# Expanding Universe but ......

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Hi all.  I wonder if anyone can help me with our little dilemma.

The universe is expanding by stretching so everything is moving apart.  So how come Andromeda is moving towards us (relative to our perspective)?

Any thoughts very welcome because we have exhausted our brains on this one. Cheers

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Gravity

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Bubble gum vs elastic band

Due to distance, you can think of close galaxies being tied with elastic band (stronger gravity), so when you exert light pull on elastic band it will stretch to a point but no further than that.

Far way galaxies are tied with bubble gum (think of kids playing with bubble gum and stretching with hand, not one of the most hygienic analogies ), so when you exert same pull, you will feel a slight resistance to pull (weak gravity at far distances), but you will be able to keep pulling and stretching the bubble gum until it breaks (big rip ).

So it is due to distance between objects and the fact that gravity falls of with square of the distance, there is expansion force of the space even between you and the earth, but it is so minute that you can't even measure it on such scales (nor within a single galaxy for that matter, even galactic clusters are kept together by gravity).

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The strain on an object due to the expansion depends on its scale but for the solar system it is some 44 orders of magnitude smaller that the gravitational force and some 7 orders of magnitude smaller for the local group of galaxies. See https://arxiv.org/abs/astro-ph/9803097v1

Regards Andrew

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If the bit of the universe between us and Andromeda was stretching at 1 meter per sec and Andromeda was moving towards us at 4 meter per sec then the result is that is it moving towards us at 3 meter per sec.

Bit like a rubber walkway belt at Heathrow. If you are walking against the direction of movement and walking faster then the belt is moving then you will eventually get to "starting" end, othewrwise you get whisked off to the other end.

Edited by ronin
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Easy - the rate at which M31 is moving towards us is faster than the rate of the expansion of the universe.

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Much thanks as always.  You guys never let the side down.  However ......

If gravity pulls them together, gravity sourced by Andromeda and the Milky Way, then they almost certainly will merge.  Even if they slip by one another presumably they will attract once more and maybe oscillate until they merge.  Does that seem logical?

As time goes by would we expect lots of galaxies living in near neighbourhoods to merge?

Cheers

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I am never quite sure if the universe expands at all, we could all still be in a singularity and only time changes which gives the illusion of space.

Alan

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8 minutes ago, Nigele2 said:

Much thanks as always.  You guys never let the side down.  However ......

If gravity pulls them together, gravity sourced by Andromeda and the Milky Way, then they almost certainly will merge.  Even if they slip by one another presumably they will attract once more and maybe oscillate until they merge.  Does that seem logical?

As time goes by would we expect lots of galaxies living in near neighbourhoods to merge?

Cheers

They will merge. Galaxies are merging all the time. Remember that galaxies are mostly empty(ish) space so they don't merge in the way that two colliding blocks of concrete merge. (Fortunately!)

Back on the expansion of the universe, it's imortant to remember that it expands 'per unit volume.' Think of the entirety of space being made up of cubes of space. Each cube expands at a certain rate. the more cubes there are between you and galaxy 'x' the faster you and galaxy 'x' are racing apart. There are not many cubic units of space between us and M31 so we ae not racing apart very fast and the gravitational pull bringing us together easily outpaces the expansion. When there are lots of 'cubic units of expanding space' between us and galaxy 'x' there is a lot of expansion and not much gravitational attraction. Expansion wins.

Olly

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3 minutes ago, Alien 13 said:

I am never quite sure if the universe expands at all, we could all still be in a singularity and only time changes which gives the illusion of space.

Alan

I trust the PhDs. If they say the universe is expanding, who am I to dispute there conclusions.

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17 minutes ago, michaelmorris said:

I trust the PhDs. If they say the universe is expanding, who am I to dispute there conclusions.

Science is a culture of doubt, so I think it's actually important to doubt the PhDs and require them to convince you with the power of their evidence and argument. That's what makes science science. (I don't think that my humble challenges will disturb them but I think they'll be glad that I aired my doubts. I'm always glad when people air their doubts in my own area of professional enquiry.)

Olly

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40 minutes ago, ollypenrice said:

Science is a culture of doubt, so I think it's actually important to doubt the PhDs and require them to convince you with the power of their evidence and argument.

Agreed but it is also for the skeptic to look at the freely available evidence. They could start here https://en.wikipedia.org/wiki/Big_Bang .

1 hour ago, Alien 13 said:

I am never quite sure if the universe expands at all, we could all still be in a singularity and only time changes which gives the illusion of space

What do you think a singularity is  and how does it explain what we currently observe?

In mathematics and physics it's normal definition is "a point at which a function takes an infinite value, especially in space–time when matter is infinitely dense, such as at the center of a black hole." In the context of GR it is a point in space-time where all local future world lines converge. This does not accord with our observational experience of the world lines of the object in the universe we observe.

Regards Andrew

Edited by andrew s
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1 hour ago, Nigele2 said:

If gravity pulls them together, gravity sourced by Andromeda and the Milky Way, then they almost certainly will merge.  Even if they slip by one another presumably they will attract once more and maybe oscillate until they merge.  Does that seem logical?

This is an update on what we think will happen http://phenomena.nationalgeographic.com/2014/03/24/scientists-predict-our-galaxys-death/

Regards Andrew

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11 hours ago, Pondus said:

Gravity

Andromeda is so close to us that the Milky Ways gravitational pull (being the larger galaxy) is pulling it towards us,even while the universe is expanding. It is ommon for 2 to collide and  become one. A prime example are the large and small MLC visible in the souther hemisphere.............galaxies collide.

