How is space expanding ?

[island]

Negative gravity, Hmm, I will consider the idea...

 

1) It's not negative gravity, but it does have an antigravity effect if the universe is expanding. This is not the case if it is contracting or static.

 

2) The "idea" is supported by the most conservative approach to the cosmological constant, as well as any other theory that's worth having, so yeah, you might want to "consider the idea"... lol ... of Einstein, et. al... with a little more than idle curiosity...

 

 

The speed of the expansion is measured by the redshifts of the light from distant stars.

 

Link to thread "Space expanding FTL": http://www.scienceforums.net/forums/showthread.php?t=9693

 

Quote from a small part of a good post by Martin in that thread:

And here are a link to Ned Wright's own online cosmology calculator: http://www.astro.ucla.edu/~wright/CosmoCalc.html

 

Yeah, okay... the rate of expansion beyond the Hubble distance is necessarily FTL, but no violation of SR.

[Spyman]

Negative pressure still has postive energy density, so a universe that has a predominant expansive tendency also has negative gravitational curvature, which causes matter to diverge.

1) It's not negative gravity' date=' but it does have an antigravity effect if the universe is expanding.[/quote']You have to clearify this to me: a negative gravitational curvature with an antigravity effect which causes matter to diverge by "pushing" matter like normal gravity is "pulling", is what I would call negative gravity.

Do You think the "grip" of matter causing the Universe to expand, is a negative gravitational effect or not ?

 

Yeah, okay... the rate of expansion beyond the Hubble distance is necessarily FTL, but no violation of SR.

If matter is being "pushed" away from us by a gravity field then it can be compared to being pushed by a rocket engine and if faster than the speed of light, I think it is a violation.

Although I am not educated in Relativity, so I could be wrong here.

[chatlack]

I hate to be the first to break it to you' date=' but almost everybody who thinks, has at sometime pondered the atom/solar system parallel. When you look at it more closely the comparisons break down.

I once drafted a short SF story where quantum effects occured on a macroscopic level. It was crap, and fortunately its waveform collapsed on a fire.[/quote']

 

My thought does not depends on only atom/solar system. I take it as philasophic answer.

We can forget it but Big Bang doesnt make sense to me because its an unimportant theory for me. (when we go somewhere you can say "you are walking" but its not important. Because we are off for an another more important subject) (Also we may be near a "explosion in upworld" , I think there is no differences between those)

When I wrote "universe in universe" , I mean about "expanding or bigbang" that it may be an explosion on normal life and some basic organizms on electrons are wondering the reason of it.(their time is slow)

But I really wonder where comparisons break down.Can you tell me??

[island]

You have to clearify this to me: a negative gravitational curvature with an antigravity effect which causes matter to diverge by "pushing" matter like normal gravity is "pulling"' date=' is what I would call negative gravity.

[/quote']

 

This is deceiving, because the positive energy density of an expanding vacuum results in normal gravity whose cumlative characteristics are slightly outweighed by the repulsive effect of negative pressure. Which is kind of like saying that gravity is acting normally, but it's "pulling" in "wrong" direction, due to the predominant expansive tendency of the vacuum. In other words, the vacuum doesn't have negative mass, its just less than the matter density.

 

Do You think the "grip" of matter causing the Universe to expand' date=' is a negative gravitational effect or not ?

[/quote']

 

No, I think that the "grip" of whch you speak is very nearly offset by a proportional increase in gravity as explained in earlier posts... (and below)... although runaway expansion would lead to that kind of thing in terms of a "Big-Rip", since the accelerating force would increase if you dump disproportionate amounts of dark energy into the soup... from "elsewhere"... lol@ThatLoadaMalarky

 

This would lead to runaway expansion that would enable the "grip" to tear the universe apart in a "Big Rip." But this would be due to an accelerating acceleration, rather than a near equally offset acceleration.

 

If matter is being "pushed" away from us by a gravity field then it can be compared to being pushed by a rocket engine and if faster than the speed of light' date=' I think it is a violation.

Although I am not educated in Relativity, so I could be wrong here.

