What's the nothingness that our universe is expanding into?

[PeterDonis]

these discrete theories inevitably predict that the time needed for photons to get from very distant places of the Universe to the Earth will measurably depend on the photons' energy.

Do you have a reference for this? I wasn't aware that all such "discrete theories" had been ruled out by this method.

 

[cptstubing]

We all know that the universe is expanding. What I'm curious about is what it is expanding into.

I think it is inaccurate to describe the universe as expanding. It is better to think about space and time (space time) as expanding. Instead, it's possible that the space between celestial bodies is expanding. The rubber band idea, think of a thick rubber band with points on it that is stretched out.
Really though, this topic is full of unknowns.

 

[tressure]

We all know that the universe is expanding. What I'm curious about is what it is expanding into.

what a coincidence, your question doesn't differ from mine check mine out at: https://www.physicsforums.com/threads/is-the-universe-truly-expanding.804886/ ... you might find information that might come helpful to you!

 

[Murdstone]

This is not correct. In our models in relativity, "space", and more generally "spacetime", is defined as a manifold (3-dimensional for space, 4-dimensional for spacetime), with particular properties. All of this is perfectly well-defined mathematically. Physically, "points" in the manifold (for spacetime) correspond to events--physical happenings, such as "lightning strikes location X at time T by observer O's clock". The mathematical properties of the manifold correspond to the physical fact that, as far as we can tell, the set of physical happenings is continuous--there is no "minimum separation" in space or time between physical happenings. (There are speculations in quantum gravity that this may not hold at the Planck scale, but that scale is twenty orders of magnitude smaller than the smallest scale we can access experimentally, so the model of spacetime as a continuous manifold works at all the scales we can actually experiment with.)

Once you have spacetime as a manifold, "space" can be defined as some particular submanifold of spacetime, picked out according to some criterion (such as being a surface of constant coordinate time in some coordinate chart).

This is not an independent definition of space. All of the above is being assumed. LET'S ASSUME...Space enters the model by necessity.

An alternative perspective is to ponder whether space has a physical aspect or is just a theoretical construction. Most approaches to "space" are theoretical. At best the above is saying that space is continuous but its composition is undefined.

There is a need for a definition of space framed in terms of its composition.

 

[PeterDonis]

This is not an independent definition of space.

If you mean it's not "independent" of the definition of spacetime, I agree, but I don't see why that's a problem.

All of the above is being assumed.

That's true of any scientific model. You assume a model with certain properties; then you compute the consequences of the model and compare them with experiment. Again, I don't see why this is a problem.

There is a need for a definition of space framed in terms of its composition.

Some quantum gravity theories are attempting something like this: spacetime is no longer a fundamental entity, but emerges from something else (such as strings or loops). But then you just have a need for a definition of the strings or loops in terms of their composition. Such a demand never ends; so I don't see why it's any particular issue for space or spacetime as opposed to just a general property of scientific models, that there is always more to be explained.

 

[Drakkith]

I think it is inaccurate to describe the universe as expanding.

I disagree. In fact, expansion is by far the most accurate way of explaining our observations. The way that distances increase both over time and distance is exactly how an expansion process works.

 

[julcab12]

Do you have a reference for this? I wasn't aware that all such "discrete theories" had been ruled out by this method.

http://motls.blogspot.ae/2009/08/fermi-kills-all-lorentz-violating.html
http://arxiv.org/abs/1102.2784
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3270.html

 

[Chronos]

This thread has gone over the 'edge'. Philosophical 'nothingness' is not a confirmed property of the universe. It lacks observational, and even mathematical support.

 

[PeterDonis]

http://motls.blogspot.ae/2009/08/fermi-kills-all-lorentz-violating.html
http://arxiv.org/abs/1102.2784
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3270.html

The Fermi observations set fairly stringent constraints on a particular class of discrete models, yes. But that is not at all the same as ruling out all discrete models. (Yes, I know Lubos Motl makes the stronger claim--well, actually he makes a different claim, that all models that violate Lorentz invariance are ruled out, which is not the same as saying all discrete models are ruled out. But in any case, that's a blog post, not a peer-reviewed paper.)

 

[rootone]

. We can only assume a singularity.

Which actually means that in terms of our present understanding,we don't have a clue what is going on.
Need to be careful not to perpetuate the myth that a singularity is a physical object of some kind.