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1 hour ago, ollypenrice said:

Science is a culture of doubt, so I think it's actually important to doubt the PhDs and require them to convince you with the power of their evidence and argument. That's what makes science science. (I don't think that my humble challenges will disturb them but I think they'll be glad that I aired my doubts. I'm always glad when people air their doubts in my own area of professional enquiry.)

Olly

Totally agree that science has a culture of doubt.  The issue is that the subject is very complex. I don't have the tools (a training in advanced mathematics) to challenge the conclusions of experts in the field of cosmology. So, I feel quite sensibly, I differ to their greater knowledge.

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9 hours ago, michaelmorris said:

Totally agree that science has a culture of doubt.  The issue is that the subject is very complex. I don't have the tools (a training in advanced mathematics) to challenge the conclusions of experts in the field of cosmology. So, I feel quite sensibly, I differ to their greater knowledge.

So do I, really. However, there are areas where conceptual thinking can still come into it. For instance, the evidence that the current rate of expansion is increasing is largely reliant on estimations of the 'transparency' of space over large distances. This does give me pause. How good are such estimations?

Olly

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1 hour ago, ollypenrice said:

So do I, really. However, there are areas where conceptual thinking can still come into it. For instance, the evidence that the current rate of expansion is increasing is largely reliant on estimations of the 'transparency' of space over large distances. This does give me pause. How good are such estimations?

Olly

You are quite correct and an early "conceptual" objection to Newton's dynamics was that it was based on absolute space and time. This was quite soon after Newton proposed his theory and it was of course replaced by relative space and time in Einstein's theory of relativity.

In the case of Type 1a supernova there are several issues with their adoption as a standard candle ( http://phenomena.nationalgeographic.com/2014/08/28/type-1a-supernovas-cosmic-candle-mystery/ ) as well as issue of inter-galactic and inter-stellar absorption (that Olly mentioned) which they do try to compensate for.

I think the technical issues are generally well examined in the normal science process but the background assumptions are seldom examined.

Regards Andrew

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By the way, I notice I'm slipping into franglais, unconsciously. ('Estimations' instead of 'estimates.' Oh dear!)

Olly

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17 minutes ago, ollypenrice said:

By the way, I notice I'm slipping into franglais, unconsciously. ('Estimations' instead of 'estimates.' Oh dear!)

Don't think so Olly - I've always known "estimation" to be an English word that can be plural..... (I reckon the French pinched it from us cos it looks like it might be French lol)

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2 hours ago, ollypenrice said:

By the way, I notice I'm slipping into franglais, unconsciously. ('Estimations' instead of 'estimates.' Oh dear!)

Olly

Almost like my sweutch (or is that dudish?)

As for expansion of the universe, to fully explain it, you also need to introduce dark energy.

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16 minutes ago, wimvb said:

Almost like my sweutch (or is that dudish?)

As for expansion of the universe, to fully explain it, you also need to introduce dark energy.

Interestingly a pot of honey prevents any solution of the cosmological FWR equations giving a static solution. Only a Universe without mass/energy can be static! Adding a cosmological constant, contrary to what Einstein thought, does not help.

Regards Andrew

PS Normally the simplest models treat the universe as a prefect fluid and while Pooh thought honey was such a fluid (taste-wise) it is just too viscous.

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14 hours ago, LukeSkywatcher said:

the Milky Ways gravitational pull (being the larger galaxy) is pulling it towards us

That's interesting.  Is this relative to mass or dark matter or other black magic stuff which to me is all a blur?

I have always thought Andromeda to be roughly (give or take a few thousand light years) twice the size of the Milky Way, but of course that is only the physical size and number of stars I assume?  Does this change with regards to mass, meaning Andromeda is actually, in astronomical terms, smaller?

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6 minutes ago, RayD said:

That's interesting.  Is this relative to mass or dark matter or other black magic stuff which to me is all a blur?

I have always thought Andromeda to be roughly (give or take a few thousand light years) twice the size of the Milky Way, but of course that is only the physical size and number of stars I assume?  Does this change with regards to mass, meaning Andromeda is actually, in astronomical terms, smaller?

I think you are right. Andromeda is just under twice the mass of our galaxy 1.5 E12 to 8.5E11 solar masses.

Regards Andrew

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2 hours ago, brantuk said:

Don't think so Olly - I've always known "estimation" to be an English word that can be plural..... (I reckon the French pinched it from us cos it looks like it might be French lol)

Both exist in English but estimation is the process which results in an estimate. I must insist that I was wrong!!

Sorry, back to the real discussion...

Olly

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Well you guys fuelled something which led to more questions than it answered but that is the universe we know and love.

One thing it led to was the concept that the universe is just a zone of space that we perceive and that another universe maybe over there and another one over there.  [When I was very young I used to think  that we would arrive at the edge of the universe, drill a hole, and climb through ]

And maybe a question for a different thread but are we sure life was only created once on either or both earth and mars?  i.e. there is more than one common ancestor or there have been many original life forms but all but one didn't make it?  Suggesting that we are looking in the wrong place for the aliens!

Quite how my super hero The Pooh got a look in I'm really not too sure.  Cheers

Edited by Nigele2

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