# what is expanding?

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If the void between the filamentary clusters is a "vacuum" as we know it. No particles no energy i.e "absolutely nothing". Except mabey for CBR.

And I choose one cubic meter , smack bang in the middle of such a void . What is expandinfg inside of that cubic meter.

The concept of an expanding vacuum is puzzeling me, where am i going wrong?

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There are still fields in the "void". This means we may have non-zero energy.

Important for your question is the fact that we still have the gravitational field, which is a metric on space-time i.e. it gives us the local geometry and in particular the distance between two (near by) points.

The thing is this metric is time dependant. So the distance between any two points can change with time. This is what we see as an expansion.

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"There are still fields in the "void". This means we may have non-zero energy"

This statment implies that "fields" are massless particles( or waves) with energy, i.e bosons ( photons) or force carriers.

IF this is so , are they expanding .

What determines the boundary in the space they (photons) occupy.

If there are many force carriers in the cubic meter what is between the force carriers.

My origional question seems to be repeated but instead we are replacing " void" with a "Quantum field theory".

i am still puzzelled.

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If there are many force carriers in the cubic meter what is between the force carriers.

The photons are "smeared" out all over the place, which means that one photon is at many different places at the same time. The photons in a cubic meter will overlap each other, so there is no real place where the photon "ends".

I don't mean to hijack your thread, but I wonder if the probability for a photon to be at a certain place at a certain time is ever = 0? Will it not just gradually get smaller and smaller until it is reaching infinitesimal size?

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"The photons are "smeared" out all over the place, which means that one photon is at many different places at the same time" And they overlap with eachother??

That is the key to my puzzel.

How does one arrive at such a wave function?(many places at the same time).

Given only that the photons are "free moving" in a relatively flat 3 dimensional cubic meter of "vacuum" or "field"

Are there any good references for us novices , that anybody can suggest.

My mathematics is adaquate at best, but my imagination is infinite.

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That is the key to my puzzel.

How does one arrive at such a wave function?(many places at the same time).

Understaning this phenomena can be difficult to grasp, as anything else in quantum mechanics. I am really not a particle physicist, but I can tell you the way I've understood it.

When we say that a photon (or electron or other particles) are waves, we have to understand what is actually waving. For example watermolecules waves in crests, sounds waves in contraction, but particles wave in probability. Meaning that a particle is at many places at the same time, but when we look at it, it is a certain probablilty for it to be one place, and a different probability for it to be another place.

My key to accept this fact, was to read about Thomas Young double-split experiment. Are you familiar with that?

Edited by Nano
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yes i am

Its a facinating experiment , which confirms the wave nature of a photon.

But "waving in probability" is a little abstract for me.

For me the bright lines on the detector in the experiment illustrates that the photons energy is concentrated there .

Whereas the dark patches ( wave cancellation), indicates that the energy is absent there.

Why then define a "probability" stating whether it could be there or not!

If one photon indeed , went through both slits and interfered with "itself"

then its energy has been spread or "smeared out" over an area on the detector.

this leads to another puzzel , what exactly difines "ONE PHOTON"

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For me the bright lines on the detector in the experiment illustrates that the photons energy is concentrated there .

Whereas the dark patches ( wave cancellation), indicates that the energy is absent there.

Why then define a "probability" stating whether it could be there or not!

The formation of bright and dark lines on the screen has nothing to do with the photons energy. (except that the distance between the lines are dependent on the photons frequency) It is interference and probability that creates the pattern. If you sent only one photon through the slits, it could end up everywhere on the screen, but it would most likely end up near the bright lines. "Most likely" is then another way to say "high probability".

If one photon indeed , went through both slits and interfered with "itself"

then its energy has been spread or "smeared out" over an area on the detector.

A photon can be many places at the same time, but it can only be absorbed by one atom at one point in time. Therefore will it not be smeared out over the detector.

Don't know if I understood your questions right. My head is a bit slow

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Nano and Johan, please, if you want to understand "what is expanding", focus on the main thing AJB said. Read it over, decide what you don't understand, and ask him more questions.

...

Important for your question is the fact that we still have the gravitational field, which is a metric on space-time i.e. it gives us the local geometry and in particular the distance between two (near by) points.

The thing is this metric is time dependant. So the distance between any two points can change with time. This is what we see as an expansion.

This is exactly the right answer, it only needs more explanation filled in to suit the needs of whoever is reading.

There is no fixed geometry. There are no particles swimming in the fixed Euclid geometry you are told about in highschool. Geometry (distance, area, angle relations) is dynamic and changing.

Distances can increase between objects which are not in any ordinary sense going anywhere. The immediate local geometry around the earth and solar system is approximately Euclidean, so we don't see that kind of thing. But you have no right to assume that the distance between two objects which you consider for some reason to be stationary will not increase or decrease.

