If the Universe is Expanding.....

[Terrywoodenpic]

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.

Nothing moves in straight lines in space, nor does anything move at a uniform speed.
While there is a general movement to expansion, many orbits intersect.
galaxies can be seen colliding to day. so this is not just theoretical.
but what we see to day happened a long time ago....

 

[shunyadragon]

...then how can galaxies collide?

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

Still gravity is the cause, because of th ehigh mass of galaxies, no problem.

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

Earlier in the history of the universe, the expansion was the greatest force, and the galaxies were less dense, Hydrogen, Helium and Lithium.

.... how could this be predicted?

First, and for most by observation of the collision of galaxies, and calculating the collision course of galaxies. Second, simply predicted by calculation of Newtonian physics and of course modern refinement if the nature of gravity.

 

[shunyadragon]

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.

Actually in another thread it is found that he universe not only is expanding, but also rotating like a galaxy of galaxies. Our universe may be spinning like everything else.

Our universe may be spinning like everything else

 

[Polymath257]

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 of 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.

This, by the way, is the most correct answer. gravity tends to enhance random fluctuations in density, making the clumps clumpier and the spaces in between spacier.

In the individual clumps, however, the gravity between the galaxies can overwhelm the overall expansion. This is known as peculiar motion, and it is not a small effect. In fact, it was one of the big technical issues in our attempts to measure the overall rate of expansion.

One way to think about this is ants crawling on an expanding balloon. If the expansion is slow enough, the ants can move over the surface fast enough to interact with other ants that are close, even though they may never be able to get to ants farther away.

Our Milky Way galaxy has several smaller galaxies orbiting it. And, as has been noted, the Andromeda Spiral is expected to collide in a few billion years (since galaxies are mostly empty space, almost no stars will collide). We have pictures of other galaxies doing the same thing.

Also, our local cluster of galaxies is fairly small on the cosmic scale, where galaxy clusters can have thousands of galaxies.

 

[ChristineM]

...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?

It's still gravity. Although inflation is predominant local mass still has gravitational effects. More so in the past but the pull is still there.
The effect of gravity is infinite, it works on the inverse square rule over distance but that effect never actually reaches zero. So every mass in the universe is acting on every other mass to some extent. The upshot is that although generally expanding the individual masses move with a random motion. This effect is more pronounced in local galactic clusters

 

[YoursTrue]

Nothing moves in straight lines in space, nor does anything move at a uniform speed.
While there is a general movement to expansion, many orbits intersect.
galaxies can be seen colliding to day. so this is not just theoretical.
but what we see to day happened a long time ago....

I'll just say this: the chances of life happening on this earth are in the astronomical, unimaginable tables of mathematics. My conclusion is that God can protect the earth from colliding if He wants to.

 

[Hockeycowboy]

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.

What if it has white & black pin stripes? Then the engine blows up, right? Lol.

 

[Revoltingest]

What if it has white & black pin stripes? Then the engine blows up, right? Lol.

I don't know.
No one I know paints them with that color scheme.
Good experiment for a PhD thesis, eh.