dark energy or vaccum energy

[scottbekerham]

according to special relativity mass and energy are equivalent so because vaccum energy has mass so it should exert a gravitational force on matter . so , why can't dark energy be simply vaccum energy ?

[zhermes]

Short answer: the numbers don't work out. Vaccuum energy isn't enough to account for the effects of dark energy.

That is still one of the best candidates, however. I think lots of people are just trying to find something wrong with the calculations, or others factors missing, etc etc.

[Ich]

Short answer: the numbers don't work out. Vaccuum energy isn't enough to account for the effects of dark energy.

The numbers don't work out, yes. But vacuum energy is some 120 orders of magnitude too much to account for dark energy.
Which means that we most likely don't know how to correctly calculate vacuum energy. If it can have the correct density for some reason, it is the hottest candidate for dark energy, because its equation of state matches best the observations.

[zhermes]

Woops... i always get that in the wrong direction. Sorry O.P.

[John232]

Does dark energy act the same as gravitational energy? Accoring to Berkley, the universe can expand faster than the speed of light. Then the edge of the visable universe has galaxies that travel almost the speed of light. So then what if the universe was larger than the visable universe? Then wouldn't galaxies be traveling faster than the speed of light with infinite relative mass?

[physixlover]

Does dark energy act the same as gravitational energy? Accoring to Berkley, the universe can expand faster than the speed of light. Then the edge of the visable universe has galaxies that travel almost the speed of light. So then what if the universe was larger than the visable universe? Then wouldn't galaxies be traveling faster than the speed of light with infinite relative mass?

1.Gravity is an effect caused by the accelerating movement of dark energy.
and 2.yes, offcourse the galaxies will be travelling at speed of light.

[zhermes]

1.Gravity is an effect caused by the accelerating movement of dark energy.
and 2.yes, offcourse the galaxies will be travelling at speed of light.
This is completely wrong, and even more so misleading.

[zhermes]

Does dark energy act the same as gravitational energy?
NO! Dark Energy and gravitational energy (lets say, normal mass energy---along with its gravitational effects) are just about opposites, in numerous ways. The most important is that dark energy is repulsive while gravity is universally attractive. The other differences aren't especially relevant to this discussion, but they are important.


So then what if the universe was larger than the visable universe? Then wouldn't galaxies be traveling faster than the speed of light with infinite relative mass?
The universe is almost certainly much, much larger than the visible universe.

Things can be moving faster than the speed of light with respect to eachother in some situations. The most important is inflation---instants after the birth of the universe when everything was expanding ridiculously fast. The mathematical details can be fairly complicated, but the key idea is that nothing will ever be traveling faster than the speed of light in a local inertial frame, which doesn't apply to galaxies relative to each other on opposite sides of the universe.

There is tremendously more to say on the subject, but I think this is a good place to stop unless there are other, specific questions, etc.

[bapowell]

NO! Dark Energy and gravitational energy (lets say, normal mass energy---along with its gravitational effects) are just about opposites, in numerous ways. The most important is that dark energy is repulsive while gravity is universally attractive. The other differences aren't especially relevant to this discussion, but they are important.
We should be a little careful here. Dark energy isn't repulsive anymore than ordinary matter is attractive. Rather, a homogeneous distribution of dark energy will cause spacetime to accelerate -- this is a gravitational phenomenon. So it's more correct to say that dark energy is gravitationally repulsive, and ordinary matter is gravitationally attractive.

So then what if the universe was larger than the visable universe? Then wouldn't galaxies be traveling faster than the speed of light with infinite relative mass?
To add to zhermes' good answer, it's important to point out that there is a distance from earth at which objects are receding at light speed -- this is the Hubble radius. The Hubble radius exists regardless of whether there is any dark energy present (or in other words, whether inflation is happening). And yes, beyond this point, galaxies are receding superluminally. As zhermes points out, two distant galaxies do not share a local frame, and so special relativity is not applicable. In fact, each galaxy is locally at rest -- instead one can view the space between the galaxies as expanding. So distant galaxies recede faster the farther away they are from us simply because there is more space expanding between us and them.

[John232]

What bapowell mentioned about gravitational rupulsion and gravitational attraction was more or less what I was getting at. I know that dark energy increases the size of the universe by a square of the distance between galaxies. I just couldn't help but wonder if this could be caused by a gravitational force outside of the visable universe.

I try to avoid talking about objects traveling faster than light on the opposite side of the universe to avoid any addition of velocity issues. The fact of the matter is that if we where to assume that we are at the center of the universe, then a galaxy on the edge of the visable universe would be traveling close to the speed of light already relative to us.

The main question I have been haveing lately is if galaxies traveling along with the expansion of space are null and void of any relativistic effects. It seems to me that a lot of the problems scientist have with dark energy/matter could be explained just by adding in relativistic mass to these systems. There just isn't the information put out there about it to be able know about how relativity affects galaxies traveling at relativistic speeds.

What was so cosmo- -logical about dark energy? I read once that the cosmological constant was an accurate depiction of dark energy. What did Einstein know about the universe that lead him to predict the presence of a force overwhelming the universe that everyone at the time was completely unaware of? Everyone says that he was only trying to balance the force of gravity so that it created a static universe, but the universe was larger than he predicted and I read that the cosmological constant actually described an expanding universe if applied to it today.

Then it occured to me one day while I was watching a TV episode about how cosmologist measure the mass of systems by comparing its orbit to other nearby systems. What if its relative velocity made it appear more massive to other systems traveling at relative speeds and didn't have a relativistic mass to the other bodies that it was in orbit with that was not at relativistic speeds. If that was the case then it would seem that according to relativity that we should have dark matter and dark energy that I never could beleive that should be existing...

[bapowell]

I try to avoid talking about objects traveling faster than light on the opposite side of the universe to avoid any addition of velocity issues. The fact of the matter is that if we where to assume that we are at the center of the universe, then a galaxy on the edge of the visable universe would be traveling close to the speed of light already relative to us.
The addition of velocities only makes sense locally. Technically, the two objects whose velocities you wish to add must exist within the same tangent space. This is in general not true for distant objects in the universe because of the existence of spacetime curvature. Your reasoning makes sense if you apply special relativity to the cosmos -- however, this is not the correct way to formulate cosmology because special relativity is not compatible with gravity. For cosmology, we need to use general relativity.

