Dark matter

[Stephen]

What happens if say, the expansion reverses itself? There must be a point in which it comes together and stops, right?

It's just that when I read "expansion" that tells me that it is moving in one direction, away from another, and that if that direction were reversed, there would be a point in which it came back together.

Kind of like, "for every action, there is an equal and opposite reaction."

I think you're looking at this in terms of dimensions that don't quite fit. I'm not sure I have a better analogy than the balloon thought experiment.

The Universe expanding is not like say my gut expanding as I demolish my breakfast in the office. My gut gets bigger into its local surroundings.

If you run the Universe story backwards, you don't end up with a single point inside a defined area, you end up with all points in one place. That place isn't defined in regular dimensions as it would imply an outside of that point which there is (theoretically) not.

It doesn't matter where you are located in the Universe (as per the post above,) should the Universe spontaneously contract, all points would seem to have all other points contracting in on it.

Just to really confuse you (and try to get the concept of weird dimensions across,) there's equally no edge (no expanding face) of the Universe. Theory goes that if you could travel infinitely fast in one direction, you would ultimately end up back where you started

[Ponoobersie]

Im sure that one day someone will come to actually understand how all this works, as in the how the universe came about and how it all works...and when that days comes their head will explode the very second they have the understanding....I am certain this will happen as my head hurts...allot! and im only trying to get my head around the simple bits we do know! haha

[jflowers]

Im sure that one day someone will come to actually understand how all this works, as in the how the universe came about and how it all works...and when that days comes their head will explode the very second they have the understanding....I am certain this will happen as my head hurts...allot! and im only trying to get my head around the simple bits we do know! haha

Douglas Adams said:

There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.There is another theory which states that this has already happened.

This is my theory too

[username]

I have a gut feeling that the dark matter isn't there. Of course we can see the effects of the additional matters gravity on the universe but maybe or calculations are wrong, we don't understand gravity properly or the universe is in a multiverse and gravity is the big daddy of the forces that works through universe barriers.

As you may be able to tell my understanding of astro-physics isn't the best

It's just if the universe is still expanding and still speeding up then maybe there is something beyond our universe pulling it (just remember reading Parallel Worlds by Michio Kaiku and the possibilities of a multiverse)

Another thought I've had is that gravity can be thought of as a wave and waves can be scattered then are we only considering the elastic scattering of gravity and forgetting about possible inelastic scattering. I read somewhere on this forum that the gravity in galaxies can't account for why some galaxies are spiral in shape (is this right?). If the elastic scattering works how we think gravity works in space-time (I'm thinking the ball bearing on a rubber sheet analogy) then maybe inelastic gravity effects can work outside the rubber sheet.

We just normally quite good at detecting matter even indirectly (of course we see the gravity from it) some maybe the matter isn't actually there, dark or not.

[SandboxGeneral]

I think you're looking at this in terms of dimensions that don't quite fit. I'm not sure I have a better analogy than the balloon thought experiment.

The Universe expanding is not like say my gut expanding as I demolish my breakfast in the office. My gut gets bigger into its local surroundings.

If you run the Universe story backwards, you don't end up with a single point inside a defined area, you end up with all points in one place. That place isn't defined in regular dimensions as it would imply an outside of that point which there is (theoretically) not.

It doesn't matter where you are located in the Universe (as per the post above,) should the Universe spontaneously contract, all points would seem to have all other points contracting in on it.

Just to really confuse you (and try to get the concept of weird dimensions across,) there's equally no edge (no expanding face) of the Universe. Theory goes that if you could travel infinitely fast in one direction, you would ultimately end up back where you started

The expanding gut theory is one that I can readily understand! Hahaha.

But, this other stuff is still difficult to comprehend. The contracting universe ending up as all points in the same place, rather than a single point in a defined area still seems like the same thing, just explained two different ways.

Now if there is no edge to the universe, wouldn't that rule out the possibility of a multi-verse? There'd have to be a defined edge so as to pass from one universe to another.

[ollypenrice]

I agree with Stephen that we must dispense with the idea of kinetic energy and explosions. That isn't the point at all. That would have the galaxies moving through space but what is really happening is that space is expanding. Note that Stephen's buttons are glued onto the balloon so that they cannot 'move' in the normal sense but can only 'move' in a new sense.

If we run the expansion backwards, yes, we come to a point. And where is that point?

Brace yourself; that point is still everywhere. It's all there is. There is nowhere outside it.

(By which we mean that the three spatial dimensions plus time - which are the only ones we know - are everywhere there is, however large or small they may be at a given point in our history. Whatever any other dimensions may be up to at this time we don't know. Maybe they exist, maybe not. I think it reasonable that they should exist but why would the multiverse care about what I think reasonable?)

