Peanuts

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Wade Hampton III
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Joined: Fri Oct 18, 2013 10:40 pm
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Peanuts

Post by Wade Hampton III » Mon Oct 01, 2018 3:22 am

David Breaux, SysAdmin (2009-present), wants to know...

Why is it believed that the galaxies in the Local Group (including Milky Way
and Andromeda) eventually collide when it's observed that all galaxies are
moving away from each other in every direction and likely will do so forever?
57689
Andromeda
Andromeda
57689.jpg (135.9 KiB) Viewed 1266 times
My cousin asked me a similar question once. He asked if everything is moving
away from everything else due to the expansion of the universe, how could
Andromeda be getting closer? Well, at least in his case, it had to do with
a misunderstanding of scale and an improper visualization of how expansion
works. See, he was thinking of expansion like the surface of a balloon that
is being inflated. In that example, all the points on the balloon are static.
This works to visualize how space itself expands, but fails when it comes
to the objects in space. Imagine that balloon again, except instead of just
being a clean surface, there are tiny bugs all over the surface of the balloon,
skittering about in all directions. These bugs represent the stars and galaxies.
While the balloon is expanding, the surface area of the balloon is growing,
creating larger and larger gaps between any two points on the balloon. Think
again about the balloon expanding. If you were to place two points directly
opposite each other and measure the distance between them on the surface,
they would get further apart faster than two points directly near each other.
There’s more balloon to expand between the opposite points, so more distance
is created.

So, back to the bugs. The bugs that are all the way across the balloon could
never reach each other because the skin of the balloon is stretching out faster
than they can walk that far. However, if their buddy is right next to them,
it’s just a matter of a few steps to go crash into him. On the small scale,
the stretching isn’t too fast to travel across.

Also, the galaxies and and stars don’t always move in the same direction.
Billions of years of collisions, deflections, and slingshots have made different
celestial bodies move in different directions. There’s also the force of gravity
pulling things around. So, they’re all kind of roiling about haphazardly. They
aren’t all uniformly moving away from some central point in the universe.
They’re mixing and spinning and rotating and revolving in all directions.
Eventually, some of those trajectories are going to intersect and collide.

Now, we know that Andromeda is moving towards us because we can see it doing so.
Blueshift[1] is a thing that happens to light when the source of the light is
getting closer. The wavelengths of light get kind of bunched up and it shortens
the wavelength to a more blue color. Redshift is the opposite and happens when
the source of light is moving away, causing the wavelengths of light to be
stretched.

And remember. The universe is big, I mean REALLY big. The distance between us
and Andromeda is basically insignificant to the overall size of the universe.
The observable universe is about 93 billion light years across. We’re about
2.5 million light years from Andromeda, or about 0.000026% the distance across
the universe. See, peanuts in comparison.

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