We are at the center of our causal patch of the universe. And indeed, galaxies at the edge of this patch are receding at speeds approaching -- and surpassing -- the speed of light. However, the main point, is that these galaxies are not moving relative to their local frames. In fact, they can approximately be taken as being at rest locally. The recession velocity is a result merely of the expanding space between the distant objects. Consider drawing two black dots on a balloon and inflating it. The black dots will separate at a rate proportional to their distance apart. But of course, each dot is at rest with respect to the balloon.

The main question I have been haveing lately is if galaxies traveling along with the expansion of space are null and void of any relativistic effects. It seems to me that a lot of the problems scientist have with dark energy/matter could be explained just by adding in relativistic mass to these systems. There just isn't the information put out there about it to be able know about how relativity affects galaxies traveling at relativistic speeds.
Why do you think that by adding mass to astrophysical objects you can match the cosmological observations that are currently accommodated by dark energy and dark matter? Why do you think dark energy and dark matter are conceptually related? Aside from the word 'dark', they need have nothing to do with each other.

What was so cosmo- -logical about dark energy? I read once that the cosmological constant was an accurate depiction of dark energy. What did Einstein know about the universe that lead him to predict the presence of a force overwhelming the universe that everyone at the time was completely unaware of? Everyone says that he was only trying to balance the force of gravity so that it created a static universe, but the universe was larger than he predicted and I read that the cosmological constant actually described an expanding universe if applied to it today.
It's all about balance. In a closed universe with the correct amount of matter, radiation, and cosmological constant, you can achieve a static universe. Today, we know that the observable universe is very close to flat, and dark energy dominates the energy density. Put that kind of matter/energy content into Einstein's Equations and you get an accelerating universe.

Then it occured to me one day while I was watching a TV episode about how cosmologist measure the mass of systems by comparing its orbit to other nearby systems. What if its relative velocity made it appear more massive to other systems traveling at relative speeds and didn't have a relativistic mass to the other bodies that it was in orbit with that was not at relativistic speeds. If that was the case then it would seem that according to relativity that we should have dark matter and dark energy that I never could beleive that should be existing...
I like your idea, but this was all taken into account when the cosmologists did these studies. It's not as simple as just adding relativistic mass to things, since, as I mentioned above, special relativity is not relevant to non-inertial systems. However, when general relativity is used to understand galaxy rotation (which I believe is what you are referring to), one finds that indeed the angular speed of rotation does not match that expected from the matter density at the center of the galaxy. Dark matter was initially introduced to deal with this problem; however, it's important to note that it has since come to be necessary in many other parts of cosmology, notably, in the formation of galaxies and galaxy clusters. People have also tried modifying the gravity theory to understand the rotation problem, but recent evidence for particulate dark matter (see Bullet Cluster) and other theoretical shortcomings make particulate dark matter, in my opinion, the more likely explanation.

Dark energy was not introduced to understand galaxy rotation curves, so I'm not sure why you are including dark energy in this discussion. Dark energy is a whole other kind of energy. It is homogeneous and gravitationally repulsive. Ordinary matter does not have these properties. It is probably incorrect to suspect that by adding relativist mass to ordinary matter in a system that you could recreate the effects provided by dark energy.

[mr.dark]

well, what if dark energy is, in fact, the combination of smaller, simpler things and the reason they don't have an answer for it is simply because they haven't thought of it that way?? they continue looking for one, large thing to be the cause of it, so i highly doubt they have looked at smaller components adding up to dark energy.

[TrickyDicky]

NO! Dark Energy and gravitational energy (lets say, normal mass energy---along with its gravitational effects) are just about opposites, in numerous ways. The most important is that dark energy is repulsive while gravity is universally attractive.

This is simply wrong. Gravitational forces and fields are always considered to have the same sign (negative) whatever way they are produced:by "normal" matter or by any type of energy density (i.e. EM radiation etc). Perhaps your confusion comes from the fact that fermionic matter stress-energy tensor has positive pressure in its trace and the vacuum/dark energy tensor has negative pressure. But the truth is there is no such thing as a gravitational "repulsive" force, any repulsive force couldn't be gravitational in nature. When dark energy is said to accelerate spacetime is just a form of stating the equivalence principle by which a gravitational field is indistinguishible from a uniformly accelerated frame.

[bapowell]

well, what if dark energy is, in fact, the combination of smaller, simpler things and the reason they don't have an answer for it is simply because they haven't thought of it that way?? they continue looking for one, large thing to be the cause of it, so i highly doubt they have looked at smaller components adding up to dark energy.

From the standpoint of General Relativity, a cosmological constant (CC) is perhaps the simplest explanation. Physically, the CC describes the energy of the vacuum, i.e. dark energy. Your descriptor of 'one, large thing' is not very precise -- in terms of degrees of freedom the CC is ostensibly simple.

That's not to say that people haven't investigated other causes for the accelerated expansion. Modifying the equations of General Relativity has been another avenue, but has met some difficulties.

Cosmologists have been working hard on the problem of the accelerating universe, and many explanations have been offered. The physics of gravity has been scrutinized and turned upside down and shaken, and at times bent and torn. What do you think these small things are?

[mr.dark]

in my opinion, it can be many things. true, the cosmological constant is the simplest explanation; however, as stated in many reports, the cosmological constant is still a ways of by a power of about twenty one of ten i believe (or somewhere around there), meaning that instead of coming up short as some theories do, it overshoots, like the vaccum energy.

what i was thinking, however, was if they can find something that can take away mass or if they added dark matter to vaccum energy and found some type of molecule or atom or such that is able to reduce the total mass of an energy or form of matter, they may find the answer.

true, it is a long shot and may not even work, but that's one of the joys about theories and cosmology: not everything works out the way one has planned.

