space expansion question

[killium]

***(Warning: a lot of thoughts experiments here, i know we can't accelerate an whole galaxy lol!...)***

As i understand it, space is expanding from everywhere at the same time. Any part of space-time is expanding in every directions. It's like if new space was created everywhere all the time.

Objects that are standing still in space will in fact be carried away from each other because of balooning space between them. We know in fact that some far away galaxies are moving away from us faster than the speed of light. It is possible cause those galaxies are not moving thru space. It's the space itself that is between us and those galaxies that is growing.

All this means that if we wanted one of those galaxies to stand still compared to us, we would need to accelerate it. It would need to move thru space in order to stand still compared to us. So, in order to have two objects in space that would stand still compared to each others, the "system" would need energy.

Where does that energy comes from in the special case where our two example objects are, a nucleus and an electron ?

 

[Saiph]

actually, the energy is required to move them apart, as the 'binding' force is gravity, an attractive force. Gravity is already trying to accelerate the objects together.

The answer to where the energy required to move them apart originates is....either left over from the BB, or from dark energy (thus the actually accelerating rate).

Very good question (and good presentation of the fundamentals too!)

 

[FlatEarth]

Where does that energy comes from in the special case where our two example objects are, a nucleus and an electron ?

Take a look at this video, particularly at 20:00. It's a lecture given by Lawrence Krauss for an atheist group, so if you are a theist like me, you may find him a bit annoying, but it is very interesting nonetheless. The entire video is over an hour long, but it's an hour well spent. Thanks to Jeters_Boy for posting it in another forum.

[youtube]http://www.youtube.com/watch?v=7ImvlS8PLIo[/youtube]

 

[MeteorWayne]

I actually found the "expansion of the Universe" graphic about 10:00 worth the price of admission

 

[killium]

Wow! I watched the whole video, thank you. Very interesting.

This is a link to a post i made here back in 2004 where i'm telling the exact same thing as this guy says about empty space....

viewtopic.php?t=11641#p222293

 

[LKD]

Watching that was a great use of one hour of my day. I cringed whenever he got off topic, but the information was very well presented and straightforward enough to fully comprehend. Thanks FlatEarth for the link.

 

[FlatEarth]

I actually found the "expansion of the Universe" graphic about 10:00 worth the price of admission

Yeah Wayne, that graphic explains expansion really well. I may watch the whole thing a few more times. Maybe some of it will stick.

You're welcome LKD and killium. This forum is a great resource for shared info like this video.

 

[FlatEarth]

In my model of the universe, the space itself is energy. There cannot exist a place where there is no energy. The very fact that a volume exist (even when totally empty) is the manifestation of energy.

A required minimum of energy is needed for any volume of empty space to exist.

Now, if the energy and the matter are related (e=mc2), it would be easy to calculated how "matter equivalent" there is in a given volume of empty space (if we know the "density" of pure, empty space, energy).

That said, empty space would exert some gravity.... The infamous dark matter could just be the space fabric itself, even if empty.

Comments....

I recently posted what I believed to be an original idea about the cause of the expansion of spacetime being simply a property of spacetime itself. How sad to see I was not the first, and because of my ignorance of quantum physics I had no idea how to properly express it. You did it well in you post, killium.

Still, I'm very disheartened by all of this. I wonder how Chuck Norris would handle it?

 

[vividasday]

nucleas vs. electron....mmmlets gather a little; rather a very large and magnetic force to create a magnificent shield.

 

[indepth]

Yeah I took a look at this video last night and his lecture was very good and detailed, yet easy enough to follow. This may be a silly question but are galaxies themself expanding? Obviously we know the universe is expanding and galaxies are getting further from eachother, but are the stars within the galaxies getting farther from eachother causing the actual galaxy to expand? Just a thought I had, thanks.

 

[killium]

indepth, yes, interresting. It's the same thing going on.... inside atoms or inside galaxies, energy is needed to counteract expansion.... the larger the scale, the larger the energy needed. especially at scales of stars in a galaxy cause you don't have the effect of the other forces to mask this phenomenon, as you would inside an atom.... so the question remains, and is even more relevant at that scale : What keeps galaxies from flying apart due to space expansion ?

 

[Saiph]

At the moment the mutual gravitation amongst all the stars in the galaxy is sufficient to hold it together against the expansion of spacetime. But if the expansion really is increasing, then at some point it won't be enough...but that's a long time from now.

 

[indepth]

indepth, yes, interresting. It's the same thing going on.... inside atoms or inside galaxies, energy is needed to counteract expansion.... the larger the scale, the larger the energy needed. especially at scales of stars in a galaxy cause you don't have the effect of the other forces to mask this phenomenon, as you would inside an atom.... so the question remains, and is even more relevant at that scale : What keeps galaxies from flying apart due to space expansion ?

