Expansion of space vs stuff just moving away

[Drakkith]

I was with you until "So the reason why objects appear to move apart is due to changes in volume, not due to momentum."

I don't see how that is explanatory; what is the mechanism?

Here's my understanding. Someone tell me if I'm wrong.

The answer you're seeking is geometry. First, consider that gravity itself is the result of the geometry of spacetime and that an observer cannot use an accelerometer to find out whether they are in free fall due to gravity or whether they are motionless in space. An inertial frame and one in free fall under the influence of gravity are identical in accordance with the equivalence principle of General Relativity. In other words, objects in free fall are not accelerating, where acceleration here means "proper acceleration", which is what accelerometer measures. This is very similar to what happens to galaxies far apart under the influence of expansion.

A proper understanding requires us to discuss what a "metric" is.
Per wiki: A metric defines how a distance can be measured between two nearby points in space, in terms of the coordinate system. Coordinate systems locate points in a space (of whatever number of dimensions) by assigning unique positions on a grid, known as coordinates, to each point. The metric is then a formula which describes how displacement through the space of interest can be translated into distances.

Metrics are used in something called a metric tensor which is used to describe the overall properties of a surface of any number of dimensions. (Even normal geometry itself is a 2d metric tensor)

Metric tensors are required in order to find the shortest distance between two points. A good example is Earth's surface. Consider that the shortest distance between two points on the Earth's surface is not a straight line, but by a curved line. In addition, the standard rules of flat geometry don't apply. For example, if two observers start out from different points on the equator and head north, they will eventually run into each other at the north pole. In other words, two lines that were initially perpendicular to the equator and parallel to each other end up crossing.

Similarly, the presence of energy and mass causes the geometry of spacetime to shift away from the flat euclidean geometry we are used to. And just like we can have planets of different sizes, which would require changes in the metrics describing them, the metric of space changes under the influence of mass and energy.

Now, here's the key point to all this. Let's say that you take off in a spaceship and travel through the solar system. As you travel, you will notice the geometry of space changing as you move relative to other objects. So you can say that the metric changes as your position in space changes. But, what about time? If we were able to freeze everything so that nothing in the solar system was moving, we would see that the metric does not change with time. However, the situation is different when we get to the very large scale of intergalactic distance. It turns out that when the curvature of space due to gravity is small enough, the metric itself DOES change over time.

Thus, the expansion of space is a result of the metric changing with time. In other words, the very geometry of the universe is dynamic and, just like gravity, this changing geometry does not result in a proper acceleration. No force is required to hold you in place at your current location in space, nor is anything required to force galaxies to "stick" to space as the geometry changes. Remember that geometry itself is used to describe the distance, shape, and position of real objects relative to other objects, and all the math and grids are tools used to understand the real world. If you are imagining some sort of underlying "grid" that objects need to "stick" to, then that is an incorrect understanding of what geometry is.

To conclude this long post, remember that there are no forces at work here. A force would result in proper acceleration, which is measurable. We are not accelerating. Other galaxies are not accelerating. We are all afloat on our little islands of stability in an ever changing universe.

 

[Mordred]

This article has a decent coverage of dark energy in terms of perfect fluid equations and the FLRW metric.

The Cosmological Constant and Dark Energy by Peebles. keep in mind its an older article, however still informative.

http://arxiv.org/pdf/astro-ph/0207347v2.pdf

 

[bahamagreen]

Let me get caught up... see where things stand.

Mordred: "...relation of energy density to pressure is determined by the equation of state..."
This looks like another restatement. Repeating statements does not reveal the mechanism to me.

Nugatory: "The only difference is that the worldlines are diverging instead of converging."
The lack of proper acceleration of expansion looks similar to gravitational free fall, but we don't think of objects moving toward each other because the space is contracting between them. We think of the objects as translating through the space between them. This suggests to me the possibility of a different mechanism.

Mordred: "...much of cosmology can be described as a form of gas..."
The distinguishing thing to me about gas is that the objects which comprise the gas have a frequency of collisions based on their average velocity and mean free path, which for the oxygen and nitrogen molecules in my room right now is on the order of 10^10 collisions per second. Pressure is the result of these collisions. Cosmological collisions are a rarity, so I don't see the possible mechanism for a "pressure". The objects moving apart are doing so without making contact collisions.

Mordred: "As added reading..."
If you substitute "geometry", then the question becomes, "How does geometry cause the separation of objects as the space is expanded?"

Drakkith: "The answer you're seeking is geometry."
If so, I'll need to understand by what action geometry moves masses.