[mr.dark]

This is simply wrong. Gravitational forces and fields are always considered to have the same sign (negative) whatever way they are produced:by "normal" matter or by any type of energy density (i.e. EM radiation etc). Perhaps your confusion comes from the fact that fermionic matter stress-energy tensor has positive pressure in its trace and the vacuum/dark energy tensor has negative pressure. But the truth is there is no such thing as a gravitational "repulsive" force, any repulsive force couldn't be gravitational in nature. When dark energy is said to accelerate spacetime is just a form of stating the equivalence principle by which a gravitational field is indistinguishible from a uniformly accelerated frame.

this is true. dark energy is able to travel and dispel gravity, allowing it to continue to travel through objects. anything that has the opposite affect of gravity's attractive nature is not considered gravitational. the very purpose of gravity is to attract smaller objects to a heavier one and either cause it to collide with that object or position it due to the other object having a certain amount of gravity of its own almost equal to the other object (i.e. the moon and earth)

[Imax]

What if dark energy = black hole spin?

[bapowell]

in my opinion, it can be many things. true, the cosmological constant is the simplest explanation; however, as stated in many reports, the cosmological constant is still a ways of by a power of about twenty one of ten i believe (or somewhere around there), meaning that instead of coming up short as some theories do, it overshoots, like the vaccum energy.
The cosmological constant is simply a constant in Einstein's Equations, and can have any value whatever. I believe what you are referring to is the vacuum energy as calculated from the Standard Model of particle physics, which gives a CC 120 orders of magnitude too large. This is mitigated somewhat when supersymmetry is taken into account. Correct, this is a problem, but my statement was simply that the CC behaves like the energy of the vacuum -- which vacuum is unclear at this point. Quintessence, in which the dark energy is due to the vacuum energy of a rolling scalar field, could still explain dark energy. Granted, the potential would need to be fine tuned (although the degree of tuning and whether or not the potential is stable to radiative corrections is a model dependent factor) but a quintessence field with a simple potential is still 'simple' in my opinion. I still don't know what about dark energy you think makes it such a 'big thing'.

what i was thinking, however, was if they can find something that can take away mass or if they added dark matter to vaccum energy and found some type of molecule or atom or such that is able to reduce the total mass of an energy or form of matter, they may find the answer.
Why do you think reducing mass will explain the current accelerated expansion of the universe?

[bapowell]

What if dark energy = black hole spin?
Because the stress-energy associated with black hole spin does not lead to accelerated expansion.

[bapowell]

this is true. dark energy is able to travel and dispel gravity, allowing it to continue to travel through objects. anything that has the opposite affect of gravity's attractive nature is not considered gravitational.
This is not true. Dark energy is just as gravitational as ordinary matter. It is gravitationally repulsive whereas ordinary matter is attractive.

[Chronos]

I encourage everyone interested to read this paper - http://arxiv.org/abs/astro-ph/0703191

[bapowell]

I encourage everyone interested to read this paper - http://arxiv.org/abs/astro-ph/0703191
Thanks for the reference Chronos. This is a much more rigorous way to talk to about model simplicity.

[TrickyDicky]

Dark energy is just as gravitational as ordinary matter.

Correct.
It is gravitationally repulsive whereas ordinary matter is attractive.
Are you proposing a modified gravity that is repulsive instead of atractive? I'd say that would be ATM as well as wrong.

[bapowell]

Are you proposing a modified gravity that is repulsive instead of atractive? I'd say that would be ATM as well as wrong.
No idea what this has to do with Automatic Teller Machines. I am not proposing a modified gravity -- just GR. In the weak field limit in the presence of a CC, \Lambda, one has for the Newtonian potential \Phi:

\nabla^2 \Phi = -\nabla g = 4\pi G \rho - \Lambda

where g is the gravitational acceleration. This becomes

g = -\frac{GM}{r^2} + \frac{\Lambda r}{3}

and you can see the \Lambda is a repulsive contribution.

[TrickyDicky]

No idea what this has to do with Automatic Teller Machines. I am not proposing a modified gravity -- just GR. In the weak field limit in the presence of a CC, \Lambda, one has for the Newtonian potential \Phi:

\nabla^2 \Phi = -\nabla g = 4\pi G \rho - \Lambda

where g is the gravitational acceleration. This becomes

g = -\frac{GM}{r^2} + \frac{\Lambda r}{3}

and you can see the \Lambda is a repulsive contribution.

Actually I was assuming the thread's hypothesis that dark energy was actually the vacuum energy, in that case the the gravitational force would be atractive as it happens with any other energy.
You are making different assumptions as I can see, first that dark energy is Lambda, second that GR field equations must contain Lambda. Both are valid assumptions but not necesarily correct, especially if the first is not right,decreases the probabilities of the second to be right. Besides, Lambda repulsive contribution might not be gravitational, but some yet unknown interaction type if we must speculate further about this speculative "dark energy" thing.
The hypothetical assumption I used surely needs not be correct either.

[bapowell]

Actually I was assuming the thread's hypothesis that dark energy was actually the vacuum energy, in that case the the gravitational force would be atractive as it happens with any other energy.
You are making different assumptions as I can see, first that dark energy is Lambda, second that GR field equations must contain Lambda. Both are valid assumptions but not necesarily correct, especially if the first is not right,decreases the probabilities of the second to be right. Besides, Lambda repulsive contribution might not be gravitational, but some yet unknown interaction type if we must speculate further about this speculative "dark energy" thing.
The hypothetical assumption I used surely needs not be correct either.
Now I'm confused. Dark energy is vacuum energy. The cosmological constant that I am considering (\Lambda above) is of course only one special example of dark energy -- that of constant density. However, whether dark energy is dynamical or constant, the effective equation of state is still such that \rho \sim -p, and my above example is valid (give \Lambda a time dependence -- you'll reach the same conclusion as the constant case.) Whatever the form of dark energy, the gravitational forces are repulsive as I have just demonstrated. It should be well known to you that vacuum energy leads to an accelerated expansion of spacetime. I am not assuming that Einstein Equations necessarily contain \Lambda, merely demonstrating that if they do, then there is a repulsive contribution.

