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Hi all, im not a physicist, but have an interest. Now given that most agree in the big bang theory, means everything is moving away from each other in a ever expanding universe. That being the case how can the milky way galaxy be going to crash into the Andromeda galaxy? Albeit in a few years time. 

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Because the gravitational attraction between Andromeda and the Milky Way is greater than the expansion.  All the galaxies in our local group are moving towards each other.

 

8 minutes ago, wildie9 said:

Albeit in a few years time. 

About 4 billion years.

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3 hours ago, wildie9 said:

Hi all, im not a physicist, but have an interest. Now given that most agree in the big bang theory, means everything is moving away from each other in a ever expanding universe. That being the case how can the milky way galaxy be going to crash into the Andromeda galaxy? Albeit in a few years time. 

Just to build on what has already been said.  "Everything" is not moving away from each other.  The atoms making up the Earth aren't, The planets are not moving away from the Sun (or at least not due to reasons tied to universal expansion),  the stars in the galaxy aren't flying apart...

All these structures are held together by interactions that are much stronger than universal expansion.*  Our galaxy, along with Andromeda and a few others are part of a local group bound together by mutual gravitation.   Our local group is one of many in a larger cluster.  You have to go to scales beyond that for expansion to take hold.  So when we say that the expansion of the universe is causing galaxies to move apart, what is really is that the larger bound clusters are moving apart from each other.

 

* One way of imagining this is to think of yourself as standing on a polished tiled floor in you stocking feet. The tiles are expanding so that the center of the tiles move apart. But the bottoms of your feet, being held together by molecular force, don't expand along with them.

If you have a friend on an adjacent tile he, would, over time, move further and further away from you as the floor expands.  Unless: You reach out and grasp hands. Now, as long as your grip is stronger than the friction between your feet and the floor, you will stay the same distance apart. Gravity acts like your grasped hands. The difference is that gravity gets weaker over distance.  So, as long as galaxies are close enough to each other, gravity can keep them bound together, but when they are further part,  it can't.

 

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3 hours ago, wildie9 said:

Thank you, it sort of makes sense now. please forgive my ignorance. 

That's not how ignorance works. If it makes sense now that your question was answered, as swansont said, there's literally nothing to forgive.

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3 hours ago, wildie9 said:

Thank you, it sort of makes sense now. please forgive my ignorance. 

As others have said, a great question.

In effect it is only space expanding, at the most observable distances of our universe...our solar system is gravitationally bound...our galaxy is gravitationally bound, our local group is gravitationally bound [including Andromeda] and our cluster of galaxies is gravitationally bound. Only galaxies beyond our cluster will appear to be moving away due to the expansion of space.

https://en.wikipedia.org/wiki/Galaxy_cluster

8 hours ago, wildie9 said:

Albeit in a few years time. 

In a few billion years. 😉 And by that time, our Sun will have reached the giant red phase, and become a White Dwarf, the Moon will be double its distance from Earth then it is now, and a day on Earth [if we are not swallowed up by the Sun] will be equal to a lunar month.

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3 hours ago, beecee said:

In effect it is only space expanding, at the most observable distances of our universe...

Can I modify the above from my last post.....should read, "it is only space that is seen to be  expanding, at the most observable distances of our universe, and the distant galaxies taken along for the ride"

In effect, all of space is expanding, but regions where the matter/energy density is high, [due to the strong, weak nuclear forces,  EMF and gravity] is "decoupled" from the expansion rate. eg; Us, Earth, solar system, galaxy, galactic group, and galactic cluster.

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12 minutes ago, beecee said:

In effect, all of space is expanding, but regions where the matter/energy density is high, [due to the strong, weak nuclear forces,  EMF and gravity] is "decoupled" from the expansion rate. eg; Us, Earth, solar system, galaxy, galactic group, and galactic cluster.

This part always confuses me and perhaps there is not yet any good explanation, but...

Assuming ALL of space is expanding and we are not taken along for the ride away from, say, the moon, then this seems to imply that the expanding space between us and the moon is flowing past us. But as space is not a 'thing', how could it 'flow'?

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3 hours ago, zapatos said:

This part always confuses me and perhaps there is not yet any good explanation, but...

Assuming ALL of space is expanding and we are not taken along for the ride away from, say, the moon, then this seems to imply that the expanding space between us and the moon is flowing past us. But as space is not a 'thing', how could it 'flow'?

