Measuring the expansion of the Universe is a tricky business. It
involves a lot more than simply working out the speed that distant
galaxies are receding from us by measuring the redshift in their
light. First of all, it is hard to know for sure exactly how far away
they are. And because they are so far away they tend to be, on
average, younger galaxies—remember the light from them set off
millions, even billions, of years ago—and younger galaxies tend
to be bluer in colour and brighter because their stars are younger.
On the other hand they are very dim because they are so far away.
In addition to all this, galaxies come in all shapes and sizes and,
while it is true that if enough of them are studied then we can reliably extract an average, all in all measuring the redshift of
whole galaxies is not the best way of inferring the expansion rate.
There is a more reliable method. Remember from Chapter 2
that supernovae are so bright they briefly outshine the rest of their
galaxy. In particular, type Ia supernovae (the result of a complete
destruction of a star in a binary system after it has gained a critical
mass by sucking matter from its partner) all shine with a certain
brightness, or luminosity. They also flare up and die down within
a certain time. This means they can be used as reliable standards
for measuring distances. Recently, type Ia supernovae have been
used to determine the rate at which the Universe is expanding;
certainly the most exciting result in astronomy in 1998.
Detecting a supernova explosion of a star in a distant galaxy
is extremely difficult because they are incredibly faint. What is
even more incredible about the recent result is that very distant
supernovae seem to be even fainter than they should be based
on their distance. One reason for this could be because space is
negatively curved (hyperbolic) which has the strange property of
making distant objects faint because of the way their light spreads
out in such a universe. But there is another more intriguing
possibility. Maybe these supernovae are fainter because they are
further away than we think. But that would mean that they should
be receding faster than their measured redshift suggests. In other
words they don’t have a high enough redshift for their distance.
Since the light reaching us from these supernovae set off when
the Universe was much younger, their less-than-expected redshift
indicates a slower expansion rate in the past! I know you may
need to read this paragraph again to follow the logical order of
arguments, but if the observations are correct then the bottom line
is that the expansion of the Universe is NOT slowing down, but
speeding up!
The only way for this to be possible is if a force of antigravity
is driving the expansion, pushing the galaxies apart and stretching
space. While gravity’s influence gets weaker the further apart the
galaxies are, antigravity gets stronger with distance, and so will
drive the expansion even faster. The existence of this strange force
is just another way of saying that the cosmological constant is not
zero. But where does it come from? The usual answer is that it is has to be due to some strange
new form of invisible energy that is spread throughout the whole
of space. This energy has the paradoxical effect of driving the
expansion of space while at the same time contributing towards
closing the Universe round on itself. That is, it would help to make
up the missing fraction of omega to make it up to one, which is
what many theoretical physicists would prefer. In fact, omega may
even be a little more than one, making the Universe closed, even
though it could expand forever. This makes the simple arguments
based on Friedmann’s model universe wrong. We can no longer
say that an open universe is one that will expand for ever, while
a closed one must one day collapse in a big crunch. The shape of
the Universe and its destiny are no longer linked.
As for the origin of this energy of empty space, physicists are
still working on it. It may be down to any one of a number of weird
sounding Jargonese terms (which you may wish to impress your
friends with) such as ‘quantum fluctuations’, ‘phase transitions’,
‘topological defects’ or, most wonderful of all, ‘quintessence’.
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