Spacetime: Stretching or Growing? Or

[salvestrom]

Let me try to establish two things up front. First, is why I am making this post. And that is fairly straightforward: we have a pool of knowledge here that may provide clarification/refutation on some of the things covered below. Second, why do I even have a notion that the expansion is one of the types in the thread title. This, also, is fairly straightforward. A ballon, while expanding, gets thinner. To put it another way, the balloon is made of no more material when blown-up then when deflated. The material is stretched thinner. An empire which expands, on the otherhand, will do so by acquiring more territory. It has grown.

Expansion is a general word. It can be applied to two quite different processes and yet say nothing of how a process is occurring - other than via context. And in the context of the expansion of space it would seem to say very little.

I am, or should more accurately state, was open to either form - or a third if anyone can think of a suitable analogy. I am, in a sense, still open to either, yet also feel that with only a little additional reasoning one can be laid to rest, even if the other remains with a question mark over it.

Where did I get these ideas from? The ether. I'm sure you'll be pleased to know I'm not talking about that ether. No, it's really just random thoughts colliding. I have reached the point of making this post primarily because of a series of Susskind lectures on cosmology, available on youtube. After an initial post on these forums using a thought experiment, which, while terribly erroneous, still managed to reveal alot, I was still left unsatisied about the mainstream view on how space expands.

Susskind's lectures add considerable weight to the view of a growing space. He seemed quite clearly to be telling his students that new space forms and this is why the cosmological constant stays constant, not diluting as the universe expands. Multiple times he offered an analogy of a rubberband being stretched, yet never breaking as "an invisible reservoir of atoms" filled in the forming gaps between the bands existing atoms. So how do you view this? I can find nothing wrong on the surface. Space must be expanding in some fashion and the idea of 'growth' does not preclude any of the observed phenomena, such as redshift.

Redshift, however, is much trickier to explain in a stretching space. Well, the redshifting itself isn't. But as the light enters a 'less-stretched' region of space, you would expect it to begin blueshifting. But by how much? Enough to entirely undo the redshifting (which of course would be contrary to observation)? Does redshifted light in a growing space also experience some amount of gravitational blueshift? This would make objects further away still than they are already calculated to be. As you can see, my inability to provide answers to these questions, is partly what leaves the issue so wide open.

From here we move into speculation, some mine: some not. Following Susskind's lead, rather than turn every sentence into a mouthful of "if, buts, maybes" and "it is suggested/expected" I will just say it plain and you can take it with as big a pinch of salt as you care to, as your leanings on the question of inflation go.

During inflation the vacuum energy was considerably higher than it is today. The bottom dropped out after a number of e-foldings had occured. In Susskind's words "the universe went over an edge". As well it did. Most of the VE was turned into thermal energy and other particles. The Vacuum was then left with very little "in the tank", so to speak.

So, what's this got to do with space? If new space is formed during expansion, does it come into existence containing the post-reheat energy or the inflation period energy? If the former, why? If the latter, it might suggest the energy spreads out around the region leading to the notion that the expansion has steadily been filling the tank back up, a drop at a time. The stretching space scenario seems to have a superficially simpler explanation for the constancy of the vacuum energy, via concepts such as tension increasing the potential energy. This would seem to move toward string theory.

There are questions about how either idea plays out in a matter dominated region, with stretching space coming out ahead, compressing then in the presence of gravity in accord with the curvature of General Relativity. Growing space, however, has a lurking can of worms labelled with a big question mark of ultra-speculation waiting for us beyond the more straight-forward acceptance that while it may grow, once grown, space simply acts according to GR. Part of this is related to a very simply question: does space stretch, then fill in with new space, or does new space form, pushing out the 'old' space?

This question provides a neat cut-off between a direct, potentially meaningful querying of current views, versus baseless speculation. I hope I have stayed on the rightside of this line throughout and apologise if any of it goes too far.

Having said that, one last area does remain. And I feel it shouldn't be over-stepping the mark to make some logical deductions about it. The Big Bang. If space has been expanding through stretching, then it implies there is no more space today than there has ever been. However, the growth of space would seem to lead inevitably to the idea that t=0 also means space=0, such that the undesired singularity is reduced to non-existance. I make no assertion that this explains the existence of the universe, and only reveals another in the series of Russian nesting dolls. Indeed, I'm fairly certain the general idea isn't even original.