[/quote']

 

If matter is being "pulled" away from us by a disproportionally increasing antigravity effect, then you are correct, but not if this effect is offset by proportional increases in mass energy that you get when you make real, massive particles from Einstein's vacuum energy. You will leave a real void in Einstein's vacuum if you rip out a chunk of its energy to make a real massive particle with, which is a trade-off, so the over-riding expansive tendency only exerts a very subtly predominant outward acting force on the bodies in it.

 

Special relativity only applies to the "normal" motion of objects that are moving *through* flat space, whereas the Hubble "constant" describes the recession velocity that's caused by the expansion of space, rather than motion through that space, so objects with a recession velocity that's greater than the speed of light don't violate special relativity because nothing ever moves through the expanding space faster than light.

 

Special relativity will bite you in the ass right when you think that you've got it by the throat.

[Spyman]

If matter is being "pulled" away from us by a disproportionally increasing antigravity effect, then you are correct, but not if this effect is offset by proportional increases in mass energy that you get when you make real, massive particles from Einstein's vacuum energy.

so the over-riding expansive tendency only exerts a very subtly predominant outward acting force on the bodies in it.

From my novice view, it sounds like You are trying to both eat the cake and still be able to spare it.

 

An outward acting force on bodies causing speeds higher than c sounds to me as an violation of Relativity.

 

What it really boils down to is there can't be any force at all "pushing" or "pulling" on the bodys without violating Relativity, if it causes speeds higher than c. If the "grip" is causing bodies to move in space then it seems to me as "normal" motion.

 

Whats needed to explain this is a force that is pinning the matter stuck to space,not moving it in space, which I would call friction.

[island]

From my novice view' date=' it sounds like You are trying to both eat the cake and still be able to spare it.

 

An outward acting force on bodies causing speeds higher than c sounds to me as an violation of Relativity.

 

What it really boils down to is there can't be any force at all "pushing" or "pulling" on the bodys without violating Relativity, if it causes speeds higher than c. If the "grip" is causing bodies to move in space then it seems to me as "normal" motion.

 

Whats needed to explain this is a force that is pinning the matter stuck to space,not moving it in space, which I would call friction.[/quote']

 

It appears to me like you are trying to willfully ignore that I didn't say that an outward acting force causes speeds higher than c.

[Martin]

Trying to sort this out.

In post #59 Spyman said

My question goes deeper than that. Ned Wright doesn't explain how the negative pressure of vacuum is able to grip matter and push it away from us' date=' (gravity), faster than the speed of light. Space has no friction.

I know that if strong enough it of course can and that the force of gravity from very distant galaxies is very very small, but still to defeat that the force must have a "grip". Any force must have a direction, strengh, source and a point where it acts.

Where does this force of negative pressure act on matter ?

 

In post #68 island responded

Well' date=' maybe Ned didn't directly answer your question in so many words, but the answer is there if you study it carefully:

 

Negative pressure still has postive energy density, so a universe that has a predominant expansive tendency also has negative gravitational curvature, which causes matter to diverge.

 

That means that the gravity of the vacuum "pushes matter away from us"... but I have no idea why you think that this process *currently* exceeds the speed of light.[/quote']

 

My last post in this thread was #44 and I havent been following the discussion. I will try to understand what you are talking about and if I can say anything helpful I will get back in.

[Martin]

Actually this post #59 of Spyman is a very interesting question and apparently it has not been answered yet to Spyman's satisfaction so let's focus on it. I will take the quotes off so it doesnt get lost if someone quotes this post.

 

 

------in post #59 Spyman said-----

My question goes deeper than that. Ned Wright doesn't explain how the negative pressure of vacuum is able to grip matter and push it away from us, (gravity), faster than the speed of light. Space has no friction.

 

"I know that if strong enough it of course can and that the force of gravity from very distant galaxies is very very small, but still to defeat that the force must have a "grip". Any force must have a direction, strengh, source and a point where it acts."

 

Where does this force of negative pressure act on matter ?

------end of post-----

 

this may seem very strange and paradoxical to you Spyman: the expansion of space is NOT A MOTION.

in Gen Rel the distance function, or "metric" defining distance between two places on the continuum, is dynamic, that is to say it can CHANGE.

the distance between to stationary objects can increase (or in other circumstances decrease) without anything moving.

