I love watching programs about space and time, the universe and related subjects. And I confess that I don’t understand quite a bit of it. but the one thing I just cant get anyone to explain to me is the idea that every galaxy is moving away from every other galaxy. I have been given the balloon example, where dots on a small balloon, when the balloon gets air in it, shows the galaxies all moving away from each other, and not getting closer to any other galaxy.
OK, if I take that at face value, I can understand it, however I’ve also read that Andromeda, our closest galactic neighbor will eventually collide with the Milky Way. How can that be?
Because they’re part of the Local Group of galaxies, not drifting independently of each other; part of the same gravitationally bound system. And because the expansion of the universe is an average; individual parts of it aren’t necessarily expanding. It’s much the way that the solar system holds together despite the expansion of the greater universe, or how the expansion of the universe wouldn’t stop a comet from interstellar space from flying into the inner solar system.
Well, although galaxies are in general moving away from each other, they aren’t all doing so.
So, you’ve got a big crowd in a mall, and somebody sets off a firecracker, and so everybody goes running, such that everyone is moving away from everyone else… But a scared young couple cling to each other…
At a very local, nearby scale, gravity can overcome expansion. But seen from any farther away, galaxies pretty much are all moving away from each other. Hubble’s Law is a statistical average; the bigger the sample you take, the more accurate it gets.
I am now imagining our own Galaxy reaching out to embrace Andromeda across the voids of intergalactic space. Yearning to calm her fears in the midst of the Great Chaos. Clinging in irresistible gravitational attraction.
Obviously, not all galaxies are rushing away from each other. Spacetime is expanding and so the space between places is expanding - except in the case where objects are gravitationaly bound together. So you can can have a cluster of galaxies, like the Local Group, where 2 galaxies can interact with each other.
Now will the expansion becomes rapid enough to overcome even these local concentrations of gravity? That’s the idea behind the idea of the Big Rip but that’s only a hypothesis.
i don’t know how accurate this answer is, but it makes sense to me. I am familiar with the local group of galaxies that Milky Way is a part of, and when you take that local group as the body flying away from everything else, but not its individual galaxies, the idea that the Milky Way and Andromeda intersecting at some point does make sense. They are interacting with each other, and that can include a gravitational attraction.
Thank you.
Technically, everything is interacting with everything else in the universe. It’s just that the size of the interaction is in this case powerful enough to overcome those other tinier influences.
It’s like a magnet being able to pull up a nail against the gravity of the entire earth on the other end. A hand magnet isn’t especially powerful; it just needs to be more powerful locally.
Instead of drawing dots on the balloon, use stickers. Stickers don’t expand, even if the balloon does, because they’re held together by a stronger force. The Milky Way and Andromeda are both part of the same sticker.
It’s also conceivable that MW and Andromeda just happen to be cruising towards each other due to their earlier trajectories (from goodness knows what).
So, instead of thinking of the surface of the balloon, imagine a balloon that’s getting bigger without any air being added (say, it’s in a vacuum chamber where the air is being pumped out so it expands).
On average, all the gas molecules in the balloon are getting farther apart. But individually, they can still be caroming off each other willy-nilly.
Note that I have no idea whether there could be some earlier event to give two different galaxies in a local cluster different velocities. But if this can happen, it might also happen that two galaxy clusters are approaching each other, despite the general tendency to the contrary.
Gravity is probably the easier answer, but it’s not necessarily the only one (without knowing a lot more about the history of the cosmos than I do).