Another good question. I think you answered it yourself...space is not a physical thing...it doesn't flow, per se...it just expands, unless that expansion is overcome/decoupled by mass/energy density.

Space not being a physical thing, does not though [imo] make it any the less real.

But hey! I'm here to learn also! Anyone with a better explanation?

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4 hours ago, zapatos said:

Assuming ALL of space is expanding and we are not taken along for the ride away from, say, the moon, then this seems to imply that the expanding space between us and the moon is flowing past us. But as space is not a 'thing', how could it 'flow'?

     Hi Zapatos, hope nobody minds me joining in.

    Well, yes you could think of space as "flowing" past an object that was bound in place by gravitational attraction to something else.  I've seen several animations of space that do exactly this - show a steady flow of space as if it is something like a fluid flowing.  That's ok.

    Alternatively, you can think of the expansion of space in a different way.  Instead of space being stretched or any part of it being made to "flow", you can imagine that it is a reasonably fixed thing that can't stretch at all - there is just simply more of it (more space) being injected in to the universe all the time.   In this view, nothing has to flow past anything else.... it's just that more stuff, more space is appearing (as if from nowhere).   We had a hamster once and my children wanted this amazing hamster "home" that was actually more like a hamster universe, it had tunnels that connected various little habitat areas, up ladders, hamster wheels.   Let's get a picture of something like this:

hamster-tubes-01-1024x351.png

   I used to feel sorry for the hamster because he lived in an expanding universe.   Whenever the children had a birthday or Christmas, they wanted another expansion for the hamster universe.   Some of the tubes you could buy were of the stretchy type.... you could stretch these... but this kind of tube really is the kind of thing that models the universe as something that stretches and I suppose you could imagine that there was something like a flow of the tube material as you stretched these tubes.   The more interesting extensions occurred when a whole extra section of tube was added.  Some of the tubes were not stretchy, fluid-like things at all but were quite rigid - they had to be because you could build them straight upwards and support a little habitat area on the top of them.  The rigid tubes were in short little sections of about 5cm length.   Sometimes when the hamster was asleep, we injected more space into his universe...  we didn't stretch any tubes or make anything flow past anything else... we just uncoupled a section and added in another section.   Sometimes we didn't even have to make the bigger habitat areas move at all, they didn't have to flow or move across the floor either.   We could replace a straight section with several curvy sections making a zig-zag if we wanted to.   Anyway, that's the thing in this hamster universe - space didn't always stretch or flow past anything else.... sometimes new bits of plastic tubing were just inserted in-between the existing pieces of tube.   Must have seemed amazing to the hamster.... new bits of space just being inserted in-between the existing bits of space.

   But all of this is just a model,  you can imagine space is some sort of fluid that flows, if you prefer.  Space certainly doesn't seem to be as empty as we once thought it was.  There seems to be quantum fluctuations and energy even in a vaccum - but that's a different story.

Edited by Col Not Colin
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3 minutes ago, Col Not Colin said:

In this view, nothing has to flow past anything else.... it's just that more stuff, more space is appearing (as if from nowhere). 

Right, but there is more of it between the Earth and the Moon, and yet the distance between the Earth and the Moon doesn't increase.

I know that I can think of it in several different ways, but how does science model it? Is there a model for it? Or does science go no further than just recognizing that superclusters are further apart over time?

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1 hour ago, Col Not Colin said:

      But all of this is just a model,  you can imagine space is some sort of fluid that flows, if you prefer.  Space certainly doesn't seem to be as empty as we once thought it was.  There seems to be quantum fluctuations and energy even in a vaccum - but that's a different story.

Yeah, good point actually. Have you seen this model of a BH?

https://jila.colorado.edu/~ajsh/insidebh/waterfall.html

 

and the paper.....https://arxiv.org/pdf/gr-qc/0411060.pdf

 

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1 hour ago, zapatos said:

Right, but there is more of it between the Earth and the Moon, and yet the distance between the Earth and the Moon doesn't increase.

I know that I can think of it in several different ways, but how does science model it? Is there a model for it? Or does science go no further than just recognizing that superclusters are further apart over time?

