American astronomer Edwin Hubble was the first to notice that the universe is
expanding. By “the universe is expanding” I am actually referring to the
metric expansion of space.
This means (for reasons currently unknown to us) that space is literally
expanding all the time. The average distance between the Earth and any other
object in the universe is increasing over time. This has been confirmed by many
experiments, but the fundamental cause of metric expansions is currently
Based on symmetry/randomness arguments, we assume that the Earth is not the very
center of the universe. In fact, unless we have evidence to suggest otherwise,
we should assume that the Earth just has a random location in the universe.
There is something in the universe called the
cosmic microwave background.
The cosmic background microwave radiation is basically a weak microwave signal
that we can detect by looking at any part of the universe. The reason it exists
is because certain theories posit that this type of radiation be released during
the big bang once the universe got cool enough—about 380,000 years after the
initial big bang, according to the current math. With very powerful telescopes
we can measure the cosmic microwave background radiation to a very high degree
of precision. Then with a bunch of math we can try to figure out how old the CMB
is and how strong it is. This is the oldest light that we can see.
Here’s where it gets interesting. We know the universe is expanding. Current
evidence suggests that the rate of expansion is increasing. All of the laws
you’ve read about concerning the speed of light being constant does not apply to
the metric expansion of the universe. Imagine the universe is a big sphere. That
means that if you’re sitting on the edge of the sphere trying to look at a light
pulse emitted on the other edge of the sphere, if the metric expansion of the
universe is increasing faster over the total distance than the speed of light,
you’ll never be bale to observe the signal.
This is what we mean by the observable universe. The observable universe is
what we can see, in principle, base on the metric expansion of the universe. For
us the cosmic background radiation is the edge of our observable universe. What’s
really cool is that the observable universe is shrinking due to metric
expansion. There are stars that we can, in theory, see today but won’t be able
to tomorrow due to metric expansion. The implication is that in many billions of
years, if human still exist, space will look a lot bigger and a lot emptier than
it does today.
This is sometimes contrasted to the universe at large. This means we could
live in a universe where there are things so far from us we’ll never see them.
Literally, given infinite time, the light would never reach us due to cosmic
expansion. Actually, we know this is the case. The universe is so big we’ll
never be able to see it all.
It’s interesting to think of the ancient star systems out there—made of
exotic materials, possibly hosting intelligent life—that is part of our
universe, and yet we’ll never be able to see or interact with.
The size of the actual universe is greater than the observable universe. On of
the major questions in physics today is how much they differ. If the observable
universe is mostly the size of the actual universe then the distinction is
mostly hypothetical. On the other hand, if the actual universe is much larger
than the observable universe then it’s possible that there are current problems
in Physics like