I have no idea what you mean by the repulsive nature of \Lambda might not be gravitational. The only dynamics being modeled by Einstein's Equations are gravitational. There are not other degrees of freedom that could mediate the repulsive force. Where do you suggest they are hiding?

[TrickyDicky]

Dark energy is vacuum energy.
I tend to agree, but it is currently just an educated guess. Dark energy is still puzzling for mainstream science.
But in any case vacuum energy can only produce an atractive gravitational field just like any other energy (EM radiation, etc) until proved otherwise.



I have no idea what you mean by the repulsive nature of \Lambda might not be gravitational.

I'm just saying that if we abandon wild speculations for a moment, to this day gravitational force is atractive, and is defined by being atractive, accordin to GR matter-energy Einstein tensor curves spacetime only with positive curvature (let's stick to the original GR equations without Lambda until it is completely accepted that Lambda=acelerated expansion of space) nobody can say with certainty that "dark energy" is gravitational since nobody knows as of today what "dark energy" is.

[bapowell]

I tend to agree, but it is currently just an educated guess. Dark energy is still puzzling for mainstream science.
But in any case vacuum energy can only produce an atractive gravitational field just like any other energy (EM radiation, etc) until proved otherwise.
OK, and I'm saying it's not an educated guess as to how vacuum energy behaves gravitationally. I've proven this with my above example. Please show explicitly how vacuum energy density is gravitationally attractive.

I'm just saying that if we abandon wild speculations for a moment, to this day gravitational force is atractive, and is defined by being atractive, accordin to GR matter-energy Einstein tensor curves spacetime only with positive curvature
So you're saying flat and negatively curved spaces are incompatible with GR? I hope not. I don't see how adding a CC to Einstein's Equations and working out the dynamics of such a universe is wild speculation.

(let's stick to the original GR equations without Lambda until it is completely accepted that Lambda=acelerated expansion of space) nobody can say with certainty that "dark energy" is gravitational since nobody knows as of today what "dark energy" is.
OK, but Lambda -- whether you want to call it dark energy or not -- Lambda, a constant in Einstein's Equations, gives accelerated expansion if it dominates the energy density. It's not a matter of debate or speculation. I agree that dark energy might not be gravitational, but that's not what you said. You argued that Lambda -- the constant in Einstein's Equations -- and its repulsive gravitational contribution was not gravitational. I'm still confused by that statement.

[TrickyDicky]

Please show explicitly how vacuum energy density is gravitationally attractive.
Maybe the problem is semantic, when you say that vacuum energy is repulsive, do you mean that since its presure is negative it produces negatively curved space? I could agree with that.

So you're saying flat and negatively curved spaces are incompatible with GR? I hope not. I hope not, too, I meant "normal" matter stress-energy curves positively space. Sorry I slipped that.

You argued that Lambda -- the constant in Einstein's Equations -- and its repulsive gravitational contribution was not gravitational. I'm still confused by that statement.
I was identifying dark energy" unknown nature with Lambda, and admitting we don't know yet much about it, I didn't mean to confuse, so never mind that statement.

[bapowell]

Maybe the problem is semantic, when you say that vacuum energy is repulsive, do you mean that since its presure is negative it produces negatively curved space? I could agree with that.
No. Vacuum energy has negative pressure. Dust and radiation have zero and positive pressure, respectively. From Einstein's equations in the Newtonian limit, I have shown in my previous post that the vacuum energy (represented by Lambda) contributes oppositely to the gravitational potential from ordinary matter/radiation. This is what I mean by repulsive -- the presence of vacuum energy lessens the acceleration due to gravity felt by two test masses. Taken by itself, it gives rise to a springy repulsion.

As for the curvature of space, the global geometry of spacetime depends only on whether the total energy density is more than, less than, or equal to the critical energy density. It doesn't matter whether the energy density is comprised of pressureless dust, radiation, or vacuum energy. In fact, a vacuum energy dominated universe tends towards flatness, not negative curvature.

[TrickyDicky]

This is what I mean by repulsive -- the presence of vacuum energy lessens the acceleration due to gravity felt by two test masses.

If that were the case I could actually see the conection suggested in previous posts between "Dark energy" and "Dark matter" since if the presence of vacuum energy really lessens the acceleration due to gravity felt by two test masses, that over huge space volumes would have the effect of a modified gravity and could explain unexpected galactic rotational curves and cluster dynamics that the "dark matter" hypothesis tries to fix. As this is obviously not the case I doubt very much your premise about repulsive gravitation.

[bapowell]

If that were the case I could actually see the conection suggested in previous posts between "Dark energy" and "Dark matter" since if the presence of vacuum energy really lessens the acceleration due to gravity felt by two test masses, that over huge space volumes would have the effect of a modified gravity and could explain unexpected galactic rotational curves and cluster dynamics that the "dark matter" hypothesis tries to fix. As this is obviously not the case I doubt very much your premise about repulsive gravitation.
This is not my premise. This is the standard lore. Pick up any cosmology text book and read about inflation, late-time accelerated expansion, gravitational effects of vacuum energy, etc. The reason that dark energy fails to explain galactic rotation curves is because galaxies are gravitationally bound objects. Expansion, whether it be accelerated or otherwise, does not operate on such scales.

I wish the idea that vacuum energy lead to accelerated expansion was my idea -- I might be waiting in the wings for a Nobel Prize. Alas, this was the brilliant discovery of Guth, Linde, and others in the 80's. I'm surprised that you are apparently unaware of this.

[TrickyDicky]

The reason that dark energy fails to explain galactic rotation curves is because galaxies are gravitationally bound objects.
Sure but if the gravitational effects of "dark energy" were to have influence in the outer part of the galactic disk, at that region it wouldn't be "bound" in the Newtonian sense (see below).
You would be introducing an element not accounted for in Newtonian gravity, Newtonian law of gravitation certainly knows nothing about any "dark energies" or vacuum energies producing "repulsive" gravitational forces.



