Undergrad Collision of Milky Way and Andromeda....?

[Sasho Andonov]

I do not understand how (in few bilion years) Milky Way and Andromeda will collide?
If the universe is expanding and these objects which are far away have bigger speeds than those who are closer, than Milky Way and Andromeda will just be far away every moment... (?)
Could someone help? :-)

 

[LURCH]

The trouble lies in the definition of “far away “. Andromeda is the closest galaxy to us; so close that the expansion rate is nearly nothing compared to the attraction of gravity between the two galaxies.

 

[mathman]

The Milky Way and Andromeda are part of a cluster of galaxies. Within the cluster gravity dominates over universe expansion.

 

[Vanadium 50]

Let's start with an easier problem - can two cars collide if the universe is expanding?

 

[Sasho Andonov]

By your responds I can infer that expansion of univese apply only on clusters, but inside the clusters, in the future, the galaxies will collide due to gravitational attraction... Hmmm... It does not look so scientific... :-(
Regarding Vanadium 50's respond, I can say that expansion of the universe does not means that universe constituents (earth where the cars are!) are expanding. So, the cars may collide, but this has nothing to do with expansion...

 

[DaveC426913]

The expansion we see and calculate is an average - as in: on average distant galaxies are receding faster than closer galaxies. It does not account for local, relative movement of galaxies.

Just like stars within galaxies have their own peculiar motions, so too do galaxies within clusters have their own peculiar motions.

The fact is, within our cluster, Andromeda is moving toward The Milky Way.

 

[Vanadium 50]

Hmmm... It does not look so scientific... :-(

Maybe the problem is with your inference and not with the correct answers you have received. Just sayin'.

So, why can cars collide but not galaxies?

 

[Sasho Andonov]

Maybe the problem is with your inference and not with the correct answers you have received.

I have got some answers on my question and I can agree that the problem could be "my inference", but you still did not provide arguments why my inference is wrong. You enter the discussion regarding my question with another question (?)... What does it mean?
Sorry, but you are not helping at all...

 

[Janus]

I have got some answers on my question and I can agree that the problem could be "my inference", but you still did not provide arguments why my inference is wrong. You enter the discussion regarding my question with another question (?)... What does it mean?
Sorry, but you are not helping at all...

Think of space as the surface of a rubber band that is slowly being stretched. The galaxies, etc. are objects sitting on that surface, but free to move along the surface.
If you pick any two points of the rubber band, they will be moving away from each other, the further apart, the faster they are moving apart. But objects at those points aren't required to stay fixed to those points, they can have a velocity relative to the rubber band itself. If one object has a velocity towards an other that exceeds the speed at which the point it it started at is receding, it will reach the second object. It's velocity relative to the rubber band material overcomes the tendency for the rubber band to expand. Similarly, if there is a force attracting the two objects, if they are close enough to each other, the motion it imparts on them towards each other will be greater than the expansion speed over the distance separating them. If they are already bound tightly to each other (like the atoms making up your body),the expansion of the band cannot pull them apart. Imagine two people standing on an expanding tile floor in their socks. If they just stood still, the expansion of the floor would move them apart. But if they reached out and grasped hands, their grip would prevent them from moving apart as the floor under their feet expanded. This is the gravitational and molecular bonds holding compact objects together against the overall expansion of the universe. (For this example to be more accurate, the reach of each person would be infinite but would fall off in strength with distance. They would easily be able to hold object close to them from drifting off, but past a certain distance, their grip is too weak to hold against the expansion.)

 

[russ_watters]

I have got some answers on my question and I can agree that the problem could be "my inference", but you still did not provide arguments why my inference is wrong. You enter the discussion regarding my question with another question (?)... What does it mean?
Sorry, but you are not helping at all...

His car example would help you if you would put some thought into it and respond to it. Think. You'll be more likely to accept the answer if you figure it out yourself (with us pushing you toward it).

 

[Sasho Andonov]

Dear Janus: THANK YOU VERY MUCH!
I appreciate very much your excellent, considerable and wise explanation!
I wish you excellent night/day!

 

[DaveC426913]

You enter the discussion regarding my question with another question (?)...
Sorry, but you are not helping at all...

As gently as possible:

Vanadium, and Russ are trying to guide you toward the answers for yourself. This forum's philosophy is that of tutelage, it is not intended to be an information depot.

 

[DaveC426913]

Cosmological expansion is not some physics-defying force then that slowly pull anything apart, down to cars and atoms.

Similar to Janus answer, but simpler: I often use the analogy of gluing pennies to a balloon. As you inflate the balloon, the pennies do not expand. They move apart, but the weak force of the expanding balloon is far too small to tear the pennies atom from atom.

Likewise, any ant (or car) is free to move about. An ant walking around on the penny does not feel itself dragged away by the expanding balloon. The force holding the ant to the penny vastly exceeds the force of the balloon's expansion.

Even a piece of thread tried between two pennies will not break. Think of this akin to Milky Way and Andromeda - they are distant, but still bound by gravity.

 

[zuz]

Galaxys collide all the time. We have countless examples of this from Hubble and other telescopes. The universe is expanding, but if two galaxys are moving away from a point in space, but on the same course, and one is moving faster, it could overtake, and collide with the other.

 

[Sasho Andonov]

As gently as possible:

Vanadium, and Russ are trying to guide you toward the answers for yourself. This forum's philosophy is that of tutelage, it is not intended to be an information depot.

Dear Dave, (as gently as possible!) I am fully aware about the way how these two guys tried to answer my question. Have no doubt about that!
But I am not sure how you determine that "this forum's philosophy is that of tutelage"? Even it is the philosophy, it is wrong! I asked the question and Janus answer it on the best possible way! Tutelage is for parents and children when you need to built their personality. In such a scientific forum it can be only tool to heal your frustration. Sorry, but I am tired from "tutelages" and please do not be angry on Janus who did not followed "forum's philosophy!
From my point of view: Janus earned my respect! :-)

 

[Sasho Andonov]

Galaxys collide all the time. We have countless examples of this from Hubble and other telescopes. The universe is expanding, but if two galaxys are moving away from a point in space, but on the same course, and one is moving faster, it could overtake, and collide with the other.

Dear sus, thank you very much! I catch the point and it is clear to me!

 

[alantheastronomer]

Surprisingly, the effect of the expansion of the universe on the motion between the Milky Way and Andromeda galaxies is non-negligible and in fact, appreciable! With a Hubble constant of 72 km/sec./Mpc and a distance of 2.5 Mly the velocity due to the expansion is a whopping 55km/sec.! The measured velocity of attraction between the two galaxies is about 300km/sec., so if we account for expansion, the "true" velocity between the two galaxies due to their mutual gravitational attraction is really 355km/sec.!

 

[phinds]

Galaxys collide all the time. We have countless examples of this from Hubble and other telescopes. The universe is expanding, but if two galaxys are moving away from a point in space, but on the same course, and one is moving faster, it could overtake, and collide with the other.

That is incorrect. Galaxies that are far enough apart to be affected by expansion relative to each other will always just keep getting farther apart. They will never collide.

Dear sus, thank you very much! I catch the point and it is clear to me!

That is unfortunate since his "explanation" is wrong.