Effort to get us all on the same page (balloon analogy)

bapowell

All good points. For me, the value of the balloon analogy is in showing that the big bang was not a localized explosion occurring in some pre-existing space. This is a common misconception, and despite the cited limitations of the balloon analogy, nicely shows how the big bang can be understood to have occurred everywhere at once, with the isotropic separation of galaxies (dots on the balloon) a result of the expansion of space (the balloon) itself. And yes, this does require that people understand that the singularity is not the center of the balloon, and that while the surface of the balloon exists in a higher-dimensional space, the universe need not, etc.

Perduta

Physically we observe an accelerated increase in distance between us and entities elsewhere in space. This has been experimentally determined as a function of the distance to said entities expressed in Hubble's law.

Space, however is just a mental concept. Increase brings with it the concept of time, making a 4D space time for which the metric is defined by theory of relativity. Measuring space and time and the permitted geometrical operations within it are formulated in terms of conceptual rigid measuring sticks and clocks and in "empty" space, the Lorentz transformation. These are our reference that defines geometry of reality.

Thus IMO we need to get on the same philosophical page of what is meant by "space is expanding". i.e. Evidently our rigid conceptual measuring sticks are not "expanding" and 3 meters in any direction remains 3 meters in that direction regardless.

So my question is to what extent the various cosmic distance definitions (co-moving radial distance, angular size distance, luminosity distance, redshift distance etc...) maintain geometrical correspondence with the established 4D time/space metric of relativity. Would it not be more appropriate to say there is an observed increase in distance to other entities in time space, for which the causal agent is not yet very well understood and would the equivalence principle not allow us to attribute it to a gravitational field that is "outward" bound?

DaveC426913

Yes. Every day objects are not expanding. Even distances between every day objects are not expanding. It only is measurable on intergalactic scales.

Except there is no precedent - nor any reason - to believe gravity has a repulsive counterpart.

Perduta

3 meters remain 3 meters and 3 billion light years remain 3 billion light years. They do not expand. Galaxies are just as "everyday" as anything else in the universe and I don't think anyone claims they are expanding. So the philosophical question remains: Exactly what is the thing that science claims is expanding relative to the way we measure dimensions?

Observed Hubble expansion is one reason. Einstein's equivalence theorem is another.

Returning to the balloon analogy we might consider it to be the effect of a gravitational field with it's centre of gravity diametrically opposite us on the balloon and a mass equal to everything in the universe.

Perduta

In fact the more I think about it the more sense this makes: The gravitational field is proportional to the volume integral of all the mass inside the enclosing surface focussed on it's center of gravity... which on the balloon is the entire universe centred on the opposite side of the balloon and that centre of gravity is always relative to each observer... sucking everything away from us.

DaveC426913

All true.
Also true.
The distances between things.

Galaxy A and galaxy B may be 10 billion light years apart today, but a billion years from now they might be 20 billion light years apart.


No, that's what you're trying to demonstrate. You can't use your premise as evidence that your premise is true.

I think you'll have to spell this out explicitly. I don't think it fits the way you think it does.

So, there's a special place somewhere out in the universe that's compact and has the mass of the entire universe? This seems plausible to you?

And it is exactly opposite our location? If I went to a star 5 billion light years away, would it be exactly opposite that point too?

If yes, that's impossible - the attractor can't be in two places at once.
If no, then just like in your balloon analogy extension, the Earth holds a very special place in the universe, violating the principle of mediocrity.

Perduta

No it is not impossible at all in fact it is nearly inevitable. It is quite simply the plain old concept of centre of gravity.

To understand this, reflect first on how we would calculate the Earth's gravitational force at a point inside the Earth. The answer comes from Gauss' law.

Then replace that with doing the equivalent for being inside the universe: First think of our balloon again. Pick any point you like to represent us. What would you say is going to be the centre of gravity of the entire 2D balloon for that point? How would you apply Gauss' law on the balloon? Which way does the gravity suck things?

DaveC426913

You said there's a special place on the balloon "diametrically opposite us". Which means it can't be diametrically opposite any other point. That means our spot on the balloon is special - unlike any other point on it.

Try reviewing your extension to the balloon analogy.

Where are we on the balloon? Call it point A
Where is this diametrically opposite point? Call it point A'.
How does A' behave such that it affects A? Does it contract ('suck' things)?

OK, now. Pick a point 90 degrees around the balloon from us. Call it point B.
Does it see our point (A')? Or its own (B')?
Does it see exactly the same phenomena there as we do here? (equal 'sucking' in all directions?)

If the former, then we see a unique view of the universe, shared by no other point,
If the latter, then you have two x' points. Indeed, you have infinite n' points, one for each x.

Perduta

Correct. Every point in the universe sees everything else falling away from them just like we do. That is because the centre of gravity of the universe is unique to every observer.

Observers at your point A will see point B falling towards your A'. Observers at B will see A falling towards B' ...and so on. Thus the explanation for accelerated expansion of the universe and redshift is amazingly simple and consistent with Einstein's equivalence principle. It also explains why the space/time each observer sees and the apparent relative movement is different too.

p.s. also it explains why the acceleration was greater in the past and will keep getting smaller in the future: In the past the balloon was smaller so the centre of gravity was closer and we all know that gravity decreases with distance.

Drakkith

The expansion is accelerating and will become greater in the future. However this did not start to happen until recently. Prior to this point in time the expansion was slowing down because of gravity.

DaveC426913

Then you have not described anything new at all. In order for each point to see every other point as receding, the balloon must be expanding consistently, at every point on its surface, which is exactly what the model shows in the first place.

Perduta

Meh - I read somewhere that the Hubble constant was decreasing. I.e in the future galaxies at a certain distance will be accelerating slower than the ones we can see currently at that same distance.

OTOH if it is true that the universe is breeding more and more dark matter/energy to fill it's expansion then the total mass will keep growing in proportion to the volume i.e R³ while gravity decreases with R², so that would make sense too as the acceleration would then be proportional to the Radius of the universe and eventually we would get to the point where evereything is just ripped apart. It all depends on this dark matter malarkey.

p.s. Note: In the 2D balloon model the radius of the universe woulod correspond to half the circumference of the balloon.

bapowell

Oh goodness. Are you really thinking Newtonian gravity here?

Perduta

Except that I give an explanation for what makes the balloon expand.

Perduta

Meh you're right, I can't be arsed with a forum full of know-it-alls.
I was just asking a question.

bapowell

Just want to be sure we are using the right tools to answer the questions. No offense intended. You are bound to lead yourself astray if you are applying Newtonian physics to the balloon analogy...

bapowell

In a perfectly homogeneous and isotropic universe, the net gravitational attraction from all other objects in the universe on our galaxy would be zero. We would have zero velocity relative to the expansion. This is not the case, however. Local gravitational sources (galaxies in our local group) dominate any net force on our galaxy, leading to a nonzero peculiar velocity. But this has nothing to do with the expansion of the universe.

The expansion of the universe is not due to a gravitational field in the Newtonian sense, which is why it's important not to think in those terms.