Expansion, whether it be accelerated or otherwise, does not operate on such scales.
Accelerated expansion (not just expansion as expansion itself is not related to "dark energy") if(big if) it really was vacuum energy with "repulsive" gravitational effect would certainly have to operate from the scales at wich its gravitational magnitude allowed it and that would depend on the figures attributed to its energy-density, using the upper limit of the cosmological constant, the vacuum energy in a cubic centimeter of free space has been estimated to be 10−15 Joules (source: Wikipedia), if we take that value seriously it certainly would be noticeable in the galactic outer areas and cluster scales due to its cumulative effect, don't you think?



I wish the idea that vacuum energy lead to accelerated expansion was my idea -- I might be waiting in the wings for a Nobel Prize. Alas, this was the brilliant discovery of Guth, Linde, and others in the 80's. I'm surprised that you are apparently unaware of this.

In fact IIRC it was first suggested by McCrea in the late 60's that Lambda could be interpreted as vacuum-energy, I'm perfectly aware of these hypothesis, but I'm free to consider them provisional conjectures; when I said "your premise" I was taking a rhetorical licence meaning "the one promise you are currently using", certainly, didn't mean it was your idea :biggrin:

The notion that it is the standard lore in every cosmological book is debatable to some extent but I haven't read All Cosmology books edited so I won't argue with you about this point.

[bapowell]

Sure but if the gravitational effects of "dark energy" were to have influence in the outer part of the galactic disk, at that region it wouldn't be "bound" in the Newtonian sense (see below).
You would be introducing an element not accounted for in Newtonian gravity, Newtonian law of gravitation certainly knows nothing about any "dark energies" or vacuum energies producing "repulsive" gravitational forces.
Nobody's talking Newtonian physics here. My statement regarding the expansion due to dark energy not affecting bound structures is relativistic. This is the conclusion that GR reaches on this topic.

Accelerated expansion (not just expansion as expansion itself is not related to "dark energy") if(big if) it really was vacuum energy with "repulsive" gravitational effect would certainly have to operate from the scales at wich its gravitational magnitude allowed it and that would depend on the figures attributed to its energy-density, using the upper limit of the cosmological constant, the vacuum energy in a cubic centimeter of free space has been estimated to be 10−15 Joules (source: Wikipedia), if we take that value seriously it certainly would be noticeable in the galactic outer areas and cluster scales due to its cumulative effect, don't you think?
No, this is patently false. Cosmological expansion is not operative on galactic or planetary scales because these are gravitationally bound structures (nor does it operate on the scale of atoms, which are electromagnetically bound, etc.) Another way to look at it is the following: how does one get an expanding universe out of Einstein's Equations? One conjectures a homogeneous and isotropic geometry (the Friedmann-Robertson-Walker spacetime) with a homogeneous and isotropic matter/energy density (this works for anisotropic models as well, but for simplicity we'll stick to isotropic.) The resulting expansion/contraction is uniform. Meanwhile, a galaxy is far from homogeneous and isotropic, and Einstein's Equations will not give you an expanding spacetime for such a matter distribution -- the universe expands on scales only on which it is sufficiently smooth.

If your suggestion were true, then we'd be able to observe galaxies, solar systems, etc growing in size along with the expansion. We simply don't see this.

In fact IIRC it was first suggested by McCrea in the late 60's that Lambda could be interpreted as vacuum-energy, I'm perfectly aware of these hypothesis, but I'm free to consider them provisional conjectures; when I said "your premise" I was taking a rhetorical licence meaning "the one promise you are currently using", certainly, didn't mean it was your idea :biggrin:
Sure, you are free to consider them conjectures and you needn't accept that inflation happened or that the universe is currently accelerating. That's not what this discussion is about. The whole basis of our discussion stemmed from the fact that you seemed to be suggesting that vacuum energy did not lead to accelerated expansion. You are not free to have your own opinions on this matter -- it derives directly from GR. Unless, of course, you have problems with GR. Then that's a different story.

[TrickyDicky]

Hmm... I think you are defending at the same time one thesis and its contrary and that's not fair, and you are arguing against a strawman as I'm not the one that supports the idea of vacuum energy having repulsive gravitational effects, I'm just deriving logical consequences of that IMO confusing hypothesis.


Nobody's talking Newtonian physics here. My statement regarding the expansion due to dark energy not affecting bound structures is relativistic. This is the conclusion that GR reaches on this topic.
GR reaches no conclusion about "dark energy", how could it? It was susrprisingly discovered in 1998, almost 75 years after GR formulation, you can try to fit DE in the equations but currently I believe there is no consensus in the scientific community as to how exactly do that.
Besides I mentioned Newtonian gravitation in the context of "dark matter", do you mean it's not used in the galactic rotation curves calculations?


No, this is patently false. Sure, that's my position, as I said I'm deriving consequences of a wrong approach.
BTW, I don't have any problems with GR, in fact is my favourite theory. I tend to be very cautious with unwarranted consequences attributed to GR specially when they are not coherent with the general picture.

[bapowell]

I'm not claiming to know what dark energy is. I'm simply saying that, if you put a stress energy into Einstein's Equations with \rho \propto -p, you get all the effects that I'm talking about -- including accelerated expansion and no effect on galaxy rotation curves. I'm not sure what two theses you think I'm simultaneously defending, nor am I aware of the strawman that you think I'm using. I'm saying that you can't argue that vacuum energy doesn't have gravitationally repulsive effects. This follows from two facts: that vacuum energy has \rho \propto -p and that GR is correct. It isn't a matter of opinion.

[TrickyDicky]

I'm simply saying that, if you put a stress energy into Einstein's Equations with \rho \propto -p, you get all the effects that I'm talking about -- including accelerated expansion and no effect on galaxy rotation curves. This follows from two facts: that vacuum energy has \rho \propto -p and that GR is correct.
I agree with those two facts and yet your conclusions are not the only ones possible from them, you are allowing yourself certain degree of interpretation from those facts so you must admit that there might be other interpretations of the same facts. We can agree to disagree, I'll leave it here.

[nismaratwork]

I agree with those two facts and yet your conclusions are not the only ones possible from them, you are allowing yourself certain degree of interpretation from those facts so you must admit that there might be other interpretations of the same facts. We can agree to disagree, I'll leave it here.