If Stephen's ballon's surface shrinks and shrinks and shrinks there is still no point on it which can be distinguished from any other point, however small it gets. The surface never ever gets to have a centre. If you think of it shrinking to a miniscule three dimensional atom sized ball with a centre you are simply breaking the analogy because in the analogy only the surface exists. It cannot reduce to a 3D micro ball however small because the anolgy excluded the dimension in which ball shapes exist.

Hey, this is cosmology. We have local brains for local tasks. We may need to loosen them up a bit!

Olly

[JulianO]

Place yourself on one of the buttons on the balloon - this is a really big balloon, so big you can't really see where it ends. You can only move on the surface of the balloon - for this example above and below you have no meaning.

From where you are stood, on one of the buttons, because the balloon is blowing up they are all moving away from you. It looks like you are the centre of the expansion. Now - if the balloon starts to deflate, all buttons move towards you - again it looks like you are the centre of the universe.

However move yourself to any of the other buttons and you'll observe the same effect. You are again the centre of the universe, everything is either moving away, or towards you.

So in effect you are, and you aren't at the centre of the universe. You made need to sit down now.

[OzDave]

So as the universe expands, galaxies move away from our galaxy, according to the balloon analogy.

Does this mean that within the solar system, the planets are getting further away from us as spacetime expands? Presumably the answer is yes if we follow the analogy.

Do we actually observe this kind of effect within our solar system?

David

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[George Jones]

So as the universe expands, galaxies move away from our galaxy, according to the balloon analogy.

Does this mean that within the solar system, the planets are getting further away from us as spacetime expands? Presumably the answer is yes if we follow the analogy.

Actually, according the balloon analogy that Stephen posted, the answer is "No."

Imagine a balloon with buttons stuck all over it.

Now in this example, the universe is the surface of the balloon. Nothing inside the balloon (the air) or outside (you) exist.

As you inflate the balloon, all parts of that surface expand equally. The buttons move away from each other (on the 2 dimensional plane) and the further away they are from each other, the faster they appear to move (apparent speed.)

In this analogy, the buttons get farther and farther apart as the balloon expands, but the button don't get bigger, because the forces that hold the buttons together are stronger than the expansion forces of the balloon,

Similarly, within galaxies, the force of gravity is strong enough to counteract expansion.

[OzDave]

So the spacetime between galaxies expands but the spacetime within them doesn't because gravity is too strong inside galaxies?

That sort of makes sense, but also doesn't. I think there must be some sort of expansion within galaxies otherwise the size of galaxies would be getting smaller and smaller relative to the size of the expanding universe around them. Conversely, if the universe were to contract back on itself, we would expect the galaxies to start contracting wouldn't we? Otherwise, everything ends up at one point, except galaxies that dont expand or contract??? That doesn't seem logical.

So I see your point but at the same time I think there must be more to it.

On the subject of dark matter, I think that fundamentally we just dont know enough about how gravity works and so the dark matter concept has been invented to plug the "holes" in the equations. Apparently it does that relatively well, but I think that one day we'll discover enough about gravity outside our own solar system to figure out why dark matter isn't needed anymore as a "fudge". Until we do that though, dark matter seems reasonable so that we can calculate things with sufficient accuracy.

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[ollypenrice]

Think of the expansion as being like the flow of a river. Indeed it's called The Hubble Flow. In this case the river is flowing south past your jetty at 5 kph. Someone in a conoe has a top speed in still water of 5kph. Paddling at this rate in a northerly direction they will remain in a fixed position in front of your jetty, going neither north nor south. Their effort, analagous with gravity here, overcomes the effort of the flow to drive them south.

It seems that many galaxies are themselves beating against the Hubble flow with more or less success as gravitational sources like the mysterious 'Great Attractor' pull on them (unless someone in the galaxy is paddling like mad...) We and M31 are hurtling towards each other, for instance, despite the Hubble Flow.

The closer two galaxies are to each other the more of each other's gravity they feel and the less space there is between them to expand and drive them apart. At a certain greater separation, though, the mutual gravity they feel becomes small enough, and the amount of expanding space large enough, for them to recede rather from, rather than approach, each other.

Olly

[Daniel-K]

There is no centre of the Universe because space all expands equally (back to the 2D balloon example.)

if it expands equally it must have a center.

i like this and believe that the universe is spinning after all huge galaxy's spin stars spin planets spin and im even sure i read somewhere atoms spin so why not the universe?

Is the Universe Spinning? : Discovery News

[AWR]

if it expands equally it must have a center.

No, that's the point, there doesn't have to be a centre.

Andrew

[ollypenrice]

Exactly. The surface of a sphere does not have a centre. A sphere has a centre but its surface does not.

This is the part of the balloon analogy which is often misconstrued.

