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

[slatts]

Oh, right. I'd forgotten about the acceleration studied through the Cepheid variable stars. I guess you've answered my question. I'd hoped for a smoother curve to the funnel, but a wavy one will do the trick for this hand-waver. (Some of us are interpreters, not scientists.)

 

[Snerdguy]

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

And the answer is: WE REALLY DON'T KNOW. Anytime science desribes something in an infinitive such as nothingness or O or omega or such, it just means we don't know. We have no way of observing the edge of the universe because it to far away for any means we have of detecting it. We really can't define what nothing is because, no matter how small we can measure, something is still there. We know that empty space isn't empty because something is there. They are terms that stand in for our lack of knowledge so we move on with our thought processes. So, "we really don't know" is the most accurate explanation of what is out there.

 

[phinds]

And the answer is: WE REALLY DON'T KNOW. Anytime science desribes something in an infinitive such as nothingness or O or omega or such, it just means we don't know. We have no way of observing the edge of the universe because it to far away for any means we have of detecting it. We really can't define what nothing is because, no matter how small we can measure, something is still there. We know that empty space isn't empty because something is there. They are terms that stand in for our lack of knowledge so we move on with our thought processes. So, "we really don't know" is the most accurate explanation of what is out there.

I disagree completely. Modern cosmology rejects the concept of an "edge" to the universe for a number of reasons. We don't know the size or shape of the universe (it could be finite but unbounded or it could be infinite) but no one proposes that it has an edge, and it is known empirically that it doesn't have a center (which would be implied by an edge)

 

[Hellmut1956]

I believe this exchange about linguistics and their impact when expressing physical concepts is touching an important field that the person not really expert in physics run into traps due to it.

First I learned many years ago something that has a huge impact when trying to "understand" concepts" in physics! Physics does not claim to say what is real! Physic theories just represent a model of a possible reality. If this model is good in explaining and forecasting aspects that are reflected in experiments and observation, fine! This has helped me to be cautious when trying to grasp theories in physics!

A second aspect of what was written here about linguistics is the reason why there is a mathematical "language" and this is really reflecting the strength of mathematics. If we look into any science in this days we will find that nearly everywhere the scientist use the "mathematical language". If you write a mathematical formula using mathematical annotation, what this is saying can be understood from a mathematically trained person anywhere on the planet, independently what "human language" he speaks natively!

Now back to what the thread dealt with at the beginning. The BigBang created this "universe". This means it did not just create the space, it also created time. So the question what was there before is meaningless. It comes from our day to day understanding of time as an infinite flow of something that separates before, from now, from next! When there was no time, how an there be something before it! This is the result of our thinking along causal paths!

Same is true about where the universe is expanding too! The best model, analogy is the surface of a balloon being inflated. Our spatial universe in this analogy is the surface of the balloon! Nobody would have a problem understanding that at the surface of a spherical balloon there is no edge, there is not something behind it. The surface is just this, the surface of a spherical object which is growing in "diameter" in a dimension that does not exist but which has the effect that the surface expands!

Remember is just an analogy, is just a model, has no other reason to be than just in being an analogy that helps us to grasp, what using mathematics is trivial! I have sometimes the impression myself, that I am a mathematical analphabet or legasthenic person! being interested in science I also try to grasp the concepts expressed in the different fields of science. But it is like being limited to watch the picture in an illustrated book and not being able to read. As soon as you try to get deeper into concepts presented, as soon as you try to grasp it to a level were you are able to understand the differences between string theories and such around quantum gravity, I already have trouble to remember if I using the right names! But to understand why one or the other theory have their supporters and their opponents, mathematical skills are required that I confess are beyond my abilities!

 

[andrewkirk]

Unknown. The Big Bang Theory has nothing to say about what came before one Plank time.

Why one Planck time specifically? I understand that the Big Bang theory says nothing about why the expansion commenced - ie nothing about 'time zero', or whether there even was a time zero - but I was not aware of the theory identifying a cut off at a positive cosmic time coordinate.