Um... I won't. I've been reading this, and I don't see this other possible interpretation THAT WORKS. You claim it exists, would you please show me, or link or otherwise support that claim?

[TrickyDicky]

Um... I won't. I've been reading this, and I don't see this other possible interpretation THAT WORKS. You claim it exists, would you please show me, or link or otherwise support that claim?

Well, bapowell made it clear that his interpretation is the one followed by most people in the community, I'm not completely sure about that but just in case that is the opinion of PF mentors too, I can't support it here any more than I've done, I don't wanna risk an infraction, (still got many questions to ask:wink:), sorry.

[nismaratwork]

Well, bapowell made it clear that his interpretation is the one followed by most people in the community, I'm not completely sure about that but just in case that is the opinion of PF mentors too, I can't support it here any more than I've done, I don't wanna risk an infraction, (still got many questions to ask:wink:), sorry.

Ahhhh... I see. Well, thanks for being up-front about that, I'm not trying to get you into any kind of trouble.

[Delta²]

No idea what this has to do with Automatic Teller Machines. I am not proposing a modified gravity -- just GR. In the weak field limit in the presence of a CC, \Lambda, one has for the Newtonian potential \Phi:

\nabla^2 \Phi = -\nabla g = 4\pi G \rho - \Lambda

where g is the gravitational acceleration. This becomes

g = -\frac{GM}{r^2} + \frac{\Lambda r}{3}

and you can see the \Lambda is a repulsive contribution.

Dont know much about GR and cosmological constant but cant CC be negative?

[bapowell]

Dont know much about GR and cosmological constant but cant CC be negative?
Yes. That geometry, called anti de Sitter space, is of particular interest to string theorists.

[Imax]

Dark energy is assumed to exist because of observations made on distant galaxies. The more distant a galaxy, the greater the red shift. So, for example, a galaxy at 10 billion light years has a greater red shift that a galaxy at 1 billion light years. This is interpreted to mean that the farther a galaxy is from our point of observation, the earth, the greater its’ speed, leading to the conclusion that the expansion of the Universe is accelerating.

What if instead of interpreting the observed red shift as a function of distance, it was interpreted as a function of time? So, for example, 10 billion years ago (i.e. a galaxy at 10 billion light years) a galaxy would have a greater red shift than a galaxy 1 billion years ago (i.e. a galaxy at 1 billion light years). That would mean that 1 billion years ago, the speed of a galaxy was less than that of a galaxy 10 billion years ago. This could be interpreted to mean that the expansion of the Universe is decelerating with time.

[nismaratwork]

Dark energy is assumed to exist because of observations made on distant galaxies. The more distant a galaxy, the greater the red shift. So, for example, a galaxy at 10 billion light years has a greater red shift that a galaxy at 1 billion light years. This is interpreted to mean that the farther a galaxy is from our point of observation, the earth, the greater its’ speed, leading to the conclusion that the expansion of the Universe is accelerating.

What if instead of interpreting the observed red shift as a function of distance, it was interpreted as a function of time? So, for example, 10 billion years ago (i.e. a galaxy at 10 billion light years) a galaxy would have a greater red shift than a galaxy 1 billion years ago (i.e. a galaxy at 1 billion light years). That would mean that 1 billion years ago, the speed of a galaxy was less than that of a galaxy 10 billion years ago. This could be interpreted to mean that the expansion of the Universe is decelerating with time.

AFAIK: Because this would be inconsistent with observations, and experiment since Hubble first chewed on a pipe.

[bapowell]

What if instead of interpreting the observed red shift as a function of distance, it was interpreted as a function of time? So, for example, 10 billion years ago (i.e. a galaxy at 10 billion light years) a galaxy would have a greater red shift than a galaxy 1 billion years ago (i.e. a galaxy at 1 billion light years). That would mean that 1 billion years ago, the speed of a galaxy was less than that of a galaxy 10 billion years ago. This could be interpreted to mean that the expansion of the Universe is decelerating with time.
10 billion years ago, a galaxy 10 billion light-years away would have had a redshift of around 0. The key is that today that galaxy has a greater redshift than a galaxy 1 billion light-years away. The galaxy at 10 billion light-years has a greater recession velocity today than the galaxy 1 billion light-years away simply because it is further away from us. This is a general trend that follows from Hubble's Law, and doesn't by itself tell us anything about the expansion history of the universe. What matters here is the detailed dependence of redshift on distance -- this is provided by observations of type 1a supernovae.

[Imax]

AFAIK: Because this would be inconsistent with observations, and experiment since Hubble first chewed on a pipe.

Not necessarily. The observation is the same (i.e. red shift), but the interpretation is different.

[nismaratwork]

Not necessarily. The observation is the same (i.e. red shift), but the interpretation is different.

That still conflicts with observations (see bapowell's post!).

[71STARS]

Gravity has nothing to do with so-called dark energy, in my opinion. Gravity is the attraction between two core entities. Its function in Empty Space diminishes greatly. When dark matter is found to be a steam/mist/quantum foam, the energy associated with it may be minimal as to be nil. However, the quantity of dark matter (aka pressure ether in Dwyer's), the sheer volume of dark matter, this would be the factor of force, but one that is completely non-hindering. Einstein's CC was strangely on point, even though it was not created for such.

[nismaratwork]

Gravity has nothing to do with so-called dark energy, in my opinion. Gravity is the attraction between two core entities. Its function in Empty Space diminishes greatly. When dark matter is found to be a steam/mist/quantum foam, the energy associated with it may be minimal as to be nil. However, the quantity of dark matter (aka pressure ether in Dwyer's), the sheer volume of dark matter, this would be the factor of force, but one that is completely non-hindering. Einstein's CC was strangely on point, even though it was not created for such.