"Consider that the shortest distance between two points on the Earth's surface is not a straight line, but by a curved line."
I would consider that it is a line through the interior of the surface. If you stipulate that the line must curve with the sphere's surface, that is fine, but that is a longer distance.
Does this mean I won't be satisfied with a geometric answer?

 

[Drakkith]

Drakkith: "The answer you're seeking is geometry."
If so, I'll need to understand by what action geometry moves masses.

We're getting into the realm where words need to be used very carefully. I would say the word "action" suggests a force, which suggests a proper acceleration. There is no force. I expect you are using it differently.

The only thing I can say is that there is no action. At least not in any sense of the normal use of the word. No force is required to move these masses, and they never undergo proper acceleration. They also do not gain inertia. I believe I remember reading somewhere that if you could magically teleport to a galaxy 20 billion light years away, while retaining the velocity you had prior to the teleport, the galaxy would not be moving at a significant fraction of c relative to you like it is relative to us back here on Earth.

"Consider that the shortest distance between two points on the Earth's surface is not a straight line, but by a curved line."
I would consider that it is a line through the interior of the surface. If you stipulate that the line must curve with the sphere's surface, that is fine, but that is a longer distance.

Yes, I mean a line on the surface of the Earth, not through the Earth.

Does this mean I won't be satisfied with a geometric answer?

You may or may not be satisfied with the answer, yet, as far as I know, it is the correct one.

 

[Jonathan Scott]

After reading various opinions about the usefulness of the concept of expanding space (including those of J A Peacock in "A Diatribe on Expanding Space" and Sean Carroll's comments in his 2008 blog entry "Does Space Expand?") I think a simple way to understand it is to consider a universe model consisting of a cone, where the distance from the point is the time since the Big Bang and the tangential distance gives the one-dimensional separation in space.

It is then clear that there is more space as time elapses, but nothing actually stretches, so saying it is expanding could be misleading.

If two objects remain a fixed fraction of the size of the universe apart, they then diverge at a constant rate. However, if two objects are locally moving in parallel, they remain moving in parallel, at least if the rate of expansion is approximately constant.

I think this is a reasonable analogy, but this isn't something I've looked into in great depth, so feel free to correct me if I'm wrong.

 

[phinds]

"Consider that the shortest distance between two points on the Earth's surface is not a straight line, but by a curved line."
I would consider that it is a line through the interior of the surface. If you stipulate that the line must curve with the sphere's surface, that is fine, but that is a longer distance.
Does this mean I won't be satisfied with a geometric answer?

Well, if you use an analogy half-way and then cheat on it, yeah, you will be dissatisfied. You either accept that the surface analogy REQUIRES you to ignore the "through the interior" or you reject the analogy. You can't have it both ways, you can only try to do so and be frustrated.

 

[Mordred]

How does one determine pressure from the energy density without the ideal gas laws classical "particles bouncing off the container walls" is a diffficult concept. So I fully understand your difficulty. I've been poring over my cosmology thermidynamic literature for an accurate way to define the 3 ideal gas laws in terms of cosmology usage but the books I need are packed. Middle of moving.

The mean free path of particles and their interactions is involved in determininig pressure. In cosmology a region of a uniform (homogeneous and isotropic) distribution can and is described as a perfect fluid. Whose boundary (volume) is determined by surrounding regions that cannot be described by the same metrics as the region being described.

Off hand without the proper interpration that
is as close as I can describe it. I did come across a better interpretation but as I mentioned cannot locate it.

 

[Mordred]

I haven't been able to locate the article I was hunting however did find this one.

http://arxiv.org/abs/1402.4522

this article covers numerous perfect fluid solutions to EFE. Including dark energy. It should provide some far better detail

 

[Chronos]

I suspect ideal gas laws may not be reliable at the quantum level.

 

[Mordred]

I suspect ideal gas laws may not be reliable at the quantum level.

Good point. Ideal gas laws are at best an approximation. In order for an ideal gas law to be applied you must work at a scale where a uniform thermaldynamic equilibrium can be described. This is true in both the classical description of particles as well as the quantum descriptive. Much like the FLRW metric itself utilizes the homogeneous and isotropic descriptives in cosmology.

