Big Bang - No Single point of Expansion

S.Vasojevic

So it is expanding into its future self. There must be some propagation front that we can describe. Does GR strictly implies that it does not exist?

sylas

The term "propagation front" is not well defined. In the normal sense of the word, no there does not have to be a propagation front.

But I guess you might be able to think of "time" as a "propagation front". There is no location of a front in space; but all of space is expanding as time passes. This just means there is more space in between things; subject of course to the fact that objects are also moving with their own peculiar motions and gravitationally bound conglomerations like a galaxy don't expand.

I suspect you are still holding on to misleading metaphors and analogies with an explosion of material out from a point. You have to drop that idea; it's wrong. And you're probably best to forget about trying to shoehorn the idea of a propagation front into cosmological expansion. It's terminology that is bound to mislead.

Felicitations -- sylas

S.Vasojevic

Thanks sylas. If I understand this correctly galaxies are not moving apart because expanding space is exerting force on them which overcomes gravity, but simply because more space is being "stuffed" in between them?

sylas

There are better people here than me to answer this. Comment from Wallace would be welcome; and there are a number of others well placed for this also.

I shall try to go a bit further; and I will welcome informed instruction and criticism from experts here present.

Any answer in these terms is necessarily an analogy, or metaphor, for the underlying mathematical descriptions; and I am not sufficiently strong on those to be confident about all the pitfalls, or about identification of exactly how far an analogy works and where or how it breaks down.

My understanding is that things are moving apart because they are moving apart. That is, there's no force required; only an initial impulse. Additional pushes only slow it down or speed it up.

Expansion of the universe kicked off with a burst of inflation in the very very early universe, and since then (for the most part) everything has continued expanding, but gradually being pulled together again by gravity. Recently it has been discovered that the expansion of things is accelerating again; and this corresponds to a small additional kick from "dark energy", which pushes things apart rather than pulling them together as gravity would do.

The key point is that there is no indication of any boundary or edge, and no need for an edge. We can't see all the universe to be sure of what it looks like beyond the limits of telescopes. But the simplest and most straightforward models have all the universe pretty much like the observable universe, with everything expanding away from everything else, and with no limit or edge. If indeed everything is pretty much the same as the observable universe, then whether the universe is finite or infinite depends on the large scale "curvature" of the universe. A positively curved universe would be finite... a bit like the curved 2d surface of a planet is finite without having any edge. Except, of course, that space is 3d rather than a 2d surface.

The thing about relativity is that "Matter tells space how to curve, and space tells matter how to move." (Wheeler) There are small scale examples of how movement of matter leads to changes in spacetime which leads to movement of matter. For example: the "frame dragging" effect means that a satellite in orbit around a rotating body (like the Earth) will experience a small "force" pulling it along with the rotation of the planet. I don't know how good a parallel that is; but with an expanding universe, I think that to say there is additional space between things another way of saying they are moving apart.

On the scale of the whole universe you have to deal with space in terms of general relativity; but it reduces on smaller scales to things just moving apart from one another, like a cloud of expanding gas becoming less dense -- although the cloud has no edge or boundary.

Cheers -- sylas

jdz

(this is a first post, I hope that it is on-topic; no thread-jack intended...)

I have a question about the curved 3-D space and "no edge and no boundry of the universe" idea.

Would it be possable to 'see' far enough to 'see' an edge or front of the universe, or does the curved 3-D space mean that any/every point in the universe is surrounded by the same (aprox the same) observable universe? That is, would the 'sky and stars' be the same, or look the same, from every where in the universe, whether the universe is open or closed?

thanks

WhoWee

A central point would be possible if the universe first collapsed to a single point - then exploded in a Big Bang (once or cyclically).

sylas

No; there is no distinguished central point in this case. The collapse to a point refers to all of space being reduced to a point; not collapse of all matter into a point in space.

There is no "central" point, in the sense of a point distinguished from other points being in the center. The collapse is when there is no space left between anything; all points are the same point; a singularity.

Felicitations -- sylas

WhoWee

With no space left to fill after a rapid contraction - the only possible outcome (other than a static space-time) would be an outward expansion.

S.Vasojevic

Shouldn't that imply that Universe is gaining gravitational potential energy, at expense of violation of first law of thermodynamics?

sylas

Energy in GR is a bit subtle, but the short answer is no.

Gravity slows the expansion down, as I indicated previously, and this is a bit like a gain in potential energy as things are further apart, and a loss in kinetic energy as the expansion slows. The problem is really that energy depends on the frame of an observer, and you run into difficulties defining energy for the universe. That get beyond what I can explain well.

But there is a correspondence with the normal exchange of potential and kinetic energy for objects moving in a gravitational field.

Cheers -- sylas

sylas

There's still no central point. The expansion of the universe is not really "outward", but an expansion of matter and energy that fills all of space, at any time slice you consider. No point in space can be distinguished as central. This applies before the hypothetical bounce while the universe contracts, or after as it expands with a hypothetical bounce.

S.Vasojevic

Yes, but I think that expansion is speeding up.

WhoWee

There really is no way to continue the conversation without re-defining the word "point" to mean a place/area/location of an unspecified/unknown size/shape/dimension.

sylas

Quite right... and that is from an additional energy term, called "dark energy". But how this relates to the conservation of energy -- or rather, to the conservation results in general relativity which involve stress-energy-momentum pseudotensors -- I do not know. At this point the technical details are beyond me. Sorry.

sylas

The word point refers to a specific location, without size or dimension or shape. No redefinition is required. It is a point like you are used to.

The only likely ambiguity is whether we mean a point in spacetime, or a location in space which persists through time. In either case there's no shape or size involved.

I explained previously what I mean by "no central point". Here it is again with a few additional words to see if it is more clear.

There is no "central" point, in the sense of a point that is distinguished from other points by being in the center of space. The collapse is when there is no space left between anything; all points are the same point; a singularity. This is not a "center", in the sense of a being in the middle of other points.

You can identify a distinguished point in spacetime, and you don't need to postulate a cyclic universe for that. It's called the singularity. But you can't give the location of the singularity in space... only in spacetime.

You can identify a point in space as a "timelike worldline". It means you can integrate proper time along the world line and give a unique event for each proper time value along the world line. This is how you trace the trajectory of a particle.

If you do this, you find that at the singularity, all world lines converge. There is no distinguished world line to give a spatial center to expansion. Only a singularity to give a temporal origin. At every instance after the singularity (or before, in a cyclic case) the universe is roughly homogeneous, with no distinguished center in space.

The reason for the thread is to explain this very thing.

Felicitations -- sylas

Dmitry67

No. A boundary is something between one region of space and another region of space.

There are no boundaries. Note that if Universe is finite it still has no boundaries.

HowardS

Hello. I can understand that there is no central point for the BB as it is the only point that exists. But it does not make sense that all of the universe's mass came into existence, from one point - whether that point IS the universe or is a point in eternal, empy space. You would have a continuous "sphere" of particles (mass), or equivalent energy, ejecting from this point, in all directions, for as long as it takes to make up the finite? amount of matter in the universe.

Howard