Distances must change dynamically according to the law of GR, the einstein field equation. This is our law of gravity. It has been tested repeatedly and verified out to great accuracy, we have no alternative description of gravity that works as well---and it says that gravity = geometry. The grav. field is no more and no less than the geometry of the world. So it is dynamic because as we know it must be affected by matter. As matter moves the geometry must change.

It is the geometry that is expanding, to answer your question. This is what AJB post says.

Please take a minute or two and assimilate what this means, and ask further questions.

Don't get off on some other topic like photons or the double slit experiment quite yet.

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Distances must change dynamically according to the law of GR, the einstein field equation.

my question was not to describe how interpret the "gravitational field between points or mass " as their position changes.

But rather why we believe that our universe , the very fabric of space is expanding ,

and try and isolate what this dynamic quantity really is.

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There is no fixed geometry. (...) Geometry (distance, area, angle relations) is dynamic and changing.

Distances can increase between objects which are not in any ordinary sense going anywhere. (...) you have no right to assume that the distance between two objects which you consider for some reason to be stationary will not increase or decrease. Distances must change dynamically according to the law of GR,(..) . This is our law of gravity. (...) gravity = geometry. The grav. field is no more and no less than the geometry of the world. So it is dynamic because as we know it must be affected by matter. As matter moves the geometry must change.

(emphasis mine) I am amazed of such a dynamical description. I support & agree 100%

It is the geometry that is expanding, to answer your question. This is what AJB post says.
That is the point I cannot swallow.

Please take a minute or two and assimilate what this means, and ask further questions.
Can you from your side at least wonder a few instants how it comes that we can agree on so many things and disagree on the conclusion ? There is a huge gap between our conceptions, and I'd like to bridge them. This is not intended to highjack the thread, I have the feeling Johan has a problem too.

Don't get off on some other topic like photons or the double slit experiment quite yet.

For sure.

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(emphasis mine) I am amazed of such a dynamical description. I support & agree 100%

...

That is the point I cannot swallow...

Well you got me puzzled and intrigued. It's really up to AJB to clarify and settle any mismatch. He answered up front exactly right AFAICS, and he studies this stuff at grad level. If I remember right.

But I have to admit that what you say interests me. I don't have time now to read a lot of past posts, but maybe you can summarize in brief.

You are fine with the idea that geometry is dynamic---that distances change in response to matter.

Then somebody says "I heard about something expanding, what is expanding?" Well it is actually that distances are increasing according to some diff. eqn. rule, the Einst. Field Eqn. (which governs the gravitational field=the metric that defines distance). But what can I say? He wants to know what is expanding? OK, so I give him a cheap answer. I say it is the geometry which is expanding. And you don't like this! Well, can you suggest a different simplified short answer that the guy can remember easily?

Or is there some other problem?

This is really AJB's baby, but I am curious what's bugging you.

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Thank you for all your inputs and viewpoints which I appreciate.

I understand the concept of the gravitational field and the metric that defines the distance. This is good.I am not interested in the mathematics however.

The way in which I interperated the response from AJB is that , Gravitational field theory is what we currently have to describe our interpretation for "fabric" of space. Hence he's answer

"The thing is this metric is time dependant. So the distance between any two points can change with time. This is what we see as an expansion"

And obviously many people have the same viewpoint. Quantum Gravitational field theory is one of the great advancements to better describe our classical view of gravity and even space itself.

But I am not yet convinced that it is the fundemental principal describing expansion.

The one is the canvas and the other, the paint on which this dynamic canvas ,changes. It is the expansion of the former that puzzels me.

I chose a hypothetical cubed meter , trying to eliminate any local effects.

The reason why I divulged into the wave properties of all photons , is because they seem to have the property to "occupy" all that "space" simoultaneously, since the dawn of radiation.Until something actually interacts with them , collapsing their wavefunction, back down to our reality.

All these concepts lead to a deeper understanding of our universe , but it is far from complete.

If it was the complete truth we would all agree 100%.

Anybodies viewpoint can still be considered as a cheap answer, until then.

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What is bugging me:

On one hand you have matter, radiations, atoms, quarks, etc.

On the other hand you have a concept: geometry.

What you say is that geometry is not a concept, geometry is something, and that "something" is expanding.

What I say is that geometry is a human concept, and geometry cannot expand. But matter, radiation, atoms, quarks can.

You could say I am looking in the left hand, you are looking in the right hand.

But I think I am looking in the right one, and you in the wrong one.

Is that cheap enough?

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What is bugging me:

...

Is that cheap enough?

OK, that is a good answer. I was just curious what is bugging you. It will have to go on bugging.

When you ask what is geometry made of, you are at limit of knowledge and you stand around waiting, with the rest of us. Like at a bus stop, except we don't queue up in a line like Brits.

There are very interesting papers recently by Eric Verlinde (Amsterdam) and Thanu Padmanabhan (from Poona, India).