I read on Wikipedia that the concepts of Planck time and Planck length have specific significance only within hypotheses like loop quantum gravity that are designed to try to unite QM and GR, but which do not have theory status.

Is the reference to the Planck time as a lower bound for the reach of the Big Bang theory, a reference to what is suggested by some of those hypotheses? If not, what aspect of accepted QM (or GR, although that seems unlikely) is it that identifies the Planck time as a specific lower limit for what the Big Bang theory says about the history of the universe?

Thank you

 

[phinds]

Why one Planck time specifically?

I have always taken that to be an approximation, not an exact specification. It's more specific and useful than saying "a REALLY, REALLY small amount of time after the singularity" and as I understand it, it is at least approximately correct.

 

[magneticnorth]

I think the best answer to what is the Universe expanding into , is " We don't know " .We may know at some future time but at the moment we know just about the same as what we know about the inflation a.k.a. Big Bang . Physics can extrapolate back to 2 or 3 Planck time segments after the expansion , but we know absolutely nothing of what was or what came before . We can only assume a singularity ,as there is not any mathematics to describe such a thing or physical laws to allow it .

 

[andrewkirk]

I have always taken that to be an approximation, not an exact specification. It's more specific and useful than saying "a REALLY, REALLY small amount of time after the singularity" and as I understand it, it is at least approximately correct.

Sorry but I'm afraid I still don't follow you. If it's approximately correct, however rough, it must be an approximation to some quantity. Presumably that quantity is specified by some theory. If so, I'm wondering what the theory is, because I can't see anything in the current accepted theories of QM or GR per se that implies there will be a particular very small but nonzero unit of time or length that has a special significance.

If all the reference is trying to communicate is that neither GR nor QM tells us anything about time zero, or whether there was one (ie is the set of spacetime points with cosmic (FLRW?) time coordinate zero non-empty?), then that's quite clear. It's only if it's trying to say something more than that that I can't see what that something more would be.

 

[julcab12]

, I'm wondering what the theory is, because I can't see anything in the current accepted theories of QM or GR per se that implies there will be a particular very small but nonzero unit of time or length that has a special significance.

... Planck time is just the implication or perhaps computational threshold by combining such constants. It's the lowest(not absolute) they can possibly make as scale of time when combining such values using power series and dimensional analysis. They can speculate that during that era http://hyperphysics.phy-astr.gsu.edu/hbase/astro/grav.html#grav force begins to differentiate from the other three forces. It is not actually understood what really happened during that moment but it is used/integrated (at least they are trying^^) as a part of the model for High energy physics mainly fundamental forces.

 

[magneticnorth]

It's like saying I trillionth ,of a trillionth , of a trillionth of a second AFTER the expansion began .Physics can look back , extrapolating of course, using known physical laws and mathematics to such a point in time . Whatever happened looking back beyond that time is at present totally unknown . I'm a layman , so from my purview , I liken it to being catapulted with my back facing the direction of the acceleration , and all my observations are made looking back towards the point of acceleration , however I can only see up to a certain point after, or slightly after which I was catapulted - not being able to see that mechanism [catapult ] nor the source of it's energy. We look out into the Universe and can measure that acceleration , based on our estimates of gravity , observed [or unobserved ] mass ,combined with known Physics according to Relativistic, Mathematic , and Quantum calculations.

 

[Murdstone]

I agree with the statement advanced by Snerdquy and reinerated by magnetinorth - "We Really Do Not Know."

Not only is it unknown as to what matter is expanding into - we have no definition of space. Space is just assumed.

It is a carnard to suggest if one is not sufficiently versed in the mathematics then they just can not understand what is going on.

Use any mathematics you like, it would be most welcomed if using your tools you could construct a definition of space.

 

[PeterDonis]

we have no definition of space

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).

 

[andrewkirk]

... Planck time is just the implication or perhaps computational threshold by combining such constants.

Implication from what equations?
Computational threshold estimated by what equations?
Deduced from what postulates?