I came here to learn. Not to teach.

 

[alphachapmtl]

What we call space expansion can as well be viewed as matter contraction.
As particles and galaxies shrinks wherever they are, they appear to be moving further apart.
Are space expansion and matter contraction two exactly equivalent ways to describe the same phenomenon?
Maybe they are, but maybe not.
Of course I don't know the answer.
In any case seeing things from another viewpoint can help.

Just look at a computer simulation showing a comoving volume of space.
We see matter contracting and coalescing into a 3D spiderweb like network.

 

[Spourk]

Here, have a read:

http://arxiv.org/abs/0707.0380

"Expanding Space: The Root of all Evil?"

It doesn't matter whether or not you think space is expanding or that we are shrinking, as long as you understand the thoughts behind it from those who actually understand what is going on. After that, you can call it what you want, but there is an excepted terminology for the sake of causing less confusion, which is kind of what is going on here.

 

[Drakkith]

What we call space expansion can as well be viewed as matter contraction.
As particles and galaxies shrinks wherever they are, they appear to be moving further apart.
Are space expansion and matter contraction two exactly equivalent ways to describe the same phenomenon?
Maybe they are, but maybe not.
Of course I don't know the answer.
In any case seeing things from another viewpoint can help.

Just look at a computer simulation showing a comoving volume of space.
We see matter contracting and coalescing into a 3D spiderweb like network.

I can't see the picture from this computer, but I'm pretty sure a shrinking universe cannot fit the observed evidence without some pretty severe asumptions that break current known laws.

 

[salvestrom]

I can't see the picture from this computer, but I'm pretty sure a shrinking universe cannot fit the observed evidence without some pretty severe asumptions that break current known laws.

It's a really good picture. Insta-saved it.

As for the paper that was linked. Dubious. They say space has no more a meaningful existence than a model of a magnetic field, then started proposing it only be thought of in terms of cosmological fluid, which is essentially a GR model... In some respects I was on board with what they had to say - space expansion isn't really a well-defined description, but they argue Wheeler's well known description of spacetime can be shortened to simply "matter tells matter what to do".

It had the merit of being quite straightforward, though, particularly with regard to explaining why an individual galaxy doesn't fly apart. They even mention the formation of new space at one point.

 

[alphachapmtl]

Here, have a read:

http://arxiv.org/abs/0707.0380

"Expanding Space: The Root of all Evil?"

It doesn't matter whether or not you think space is expanding or that we are shrinking, as long as you understand the thoughts behind it from those who actually understand what is going on. After that, you can call it what you want, but there is an excepted terminology for the sake of causing less confusion, which is kind of what is going on here.

I basically agree with your comment.
Thanks for the reference, it is very interesting.
It deals directly with the many issues I have trying to understand expanding space.

 

[Drakkith]

Ok, I'm at home now and can view this picture. That picture is not of "matter contraction". It is normal gravitational attraction between matter and possible dark matter with the result being the clumping of galaxy clusters into filaments that we see today.

 

[salvestrom]

Ok, I'm at home now and can view this picture. That picture is not of "matter contraction". It is normal gravitational attraction between matter and possible dark matter with the result being the clumping of galaxy clusters into filaments that we see today.

In his defence he stated it was images from a computer simulation of a comoving volume of space. I assume his point was to say that with the individual images fixed at one size the matter gathering and appearance of voids cannot readily be discerned as either purely space expanding or matter contracting. Of course, the process being modeled actually covers both, but without the added information that the images are comoving volumes you cannot, by the images alone, say space is expanding. In my defence, I'm not suggesting he's right.

In another thread in which he posted much the same I inquired if a matter-contraction model actually required the opposite of a big bang: a large volume of existing space within which diffuse matter contracts under gravity, becoming ever denser.

EDiT: Actually, looking over those images I'm not sure they actually show any spatial expansion - the filaments don't adjust. The basic structure is clear in all 6 images, the rest of the material is just gravitationally drawn in.

 

[alphachapmtl]

What we call space expansion can as well be viewed as matter contraction.
As particles and galaxies shrinks wherever they are, they appear to be moving further apart.
Are space expansion and matter contraction two exactly equivalent ways to describe the same phenomenon?
Maybe they are, but maybe not.
Of course I don't know the answer.
In any case seeing things from another viewpoint can help.