Olly

[Daniel-K]

Exactly. The surface of a sphere does not have a centre. A sphere has a centre but its surface does not.

This is the part of the balloon analogy which is often misconstrued.

Olly

no iget it durrrr

[SandboxGeneral]

Ok, I think I'm starting to understand the balloon analogy, with the differences of the surface having no center, but there being a center of the balloon.

I have another question though about gravity. Can gravity be measured in an area of space where there is no known matter, planets, stars, galaxies, etc?

[ollypenrice]

Ok, I think I'm starting to understand the balloon analogy, with the differences of the surface having no center, but there being a center of the balloon.

I have another question though about gravity. Can gravity be measured in an area of space where there is no known matter, planets, stars, galaxies, etc?

The anaolgy is just that, an analogy, so we begin by saying, 'Let's pretend we have a balloon which has a surface but no centre...' After that, it's important to remember the pretence as you develop the analogy... The danger is forgetting that we mean only the surface.

Gravity's range is infinite and so there are no regions exempt from gravitational effects. There may be no local matter but non-local matter will be producing gravitational effects.

You can be in 'free fall' and feel no effects, I suppose, but we need a physicist to explain what happens in complex gravitational fields where someone in free fall towards a dominant gravitational source might still be affected by weaker sources from other directions. I have no idea how that plays out! Off the top of my head someone in free fall towards a dominant gravitational source does not feel that source's gravity, but if falling past a gravitational source to right hand side he feels a pull to the right because he has his inertia downwards resisting a true free fall to the right. This may be total rubbish!

Olly

[Stephen]

Ok, I think I'm starting to understand the balloon analogy, with the differences of the surface having no center, but there being a center of the balloon.

I have another question though about gravity. Can gravity be measured in an area of space where there is no known matter, planets, stars, galaxies, etc?

I would suggest yes it can. Olly has pretty much covered it, but gravity works over all distances. Every particle in the Universe experiences a force of gravitational attraction to every other particle.

It's just so weak as to make very little impact unless masses are huge or distances are close!

Force due to gravity = Gravitational constant x Mass Object1 x Mass Object2 / Seperation squared

You can see that the force between two helium nuclei (very very VERY small masses) at very large distances will have virtually no (but not zero) gravitational attraction between them.

I'm not delving into general relativity here, but Newton should convey the gist of it?

[ollypenrice]

...in fact, of course, one thing your gravity measuring device will measure, however remotely you situate it, is its own graviational field. You'd need to calibrate with 'darks' but what would you call them? 'Heavies?'

Getting silly.

Olly

[Stephen]

...in fact, of course, one thing your gravity measuring device will measure, however remotely you situate it, is its own graviational field. You'd need to calibrate with 'darks' but what would you call them? 'Heavies?'

Getting silly.

Olly

[Macavity]

Randomly: There seems often to be slightly "contradictory" aspects to these things e.g. in the notion that space only "expands" outside gravitationally-bound systems. Especially given the idea that (inverse square) the force of gravity is infinite. The macroscopic versus the microscopic? - The continuous nature of these? Never easy to conceive... maybe convenient in providing loopholes?

I would imagine (hope!) you could perform the Cavendish Experiment, reproducibly at ANY point in the universe. "Gravity", is present, at some level, everywhere, except perhaps at some "node point", equidistant from all masses, but even then...

[ismangil]

Prof Jim Al-Khalili's new book Paradox delves into things like Relativity in a gentle and fun way. I'm only halfway through it, already I can read this thread better than if I hadn't read it - even if I'm a loyal fan of PBC TV shows, in which he has covered expansion, gravity and the likes!

[AWR]

...in fact, of course, one thing your gravity measuring device will measure, however remotely you situate it, is its own graviational field. You'd need to calibrate with 'darks' but what would you call them? 'Heavies?'

Getting silly.

Olly

I like the way that this is dealt with:

Gravitational Field Strength is defined as the force per unit mass acting on a small, point test mass. Words like small and light have such subtle meanings to physicists.

Andrew

[George Jones]

Similarly, within galaxies, the force of gravity is strong enough to counteract expansion.

My language above is somewhat loose.

I'm not delving into general relativity here,

In order to correct my loose language, I would like to have a go at this when my plate become emptier.

[Mr Spock]

I feel like a lot of theories dark matter was invented to explain something we don't yet understand. It's the new ether.

I like to think of a relationship between gravity and time, and I don't mean relativity. I tend to refer to it as non-linear time, where, time varies with the amount of gravity. So two objects together could be in different time zones depending on the amount of gravity. This would explain how motion in galaxies comes about rather than inventing dark matter to fit the motion.

I wish I had the knowledge to pursue this further, along with some other ideas I have (my brain is full of nonsense).