 

the evolution of the metric is governed by the main equation of Gen Rel which is called the Einstein equation. this is a differential equation according to which the metric changes in a way determined by its prior history and the flow of matter. one has to solve the equation to get the dynamically changing metric

 

nothing needs to MOVE, the continuum just sits there, with galaxies on it, and the distances between points change. there are no FORCES exerted on the metric (at least not any familiar Newtonian F=ma forces, no forces in any straightforward sense)

 

the metric is the geometry, the geometry does not "move", it just changes, like the distance between two things is discovered next year to be more than it was today.

[Martin]

continuing to reply to Spyman's question

 

------in post #59 Spyman said-----

My question goes deeper than that. Ned Wright doesn't explain how the negative pressure of vacuum is able to grip matter and push it away from us, (gravity), faster than the speed of light. Space has no friction.

 

"I know that if strong enough it of course can and that the force of gravity from very distant galaxies is very very small, but still to defeat that the force must have a "grip". Any force must have a direction, strengh, source and a point where it acts."

 

Where does this force of negative pressure act on matter ?

------end of post-----

 

this idea of "grips" or "hooks" is intriguing. there are no hooks holding the galaxies in place as space spread out BECAUSE SPACE ISNT MOVING so there do not need to be any "friction" or "grips".

 

but it is an intriguing image. I like it and the closest thing to such a friction or a grip that I can think of is INERTIA

suppose some galaxy is roughly stationary, moving only some negligible random amount, with only negligible forces acting on it, and suppose the distance to it is increasing at, say, 3 times the speed of light (that would be fairly normal, I can tell you what the redshift would be)

and suppose that the recession is accelerating, so that after a while the distance will be increasing at 4 times the speed of light

 

remember however that nothing is moving, the galaxy is only drifting in the space around it by a few miles per hour---some trivial random speed that doesnt count---not subject to any significant forces

 

you may ask, what keeps the galaxy from "falling off" , what keeps it perched peacefully in its astronomical neighborhood while seemingly the space around it is receding from us at a blinding rate and indeed ACCELERATING.

 

well space doesnt move, but if you want to picture it like a conveyor belt then what makes the galaxy stay put is INERTIA. In order to make it move significantly it would need to be subjected to some force and I am assuming the the forces on it are approximately negligible (dont count in the big picture).

 

Lots of people dont like the way Gen Rel uses a dynamic metric but it is the most accurate model of gravity we have and it explains a lot of stuff like pulsars and quasars and light bending and it is the basis of contemporary ideas of big bang and black hole and it explains the redshifts etc etc. So it is very hard for someone to actually get rid of, although the idea of a changing distance functions may offend one's prejudices.

 

in the end one's intuition adapts and gets used to it and hopefully it isnt so unintuitive

[island]

heheh... I don't think he's gonna buy that either, but they don't call Relativity "counter-intuitive" for nothin

[Johnny5]

I like it and the closest thing to such a friction or a grip that I can think of is INERTIA

 

What do you mean here Martin?

 

 

you may ask' date=' what keeps the galaxy perched peacefully in its astronomical neighborhood while seemingly the space around it is receding from us at a blinding rate and indeed ACCELERATING.[/quote']

 

What do you mean that the space around it is receding from us, at an accelerated rate?

 

Thank you

[Martin]

What do you mean here Martin?

=====

I mean no "grips" or "hooks" are needed because it would take some force to make the galaxy NOT conform to the space around it and pick up some proper motion of its own

===========

 

What do you mean that the space around it is receding from us, at an accelerated rate?

==============

I mean, by receding, that the distance to it is getting larger.

It is not moving, but the distance, as measured by the metric, keeps increasing.

And the RATE OF INCREASE of distance is itself increasing

 

if a(t) is the scale factor in the FRW metric (standard metric for cosmology) and a' is the first deriv. and a'' is the second deriv. then

a'/a is positive and also (this is the acceleration part) a''/a is positive.

