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modsarebrainstems OP t1_j2np4pe wrote

Reply to comment by [deleted] in How do galaxies move? by modsarebrainstems

Thank you. I'm a little puzzled, however in that wouldn't the attractive forces weaken with distance? That is to say, shouldn't, say, Andromeda and the Milky Way have collided a long time ago?

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ExoticSwan8523 t1_j2o7haz wrote

Gravitational forces do indeed weaken with the square of the distance, but they strengthen between more massive objects. You also need to factor in the relative velocity between the objects.

Just for a complete picture, the Andromeda galaxy is about 2.5 million light years away from the milky way (~0.9 Megaparcecs), and moving towards us at about 110 km/s. The expansion rate of the universe is about 73km/s per megaparcec.

While this is probably a massive oversimplification, we can think of the Andromeda galaxy receding from us at about 73 * 0.9 = 65km/s, but is moving towards us faster than it's receding, at 175km/s, but the expansion rate of the universe brings its net speed down to 110km/s. This is just a snapshot in time, since as the distance gets smaller, the force of gravity gets stronger, and there's less expansion of the universe to deal with. In other words, Andromeda should be accelerating towards us over time, assuming just gravitational force and the expansion rate of the universe.

Why didn't Andromeda and the Milky way already merge? Basically, both galaxies were originally two separate denser regions in space that were far enough apart to form two independent galaxies, but close enough to not recede over time thanks to the expansion of the universe.

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ScootysDad t1_j2oe84f wrote

Yup. Except that there is no expansion of space between our two galaxies. As miniscule as it is, the gravitational "force" is much stronger than the expansionary forces so that rate is 0.

We are in the Laniakea Supercluster and the space between our Supercluster and the next one, Perseus–Pisces Supercluster, are expanding at 73km/s. Within Laniakea, the gravitational "force" keeps us together in the same orbit. Everything is orbiting something.

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Edit: 73km/s/megaparsec

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Aseyhe t1_j2q6qrm wrote

Superclusters are still expanding, they are just overdense regions that would eventually collapse if there were no dark energy. Assuming dark energy persists, only our Local Group will remain nearby.

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ScootysDad t1_j2r15zu wrote

That's a yes an no answer mostly because the space between the local clusters are too large. There's a region around the supercluster where objects are gravitationally bound to the center of gravity and outside of that radius the local clusters will eventually escape. Much like the orbital mechanic of our solar system. So from that region outward the dark force appears to dominate and expand the space.

Edit: With our current understanding of the universe, within the Supercluster the dark force responsible for the expansion of the universe is too weak to overcome the gravitational "force" within the bounded section of the supercluster.

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Aseyhe t1_j2r1iul wrote

If a cluster of galaxies is virialized (its constituents are orbiting stably), we call it a cluster, not a supercluster. Superclusters are expanding with the Hubble flow by definition. A supercluster could certainly have a virialized cluster at its center though!

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Krail t1_j2p4eko wrote

So, are you saying that past a certain magnitude threshold, the force of gravity effectively causes the expansion of space in that region to stop?

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omgwtfbbqgrass t1_j2pb1nu wrote

It's not that gravity causes the expansion of space to stop, it's just that on relatively "small" scales we can safely ignore the expansion of space. Gravity still dominates even at the scale of galactic superclusters (for now). But increase the scale by comparing entities billions of light years away, and it's the expansion of space that dominates over gravity.

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ScootysDad t1_j2pq816 wrote

Forces have an effective range. At the sub-atomic range, the Strong and Weak forces act on particles like quarks. Above that is is the electromagnetic force which works at the atomic level to the macroscopic level (normal everyday experiences). After that is the gravitational "force" which works at the normal everyday objects like apples, cars, rockets, and people to galaxies, local clusters, and superclusters. All of the above forces are orders of magnitude stronger than the dark force that caused the expansion of the universe.

So, the space between superclusters is vast and gravity no longer hold sways over the space fabric so it stretches.

One posit is that gravity is not a force but rather a time gradient around mass. The closer you are to the central mass the greater the time curvature so the differential time difference causes you to spiral downward through space instead of an actual field that interacts with a particle (like photon to the electromagnetic field or the Higgs boson with the Higgs field to give us mass).

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omgwtfbbqgrass t1_j2pb29x wrote

It's not that gravity causes the expansion of space to stop, it's just that on relatively "small" scales we can safely ignore the expansion of space. Gravity still dominates even at the scale of galactic superclusters (for now). But increase the scale by comparing entities billions of light years away, and it's the expansion of space that dominates over gravity.

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ScootysDad t1_j2ockk7 wrote

There's amble evidence that the Milkyway has collided with another galaxy and currently is in the process of incorporating another galaxy into its structure. The other galaxies are drawf galaxies so we maintained out spiral structure. With Andromeda we will not be a spiral any more.

Andromeda is slightly larger and it and 29 other galaxies (including the Milky Way) are part of the Local Group of galaxies. There may have been many more galaxes but they have since been incorporated through glactice mergers. Andromeda and the Milky Way's orbits around our center of gravity will bring about a merger in the distant future. By that I mean one of these galaxies are not in the stable orbit (on galactic time scale). Even after the merger the combined mass and velocity of the the merged galaxies will put us into a different orbit around our center of gravity.

You know what they say: If Andromeda doesn't come to us, we will come to Andromeda.

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modsarebrainstems OP t1_j2oglge wrote

So you're saying that because our mass isn't fixed, every time it changes we more or less move to a new orbit relative to our common center of mass? Is that basically correct?

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