    How does science model it?   Well, that one can be answered.   The usual model of the universe (on the really big scale) is General Relativity with the FLWR metric (given various names but LeMaitre, Friedmann, Robertson and Walker contributed and at-least a couple of these names get used).   All that's important is that the spatial contribution to any proper distance measurement is multipled by something called the scale factor and this grows with increasing time.  How you explain or visualise that is, as you say, largely up to you.  It is a mathematical model and not any other sort of model.  In particular, it doesn't say where some extra space comes from but only that there is more space as time evolves. Depending on how you look upon it, it either side-steps the issue or else indicates to us that this doesn't even have to be an issue.  There are other areas of science that do make some attempt at explaining this phenomena, if you were interested, but for general relativity it doesn't need explaining, it just happens.

    Does the amount of space between the Earth and the Moon increase?   Well I suppose there's three reasonable answers (two I can think of and one more because I'll probably have something else after I finish writing the first two):

1.  Maybe not.  The FLWR metric doesn't have to apply to such small regions as the bit of space between the Earth and the Moon.   In practice, that small region of space is better modelled with the Schwarzschild metric using the Earth as the dominant mass.  If you want to get a better metric you can create a customised metric by solving the Einstein Field Equations directly given the distribution of mass you have (let's say having the earth and the moon as perfectly round spheres of uniform density etc.).  These customised metrics usually all tend to be just approximations because the equations you have to solve to find the metric are just too complicated to get exact solutions.   However, the main point is that the FLWR metric (the one we use for very large scale universe models) is itself an approximation that is reasonable ONLY on the large scales.  There almost certainly are some regions of space that are not expanding as fast as the FLWR metric would predict but then there are some regions that may be expanding a bit faster.  On the very large scales, the FLWR metric is a reasonable metric to use but it never claimed to be an exact solution for that tiny region of space.

2.   Yes it does.  Here's a quote from the NASA website:

The Moon is slowly moving away from Earth, getting about an inch farther away each year.      (    https://solarsystem.nasa.gov/moons/earths-moon/in-depth/    )

However I should make it clear the main reason to explain this does NOT involve the expansion of space.  None-the-less it is quite possible the expansion of space causes a tiny portion of that.

3.   Umm.......  Nope, can't think of anything else at the moment....  I  guess point no.3 was a spare.

Best wishes to you.  Bye for now.  

 

        ----- Wish I could separate these two replies here ----

 

9 minutes ago, beecee said:

Yeah, good point actually. Have you seen this model of a BH?

  I like the waterfall image, thanks.   I'll check the paper later, thanks again.

Found this YouTube video with a good waterfall representation of a Black Hole:

Brian Greene, daily equation.   Jump to  about time stamp  4:40   for the waterfall.

 

Edited by Col Not Colin
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2 hours ago, Col Not Colin said:

Does the amount of space between the Earth and the Moon increase?

How do you define “amount of space”, exactly?

2 hours ago, Col Not Colin said:

1.  Maybe not.  The FLWR metric doesn't have to apply to such small regions as the bit of space between the Earth and the Moon.  

The FLRW metric is not a vacuum solution to the field equations (unless in the trivial case of a(t)=const.), so it doesn’t apply to a 2-body system in vacuum, such as the Earth-Moon system.

2 hours ago, beecee said:

Yeah, good point actually. Have you seen this model of a BH?

I’d just like to stress - because I think that it is important to make this very clear - that this is an analogy, a way to look at the situation that can be used as a helpful conceptual aid under certain circumstances, just like the “rubber sheet” analogy. It is not to be taken literally, however, as spacetime isn’t a medium that “flows” somehow.  

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Hi All, again feeling a little out of my depth here, but, given what's been said, i.e. gravitational pull on our galaxy cluster, does that then mean that in time all the galaxies in our cluster will converge and make one massive 'galaxy' or will the resulting chaos mean that new galaxies will form  from the debris that is a result of the collisions. Also if this is happening in our galaxy cluster, is the same thing happening in other clusters, and if so, does this mean that  that we will have fewer but larger galaxies, or that all galaxies will reform on a smaller scale and the whole process will start again. 

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6 minutes ago, wildie9 said:

Hi All, again feeling a little out of my depth here, but, given what's been said, i.e. gravitational pull on our galaxy cluster, does that then mean that in time all the galaxies in our cluster will converge and make one massive 'galaxy' or will the resulting chaos mean that new galaxies will form  from the debris that is a result of the collisions. Also if this is happening in our galaxy cluster, is the same thing happening in other clusters, and if so, does this mean that  that we will have fewer but larger galaxies, or that all galaxies will reform on a smaller scale and the whole process will start again. 