 

[julcab12]

The Fermi observations set fairly stringent constraints on a particular class of discrete models, yes. But that is not at all the same as ruling out all discrete models. (Yes, I know Lubos Motl makes the stronger claim--well, actually he makes a different claim, that all models that violate Lorentz invariance are ruled out, which is not the same as saying all discrete models are ruled out. But in any case, that's a blog post, not a peer-reviewed paper.)

I know he came way overboard. Trust me I'm not a big fan of Lorentz or CPT invariance breaking besides we still don't see any violation (atleast a clear violation) BUT I'm also open to such scenarios/models (Although I'm quite aware that it goes beyond cosmology OT). String guys are hoping thought. I'm just pointing out the aspect of which such diversification of ideas also has it's specified merits. flavors, realizations and predictions. Here is a fine paper on scenarios for quantum gravity minimal length scale.

http://arxiv.org/abs/1203.6191

"...These models have entered the literature as the generalized uncertainty principle or the modified dispersion relation, and have allowed the study of the effects of a minimal length scale in quantum mechanics, quantum electrodynamics, thermodynamics, black-hole physics and cosmology. Finally, we touch upon the question of ways to circumvent the manifestation of a minimal length scale in short-distance physics."At the end of the day it solely depends whether our way of making mathematical models is consistent with our data in lieu of whatever impression or interpretation we make of things particularly wavefunction.. (This is beyond cosmology but i think it is crucial).

 

[bahamagreen]

There are lots of simple questions about expanding space it might help to be clarified.
Most have to do with what exactly is expanding, what is moving relative to expansion, and how space itself might be measured.
Maybe clearing a physics path through these "naive" but usual questions would help.

How is space measured with respect to neighboring regions?
- is it measured so that all the regions come to occupy their neighboring regions' prior occupied space?
- is it measured so that regions move out of each other's way taking their prior occupied space with them?
- something else?

What are the assumptions about expanding space?
- new "same-metric" space being created and displacing the regions apart?
- existing space itself expanding and carrying embedded regions with it?
- existing space metric itself getting smaller enlarging the apparent distance between regions?
- existing "same-metric" space being annihilated and enlarging the apparent metric between regions?

How is space different from vacuum?
- how does moving space move anything, including itself?
- how would new space displace existing space?

 

[Murdstone]

If you mean it's not "independent" of the definition of spacetime, I agree, but I don't see why that's a problem.
That's true of any scientific model. You assume a model with certain properties; then you compute the consequences of the model and compare them with experiment. Again, I don't see why this is a problem.
Some quantum gravity theories are attempting something like this: spacetime is no longer a fundamental entity, but emerges from something else (such as strings or loops). But then you just have a need for a definition of the strings or loops in terms of their composition. Such a demand never ends; so I don't see why it's any particular issue for space or spacetime as opposed to just a general property of scientific models, that there is always more to be explained.

The crux of the argument is why some find it so difficult to just say we do not know what the universe is expanding into.

When one starts using manifolds and other pedagogical devices, the implication is that they know.

It would be most refreshing if one were to say - "At present we do not know what the universe is expanding into but we are working very hard on it.

 

[PeterDonis]

The crux of the argument is why some find it so difficult to just say we do not know what the universe is expanding into.

When one starts using manifolds and other pedagogical devices, the implication is that they know.

When one phrases the question the way you have phrased it, the implication is that we know the universe is expanding into something, we just don't know what. The problem is that we don't know that the universe is expanding into anything; as best we can tell, that question does not even have a well-defined meaning, i.e., as best we can tell, there is nothing else besides the universe, so the concept of it expanding into something else is meaningless.

 

[K. Hamze]

we just don't know what. The problem is that we don't know that the universe is expanding into anything; as best we can tell, that question does not even have a well-defined meaning, i.e., as best we can tell, there is nothing else besides the universe, so the concept of it expanding into something else is meaningless.

The concept of "requirement for a media to expand into" initiates from our common sense feelings, not from a scientific logic. If we accept that any elementary particle and any radiation do not need any media to exist (inside that media) and accept that they can move with respect to each other, then answer to the question "what is Universe expanding into?" becomes clear.
The point is that the space and absolute vacuum are not physically existing entities. Our Universe is not located at specific place in a larger surrounding, The physical existence is limited to the collection of finite existing particles an radiations only.
Metaphysics is a different issue.

 

[Murdstone]

PeterDomis - "Universe is not expanding."
K. Hamze - "Space does not exist."

These two states, in conjunction, pretty much abnegates the field of Cosmology.

 

[PeterDonis]

PeterDomis - "Universe is not expanding."

