity
as an undergraduate, argue that the redshift can be explained
by something known as a transverse Doppler shift. This is the
Doppler shift observed in the light from objects moving at high
speed across our field of vision and not away from us. This is
quite correct. However, I will show that the redshift is not the
only evidence we have of expansion.