If the Universe is Expanding.....

[Hockeycowboy]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

 

[YoursTrue]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

Yeah. That's a good point.

 

[sun rise]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

Because in the universe galaxies are not all moving out uniformly. Some are moving in other directions as well.

 

[Twilight Hue]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

Well what you see out there isn't exactly the same material form that was originally expulsed. New stars are born and old Stars die and planets and such, nebula change shape and so forth the list is endless. Explosions, Black holes, gravity slingshoting everything everywhere.

Basically it's to say that it's not surprising to see collisions occur. We may be expanding but physics is still physics.

Think running of the bulls or a good NASCAR pile up with debris and blood flying everywhere!

 

[Hockeycowboy]

Some are moving in other directions as well.

I get that. But expansion in motion is still expansion, right?

 

[YoursTrue]

Because in the universe galaxies are not all moving out uniformly. Some are moving in other directions as well.

It is possible, isn't it, that the gravitational forces could be elliptical rather than let's say, a straight line, using layman's terminology? That could mean that the galaxies would not collide but evade one another.

 

[Hockeycowboy]

Well what you see out there isn't exactly the same material form that was originally expulsed. New stars are born and old Stars die and planets and such, nebula change shape and so forth the list is endless. Explosions, Black holes, gravity slingshoting everything everywhere.

Basically it's to say that it's not surprising to see collisions occur. We may be expanding but physics is still physics.

Think running of the bulls or a good NASCAR pile up with debris and blood flying everywhere!

Did you mean “running of the bulls,” like in Pamplona?
OK.

But how does expansion explain collision?

 

[Left Coast]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

Here's a cosmologist's explanation:

If galaxies are all moving apart, how can they collide?

 

[Hockeycowboy]

Here's a cosmologist's explanation:

If galaxies are all moving apart, how can they collide?

Interesting! I see the point, don't know if it's a valid explanation, but it may be.
Thanks.

When is Andromeda expected to intersect the Milky Way?

 

[Yerda]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

You could extend that question and ask if the universe is expanding then how can anything collide?

I'm not a cosmologist or anything but the answer is that the rate that any two objects move away from each other due to expansion depends on the distance between them. Andromeda is close enough that the pull between it and the Milky Way, due to gravity, is greater than the motion pushing them apart due to expansion.

That make sense?

 

[Left Coast]

Interesting! I see the point, don't know if it's a valid explanation, but it may be.
Thanks.

When is Andromeda expected to intersect the Milky Way?

Looks like 4.5 billion years.

Andromeda–Milky Way collision - Wikipedia

 

[exchemist]

Here's a cosmologist's explanation:

If galaxies are all moving apart, how can they collide?

Yes.

The same thing occurs at smaller scales too. If it did not, someone might be tempted to think that, because "everything is moving away from everything else", it would be impossible for stars or planets to form!

The key thing it seems to me is that because gravity gets stronger as any 2 objects get closer, you have an intrinsically unstable situation. This will will turn any initially homogenous distribution of matter into clumps, as soon as the tiniest disturbance in complete homogeneity occurs.

This is not inconsistent with the idea of an expanding universe, as it is spacetime itself that is expanding, which means there is a general background tendency for objects to move apart, on which individual gravitational attractions are superimposed.

An expanding universe does not mean that every object in it has to be moving away from everything else.

 

[Hockeycowboy]

You could extend that question and ask if the universe is expanding then how can anything collide?

I'm not a cosmologist or anything but the answer is that the rate that any two objects move away from each other due to expansion depends on the distance between them. Andromeda is close enough that the pull between it and the Milky Way, due to gravity, is greater than the motion pushing them apart due to expansion.

That make sense?

Yep. I was thinking that 2.5 million ly distance from each other, and given their apparent size and density, that gravity wouldn't be much of a factor. Their outskirts really aren't that dense.

IDK. But thanks for your input.

 

[Hockeycowboy]

Looks like 4.5 billion years.

Andromeda–Milky Way collision - Wikipedia

Yeah, I just found that out! Thanks, Left Coast!

Ah, GOOGLE...what a tool!

 

[Heyo]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

If someone shoots at you, just move away. Any arrow or bullet is moving at decreasing speed. As long as you move at constant speed, no matter how slow, you will outrun the projectile eventually.
Does that make sense?

 

[Brian2]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

I had a similar thought when recently I heard that Andromeda and the Milky Way galaxies would collide in 4.5 Billion years give or take. Then I realised that gravity and explosions etc would still happen even in a generally expanding universe. ie movement is in more directions than away from the initial point of the big bang.

 

[Hockeycowboy]

I had a similar thought when recently I heard that Andromeda and the Milky Way galaxies would collide in 4.5 Billion years give or take. Then I realised that gravity and explosions etc would still happen even in a generally expanding universe. ie movement is in more directions than away from the initial point of the big bang.

Yeah. Apparently, the expanding part, because of the universe’s vastness, affects basically the clusters, not the individual galaxies within them.

 

[Brian2]

Yeah. Apparently, the expanding part, because of the universe’s vastness, affects basically the clusters, not the individual galaxies within them.

And on a smaller scale we are not affected and we can go in any direction.
But yes gravity would have it's effect over time even if both the Milky Way and Andromeda are moving away from the big bang............or, as they say, the space time universe is expanding.

 

[Revoltingest]

I get that. But expansion in motion is still expansion, right?

An analogy....
Imagine a burning gas inside an engine.
(The engine is red, with white pin stripes.)
As the piston moves, the volume expands.
The gas is therefore expanding, but the individual
molecules still collide with each other. It just
becomes less frequent, so the temperature drops.

 

[Terrywoodenpic]

...then how can galaxies collide?

I mean, gravity would have less and less an effect.

Earlier (billions of years ago), when the universe was denser, maybe I could see how. But with expansion continuing.... how could this be predicted?

Any Thoughts?

because although the universe is expanding, galaxies have individual orbits with in it. Just as stars do within galaxies. at one time when every thing was somewhat closer collisions would have been statistically more likely.
But the universe is composed mostly of nothing. in the same way as atoms are. distances between objects are unimaginable.

However at a local level, the Earth is bombarded by millions of objects every year, adding tons to the mass of the Earth. So collisions are not exactly rare.