Why is it dark at night?

You might think that this is a rather trivial, even silly, question to
ask. After all, even a child ‘knows’ that this is because the Sun
sets below the horizon, and since there is nothing else in the sky
anywhere near as bright as the Sun we have to make do with the
feeble reflected light from the Moon and even more feeble light
from the distant stars. Well, guess what? It’s not as simple as that!
We have good reason to believe that even if the Universe is not
infinite in size, it is probably so enormous that, for all intents and
purposes, it is infinite. If so, then we come up against something
known as Olbers’ paradox. Simply stated, this says that the night
sky has no right being dark at all. It should be even brighter than it
normally gets during the day. In fact, the sky should be so bright,
all the time, that it should not even matter whether the Sun is up
in the sky or not.
Imagine you are standing in the middle of a very large forest.
So large in fact that you can assume it is infinite in extent. Now try
shooting an arrow in a particular (horizontal) direction such that it
does not hit a tree trunk. In this idealized situation the arrow must be allowed to keep on going in a straight line without ever dipping
down to the ground. You find, of course that it is impossible. Even
if the arrow misses all the closer trees, it will eventually always hit
one. Since the forest is infinite, there will always be a tree in the
flight path of the arrow, however far away that tree is. It doesn’t
matter how dense the forest is either. If you were to chop down
ninety per cent of all the trees, this would simply mean that the
arrow will, on average, travel ten times as far before it encounters
a tree trunk.
Now consider a simple model universe that is infinite, static
(not expanding) and with stars evenly spread out. The light that
reaches us from the stars is like the example of the arrow. It does
not matter where we look in the sky, if the Universe is infinite we
should always see a star in our line of sight. So there would not be
any gaps in the sky where we do not see a star and the whole sky
should be as bright as the surface of the Sun, all the time!
The real Universe may also be infinite, but in other respects it
is not quite like the above simple model. First of all, the stars are
not spread out evenly but clumped together in galaxies. This does
not matter. It just means that the night sky should be as bright
as an average galaxy, which is not quite as bright as the surface
of an average star but still blinding. Secondly, our Universe is
expanding. Does this make a difference? Physicists have carried
out detailed calculations that have shown that this does not solve
the problem, it just reduces it. So what is the answer?
It was thought that maybe space is filled with interstellar dust
and gas that would block the light from the more distant galaxies.
But if the Universe has been around for long enough, then even
this material would slowly heat up, due to the light it has absorbed,
and will eventually shine with the same brightness as the galaxies
it obscures.
The true answer, the one which finally lays Olbers’ paradox to
rest, is that the Universe has not been around forever, so light from
very distant galaxies has simply not had enough time to reach us.
If the Big Bang happened 15 billion years ago, the galaxies that are
further away than 15 billion lightyears (remember a lightyear is
the distance covered by light in a year) are invisible to us because their light is still in transit and has yet to reach us. Admittedly,
the discussion is complicated a little due to the expansion of the
Universe, but what we can see in the sky is just a tiny fraction
of the whole Universe. It is called the Visible Universe and we
cannot, even with the most powerful telescopes, see beyond a
certain horizon in space. Thus, the Visible Universe (our tiny
corner of space) does have an edge even if the Universe as a whole
does not.
Finally, we can turn Olbers’ paradox on its head and say that
the real proof that the Big Bang happened is that it gets dark at night!