Expansion of space vs stuff just moving away

[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?

 

[No-where-man]

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.

Than all the stories where it says that gravity affects space is also wrong?
If space is not tangible and if it's not physical, how does the black hole affect space (and time for that matter)?
Now, I'm a little confused.

 

[phinds]

Than all the stories where it says that gravity affects space is also wrong?
If space is not tangible and if it's not physical, how does the black hole affect space (and time for that matter)?
Now, I'm a little confused.

Your problem here is that you are thinking of space and time as separate things, but in GR in particular, they cannot be separated. Gravity affects spacetime. The extreme gravity of a black hole has an extreme affect of spacetime.

 

[No-where-man]

Your problem here is that you are thinking of space and time as separate things, but in GR in particular, they cannot be separated. Gravity affects spacetime. The extreme gravity of a black hole has an extreme affect of spacetime.

I wonder if enough extreme gravity can truly destroy space-time-i wonder if this is true?

But you also said that space itself is not anything tangible or physical, so what's the difference if you say it affects space-time, and it does not affect space alone?

 

[phinds]

I wonder if enough extreme gravity can truly destroy space-time-i wonder if this is true?

Well, "destroy" might not be the right description, but an infinite mass would likely cause some problems, but not to worry, there is not and never will be any such thing.

But you also said that space itself is not anything tangible or physical, so what's the difference if you say it affects space-time, and it does not affect space alone?

Space is a classical concept. In the context of what you are talking about, it cannot be considered "alone" because it ISN'T "alone", it is the spatial dimension part of what we call spacetime.

You can ask "if the laws of physics don't apply, then what do the laws of physics say about <put in any statement you like>?" and you won't get a meaningful answer.

 

[Drakkith]

I wonder if enough extreme gravity can truly destroy space-time-i wonder if this is true?

This cannot be true since gravity is the result of curved spacetime.

But you also said that space itself is not anything tangible or physical, so what's the difference if you say it affects space-time, and it does not affect space alone?

This really gets bogged down into a discussion over what certain words mean. You're best, most absolute guaranteed way of understanding spacetime as we know it is to study what General Relativity says.

 

[No-where-man]

This cannot be true since gravity is the result of curved spacetime.

I thought it's the opposite, that gravity affects spacetime, that the gravity is directly responsible for creating curves of spacetime?

This really gets bogged down into a discussion over what certain words mean. You're best, most absolute guaranteed way of understanding spacetime as we know it is to study what General Relativity says.

OK.

 

[No-where-man]

Well, "destroy" might not be the right description, but an infinite mass would likely cause some problems, but not to worry, there is not and never will be any such thing.

OK, but I truly hope it's 100% impossible than for gravity to destroy space-time.

Space is a classical concept. In the context of what you are talking about, it cannot be considered "alone" because it ISN'T "alone", it is the spatial dimension part of what we call spacetime.

OK, understood.

You can ask "if the laws of physics don't apply, then what do the laws of physics say about <put in any statement you like>?" and you won't get a meaningful answer.

OK.

 

[Mordred]

Drakkiths comment on learn what GR teaches, is a good solid point. Here is a statement from Einstein that applies.

"According to the general theory of relativity, the geometrical properties of space are not
independent, but they are determined by matter. Thus we can draw conclusions about the
geometrical structure of the universe only if we base our considerations on the state of the
matter as being something that is known."

keep in mind this line was written prior to the cosmological constant. Which is in essence a negative pressure influence, gravity being a positive pressure influence.

However the above statement is still true provided you also consider the cosmological constants added influence on the average densities of matter.

this isn't the best book to study GR and SR even though its written by Einstein, however it is a free for distribution book. As well as a good historic reference article. Much of the explanations in it are easily read so its a good starting point.

http://www.marxists.org/reference/archive/einstein/works/1910s/relative/relativity.pdf

 

[Drakkith]

I thought it's the opposite, that gravity affects spacetime, that the gravity is directly responsible for creating curves of spacetime?

Nope. Gravity is the result of spacetime being curved by the presence of energy (which includes the energy matter has) and stress. Hence the term stress-energy tensor, which is what is used to calculate the curvature of spacetime.

 

[No-where-man]

Nope. Gravity is the result of spacetime being curved by the presence of energy (which includes the energy matter has) and stress. Hence the term stress-energy tensor, which is what is used to calculate the curvature of spacetime.

Ok, big thanks for correcting me in my obviously completely false understanding of gravity and curved space-time, until now.