Just look at a computer simulation showing a comoving volume of space.
We see matter contracting and coalescing into a 3D spiderweb like network.

References:
http://www.astro.up.pt/~asilva/CLEF_SSH/public/images/clef_tau_6.jpg
http://www.astro.up.pt/~asilva/CLEF_SSH/public/images/clef_tau_6stretch2.jpg
http://www.astro.up.pt/~asilva/CLEF_SSH/public/sim_runs_pub.html

The main simulation generated by the CLEF-SSH collaboration is the CLEF hydrodynamics run, which features a flat Omega_m=0.3 cosmology and about 158 million particles (2x(428)^3) of gas (baryons) and dark matter inside a comoving volume of (200 Mpc/h)^3. This simulation was run at the French national parallel computing centre of CINES, and consumed about 66000 cpu hours on 128 (SGI R14k) processors. This corresponds to a total wall-clock time of about one month.

The CLEF hydrodynamics run is presently one of the largest large scale structure simulations including models that account for the effects of radiative cooling and energy feedback in the formation history of Galaxy Clusters. It was run using a modified version of the parallel code package Gadget II (Springel, Yoshida & White, 2001), with an energy feedback model by (Scott Kay, 2004).

 

[Drakkith]

Yes, but that picture is not showing "Matter Contraction" which you said could be what expansion is.

 

[Helios]

I don't get space stretching ( or growing ). Take the Bohr radius for example. It's a constant length, the product of universal constants. How does the space inside the radius know not to expand and the space outside the radius know to expand? This doesn't make sense to me.

 

[Drakkith]

I don't get space stretching ( or growing ). Take the Bohr radius for example. It's a constant length, the product of universal constants. How does the space inside the radius know not to expand and the space outside the radius know to expand? This doesn't make sense to me.

Inside of and around most matter the gravity is much too strong for space to expand. This is analogous to how you can pick up one magnet with another in the sense that one force simply dominates over another. The strength of the magnetic field is too much to allow gravity to pull the magnets apart. It's simply a result of the different forces adding up as they do. So space never "knows" when to expand or when not to it simply happens when the strength of gravity drops off enough.

 

[Helios]

So what about a lone atom in a very very void part of space? You say space expansion "simply happens" and yet the Bohr radius is the Bohr radius. The space inside the radius knows not to expand and the space outside the radius knows to expand. I don't get it.

 

[Drakkith]

So what about a lone atom in a very very void part of space? You say space expansion "simply happens" and yet the Bohr radius is the Bohr radius. The space inside the radius knows not to expand and the space outside the radius knows to expand. I don't get it.

Are you sure space doesn't expand inside the Bohr radius? It is a given that the atom doesn't increase in size, but this is because the electromagnetic force holds the electron to the nucleus. Besides, just because space expands doesn't mean that it simply carries everything with it. If the various forces are strong enough they will hold things together against expansion. On large scales this is why galaxies stay grouped into clusters and superclusters.

As an analogy, consider the radiation pressure from a star pushing on a planet such as Earth. This pressure doesn't force us from our orbit because gravity simply overwhelms it. The force on the Earth is like 0.0000000001% the strength of the Suns gravity, much to weak to affect us that much. It does, but the effect is so small as to be unnoticeable on everything but the most precise measurements.

 

[Helios]

The Bohr radius is the Bohr radius. It's a forever constant length, the product of universal constants that never change. The space inside the atom knows not to expand and the space outside the atom knows to expand. Is this an unanswerable paradox of nature?

 

[alexg]

The space inside the atom does expand, but the electromagnetic forces holding the electron and proton at the Bohr radius easily overwhelm the new space being formed and maintain a constant distance.

Just as gravity does to everything closer than 200 million light years.

 

[Helios]

"Overwhelm?" or completely, absolutely, and perfectly negates all traces of space expansion? If the Bohr radius is the Bohr radius, a forever constant length, the product of universal constants that never change, then there's no compromise. The space inside the atom must not expand one bit. Otherwise, the Bohr radius is not the Bohr radius.

 

[salvestrom]

"Overwhelm?" or completely, absolutely, and perfectly negates all traces of space expansion? If the Bohr radius is the Bohr radius, a forever constant length, the product of universal constants that never change, then there's no compromise. The space inside the atom must not expand one bit. Otherwise, the Bohr radius is not the Bohr radius.