At the moment the mutual gravitation amongst all the stars in the galaxy is sufficient to hold it together against the expansion of spacetime. But if the expansion really is increasing, then at some point it won't be enough...but that's a long time from now.

That is very interesting, thanks for clearing that up a bit.

 

[FlatEarth]

Yeah I took a look at this video last night and his lecture was very good and detailed, yet easy enough to follow. This may be a silly question but are galaxies themself expanding? Obviously we know the universe is expanding and galaxies are getting further from eachother, but are the stars within the galaxies getting farther from eachother causing the actual galaxy to expand? Just a thought I had, thanks.

indepth, yes, interresting. It's the same thing going on.... inside atoms or inside galaxies, energy is needed to counteract expansion.... the larger the scale, the larger the energy needed. especially at scales of stars in a galaxy cause you don't have the effect of the other forces to mask this phenomenon, as you would inside an atom.... so the question remains, and is even more relevant at that scale : What keeps galaxies from flying apart due to space expansion ?

Expansion of space occurs only between objects that are not gravitationally bound to each other, and the expansion rate is actually rather small. It's only over vast distances, at the limits of what we can see, that galaxies recede at super luminal speeds from each other. The fact that galaxies are known to move apart at such speeds makes it seem as though the universe is expanding at a furious pace, but it is not. The stars and material in galaxies are gravitationally bound, even as space expands. The subtle expansion of spacetime does not affect galaxies, or gravitationally bound groups of galaxies.

At the moment the mutual gravitation amongst all the stars in the galaxy is sufficient to hold it together against the expansion of spacetime. But if the expansion really is increasing, then at some point it won't be enough...but that's a long time from now.

Is the accelerating rate of expansion of the observable universe driven by a changing rate of expansion of spacetime per a given volume, or is it really the expansion of space over great distances that make it seem to be accelerating? (Ref. 10:00 on the video.) A constant expansion rate of a small volume, when compounded over great distances, would produce an accelerating expansion rate overall, wouldn't it? It's like compounding interest in a bank account. I tend to think this may be what is happening, but I of course I may be wrong.

 

[SpeedFreek]

At the moment the mutual gravitation amongst all the stars in the galaxy is sufficient to hold it together against the expansion of spacetime. But if the expansion really is increasing, then at some point it won't be enough...but that's a long time from now.

Is the accelerating rate of expansion of the observable universe driven by a changing rate of expansion of spacetime per a given volume, or is it really the expansion of space over great distances that make it seem to be accelerating? (Ref. 10:00 on the video.) A constant expansion rate of a small volume, when compounded over great distances, would produce an accelerating expansion rate overall, wouldn't it? It's like compounding interest in a bank account. I tend to think this may be what is happening, but I of course I may be wrong.

It is true that, with a constant expansion rate, the further away a coordinate moving with the expansion is, the faster that coordinate recedes over time. The relationship between distance and recession speed would be linear.

But we think the rate of expansion was decelerating from great speed for 8 billion years or so, and has been very slowly accelerating for the past 5 billion years. It is still the case (as always, in an expanding universe) that the further away something is, the faster it recedes, but the relationship is not linear.

It is confusing, as whatever the rate of expansion at any given time, objects at large distances separate equally across the universe. So, whether the rate is decelerating, constant, or accelerating, if coordinates comoving with the expansion were all equidistant, they would always remain equidistant at any given time, across the universe, but the further away a coordinate is from another, the faster the apparent recession speed.

But we cannot take a snapshot of the universe showing the whole thing at any given time, we see light from different times at different distances, due to the finite speed of light.

The upshot of all this is that, in the time since the rate of expansion started to accelerate, our view of the universe is that everything is a little further away than it would have been if the universe had continued to decelerate. The closer a galaxy (or supernova) is to us, the faster the rate of expansion when that light was emitted, for anything with a light-travel time of less than 5 billion years. Cosmological distances have been increasing at an accelerated rate recently, so when we see a supernova 1 billion light-years away, it is dimmer and therefore further away than we would expect, if the redshift-distance relationship between that supernova and more distant objects were linear. The gaps between the galactic clusters have been getting bigger at an increased rate recently, so we should see slightly larger gaps between clusters the closer to home we look, relative to what we see at great distances (which are distances backwards through time).

 

[FlatEarth]

It is true that, with a constant expansion rate, the further away a coordinate moving with the expansion is, the faster that coordinate recedes over time. The relationship between distance and recession speed would be linear.

Yes, I agree with your explanation. If the observations are good, then expansion is now accelerating.

