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Halo, halo, halo
Welcome to this Galaxy
Halo, halo, halo
Look at the mess you'll make
Icon - Midge Ure's raincoat.
Andromeda sports a ginormous halo of gas with a mass greater than 100 billion Suns that stretches from its outer edges up to two million light-years – a distance that reaches more than halfway to our own Milky Way galaxy. If the structure was visible to the naked eye, it would appear three times the width of the Big Dipper, …
The simple answer is that if you're close enough (and massive enough) that gravity is drawing you closer faster than the Universe expands for the distance apart, then you are 'gravitationally bound' and a collision/merger is inevitable.
So the Milky Way and Andromeda are massive enough and close enough that gravity has won out over the expansion rate of the Universe.
Go check out Starts With A Bang (on Medium or Forbes) for plenty of articles on this sort of stuff.
An honest question should not be downvoted... You're correct: on the large scale of the universe, everythings seems to drift apart, like a helium weather balloon floating up in the sky pushing individual heliums atoms further apart as the balloon grows in size. These helium atoms still bounce against eachother though, but that frequency drops as the balloon gets higher up: the internal pressure drops. It's just that individual helium atoms have speeds greater than the speed with which the balloon extends.
It's the same for stars and star systems: if their individual movement is with higher speed than the speed with which the universe is expanding, that individual movement will prevail.
Think of a cannon ball on the rubber sheet. We're heating the sheet so it stretches and expanding the outer frame to keep the tension the same. None of which changes the dimple created by the cannon ball. If there was a marble spinning round the cannon ball, the marble would move slightly higher round the well as the sheet was tugged outwards, but providing it didn't break free, it would carry on spinning and return to it's original height.
More simply: you can model metric expansion as a repulsive force and in the modern universe it's much smaller than the correctional gravitational attraction.
Galaxies are in clusters which are gravitationally bound to each other.
The expansion of the universe means that clusters are moving apart from other clusters but movement within the clusters themselves will continue.
Eventually the clusters will start to spread out and the stars fade away but that is zillions of zillions years in the future.
First, the expansion of the universe doesn't affect the movement of galaxies. Space itself is expanding. The perceived movement of galaxies due to space expanding is such that distant galaxies are perceived to be moving at more than the speed of light (which they can't do).
That said, galaxies (and solar systems, and planets) do move due to gravity. In our galactic neighborhood gravity, electromagnetism, and other forces are much more noticeable than the expansion of space which only becomes noticeable for objects millions of light years away. Thus we have drawn in and will continue to draw in dwarf galaxies which orbit the Milky Way and Andromeda will continue to draw in our smaller galaxy.
One other correction for a subsequent post - the sun is predicted to start expanding about a billion years after the galactic merger so whatever life is surviving on Earth will see a great show.
Congrats to the boffins who worked this out and I look forward to an analysis of what the interactions of the halos will do to star formation.
"Space itself is expanding.... perceived movement of galaxies due to space expanding is such that distant galaxies are perceived to be moving at more than the speed of light (which they can't do)... etc etc etc"
You assume that the gravitational constant is actually a constant, but there's no real proof of it. Measuring it locally, then extrapolating it universe wide does not work. It needs dark-matter and inflation to fix it up. Yet you cling to the 'constant'!
A more obvious observation is that gravity cannot be a constant, that gravitational 'constant' must be stronger locally than distantly.
I can show you why that must be the case quite easily with a thought experiment.
1) Take a bunch of magnets, shake them together, they will align to clump together. Like-polls push each other apart, opposing-polls pull each other together, add motion and they will order themselves to form a net-attraction force. Lets call this 'magnetic clumping'.
2) You see the same "attraction only" style forces throughout nature, e.g. water molecules arrange themselves to clump together. Crystals clump together and so on.
3) This clumping effect does not magically disappear at distance, so it MUST form part of gravity. The overall magic clumping force, gravity, must be the sum of lots of these clumping forces, including our magnetic clumping, water clumping, crystal clumping effects and many many many others.
4) Our magnets are now stuck together in one place, locally the clumping force is higher, and since the magnets are now concentrated in one place and less spread out across the galaxy, the universe clumping force is slightly lower.
5) So the act of clumping, causes *local* clumping to be stronger than universe wide clumping.
6) And since our clumping force forms part of gravity, this also applies to gravity.
7) Hence local gravity is stronger than distant gravity and "gravitation constant" cannot be an actual constant.
8) So the internal gravitational 'constant' inside a galaxy is stronger than the gravitational 'constant' between distant galaxies.
So I haven't gone into the nature of peasoup, the resonant oscillating electric universe, or any kind of deep dive here. It's all very simple and easy to understand and there's nothing in it that isn't logically consistent, yet it shows you why the clumping force, gravity, cannot be a constant and must be locally stronger than at a distance.
I'm currently asking you (well cosmologists) to find tumbling galaxies, it's really a no-brainer to see why they keep their structure given the above.
1) Take a bunch of magnets, shake them together, they will align to clump together. Like-polls push each other apart, opposing-polls pull each other together
I see that in the campaign for the Brexit-poll, Farage told you not to like Poles. This has led to your being pushed apart from European like-poles. Meanwhile the hot gas halo drifting out of BoJo's mouth, is magnetically pulling you together into a chlorinated-chicken 51st state of Andromeda fantasy.
The only problem is the fact that our planet is going to be an roasted husk in about a billion years, due to the Sun's increased activity as it goes through its hydrogen reserves.
Humanity will have to have migrated from Earth by that time if we want to have a chance of being around when the merger actually becomes visible.
1 billion years ago life was single celled. The Cambrian explosion was 500 million years ago. If humans have descendants a billion years from now evolution will make them very unlike us - and that is ignoring genetic engineering. If anything remotely human exists that far in the future it will be because archaeologists digging up something like a future Jurassic Park get raided by time travellers.
Good idea, but needs a change to the passenger selection - as per above the 'influencers' should get priority, and since we know the lesson of it we should keep the telephone sanitisers here because their skills are needed right now.
And being unable to mention the subject without one of my favoured remarks "social media influenzas are a plague on the internet" as it has that magical combination of both punnage and accuracy on how their infection spreads.
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Due to lockdown and also not owning a TV I've watched too many YouTube videos. Unfortunately I've watched a whole load of flat-earth debunking videos. Most of these are amusing but there is something really wrong with these flat-earthers. A complete lack of imagination. I can imagine galaxys colliding, it is sort of mind blowing. But these poor flat-earthers who believe that they live in a snow dome are very sad. Our knowledge of the Universe has expanded exponentially in the last few hundred years. How can people still be so dim?
How can people still be so dim?
Probably has to do with not wanting to confront the reality that they're an amazingly primitive ape-descendant life form on an utterly insignificant little blue-green planet some ninety-two million miles out from a small unregarded yellow sun in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy.
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If the light from Andromeda's quasars gives our scientists an understanding of the material it has passed through on its journey to Earth, and they have no means to measure the Milky Way's halo gases, then surely their findings are the sum of both galaxies' halos.
I guess the only solution is to measure it again after a few million years have passed, and then calculate the influence of our own halo based on the assumption that Andromeda has remained constant, while the distance travelled through the Milky Way to get to us has changed as we orbit our galactic black blob.
Then again, I'm sure that has already been factored into their findings, and my argument is based solely on the article presenting the juicy information while not getting bogged down in the technical whys and ways.
Rogue stars! That's the answer. Stars that had slipped their galactic chains long ago, and therefore their light arriving at Earth is far more influenced by our own halo than anything else, give or take the odd intergalactic gas cloud.
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