How big is the Universe?

I am tempted to just say VERY BIG! and leave it at that. In fact,
according to the most recent astronomical evidence, it may well
turn out that the Universe is infinite. This means that it just goes
on forever. However, we can only ever see a small part of it, even
with the most powerful telescopes we could hope to build. There
exists a sort of horizon out in space beyond which we can never
see that marks the limit of what is known as the Visible Universe.
This is not a real edge but has to do with the fact that the Universe
hasn’t been around forever and light takes a certain time to reach
us. I will go into this a little more when I discuss something called
Olbers’ paradox.
The Earth orbits the Sun at a distance of 150 million kilometres,
which is equivalent to almost 4000 times round the Equator.Together, the Sun and its planets form the solar system. Earth’s
orbit takes 365 days and six hours, which is why we need leap
years of 366 days, since four lots of six hours will make up the
extra day.
Of course it makes no sense to measure vast astronomical
distances in kilometres. Instead they are measured in terms of
the distance light travels in one year. In the chapter on special
relativity we will see that the speed of light is the fastest speed
attainable by anything in the Universe1. But it nevertheless takes
light a certain time to get fromAto B; it just depends how far away
B is. This may not be obvious to us when we flick a light switch in
a room. To us, the whole room is instantly bathed in light, but this
is only because the distance light needs to cover from the bulb to
the four corners of the room is so small. In fact, it takes the light
typically only ten billionths of a second to get from the bulb to the
walls of a room.
Over astronomical distances, the time taken by light to travel
from one place to another becomes appreciable. For instance, it
takes light from the Sun eight minutes to reach the Earth: just
eight minutes to cover 150 million kilometres. But it takes five
hours to reach the outermost planet, Pluto. In a whole year light
could cover the distance from the Sun to the Earth sixty thousand
times. This distance that light can travel in one year is known,
imaginatively, as a lightyear. (Well, what else would you call it?)
It is nevertheless a little confusing to use a term containing a time
span to define a distance, but there you go.
These vast cosmic distances mean that cosmology has a clever
trick up its sleeve. When we look through our telescopes at a star
that is one lightyear away, we must remember that what we are
seeing is the light that left the star one year earlier. So we are
not seeing the star as it is now but a slightly younger version
of itself. In effect, we are looking into the past. In geology and
archaeology, scientists look at the evidence around them (rocks
and fossil remains) and try to infer what things were like in the past. Astronomers, however, can look directly into the past. The
further out in space they look, the older the light their telescopes
are picking up and the further back in time they are probing. The
very furthest objects that can be detected from Earth are billions
of lightyears away and show what the Universe was like when it
was very young.
Apart from the Sun, the closest star to us is a much fainter and
smaller dwarf star called Proxima Centauri which is just over four
lightyears away. Relatively close to this star is the Alpha Centauri
binary system, which is a pair of stars similar to the Sun that orbit
each other once every eighty years. Incidentally, Beta Centauri is
nowhere near Alpha Centauri but a hundred times further away. It
is just that, being a very bright giant star, it shines with a similar
brightness in the same region of the night sky and so they look, to
us, to be close together.
Stars are so far apart that you would be right in thinking
that most of space is just that: space. But you would be wrong
in thinking that the stars are spread out evenly throughout the
Universe. The distances to our closest neighbours that I have
quoted above are quite typical between stars in our neck of the
woods, but elsewhere stars can bunch up much more closely, and
there are vast stretches of the Universe which contain no stars at
all. Without exception, all stars congregate in large groups called
galaxies. We live in the Milky Way Galaxy (with a capital G to
distinguish it from other galaxies), which is shaped like a flat disc
with a bulged-out central region. The visible outer region consists
of spiral arms which give it its name: a spiral galaxy. It is eighty
thousand lightyears across and, to give you some idea of this size,
there are more stars in our Galaxy (about a hundred billion) than
there are people living on Earth (about six billion). The Sun is
situated towards the edge of the Galaxy, on one of its spiral arms,
and orbits the centre of the Galaxy once every 255 million years.
The galactic centre is much more densely populated and contains
stars which are older than the Sun.
It is helpful to think of the Galaxy as a great star city, with
the Sun situated out in the modern suburbs, far from the hustle
and bustle of the downtown galactic centre. All the stars we see in the sky with the unaided eye are in our Galaxy, but there are
many billions of other galaxies, each with its own population of
stars. Very few of these stars, even in neighbouring galaxies, can
be singled out even with a telescope. The only time one can be seen
with the naked eye is if it undergoes a supernova explosion when
it briefly outshines the rest of the stars in its galaxy put together.
Not only do stars cluster together in galaxies, galaxies also
group together in clusters. Our Galaxy is one of a motley collection
of about forty, known as the Local Group. Closest to us are a number
of dwarf galaxies hanging on to the coat tails of our own. The
nearest large galaxy to us is the Andromeda Nebula, which is about
two million light years away and is the only galaxy, beyond our
own, that is clearly visible from Earth with the naked eye.
Astronomical measurements have reached such a degree of
precision and sophistication with ever more powerful telescopes
being built, allowing us to probe ever deeper into space, that we
now know that galaxy clusters are themselves grouped together
into what are known as superclusters. Our Local Group is in
fact part of the Local Supercluster. What next? A cluster of
superclusters?
What does all this tell us about the Universe? For one
thing, it is very lumpy. On every scale: from stars to galaxies to
clusters to superclusters, matter tends to clump together unevenly.
This is, of course, due to the all conquering force of gravity
which dictates the structure of the whole Universe. The mutual
gravitational attraction of all the stars in the Galaxy keep them
bound together. It is gravity that causes galaxies to clump into
clusters and superclusters, and the gravitational pull of all the
matter in the Universe that dictates its overall shape.