The electron shell isn't a solid barrier. A spatial expansion within the shell radius might simply flow outward without disturbing the electon. The electron will only move if supplied with the energy to do so.

You also need to understand that if spatial expansion did cause an atom to grow - let's exaggerate and say it doubles in size - then everything, including you and your measuring device that tells you the Bohr radius is the Bohr radius will grow equally. You will continue to measure that the Bohr radius as the Bohr radius, utterly unaware of what has happened.

Finally, as others and yourself have point out, the attractive forces of the atom may prevent the space from actually expanding in the first place.

In all three cases the measure of the Bohr radius is unaffected.

 

[Drakkith]

"Overwhelm?" or completely, absolutely, and perfectly negates all traces of space expansion? If the Bohr radius is the Bohr radius, a forever constant length, the product of universal constants that never change, then there's no compromise. The space inside the atom must not expand one bit. Otherwise, the Bohr radius is not the Bohr radius.

No, not if it doesn't carry the electron with it. To be honest if space is expanding inside the Bohr radius then it might apply a "counter force" and push the electron out a little bit, increasing the Bohr radius. However this increase would be so tiny that we wouldn't be able to measure it. So our constant would still be just as useable either way. Just because we call it a constant doesn't mean that it actually IS a constant.

 

[alphachapmtl]

Yes, but that picture is not showing "Matter Contraction" which you said could be what expansion is.

I agree with you. This image doesn't show or prove "Matter Contraction".
My point is just that "matter contraction" has exactly the same observational effects as "space expansion": redshifted galaxies, the more redshifted the further away they are.
Of course, if both descriptions are equivalent, then we might as well stick with a standard viewpoint like "space expansion". Nothing wrong with that.

A comoving volume basically always contains the same portion of the universe (the same galaxies, etc...).
We have the redshift formula 1 + z = d(0) / d(z).
d(0) is the distance to a given galaxy today.
d(z) is the distance to a given galaxy when the light we see was emitted.
So, in the usual description:
the comoving volume at z=4.0 is 5 times smaller than at z=0.0.
the comoving volume at z=3.0 is 4 times smaller than at z=0.0.
the comoving volume at z=2.0 is 3 times smaller than at z=0.0.
the comoving volume at z=1.0 is 2 times smaller than at z=0.0.

Because a comoving volume basically always contains the same portion of the universe,
I find it is more natural to consider such a comoving volume as dimensionnaly fixed, which also means we have to consider its content (galaxies and atoms) as shrinking.

What is gain in practice by adopting this viewpoint?
Really nothing concrete I must say.

So it's not like "matter contraction" is any better than "space expansion".
But I still think it is good to be aware of both interpretation.

We always have to be careful
1-to accept the observational evidences (of course)
2-to be critical of our own model build upon them

For example, the observed redshifts can not be denied.
What they really mean is another matter, open to debate.

Stretchy Space? http://www.chronon.org/Articles/stretchyspace.html

Is space expanding? Is space stretching? Is is thinning as it stretched?
Is matter shrinking?
(if we could see our bubble universe from some other bubble universe, whatever that may mean,
then maybe space expansion would be different than matter contraction,
cause we would have an external reference point)

Consider this interesting problem, the tethered galaxy problem.
http://arxiv.org/abs/astro-ph/0104349
http://arxiv.org/abs/astro-ph/0511709
This is a quite perplexing problem.

If we could hold a galaxy at a fixed distance for a while, using some strong rope, what would happen if we release the rope?
Would it stay at the distance it is because it has no recessional velocity to begin with?
Would it (suddenly?) be dragged away by the underlying expanding space? by some kind of frictional force?
Try the same problem in the contracting matter interpretation. It's no simpler.

Clearly I don't really understand space expansion, and the somewhat recent discovery that it is accelerating is not helping.

I also recommend the reference provided by Spourk in a previous post:
http://arxiv.org/abs/0707.0380
"Expanding Space: The Root of all Evil?"

 

[Drakkith]

How does contracting matter explain redshift at all? And especially redshift increasing with distance?

 

[salvestrom]

How does contracting matter explain redshift at all? And especially redshift increasing with distance?

