Physicists tell us that the universe is expanding, but does that mean we expand with it?
I mean, if so, wouldn't no one notice because everything would scale up at the same relative
size and it would seem like nothing had changed?
Well, the truth is, the universe isn't expanding.
Space is.
Of course, everything else in the universe is "in" space, and ignoring the influence
of any other forces, when the space between things expands, those things will appear to
be carried farther apart.
But there are other forces in the universe, and their strengths aren't directly affected
when space expands.
For example, gravity and electromagnetism are able to hold atoms and solar systems together
because these forces are too strong for the slow expansion of space to overcome.
Think of it like this - if you and your cat are standing on a piece of ground that starts
to stretch horizontally between you, the ground will pull you and your cat away from each
other.
But if the leash is strong enough - in particular, if it's stronger than the friction force between
your feet and the ground - it'll hold you and your cat together as space expands beneath
you.
In the real universe, it's the strength of the electromagnetic force that determines
the effective sizes of particles, atoms, and molecules so that even if the space they're
in expands, the molecules don't.
Electromagnetism is basically the leash that holds atoms and molecules together - and gravity
is the leash that holds solar systems and galaxies together.
Only over super super long distances is gravity's attraction weak enough and the cumulative
expansion of all that space big enough to cause galaxies and galactic clusters to actually
grow more distant from one another over time - we know this because the light from distant
galaxies gets red-shifted as it's stretched by the expansion of the space it's traveling
through.
Gravity certainly still pulls distant objects towards each other, but space expands faster.
Basically, the cat is
off
the leash.