What the HELL are you talking about? Gravity is, currently, believed to be the geometry of spacetime; you're describing Newton's view of gravity. Then you start rambling...
:rolleyes:

[TrickyDicky]

Not necessarily. The observation is the same (i.e. red shift), but the interpretation is different.
Simplyfying a lot. What is known as accelerated expansion is just an observation made from 1997 and well confirmed since, that a type of Supernovae, the type Ia, that due to its features is considered a good "standard candle" for large distances doesn't look as bright from here as the models predicted. This is done by comparing observations in the two main sources of information in astrophysics:spectrometry (redshift) and photometry (brightness).
Redshift can be used as an independent measure of the distant an object such a galaxy is, and if we have a good standard candle, that is, a reliable way to guess the intrinsic brightness of a distant object we can relate that supposed intrinsic brightnes with the brightnes we measure with our photometers.
And what was observed was that certain Supernovae redshifts when converted to distance didn't fit with the expected brightness, it appeared less bright than models usually predicted, and that could logically be interpreted as the Supernovae being farther away for a given redshift than the distance the model predicted for that redshift.
Considering redshift as velocity it leads to think that the Supernovae is accelerating from us, thus accelerated expansion. As simple as that, although the implications for cosmology are not as simple.

[PhilKravitz]

We should be a little careful here. Dark energy isn't repulsive anymore than ordinary matter is attractive. Rather, a homogeneous distribution of dark energy will cause spacetime to accelerate -- this is a gravitational phenomenon. So it's more correct to say that dark energy is gravitationally repulsive, and ordinary matter is gravitationally attractive.


So as the universe gets bigger there is more dark energy and the rate of expansion increases. What does this feedback lead to? Does it create a BANG!? If so, what happens next? If not, how fast will the universe be expanding in the future? Is there a limit to the expansion rate?

[nismaratwork]

So as the universe gets bigger there is more dark energy and the rate of expansion increases. What does this feedback lead to? Does it create a BANG!? If so, what happens next? If not, how fast will the universe be expanding in the future? Is there a limit to the expansion rate?

Less than 'c' in their own frame...

[PhilKravitz]

Less than 'c' in their own frame...

If I understand correctly during inflation points in the universe had the space between them increase faster than the speed of light. Can this happen at some point in the future due to dark energy repulsive force?

[bapowell]

So as the universe gets bigger there is more dark energy and the rate of expansion increases. What does this feedback lead to? Does it create a BANG!? If so, what happens next? If not, how fast will the universe be expanding in the future? Is there a limit to the expansion rate?
Good question Phil. The destiny of the universe depends on the nature of the dark energy. In its simplest incarnation, dark energy can be chosen to have a constant energy density (a cosmological constant.) In this case, as the universe expands, the dark energy in a comoving volume increases. The (logarithmic) rate of expansion given by the Hubble parameter,

H = \frac{\dot{a}(t)}{a(t)}.

where a(t) is the scale factor (governing in the growth of length scales in the universe. ) In this case, when the dark energy is constant, the Hubble parameter is constant as well. The universe goes right on expanding.

However, consider the case in which the dark energy density grows, \dot{\rho} > 0. Then, it can be shown that a future singularity is hit (see: http://prl.aps.org/abstract/PRL/v91/i7/e071301) because the scale factor goes to infinity in finite time. This has been termed the Big Rip, and occurs at a time determined by

t_{\rm rip} - t_0 \propto |1+w|^{-1}H_0^{-1}

where the subscript '0' denotes present values, and w=p/\rho, with w<-1 for \dot{\rho} > 0. Dark energy that behaves in this way is called phantom energy, and has met with serious theoretical difficulties. In any case, as an effective equation of state, it leads to a cataclysmic dooms day.

[nismaratwork]

If I understand correctly during inflation points in the universe had the space between them increase faster than the speed of light. Can this happen at some point in the future due to dark energy repulsive force?

bapowell gave the good answer... I was just going to say, "nobody knows... we just know some options."

It is a very good question indeed.

[bapowell]

If I understand correctly during inflation points in the universe had the space between them increase faster than the speed of light. Can this happen at some point in the future due to dark energy repulsive force?
This is a popular misconception. Even during ordinary, decelerated expansion, there exist points in the universe that separate at speeds surpassing that of light. This follows simply from Hubble's law: v=Hr, where v is the relative velocity of the two points and r their separation. When their separation reaches a value of r=c/H then you can see that the relative velocity surpasses that of light. Nothing funny going on -- this is what defines an important distance known as the Hubble radius.

The main distinction to be made with inflation is that during decelerated expansion, the Hubble distance grows faster than the expanding spacetime, gradually illuminating (literally) distant regions of the universe. During inflation, the background spacetime is expanding more rapidly than the Hubble distance is increasing with the result that the boundary of the observable universe becomes an event horizon. During inflation, the expansion can (and did) pull things outside of the observable universe.

[nismaratwork]

This is a popular misconception. Even during ordinary, decelerated expansion, there exist points in the universe that separate at speeds surpassing that of light. This follows simply from Hubble's law: v=Hr, where v is the relative velocity of the two points and r their separation. When their separation reaches a value of r=c/H then you can see that the relative velocity surpasses that of light. Nothing funny going on -- this is what defines an important distance known as the Hubble radius.

The main distinction to be made with inflation is that during decelerated expansion, the Hubble distance grows faster than the expanding spacetime, gradually illuminating (literally) distant regions of the universe. During inflation, the background spacetime is expanding more rapidly than the Hubble distance is increasing with the result that the boundary of the observable universe becomes an event horizon. During inflation, the expansion can (and did) pull things outside of the observable universe.

That was... elegantly put!

[Imax]

Redshift can be used as an independent measure of the distant an object such a galaxy is, and if we have a good standard candle, that is, a reliable way to guess the intrinsic brightness of a distant object we can relate that supposed intrinsic brightnes with the brightnes we measure with our photometers.
And what was observed was that certain Supernovae redshifts when converted to distance didn't fit with the expected brightness, it appeared less bright than models usually predicted, and that could logically be interpreted as the Supernovae being farther away for a given redshift than the distance the model predicted for that redshift.

OK, maybe I’m a fuzz brain (i.e full of dark matter), but I don’t understand why differences in photometric (i.e. brightness) and spectroscopic (i.e. red shift) measurements of type 1a supernovae imply an acceleration in the expansion of the Universe.