 

You should read the popular explanations by Lineweaver and Davis in a recent Sci Am. Lineweaver and Davis make a useful distinction between

RECESSION SPEED (where nothing is moving but the distance increases at a certain rate called recession speed) and ORDINARY speed.

 

J5 I cant field all the questions about this. See if you can find the Lineweaver and Davis article called "Expanding Confusion, Popular Misconceptions about the Expansion of Space"

 

Lineweaver is a worldclass mainstream cosmologist and one of the few who cares about clarifying issues like this for the public and who can write clearly

[island]

I like it and the closest thing to such a friction or a grip that I can think of is INERTIA

 

What do you mean here Martin?

 

Einstein's interpretation of Mach's Principle?

[Martin]

Einstein's interpretation of Mach's Principle?

 

I didnt make the connection. Had nothing grand in mind, just trying to picture what would serve for "grips" in Spyman's mental picture and it seemed like inertia was it.

 

I think I will wait til Spyman gets back and see if he thinks his question is answered, or what. this might be a good place for me to leave it til we get his reaction

[Johnny5]

You guys seem kinda bright. I wonder how that happened.

 

I will read this thread carefully later... it's interesting.

 

Regards to you both

[Spyman]

It appears to me like you are trying to willfully ignore that I didn't say that an outward acting force causes speeds higher than c.

Sorry if I have misinterpreted/not understanded You.

 

But using words like "an outward acting force on bodies", "antigravity", "That means that the gravity of the vacuum 'pushes matter away from us' ", and accepting the fact that matter is moving away with speeds higher than c, which also according You, (Ned Wright), is due to this force, maybe made me take a wrong turn somewhere...

 

That doesn't mean that I am convinced, just that I will try to interpreter Your words again, in another way.

 

I will read through Martin's posts also and then reply further.

[Spyman]

this may seem very strange and paradoxical to you Spyman: the expansion of space is NOT A MOTION.

in Gen Rel the distance function' date=' or "metric" defining distance between two places on the continuum, is dynamic, that is to say it can CHANGE.

the distance between to stationary objects can increase (or in other circumstances decrease) without anything moving.[/quote']This part I can accept, (and already have accepted), it's strange but not paradoxical, if "the fabric of space-time", (=space), streach then the distance between to stationary objects increases without anything moving through space, if they move with space, "holds on" to their position.

 

I will continue reading and reply further...

[Spyman]

the metric is the geometry, the geometry does not "move", it just changes

This is like saying: space is not expanding, it's matter that is shrinking.

 

Maybe I should try to think about it from that view and then switch forth and back between the views.

 

Continuing reading and adding more replies...

[Spyman]

this idea of "grips" or "hooks" is intriguing. there are no hooks holding the galaxies in place as space spread out BECAUSE SPACE ISNT MOVING so there do not need to be any "friction" or "grips".

 

but it is an intriguing image. I like it and the closest thing to such a friction or a grip that I can think of is INERTIA

you may ask' date=' what keeps the galaxy from "falling off" , what keeps it perched peacefully in its astronomical neighborhood while seemingly the space around it is receding from us at a blinding rate and indeed ACCELERATING.

 

well space doesnt move, but if you want to picture it like a conveyor belt then what makes the galaxy stay put is INERTIA. In order to make it move significantly it would need to be subjected to some force and I am assuming the the forces on it are approximately negligible (dont count in the big picture).[/quote']"Space is not moving" and "inertia", these words are important, and togheter with a "shrinking" metric...

 

I need to digest this, I will come back and post further...

[Spyman]

I will read this thread carefully later... it's interesting.

Then maybe You will find this post by Yourself:

Now, I do not know GR, so I am not the best person in the world to answer this. However, I do have execellent spatial reasoning ability, and I think I understand your question, but first I have to make sure. I want to try and answer it, because the subject matter interests me.

and maybe this time tries to answer the question ?

[Johnny5]

Spyman, not all questions can have their answers figured out.

 

 

If i knew the answer I'd tell it to you. Right now, i think the notion of space expanding is wrong. The idea that space grips matter, and then accelerates in its expansion conflicts with a few things like...

 

1. Newton's second law

2. Law of inertia

 

Let me explain...

