Space is stretching

Let us take a closer look at what Hubble’s discovery means. How
can all the galaxies be receding away from ours? Surely this means
we must occupy a privileged position in the Universe. We must
be sitting exactly at its centre. If galaxies on all sides of us, and
which are the same distances away from us, are travelling away at
the same speed then we might conclude that we are not moving at
all. It is as though all the matter in the Universe originated from
our little corner of it.
It may be that we are unique in being the only life in the
Universe, although even this seems pretty unlikely given the sheer
size of the Universe. But we most certainly have no reason for
believing that we occupy a privileged location in the Universe. In
fact, an important tenet in cosmology, known as the cosmological
principle, says that there is no preferred place in the Universe. That, on the very largest scale, the Universe looks the same
everywhere. So how does everything appear to be moving away
from us?
The answer is deceptively simple. It is not that the galaxies
are flying through space away from our own, but rather that the
space in between is stretching. Imagine a large sheet of rubber on
which you place markers on square grid points so that they are
all at equal distances from each other . If the sheet is
stretched equally in all directions the distance between any two
markers will increase. Every marker would see all surrounding
ones moving away from it and no one marker is any more special
than the others. Of course I am assuming the sheet is very large,
otherwise we would have to worry about those markers that are
sitting out on the edge.
When I lecture on this topic I almost always get some bright
spark in the audience who asks the following question: if space
is expanding and everything is imbedded in space then surely
everything should be expanding together, including us and all
our measuring equipment on Earth. If the distance between our
Galaxy and another one doubles over a certain period then, surely,
the distance between all the atoms in our bodies, measuring tapes and rulers will also double. So how would we ever ‘observe’ the
expansion?
Once they have asked this question they tend to turn
triumphantly to the rest of the audience as if to say ‘there, let’s
see him get out of that one!’
However, the answer is surprisingly simple. Remember that
gravity acts to slow down the expansion of space, and if gravity
were strong enough it would win over the expansion completely.
At the level of the whole Universe the expansion rate is high and
the density of matter very low. But at the level of our Galaxy,
the space within it will not be affected since gravity is strong
enough on this scale not to permit any expansion. Down at the
level of humans and our measuring devices, matter is densely
packed together and the atoms that everything is made of are held
together by a force much stronger even than gravity. It is called
the electromagnetic force and is the glue that binds atoms together.
Space is most certainly not allowed to expand at this level and so
we, and everything else on Earth, remain the same size.
Time for an everyday example of this (please skip this
paragraph if you are already convinced by the previous one).
Consider the air bubbles that rise up from the bottom of fish
tanks. These bubbles start off small because the pressure of the
water at the bottom of the tank is high and squeezes the air inside
the bubble. As the bubble rises, the pressure decreases and the
bubble expands due to the outward push of the air molecules
inside it. Since the number of air molecules inside each bubble
does not change, they must be further apart when the bubble is
large. However, and this is the crucial point, we would not expect
each molecule of air to grow in size along with the bubble.
An interesting point to make is that the nearest galaxy to
our own, Andromeda (or M31 to give it its highly imaginative
astronomical name), is actually moving towards us! Andromeda
is two million lightyears away and, according to current estimates
of the expansion rate of the Universe, should be moving away
from us at a speed of fifty kilometres per second. Instead it is
moving towards us at three hundred kilometres per second! The
expansion of the Universe, therefore, does not even show up at the
scale of our Local Group of galaxies, let alone on Earth.Hang on a minute, you might be thinking, did Hubble get
it wrong after all? I thought he observed all galaxies moving
away from ours? The answer is that galaxies are not uniformly
distributed with equal spacing throughout the Universe. Hubble
had been observing very distant galaxies, that are moving away
from us, and not the nearby ones.
The speed at which our Galaxy and Andromeda are moving
together is equivalent to covering a distance all the way round the
world in two minutes, or a distance between the Earth and the Sun
in under a week. But before Hollywood decides to base its next
blockbuster movie on how a few brave men and women save the
Earth from an imminent collision with Andromeda I should point
out that, at the current rate, it will take several billion years for
the two galaxies to merge. Even when this eventually happens it
is highly unlikely that anything will actually hit the Earth since,
as we have seen, stars are quite far apart and the chance of a
star ploughing through the solar system is remote. Physicists
are able to build sophisticated computer simulations that show
dynamically how two galaxies behave when they merge together.
So what of Einstein’s antigravity force, his cosmological
constant that appeared in his equations to stop the Universe
from collapsing under its own weight? Did the discoveries of
Friedmann and Hubble consign it permanently to the scientific
scrap heap?
As the field of cosmology has evolved and matured over
the twentieth century, the cosmological constant has proved to
be rather more durable and resilient. In fact, it has made more
comebacks than the Rolling Stones3. For a while, cosmologists
decided that it would not and should not be entirely ruled out in
Einstein’s equations. Maybe it should be left in but be given a very
small value so that it did not conflict with Hubble’s observations.
Remember I am talking here about an abstract mathematical
model of the Universe that is predicted when Einstein’s equations
are solved. By varying the value of the cosmological constant,
cosmologists could then study the properties of the different model universes predicted. These properties could then be compared
with those observed in the real Universe.
Upper limits were computed and they turned out to be so
small that most cosmologists felt that it made sense to simply
remove it from the equations, as Einstein had done. Other reasons
for wanting a cosmological constant have come and gone. But
today, we have good reason for believing that it is not zero.
Current thinking is that Einstein did not make a blunder when
he introduced it in his equations. First, let us take a closer look
at the evidence for the Big Bang itself. After all, if the Universe is
getting bigger then it must have had a definite moment of creation
when it first started expanding.