They say that geometry (the grav field) behaves according to thermodynamics and as if geometry had a temperature and entropy. They derive Einstein field equation from the laws of thermo. They say if you can heat something then it must have atoms.

This was Ludwig Boltzmann's great insight. If you can heat something then it must have microscopic degrees of freedom that you can't see---so he realized there were atoms and molecules explaining the behavior of a gas, even before these were seen.

Look up E. Verlinde on arxiv. Look up T. Padmanabhan. It is completely leading edge by so-far reputable people.

Geometry is real. The metric is just our mathematical description of a real thing. (Like our math description of an electron or a photon.)

But there may be deeper microscopic degrees of freedom---"atoms of geometric relationship"---which we cannot yet see, and do not yet know. Atoms of angle, of area, of curvature.

And these can be heated and they can have entropy. (You know that DeSitter space has a temp? That a black hole has a Hawk temp?

that an accelerating frame has an Unruh temp? Geometry is able to have temperature, in principle that you can measure.)

Verlinde has derived Newtonian gravity law from "entropic dynamics". Gravity for him is an "entropic force". Read the January paper. It is all simple math and clear reasoning. This is where respected reputable people say things that sound crazy.

So look up Verlinde and Padma on arxiv. Or don't. Don't think about it. Just wait.

I think you will continue to be bugged. Why? The situation is clear: there must be something deeper than geometry, from which it emerges, but we do not yet know the correct way to imagine it. Personally the situation gives me pleasure:D, it is is the right position for humans to be in---to desire and hope, but not have. But you are equally right to be bugged by the situation.

Verlinde is a former string theorist who now says adamantly that string is not the way to go and his way etc etc. Which adds a slight frisson. A whipped cream topping with chopped nuts, so to speak.

Oddly enough an American, Ted Jacobson, published a proof around 1995 where he derived the Einstein equation (governing geometry) from thermodynamics. Just google "jacobson thermodynamics" and take the first hit.

Padmanabhan is so excited that when he gives slide lectures about it he absolutely refuses to cite the work of Jacobson and Verlinde. He cites only his own papers! This in itself is a bit sensational, a histrionic gesture outside of normal behavior. He just gave a talk yesterday about it at Perimeter Institute, available online video. Such is the extraordinary ferocity that comes out in the good times. Passions are aroused.

Whole thing is fun to watch. Better than if the bus would actually arrive.

Edited by Martin
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As always, I expected another answer. But this one is better.

No I was not aware that geometry is supposed to be able having a temperature, and yes I will look further at Verlinden and Co.

Thanks.

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thank you martin

I believe verilinde is onto something , using the holographic principal and entropy down to the planck area of the event horizon seems to bring out a deeper understanding to me on the "fabric of space" , whether we call it geometry , gravity or whatever .. it is ultimately a form of information

i like his comment

" gravity is the cost of moving information around"

whatever form of information we may choose.

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Michel and Johan, I was delighted to find you interested by Verlinde ideas and have two points:

when a good physicist has a creative idea, it is often wrong. Verlinde can be on a wrong track. But, for whatever it's worth, some other smart people have now followed him and written papers pushing this line of investigation further. I'm intrigued and stimulated by the development on that front but try to be prepared in case it turns out wrong.

Also Verlinde has a few pages of "blog" about his "gravity as entropic force" idea which gives some additional intuition, and opinion. I will try to find the URL. It is a helpful supplement to the paper.

http://staff.science.uva.nl/%7Eerikv/page20/page18/page18.html

there is also the wikipedia on Erik Verlinde, which gives link to somebody else's blog commenting on this, I don't know how usefully.

Edited by Martin
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Ough, quite technical reading.

For sure they are on something.

From Padmanabhan's paper at the end of page 79:

"At a conceptual level, this may be welcome when we note that every key progress in physics involved realizing that something we thought as absolute is not absolute."

I agree 200%.

Merged post follows:

Consecutive posts merged

The door is open. Things move on.

see

http://arxiv.org/PS_cache/arxiv/pdf/1003/1003.1625v2.pdf dated 03.10.2010

http://arxiv.org/PS_cache/arxiv/pdf/1003/1003.2510v3.pdf dated 04.05.2010

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Matter and energy expanding, will expand space-time.

Say we start with a star. It will generate a given space-time field via GR. Say we physically expand the star so it is ten time larger. Since the density is getting lower, the GR density is lowering. As such, we have expanded space-time. We expanded space-time by physically moving matter and energy. We could run an experiment.

Relative to the BB singularity, a simple big bomb will also cause space-time to expand. Is there an experiment that can demonstrate that space-time can change without matter moving?

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Verlinden & Padmanabhan's papers have all the ingredients:

_acceleration

_horizon

_full relativity

_no use of graviton

_and in watermark the principle of least action.

If I understand well, Verlinden's is a description of the scale factor, although it is not expressed in such terms.

Despitely, he missed Time. IMO it will follows.

Very exciting. Thank you Martin.

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