 

Suppose that space expands, and takes the material along with it, as if the material were gripped to an expanding space.

 

Since the expansion rate is accelerating by empirical evidence, though I've not seen it myself, the center of mass of something is accelerating while moving through the coordinate system of at least one inertial reference frame.

 

Yet, it is accelerating through that coordinate system, in absence of an externally applied force, in contradiction of Newton LawII. And hence it is changing speed, though it's in an inertial frame, and so in violation of Galileo Law I.

 

If you don't understand, don't worry about it... neither do i.

 

Kind regards

 

 

And one more thing, one could suggest that the expansion is due to an external force, the external agent being the local vacuum, surrounding the object which is accelerating, in an inertial frame, but, then from Newton Law III, there is a reaction force on the vacuum.

 

Which means that objects can interact with the vacuum, and the vacuum is conceptually, something which cannot impede the foward motion of bodies, which is what Galileo/Newton Law I suggests (when it says the speed is a temporal constant in at least one, out of many, inertial reference frames.

 

 

 

PS: Spyman you just gave me an idea, thanks.

[BlackHole]

Marc McCutcheon believes that matter expands with the space around it, therefore we don't feel any expansion. This is called the expansion theory.

 

I also think that 'expansion of space' is wrong. This could be an optical illusion. I think that the universe is like a blackbody radiator and that the CMBR is basically what's prevents all matter from collapsing into a gravitational singularity (which is also impossible). Maybe future understanding of the CMBR would give better explanations for all what's going on.

 

Another explanation is that our understanding of redshifts is incomplete or/and that the Friedmann equation is not correct at late times.

[Spyman]

Spyman' date=' not all questions can have their answers figured out.

 

If you don't understand, don't worry about it... neither do i.

 

PS: Spyman you just gave me an idea, thanks.[/quote']So Your "execellent spatial reasoning ability" can't figure it out, huh ?

 

I think that at least Martin and island are making progress in making me understand.

(And I do worry about it, even if You understand it or not.)

 

You are welcome for the idea, just leave my name out next time You start a thread with "wild" ideas.

[Spyman]

Marc McCutcheon believes that matter expands with the space around it' date=' therefore we don't feel any expansion. This is called the expansion theory.

 

I also think that 'expansion of space' is wrong. This could be an optical illusion. I think that the universe is like a blackbody radiator and that the CMBR is basically what's prevents all matter from collapsing into a gravitational singularity (which is also impossible). Maybe future understanding of the CMBR would give better explanations for all what's going on.

 

Another explanation is that our understanding of redshifts is incomplete or/and that the Friedmann equation is not correct at late times.[/quote']If matter is expanding together with space we wouldn't be able to observe any difference.

 

There is a possibility of optical illusion, yes.

 

I really don't think the CMBR is holding matter apart.

 

Further understanding of CMBR will give better explanations, yes.

 

Understanding of redshifts incomplete, maybe.

 

Friedmann equation is not correct because it can't distinguish between recession speed and ordinary speed, but is otherwise correct if BB is.

[Spyman]

Martin and island, communication between different people with different science knowledge, using advanced scientific words, in a for me foreign language, is not easy...

 

island, I apologize again for misinterpreting Your posts.

 

I have reread all island's posts in this thread and I still have trouble "connecting" them with Martin's view, maybe it's because Martin take a more common approach and island specific stick to Ned Wright's or I am still not able to understand island. (or misinterpret Martin also)

 

To clearify which is which I will try to show, how I perceive, the differences:

 

island view) Vacuum energy, vacuum have gravity which pushes bodies apart, the speed due to the push is not exceeding light but the creation of new vacuum causes that, important is that the push cancels out the normal grip of gravity between the bodies, energy don't have to be conserved since new energy is inserted from the vacuum.

 

Martin view) Inertia, the metric is changing and nothing moves, energy is conserved by also streaching gravity together with space, the gravitational energy between the objects being streached apart is constant, so gravity is sort of "amplified" relative the metric with the same rate as expansion.

 

So am I right or wrong ? Both views explains the missing "grip" but are not exactly the same.