Where did I say that? I said the question "what is the universe expanding into?" doesn't have a well-defined meaning; but that's not at all the same as saying the universe is not expanding. You need to spend some time familiarizing yourself with differential geometry: it is perfectly possible to define "expansion" in a way that does not require the universe to be embedded in any external medium.

 

[K. Hamze]

PeterDomis - "Universe is not expanding."
K. Hamze - "Space does not exist."

These two states, in conjunction, pretty much abnegates the field of Cosmology.

Please carefully notice my terminology: " the space and absolute vacuum are not physically existing entities". "Position" like "temperature" is a property of the existing material. They are not physically existing by themselves.

The term "space" which refers to empty distance between galaxies and inside atoms between elementary particles do not refer to a physically existing entity.
If all materials and radiations are removed (do not be present) , "space" would not remain in place as a physical entity.
We do not detect "space" and cannot attribute any property to it!

Distance and coordinates , as essential part of any scientific field , but they are not physically existing. They exist in mathematics and are used to describe physical relations and rules. I hope it is clear enough.

 

[julcab12]

.. It's a matter of description. Space is formulation viewed as an absolute emptiness. However, LQG postulate space as a physical quantity and a model hence, some form or idealization of a structure, properties and they have reasons (math) to be so (although we don't have any clear evidence-- experimentally). I can also argue that it goes beyond just a model. We're used to thinking that space is dynamic representation(Newtonian) like distance, coordinate or speed etc but each time we do so we are left with the bias/limit of constraint and itemization that only exist as a thought or virtual construction. We simply don't separate materials, radiations or everything else in particular to space because space is ingrained to each constraints and it is dragging along with it... What about physicality? IMO space is part of the general ensemble and 'might' have a minimal structure not just limited to our construction (math) but some physical aspect also.

 

[guywithdoubts]

I guess we can ask this in a different way: is a universe with space, but without matter, energy and radiation possible in GR?

 

[julcab12]

I guess we can ask this in a different way: is a universe with space, but without matter, energy and radiation possible in GR?

Nope. Nothing will cause the distortion or the curve so i don't know how will it work in GR. The model won't make sense at all. I bet you can figure that one out.^^

 

[PeterDonis]

is a universe with space, but without matter, energy and radiation possible in GR?

Yes. This universe is called "Minkowski spacetime", and is a valid solution of the Einstein Field Equation. This is why SR is a valid special case (or limiting case) of GR.

Nothing will cause the distortion or the curve so i don't know how will it work in GR.

This just means spacetime is flat, i.e., Minkowski spacetime. See my comment above.

 

[bahamagreen]

A usual verbal-logical approach might suggest that:

- if the universe is expanding
and
- if the universe is all there is
and
- if expansion requires expanding "into" anything
then
- the universe must be expanding into itself

There might be different ways to interpret whether that conclusion is meaningful or meaningless... I'm still thinking that a simple review of the current physics understanding of the answers to the deliberately naive questions in post #62 would help all around.

 

[PeterDonis]

if expansion requires expanding "into" anything

It doesn't. "Expansion" can be defined purely in terms of quantities intrinsic to the manifold.

 

[bahamagreen]

I find the phrase "mapping...onto itself" when searching on expanding mapping.
I find the Wiki definitions of:
homotopy - a continuous deformation...one function to another (in the same space?)
homeomorphism - a continuous deformation...one space to another

Are these what you mean by quantities intrinsic to the manifold?

 

[julcab12]

Yes. This universe is called "Minkowski spacetime", and is a valid solution of the Einstein Field Equation. This is why SR is a valid special case (or limiting case) of GR.
This just means spacetime is flat, i.e., Minkowski spacetime. See my comment above.

Yep. You're right (Sorry for that) i overstated the question thinking; 'If it has any physical meaning other than GR's mathematical model'. .

In the context of GR -- Field theory has that non-trivial vacuum solutions. Matter and energy has a gravitational potential which in term makes spacetime curved. In the absence of such gravitational effect (which we don't know if it will happen in reality or in the future) spacetime will appear flat.

 

[PeterDonis]

Are these what you mean by quantities intrinsic to the manifold?

No. I mean quantities that describe the intrinsic geometry of spacetime, or of families of curves in spacetime. One of these quantities is called the "expansion scalar" of the family of worldlines that describes "comoving" observers--i.e., observers that see the universe as homogeneous and isotropic. This quantity is positive, and provides an invariant way of specifying what "the universe is expanding" means.