In many cases the boundaries used to determine the volume of the ideal gas is the separation from that uniformity. I've read numerous and unusual applications of the ideal gas laws in a large variety of scenarios. Such examples include Black hole accretion disk regions. Different regions of a star, interstellar dust dynanics etc. If you look close enough at any uniform metric that involves temp, pressure, energy density, and entropy density, yiu will probably notice an adapted ideal gas metric

Edit.. Take as an example the region within the event horizon. Describe that region as a perfect fluid. Now take the universe itself as a separate perfect fluid.. Quantum tunnelling via Hawking radiation can then be described as an interaction between two perfect fluids whose barrier is the event horizon.

take a rock off the beach. To you and I it appears solid. However in reality it is simply a more dense perfect fluid. It has a uniform density and temperature. Due to its low rate of interactions its pressure is effectively zero. However if you compress that rock it will heat up accordingly with the gas laws

 

[OCR]

We're getting into the realm where words need to be used very carefully.

Indeed, for example...

No force is required to move these masses, and they never undergo proper acceleration.

http://en.wiktionary.org/wiki/move#Verb


Indeed, again...

You may or may not be satisfied with the answer, yet, as far as I know, it is the correct one.

To click, or not to click... the choice is yours.


http://en.wikipedia.org/wiki/Motion_(physics)^*

http://en.wikipedia.org/wiki/Motion_(geometry)

http://en.wikipedia.org/wiki/Metric_expansion_of_space
http://en.wikipedia.org/wiki/Hubble's_law

http://en.wikipedia.org/wiki/Category:Metric_geometry




OCR




^*Somewhat poorly done, in my opinion...

Universe

Spacetime (the fabric of the universe) is actually expanding. Essentially, everything in the universe is stretching like a rubber band.​

 

[Drakkith]

OCR, if you've got a point to make just say it instead of spamming a bunch of links in your post with no explanation. That doesn't really help anyone.

 

[OCR]

Here's my understanding. Someone tell me if I'm wrong.

Thus, the expansion of space is a result of the metric changing with time. In other words, the very geometry of the universe is dynamic and, just like gravity, this changing geometry does not result in a proper acceleration. No force is required to hold you in place at your current location in space, nor is anything required to force galaxies to "stick" to space as the geometry changes. Remember that geometry itself is used to describe the distance, shape, and position of real objects relative to other objects, and all the math and grids are tools used to understand the real world. If you are imagining some sort of underlying "grid" that objects need to "stick" to, then that is an incorrect understanding of what geometry is.

To conclude this long post, remember that there are no forces at work here. A force would result in proper acceleration, which is measurable. We are not accelerating. Other galaxies are not accelerating. We are all afloat on our little islands of stability in an ever changing universe.

You are not wrong, is my point...

I'm not going to copy and paste Wikipedia articles here, though.




OCR

 

[Drakkith]

You are not wrong, is my point...

I'm not going to copy and paste Wikipedia articles here, though.

OCR

Hmmm. I guess I did come off as a bit snappy. My apologies.

 

[phinds]

Hmmm. I guess I did come off as a bit snappy. My apologies.

Hey, snippy is MY business. Quit it !

 

[Drakkith]

Hey, snippy is MY business. Quit it !

Aye aye, sir.

 

[Cosmobrain]

Or, you know, everybody could just get along :)

cb

 

[phinds]

Or, you know, everybody could just get along :)

cb

Nah. Boring. Here's my answer that:

www.phinds.com/idiots

 

[Mordred]

Good one Phinds lol

 

[No-where-man]

I have one question:
How exactly can you know that is 100% wrong that galaxies are moving away from each other/expanding in space, while space itself stand-still?
And how do you measure that space is expanding, how do you prove such thing?

Actually, I think one of the answer was somewhere in phys.org website when it was posted in 2012 (I think) that scientists directly measured the real effects of gravity on both surrounding space and time around the Earth thanks to satellites-that alone directly 100% proves that space from whatever is made from is both contracting and expanding, right?

 

[bahamagreen]

If you want to say that no force is applied to the separating objects, is there a force applying to expand the space?
If so, to what exactly is the force being applied, what is it that accelerates?
What is the equal and opposite reaction acting against?

 

[phinds]

I have one question:
How exactly can you know that is 100% wrong that galaxies are moving away from each other/expanding in space, while space itself stand-still?
And how do you measure that space is expanding, how do you prove such thing?

Nothing is 100% in science but this subject has been beaten to death here. Do a forum search.

Actually, I think one of the answer was somewhere in phys.org website when it was posted in 2012 (I think) that scientists directly measured the real effects of gravity on both surrounding space and time around the Earth thanks to satellites-that alone directly 100% proves that space from whatever is made from is both contracting and expanding, right?

Expanding AND contracting? Sure it is. Me too. I've been growing and shrinking for years.