A simulation is worth a thousand words:

 

 

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Thanks Joigus,

when you haven't got a grounding in this type of thing (like me) it's all a little mind blowing, but i'm going to keep looking and learning. And ill keep asking questions to try and get some understanding.  Thanks again

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10 minutes ago, wildie9 said:

Thanks Joigus,

when you haven't got a grounding in this type of thing (like me) it's all a little mind blowing, but i'm going to keep looking and learning. And ill keep asking questions to try and get some understanding.  Thanks again

You're most welcome. Physics is the realm of common-sense wonder. Welcome to the forums.

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1 hour ago, wildie9 said:

when you haven't got a grounding in this type of thing (like me) it's all a little mind blowing, but i'm going to keep looking and learning. And ill keep asking questions to try and get some understanding.  Thanks again

And the winner of this week's "Science Perseverance Award" is wildie9! Major pat on the back for not giving up like so many do. Lots of folks just start making stuff up at this point, based on the little they know from popular science articles, and stitched together with whatever makes the most sense to them. Questions help us all adhere more rigorously to our most trusted methodologies.

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  Hi Markus, hope you are well.

5 hours ago, Markus Hanke said:

How do you define “amount of space”, exactly?

  In this context, I would define an amount of space (between the earth and the moon) as being something that is measured by the distance between the earth and the moon.

5 hours ago, Markus Hanke said:

The FLRW metric is not a vacuum solution to the field equations (unless in the trivial case of a(t)=const.), so it doesn’t apply to a 2-body system in vacuum, such as the Earth-Moon system.

Agreed.  I said...   The FLWR metric doesn't have to apply to such small regions as the bit of space between the Earth and the Moon...  and maybe that should have been    "doesn't"     instead   of    "doesn't have to".   The rest of the paragraph did discuss the better metric to use for that region of space given the exact distribution of mass that it has.

5 hours ago, Markus Hanke said:

I’d just like to stress - because I think that it is important to make this very clear - that this is an analogy, a way to look at the situation that can be used as a helpful conceptual aid under certain circumstances, just like the “rubber sheet” analogy. It is not to be taken literally, however, as spacetime isn’t a medium that “flows” somehow.

Agreed.  Space is not exactly like a river flowing and it's not exactly like a hamster cage either.

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11 hours ago, Markus Hanke said:

I’d just like to stress - because I think that it is important to make this very clear - that this is an analogy, a way to look at the situation that can be used as a helpful conceptual aid under certain circumstances, just like the “rubber sheet” analogy. It is not to be taken literally, however, as spacetime isn’t a medium that “flows” somehow.  

Certainly.

7 hours ago, wildie9 said:

Thanks Joigus,

when you haven't got a grounding in this type of thing (like me) it's all a little mind blowing, but i'm going to keep looking and learning. And ill keep asking questions to try and get some understanding.  Thanks again

 

7 hours ago, joigus said:

You're most welcome. Physics is the realm of common-sense wonder. Welcome to the forums.

 

6 hours ago, Phi for All said:

And the winner of this week's "Science Perseverance Award" is wildie9! Major pat on the back for not giving up like so many do. Lots of folks just start making stuff up at this point, based on the little they know from popular science articles, and stitched together with whatever makes the most sense to them. Questions help us all adhere more rigorously to our most trusted methodologies.

Three eloquent posts that typify the epitome of science.

Just for the information of wildie9, the collision of M31 and the Milky Way, or any other galaxy, is actually more of a merger then collision. The vast distances between stellar objects would see actual collisions fairly rare.

 

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Thanks again to you, I’m learning so much and am not afraid to ask questions, as I have been before!
Just been watching a program on the Hubble telescope, my question is, do you think that the Hubble telescope is the greatest step forward in cosmology and astronomy. Given the magnificent things that have been achieved from the Mercury missions( think that was the first) through new horizons images of Pluto to Mars exploration, the fact that Hubble can give us a look into the past at the early formation of the universe means, for me Hubble is the greatest step forward. 

would love to hear other opinions though. 

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 Hi again.

1 hour ago, wildie9 said:

would love to hear other opinions though.

  You could start another thread for what is another topic - but it's your call.

Greatest contribution to cosmology?    The Hubble Law.  Obviously,  just one opinion.

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