I'm just trying to get clear in my head whether the physical significance of Planck Time and Planck Length that one sees so often referred to in physics discussions as if it were accepted science, is deduced from the GR postulates, from the QM postulates, from the combination of the two, or whether some additional postulates, such as are used for Loop Quantum Gravity, are used.

Until recently, I had been under the impression that the significance was part of currently accepted science, and could be derived from the QM postulates, but I have never seen such a derivation. The derivation of the Heisenberg uncertainty inequality does not mention Planck time or length. Recently I read the Wikipedia article, which suggests that the Planck time and length are only significant in speculative hypotheses like LQG. I don't take Wiki as gospel, so I'm asking here to see if someone knowledgeable about the issue can confirm or deny it.

If there is a derivation from the postulates of a currently accepted theory, I would really appreciate a link to such a derivation so that I can work through it and understand it. On the other hand, if there is no such derivation, because the quantities are only significant in speculative hypotheses, I'll leave it for now because I want to learn more about the currently accepted theories of GR and QM before I start learning about speculative hypotheses.

Thank you

 

[PeterDonis]

I'm just trying to get clear in my head whether the physical significance of Planck Time and Planck Length that one sees so often referred to in physics discussions as if it were accepted science, is deduced from the GR postulates, from the QM postulates, from the combination of the two, or whether some additional postulates, such as are used for Loop Quantum Gravity, are used.

It certainly isn't deduced from GR postulates, since those postulates assume that spacetime is continuous.

You can't really deduce anything useful just from "QM postulates", because those postulates in themselves don't tell you what actual quantum particles or fields exist. Any quantum theory has to make additional assumptions about that.

The usual justification for giving Planck scale quantities physical significance is that, if we try to describe gravity by a quantum field theory, similar to the way we describe the other three fundamental interactions, we find that the strength of gravity becomes the same as the strength of the other three interactions at the Planck scale. (The other three interactions actually become unified into one, with a single strength, before the Planck scale is reached.) One way of expressing this is that, in the units natural to particle physics, Newton's gravitational constant is equal to the inverse Planck mass squared.

 

[julcab12]

Hi Andrewkirk,

According to what I've read. We get the value by combining powers constant -- ℏ (Reduced Planck), G (Newton's gravitational), and c0 (speed of light or a massless particle in a vacuum), such that the result has units of distance (metres).

Implication from what equations?
Computational threshold estimated by what equations?
Deduced from what postulates?

I'm just trying to get clear in my head whether the physical significance of Planck Time and Planck Length that one sees so often referred to in physics discussions as if it were accepted science, is deduced from the GR postulates, from the QM postulates, from the combination of the two, or whether some additional postulates, such as are used for Loop Quantum Gravity, are used.
Thank you

They really don't if has any physical meaning. The 'Planck thing' is constructed based on the assumption that all fundamental constant are equal to one. So they set a scale where C, ℏ G are relevant in their description and we get that value, this would imply we would presumably need a quantum theory of gravity to explain phenomena in that setup. Since we no have such theory, many physicist think they mark a boundary to our current understanding of nature. Of course, these ideas are speculative, but are the things we expect to find, we don't know what exactly happen at that scales.

"

Implication from what equations?If there is a derivation from the postulates of a currently accepted theory, I would really appreciate a link to such a derivation so that I can work through it and understand it. On the other hand, if there is no such derivation, because the quantities are only significant in speculative hypotheses, I'll leave it for now because I want to learn more about the currently accepted theories of GR and QM before I start learning about speculative hypotheses.

Thank you

As Peterdonis mentioned; It certainly isn't deduced from GR postulates, since those postulates assume that spacetime is continuous as a classical rule. However QM has some quantities such as angular momentum or energy of bound states, can only take "quantized" or discrete values (eigenvalues) but it doesn't mean that all observables in quantum mechanics have to possesses a discrete spectrum as already mentioned. LQG ODOH is viewed as discrete - spatial distances and temporal intervals are multiples of Planck L and time respectively. The proposition that distances or durations become discrete near the Planck scale is a scientific hypothesis and it is one that may be - and, in fact, has been - experimentally falsified. For example, 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.

 

[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?