But we think the rate of expansion was decelerating from great speed for 8 billion years or so, and has been very slowly accelerating for the past 5 billion years. It is still the case (as always, in an expanding universe) that the further away something is, the faster it recedes, but the relationship is not linear.

I have yet to run across any articles that describe the evolution of the universe with detail that includes the rate at which the expansion is thought to have varied over time. Do you have anything on that?

 

[Kessy]

I just had an idea to explain this, and I think it might have some interesting implications, I'd like everyone's thoughts on it.

Killium's original point seems valid and interesting to me. Even if the expansion of the universe is a vanishingly small effect at scales below galactic clusters, it's still there, and does require constant force to conteract, whether that force is gravitational, electromagnetic, or the strong force. That does imply a constant application of energy, so where's it come from? So I tried thinking by analogy, and realized that there's another common situation with a similar problem - an object at rest on the surface of a planet. Let me explain with a thought experiment.

Imagine two people, Sherri and Terri, are walking on the surface of a planet when Sherri falls down a very deep hole. From Terri's perspective, Sherri is gaining kinetic energy and losing gravitational potential energy. But, if you think about it from Sherri's perspective, the picture changes. Sherri perceives herself as being in an inertial frame and therefore traveling at constant velocity, due to the gravitational distortion of space time. To her, it appears that Terri is being accelerated upward and away from her, and gaining kinetic energy from seemingly nowhere. But, Sherri also perceives the gravitational field around her changing, so she sees herself as losing gravitational potential energy. So, from Sherri's point of view she's losing potential energy which is being transferred by changing space time to Terri.

If we then apply that concept to the expansion of the universe, that would mean the energy to maintain bound systems is actually being transferred to that system from the rest of the universe by the expansion of space time. This implies that the universe, as a whole, is losing gravitational potential energy. But since the universe is expanding, it should be gaining gravitational potential, right? Unless the net gravitational field of the universe is opposite in sign to what we're used to - that is, if there's something out there that produces negative gravity, and quite a lot of it. A possible explanation that occurs to me is that dark energy is actually particles with negative mass.

I think something with negative mass actually could fit the observations nicely. The thing you have to remember is that such particles would have negative inertial mass, as well as negative gravitational mass, which means they accelerate in the direction opposite to the force applied to them. So, two negative mass particles would exert positive gravitational force on each other (negative multiplied by negative equals positive) but they would actually accelerate away from each other because of their negative inertial mass.

A positive mass particle and a negative mass particle would exert negative gracitional force on each other, so the positive mass particle would accelerate away from the negative one, but the negative one will accelerate *towards* the positive - so the two particles will seem to chase each other, accelerating forever without outside force, but the positive particle's gain in energy and momentum will be exactly offset by the negative particles gain of *negative* momentum and energy.

So, the end result is that if the universe were dominated by matter with negative mass, it should be expanding as the negative particles accelerate away from each other, and normal matter will be carried along for the ride by its interaction with the negative matter.

So, what do you guys think? Does this idea have potential?

 

[MeteorWayne]

I just had an idea to explain this, and I think it might have some interesting implications, I'd like everyone's thoughts on it.

Killium's original point seems valid and interesting to me. Even if the expansion of the universe is a vanishingly small effect at scales below galactic clusters, it's still there, and does require constant force to conteract, whether that force is gravitational, electromagnetic, or the strong force.

That's just not correct. The expansion of the Universe is not a vanishingly small effect, it's a very robust one.

The acceleration of the expansion is a very small effect, but not the overall expansion.

 

[Kessy]

MeteorWayne, I know the expansion of the universe is a robust effect, I meant that for systems that are very small compared to the universe as a whole, the effect is small. So, for example, the influence of cosmic expansion on the Earth's orbit around Sol is tiny.

 

[MeteorWayne]

OK, we can agree on that. On the stellar system scale, or even the galactic cluster scale the effects are immeasurably small. So what is the point you were trying to make?

 

[Kessy]

The point is that even if the effect is very small, it's still present, and therefore the forces binding any system together must apply a constant, if minuscule, force to keep that system bound, just as your chair has to apply a constant force to you to keep you from falling in Earth's gravity. The thing is, for an observer in an inertial frame - either free falling in a gravity well or co-moving with the expansion of the universe, for an object to have a net force continually applied to it, resulting in an acceleration, there must be an input of energy, since the kinetic energy of the accelerating object will be continually increasing. Does that make the starting point of my conjecture clear now?

 

[MeteorWayne]

Not really. Gravity rules on small scales (galactic cluster and smaller) and expansion is more important on larger scales.