You are probably thinking of the effects of speed (SR) and gravity (GR) on the GPS System's timing, but I don't think that is appropriately characterized as "space-time expanding AND contracting", it's just the effects of time dilation although it is true that one adds time in the system and the other subtracts time.

 

[phinds]

If you want to say that no force is applied to the separating objects, is there a force applying to expand the space?
If so, to what exactly is the force being applied, what is it that accelerates?
What is the equal and opposite reaction acting against?

I share your confusion about how it is possible, but it is very clear what is accelerating. The distance between objects accelerates in growth. This is NOT something that can be measured with an accelerometer, which makes the use of the word "accelerating" a bit confusing, I think, even though it DOES apply properly to the growth in distance.

 

[Mordred]

An easier way to look at it may be this statement. "The work done by the cosmological constant, is performed on space itself, not the objects within it.".

Saying the objects are accelerating or moving from us is inaccurate, as no work is being performed on the objects. Due to essentially being suspended in the perfect fluid of the interstellar medium, and how a uniform perfect fluid, such as the cosmological constant would act upon objects in that state.

Another key point to remember is that recessive velocity is distance dependant.

V_recessive=H_o*D

so if you were to measure an object at say 50,100,150,200 Mpc you would measure an increasing recessive velocity. If you change your observation point and teleport anywhere else in the universe. You will measure the same distance relation.

with the use of the scale factor "a" the velocity takes the form (scale factor is a mathematical value for time in the FLRW metric)

v=\frac{\stackrel{.}{a}(t)}{a}

this article shows the distance relations vs redshift with Hubbles constant.

https://www.physicsforums.com/showpost.php?p=4687696&postcount=10

 

[No-where-man]

Nothing is 100% in science but this subject has been beaten to death here. Do a forum search.
Expanding AND contracting? Sure it is. Me too. I've been growing and shrinking for years.

You are probably thinking of the effects of speed (SR) and gravity (GR) on the GPS System's timing, but I don't think that is appropriately characterized as "space-time expanding AND contracting", it's just the effects of time dilation although it is true that one adds time in the system and the other subtracts time.

But it proves that gravity affects time (at least).
So basically you're saying in your post that space is not expanding at all, gravity is not affecting space at all?
I looked everywhere else on the forum, and you're probably the only one who is stating this-probably I misunderstood you, please correct me if I did misunderstand you.

 

[phinds]

So basically you're saying in your post that space is not expanding at all

I cannot even begin to imagine how you got that interpretation from what I said. If I HAD said that, I would hope it WAS the only such statement on this forum. It is the kind of nonsense you find on crackpot forums.

You must not even have read this thread. In post #4 I SPECIFICALLY say that it is expanding.

 

[No-where-man]

I cannot even begin to imagine how you got that interpretation from what I said. If I HAD said that, I would hope it WAS the only such statement on this forum. It is the kind of nonsense you find on crackpot forums.

You must not even have read this thread. In post #4 I SPECIFICALLY say that it is expanding.

OK, but I read your 4th post, but you also mentioned in an other post that space doesn't really move, things just get farther apart?
So space is not expanding/contracting at all, but all things in space are just moving apart?

 

[Drakkith]

OK, but I read your 4th post, but you also mentioned in an other post that space doesn't really move, things just get farther apart?
So space is not expanding/contracting at all, but all things in space are just moving apart?

Our best theory in understanding space and time is General Relativity. GR is, at its core, a theory of geometry. When we say that "space is expanding" or "space is contracting" we mean that the geometry of space is changing in a way that causes objects within space to behave certain ways relative to one another. For example, at very large scales, this dynamic geometry is changing in a way that causes objects to recede from each other. Space itself isn't "moving", because space is not a tangible object that we can apply our rules of motion to. Describing space is what geometry is for.

 

[phinds]

OK, but I read your 4th post, but you also mentioned in an other post that space doesn't really move, things just get farther apart?
So space is not expanding/contracting at all, but all things in space are just moving apart?

Yes that is correct. Google "metric expansion" and/or see the link in my signature

EDIT: by the way, this whole business of "expanding space" vs "everything just gets farther apart" is a big bone of contentious discussion but basically it is most helpful to just consider metric expansion (things just get farther apart) because otherwise you start using metaphors about space "stretching" or "tearing" and you end up in la la land.

 

[Jonathan Scott]

If the expansion of the universe is approximately linear (that is, any deceleration or acceleration is small) then does that mean that the redshift of a given galaxy should be constant with time?

That is, should the velocity/distance relationship be such that after twice the time has elapsed, another galaxy at the same distance as a previous one is expected to have only half the redshift, because it took twice as long to separate that far?