Thinking about it I'm pretty sure that matter would actually need to contract above the speed of light to create the observed redshift. The only way to avoid this is to say it is space that is contracting (presumably under gravity) allowing the superluminal velocities but not really changing much about the maths. Or the concept in general. But even so, we're back at an earlier point I had which is that such a spatial contraction requires the inverse of a big bang - a vast preexisting space filled with diffuse matter. Such a thing does not fit with observations, which show a dense, hot universe in pre-history.

 

[alphachapmtl]

How does contracting matter explain redshift at all? And especially redshift increasing with distance?

If matter contract, then atoms were bigger in the past, emitting bigger wavelength light, and so appears red-shifted when we receive it.
The further away a galaxy, the bigger it was when the longer ago its light was emitted, the more redshifted it appears to us.

 

[Drakkith]

If matter contract, then atoms were bigger in the past, emitting bigger wavelength light, and so appears red-shifted when we receive it.
The further away a galaxy, the bigger it was when the longer ago its light was emitted, the more redshifted it appears to us.

That doesn't make sense to me. The frequency of the light, the oscillations of its fields per second, shouldn't be changed unless I'm missing something, so how could it be red shifted if the frequency isn't changed? Actually, now that I think about it, I don't know expansion explain that either.

 

[salvestrom]

If matter contract, then atoms were bigger in the past, emitting bigger wavelength light, and so appears red-shifted when we receive it.
The further away a galaxy, the bigger it was when the longer ago its light was emitted, the more redshifted it appears to us.

I'm not at all sure an atom having a larger radius means it emits light with a longer wavelength. Even if it did - and there's some evidence the bohr radius was slightly bigger in the passed, I have no idea if it was ever big enough to explain expansion in terms of matter contraction. In fact, an electron would need a higher energy to be at a further distance in a larger atom. while it would emit light it would surely be of shorter wavelegth. this whole idea of matter contraction just doesn't have any relevancy to observed physics.

 

[alphachapmtl]

I'm not at all sure an atom having a larger radius means it emits light with a longer wavelength. Even if it did - and there's some evidence the bohr radius was slightly bigger in the passed, I have no idea if it was ever big enough to explain expansion in terms of matter contraction. In fact, an electron would need a higher energy to be at a further distance in a larger atom. while it would emit light it would surely be of shorter wavelegth. this whole idea of matter contraction just doesn't have any relevancy to observed physics.

A larger atom is the same thing as observing it closer: all it's constituents will be (appear) larger, strings, quarks, orbitals, wavelength. It's just a matter of scale. (by larger atom here, I do not mean uranium vs hydrogen, I mean identical but larger).
Just imagine a diagram of an atom emitting a light at some wavelength, then enlarge the whole diagram.

Or just imagine the photon traveling from a far away galaxy, eventually reaching our galaxy which in the meantime has shrunk, making us see a redshifted photon.

 

[salvestrom]

A larger atom is the same thing as observing it closer: all it's constituents will be (appear) larger, strings, quarks, orbitals, wavelength. It's just a matter of scale. (by larger atom here, I do not mean uranium vs hydrogen, I mean identical but larger).
Just imagine a diagram of an atom emitting a light at some wavelength, then enlarge the whole diagram.

Or just imagine the photon traveling from a far away galaxy, eventually reaching our galaxy which in the meantime has shrunk, making us see a redshifted photon.

http://perrybw.wordpress.com/2011/1...n-of-matter-vs-the-metric-expansion-of-space/

the above is the first link found when googling "matter contraction". while he isn't a crank and fully acknowledges that he's just musing. I hope you have something stronger to actually back up your statements.

 

[mathal]

alphachaptml "I find it is more natural to consider such a comoving volume as dimensionnaly fixed, which also means we have to consider its content (galaxies and atoms) as shrinking."

There are vague analogies you can make between a universe that began with particles in fixed positions 'relatively' inertial WRT each other but with matter particles shrinking and a universe with particles simultaneously expanding away from each other and remaining constant in size. The analogy has fewer problems at close range but at any credible cosmological distances it doesn't just break down, it implodes.

Galaxies extrapolated to positions where they would be at any z 'time' show a relatively uniform density around the observable universe, simply closer together as z increases. The distances galaxies appear to be away from us is a direct concequence of how long they had to get away from us until the light we are seeing arrived. As a result the images Hubble Telescope presents of near and far distant galaxies are enormously different in size.

