Trying to understand the Andromeda-Milky Way collision

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In summary: Galaxies_and_clustersIn summary, as space continues to rapidly expand, the Milky Way and Andromeda galaxies are on a collision course due to their combined speed being faster than the rate of expansion. Galaxies are generally moving due to competing forces, such as gravity, and there is no central point around which they rotate. In 1 trillion years, assuming the Earth and humanity still exist, there will likely be fewer galaxies in our local cluster and we may not be able to see any other galaxies beyond our cosmic light horizon.
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Alltimegreat1
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Since space is rapidly expanding between galaxies, why are the Milky Way and Andromeda galaxies on a collision course with each other? Do galaxies need to be a certain distance apart for the force of their gravity to be weak enough so that it cannot overcome the expansion of space? Or is it because the Milky Way and Andromeda galaxies are simply moving toward each other as part of their orbits around the Great Attractor?
 
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  • #2
Alltimegreat1 said:
Since space is rapidly expanding between galaxies, why are the Milky Way and Andromeda galaxies on a collision course with each other? Do galaxies need to be a certain distance apart for the force of their gravity to be weak enough so that it cannot overcome the expansion of space? Or is it because the Milky Way and Andromeda galaxies are simply moving toward each other as part of their orbits around the Great Attractor?

In general things tend to get further apart due to space expansion, as long as they do not have proper motion, gravitation ally bound to another body.
 
  • #3
Alltimegreat1 said:
Since space is rapidly expanding between galaxies, why are the Milky Way and Andromeda galaxies on a collision course with each other? Do galaxies need to be a certain distance apart for the force of their gravity to be weak enough so that it cannot overcome the expansion of space? Or is it because the Milky Way and Andromeda galaxies are simply moving toward each other as part of their orbits around the Great Attractor?
The two galaxies are approaching each other because their combined speed is faster than the rate the universe is expanding. Presently, the best estimate puts the rate of expansion of the universe at 67.80 ± 0.77 (km/s)/Mpc. Since we are measuring the approach of the Andromeda galaxy at ≈110 km/s, that must mean the Milky Way and Andromeda galaxies are actually approaching each other at a combined rate of ≈162 km/s in order to overcome the expansion of the universe which would be ≈52.18 ± 0.6 km/s at the distance of the Andromeda galaxy (0.78 Mpc).
 
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OK thanks for that explanation. When you talk about speeds that galaxies are traveling, what is meant by this? Are galaxies moving just because they're orbiting some central point of a galaxy cluster? Are all galaxies moving in a specific direction irrespective of the expansion of space?
 
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Think in terms of competing forces - clusters of galaxies, for example are gravitationally bound. The degree to which this force is larger than expansion at any given time calculates how long they remain bound. AFAIK it is not meaningful to consider a "central point" other than the combination of individual vectors.
 
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Expansion is a nonfactor for objects as near as Andromeda. It is currently thought to be about 68 kilometers per second per megaparsec - re: http://phys.org/news/2015-02-fast-universe.html - or 1 km per 2.2E+018 km per second. that translates to around 9 trillionths of a kilometer [9E-012] per second at the distance of Andromeda - disregarding any other considerations. Andromeda is currently approaching the MW at about 300 kilometers per second.
 
  • #7
Alltimegreat1 said:
... speeds that galaxies are traveling, what is meant by this? Are galaxies moving just because they're orbiting some central point of a galaxy cluster? Are all galaxies moving in a specific direction irrespective of the expansion of space?
Galaxies within a cluster are gravitationally bound and can be moving relative to each other regardless of overall expansion of space.
A cluster doesn't have a central point around which everything rotates, the movement of individual galaxies is the sum of the momentum from the gas clouds which produced them and subsequent gravitational interactions.
Afaik it's chaotic, no cluster is particularly similar to another one, other than in the fact of being a cluster.
 
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Assuming the Earth and humanity still exist in 1 trillion years, what would our observable universe look like? Would the other galaxies in our cluster still be around?
 
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Alltimegreat1 said:
Assuming the Earth and humanity still exist in 1 trillion years, what would our observable universe look like? Would the other galaxies in our cluster still be around?
Assuming the Earth is not consumed when the sun reaches its Red Giant phase ≈5 billion years from now, the Earth would still be rendered into a charcoal briquette. By the time the sun reaches its degenerate white dwarf phase it will lose ≈70% of its mass, which will effect Earth's orbit (as well as the orbits of all the objects in our solar system). Regardless of whether Earth remains in orbit around the white dwarf, or is thrown out of the solar system completely, it will become a frozen ice ball. Either way, humans will have to leave Earth within the next 500 million years, assuming we make it that long. Odds are that humans will become extinct long before that.

There will still be a few galaxies left in our local cluster a trillion years from now, but that number will be much smaller than the current ≈36. The Andromeda/Milky-Way combined galaxy will be the largest in our local cluster, and most like already include several more galaxies by that time. Where the Earth will be is anyone's guess, but there will not be anything living on it.
 
  • #10
Alltimegreat1 said:
Assuming the Earth and humanity still exist in 1 trillion years, what would our observable universe look like? Would the other galaxies in our cluster still be around?

If we extrapolate current cosmological trends, we will not be able to see any other galaxies, as they will have receded beyond our cosmic light horizon and the local group will possibly have merged into one galaxy.

See here: https://en.wikipedia.org/wiki/Timeline_of_the_far_future
 

Related to Trying to understand the Andromeda-Milky Way collision

What is the Andromeda-Milky Way collision?

The Andromeda-Milky Way collision refers to the future event in which the Andromeda galaxy and our own Milky Way galaxy will collide and eventually merge into one larger galaxy.

When will the Andromeda-Milky Way collision occur?

The collision is estimated to occur in about 4.5 billion years.

What will happen to Earth during the Andromeda-Milky Way collision?

The collision will not have a significant impact on Earth as it is expected to survive the merging of the galaxies. However, the night sky will look drastically different as the two galaxies merge and form a new galactic shape.

How do scientists know that the Andromeda-Milky Way collision will happen?

Scientists have observed the motion of stars and galaxies and have predicted their future movements. Through these observations and calculations, they have determined that the collision between Andromeda and the Milky Way is inevitable.

What will be the result of the Andromeda-Milky Way collision?

The result will be a larger, elliptical galaxy formed from the merging of Andromeda and the Milky Way. This new galaxy will contain billions of stars and may have a different structure and appearance than either of the two individual galaxies.

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