 

[SpeedFreek]

From the article I currently link in my signature:

In fact, in our universe the expansion is accelerating, and that exerts a gentle outward force on bodies. Consequently, bound objects are slightly larger than they would be in a nonaccelerating universe, because the equilibrium among forces is reached at a slightly larger size. At Earth’s surface, the outward acceleration away from the planet’s center equals a tiny fraction (10^–30) of the normal inward gravitational acceleration. If this acceleration is constant, it does not make Earth expand; rather the planet simply settles into a static equilibrium size slightly larger than the size it would have attained.

 

[Kessy]

Okay, let me see if I can explain it better. Let's take two test particles, A and B, which are of equal mass and bound together by a spring. The system as a whole, AB, is located in deep space, experiences no external forces, and is therefore in an inertial frame. Now, even though the system as a whole is in an interial frame, that doesn't necessarily mean the individual components are. For example, if I pulled A and B apart, stretching the spring, and then let the system oscillate, A and B would obviously not be in inertial frames, since the outside force of the spring is acting on each of them. However, the system still obeys conservation of energy, no matter what frame of reference you chose. Now, setting A and B at rest relative to each other again, let's consider what happens when we put them in a region of expanding space time. Now, if A and B were not bound by the spring, they would begin to move away from each other, because they would follow diverging geodesics. Because the region of space time is expanding, *all* geodesics ultimately diverge. But the spring holds them together. As Speedfreak said, both A and B will experience a force pushing them apart, which is countered by the force of the spring holding them together. The thing is that A and B are no longer following geodesics because of the force applied by the spring, which means they are no longer in an inertial frame, even though the system as a whole still is. And in any non inertial frame, you expect fictitious forces - apparent forces caused by the fact that your frame of reference isn't inertial, such as the force that appears to be pushing you back in the seat of your car when you accelerate. In this case, the fictitious force is the force pulling A and B apart due to the expansion of space. The thing is that if you then observe the system from a true inertial frame, one that is co-moving with the expansion, all fictitious forces should disappear, replaced by real accelerations in the opposite direction. So, from an inertial frame, the force pushing A and B apart disappears, but the force of the spring is a real force, so it remains, constantly accelerating A and B toward each other. In this case, conservation of momentum is satisfied because A and B are accelerating in equal and opposite directions. But conservation of energy is not! Viewed from an inertial frame, A and B are both continually gaining kinetic energy as the spring pulls them off their geodesics. The question at hand is where does that energy come from? Obviously, it must come from the expansion of space time itself. This energy is similar to gravitational potential energy, which is potential caused by the curvature of space time. So, you can look at the motions of A and B as either gaining kinetic or gravitational potential energy relative to an inertial observer, depending on how you chose to measure it. That means that either way, the expanding region of space time must be losing gravitational potential energy simply by expanding.

This is in some ways the reverse of what happens in a normal gravitational well. One difference is that expanding space is time variant while a normal gravity well is distance variant, but but I don't think that should change this analysis, due to the equivalence of space and time in relativity.

If I can return for a moment to the example in my previous post of Sherri and Terri walking on the surface of a planet when Sherri falls into a deep hole, Sherri is equivalent to the observer in an inertial frame observing A and B above, while Terri and the planet itself are equivalent to A and B. The only difference is that the forces between Terri and the planet are opposite in direction from the forces between A and B. In both cases, the inertial observer (Sherri) sees the non inertial observer (Terri) constantly accelerating, appearing to gain energy from nowhere. In both cases the inertial observer sees the curvature of space time changing at their location, Sherri because she's falling down the gravity well, the observer of A and B because space time is expanding. Therefore, in both cases, energy must be being transferred by the changing curvature of space time. I'm sorry if I'm not expressing this as clearly as possible, but I just don't have the background to express it mathematically.

Now, my speculation is that if we have two dynamic situations that differ only by some signs being reversed, (in a gravity well geodesics converge, in expanding space geodesics diverge - the force between A and B is tension, while the force between Terri and the planet is compression) it is logical to hypothesize that both situations are being caused by a similar agent that simply has a sign reversed. Expanding space looks like a reversed gravity well to me, so I ask, is it possible for expanding space to be caused by that space being filled with negative mass particles? Could this be the nature of dark energy?

~_~ I really wish I'd taken some more physics in college, it's very frustrating not being able to do even basic calculations with general relativity.

 

[FlatEarth]

Expanding space looks like a reversed gravity well to me, so I ask, is it possible for expanding space to be caused by that space being filled with negative mass particles?

If that were the case, then why is spacetime distorted by matter in such a way that it is made to wrap upon itself near a black hole? It seems that spacetime and the virtual particles that form it are attracted to matter.

According to L. Krauss, the same virtual particles that account for most of the mass of protons are the same ones that form space. That's what I get out of his lecture at the 20:00 mark.