The shrinking Universe needs to posit that the galaxies are the same distance apart at any point in time and that that distance isn't changing. The problem is not just that this model presents galaxies as not visually appearing much further away with larger z-distant galaxies but neglects the obvious point that the furthest galaxies have barely begun to shrink. They should be enormous in this model-relative to their 'relatively' closer position in space than the expanding universe model.

mathal

 

[alphachapmtl]

alphachaptml "I find it is more natural to consider such a comoving volume as dimensionnaly fixed, which also means we have to consider its content (galaxies and atoms) as shrinking."

There are vague analogies you can make between a universe that began with particles in fixed positions 'relatively' inertial WRT each other but with matter particles shrinking and a universe with particles simultaneously expanding away from each other and remaining constant in size. The analogy has fewer problems at close range but at any credible cosmological distances it doesn't just break down, it implodes.

Galaxies extrapolated to positions where they would be at any z 'time' show a relatively uniform density around the observable universe, simply closer together as z increases. The distances galaxies appear to be away from us is a direct concequence of how long they had to get away from us until the light we are seeing arrived. As a result the images Hubble Telescope presents of near and far distant galaxies are enormously different in size.

The shrinking Universe needs to posit that the galaxies are the same distance apart at any point in time and that that distance isn't changing. The problem is not just that this model presents galaxies as not visually appearing much further away with larger z-distant galaxies but neglects the obvious point that the furthest galaxies have barely begun to shrink. They should be enormous in this model-relative to their 'relatively' closer position in space than the expanding universe model.

mathal

Interesting point.
I think this effect was already observed and measured.

Angular size redshift relation
an object ... beyond a certain redshift (roughly z=1.5), appears larger on the sky
http://en.wikipedia.org/wiki/Angular_diameter_distance#Angular_size_redshift_relation
http://en.wikipedia.org/wiki/Distance_measures_(cosmology )

But I don't know are secure the experimental evidences are in this respect.
We don't know of many high redshift galaxies, and we can't see them very well.

The furthest known galaxies have redshift factor z of 8, 9 or 10.
Those galaxies are barely detectable, and we don't really know their size or shape.
http://cosmiclog.msnbc.msn.com/_news/2011/01/26/5920882-hubble-spots-farthest-galaxy-again
http://firstgalaxies.org/

At a redshift factor z=9,
- space expansion viewpoint:
the universe was 10 times smaller, so those galaxies should be packed more densely, and appear larger cause of the angular size redshift relation.
- matter contraction viewpoint:
the galaxies were 10 times larger, so they should appear larger, and so would look more densely packed.

Concerning this, it looks like both viewpoints have identical observational consequences.

If the ancient universe was smaller, should it not visually be stretched across the celestial sphere to fill it, making ancient galaxies appear larger?
(basically this is angular size redshift relation, cited earlier)

 

[mathal]

Interesting point.
I think this effect was already observed and measured.

Angular size redshift relation
an object ... beyond a certain redshift (roughly z=1.5), appears larger on the sky
http://en.wikipedia.org/wiki/Angular_diameter_distance#Angular_size_redshift_relation
http://en.wikipedia.org/wiki/Distance_measures_(cosmology )

But I don't know are secure the experimental evidences are in this respect.
We don't know of many high redshift galaxies, and we can't see them very well.

The furthest known galaxies have redshift factor z of 8, 9 or 10.
Those galaxies are barely detectable, and we don't really know their size or shape.
http://cosmiclog.msnbc.msn.com/_news/2011/01/26/5920882-hubble-spots-farthest-galaxy-again
http://firstgalaxies.org/

At a redshift factor z=9,
- space expansion viewpoint:
the universe was 10 times smaller, so those galaxies should be packed more densely, and appear larger cause of the angular size redshift relation.
- matter contraction viewpoint:
the galaxies were 10 times larger, so they should appear larger, and so would look more densely packed.

Concerning this, it looks like both viewpoints have identical observational consequences.

If the ancient universe was smaller, should it not visually be stretched across the celestial sphere to fill it, making ancient galaxies appear larger?
(basically this is angular size redshift relation, cited earlier)

I get your point, the angular size isn't the problem I suspected that it would be. You can get to the same scenario either way. How would the model handle the acceleration of the expansion-(accelerated shrinking in this model)?

mathal