[PhilKravitz]

During inflation, the background spacetime is expanding more rapidly than the Hubble distance is increasing with the result that the boundary of the observable universe becomes an event horizon. During inflation, the expansion can (and did) pull things outside of the observable universe.

This is a new idea to me. Very interesting. Thanks bapowell.

[PhilKravitz]

However, consider the case in which the dark energy density grows, \dot{\rho} > 0. Then, it can be shown that a future singularity is hit (see: http://prl.aps.org/abstract/PRL/v91/i7/e071301) because the scale factor goes to infinity in finite time. This has been termed the Big Rip, and occurs at a time determined by

t_{\rm rip} - t_0 \propto |1+w|^{-1}H_0^{-1}

where the subscript '0' denotes present values, and w=p/\rho, with w<-1 for \dot{\rho} > 0. Dark energy that behaves in this way is called phantom energy, and has met with serious theoretical difficulties. In any case, as an effective equation of state, it leads to a cataclysmic dooms day.

bapowell thanks for the reference I will go and read it. And I quite enjoy the terminology "Big Rip" and "phantom energy".

[Janus]

OK, maybe I’m a fuzz brain (i.e full of dark matter), but I don’t understand why differences in photometric (i.e. brightness) and spectroscopic (i.e. red shift) measurements of type 1a supernovae imply an acceleration in the expansion of the Universe.

1a supernovae all put out the same amount of light. By measuring their brightness we can tell how far away they are. Red-shift tells us how fast they are receding. So if we plot brightness against red-shift we are plotting distance against recession. Also, since light travels at a set finite speed, we are looking at them as they were and not as they are. The further the supernova, the further in the past we are looking. It's like taking snapshots of the universe at different points of time.

If the universe were expanding at a constant speed, we would expect to see a one to one match of distance and recession. Double the distance and double the recession speed.

But we don't see this, instead, we see a pattern that indicates that, in the past, the universe did not expand as fast as it does now.

The initial study expected to find the opposite. They expected that the universe would slow its expansion over time due to gravitational attraction. What they were trying to determine if it was slowing fast enough to ever stop the expansion and cause the Universe to collapse back on itself. The results they got surprised them.

[Imax]

Hi Janus:

Thanks for your reply. It’s given me a greater appreciation for the relationship between distance and recession. But (and I like buts) I can see it only for nearby galaxies. The problem with galaxies at a distance of 10 billion light years is that those photons are 10 billion years old.

[nismaratwork]

Hi Janus:

Thanks for your reply. It’s given me a greater appreciation for the relationship between distance and recession. But (and I like buts) I can see it only for nearby galaxies. The problem with galaxies at a distance of 10 billion light years is that those photons are 10 billion years old.

OK... I'll bite: Why would that matter, assuming a photon that was never absorbed and re-emitted (read: new photon!) to begin with?

[Imax]

When trying to gauge the current expansion of the Universe, points in the data set from millions to billions of years ago need to be treated carefully.

[nismaratwork]

When trying to gauge the current expansion of the Universe, points in the data set from millions to billions of years ago need to be treated carefully.

When making a statement, you have to actually say something, not intimate an unnamed caution like an old man wagging finger. This PF, not Dagobah.

To put it in better terms: what do you mean by, "careful", and what are you cautioning against?

[Imax]

When making a statement, you have to actually say something, not intimate an unnamed caution like an old man wagging finger.

Sorry nismaratwork if I sounded like an old man wagging my finger (naughty, naughty, naughty). That wasn’t the intent of my post. I used the word “carefully” because what’s happening now may not be what happened a long time ago. As a poor analogy, what’s the average speed of vehicles on an interstate? If you include old data from Model-T Fords, then it could bias results.

[Janus]

Sorry nismaratwork if I sounded like an old man wagging my finger (naughty, naughty, naughty). That wasn’t the intent of my post. I used the word “carefully” because what’s happening now may not be what happened a long time ago. As a poor analogy, what’s the average speed of vehicles on an interstate? If you include old data from Model-T Fords, then it could bias results.

But that's the whole point. By looking at more distant galaxies we are looking into the past, and this is how we know that the universe is expanding faster now than it was then.

[Imax]

So, vaccum energy < dark energy ?

[nismaratwork]

So, vaccum energy < dark energy ?

Bingo!

[scottbekerham]

I think General Relativity should be modified and the current theory is merely an approximation . If we arrive at the correct theory for gravitational physics that can be incorporated with quantum physics in a unified manner then the theory should be able to predict accelerated cosmic expansions , dark energy . and Inflation .May be Supergravity theories should be able to predict it

[71STARS]

"Dark Energy" per se can be summed up as not existing simply because "Dark Matter" when it is in an overabundance state will naturally nudge galaxies apart. That does not imply energy. That is merely an accumulation of the unseen mass. When the cause of Dark Matter can be determined, the functions will show that an overabundance of this unseen mass is the reason for any distance fluctuation in galaxies (or suns and planets). The timeframe would be sooooo small, but simply a natural process. Yes, this is stictly my opinion. And yes, Einsteins cosmological constant can be equated to the term "Dark Matter" although he did not think in those terms.

[bapowell]

Einstein's cosmological constant in no way behaves like dark matter. Dark energy and dark matter are logically (and physically) independent concepts.

[mikejr82]

according to special relativity mass and energy are equivalent so because vaccum energy has mass so it should exert a gravitational force on matter . so , why can't dark energy be simply vaccum energy ?

This is something that has intrigued me for a while. When a matter and anti-matter colide,they destroy each other in a massive burst of energy.(cassimir effect).
Is it possible that these collisions create space between objects?
I believe this would better explain the expansion of the universe and how galaxies collide even tho everything in the universe is supposed to be moving away from each other.
No one has figured out how to calculate the true force generated by vacuum energy.

[twofish-quant]

This is something that has intrigued me for a while. When a matter and anti-matter colide,they destroy each other in a massive burst of energy.(cassimir effect).
Is it possible that these collisions create space between objects?

Casmir effect is something different, and no matter-antimatter collisions don't create unusual amounts of space. Anti-matter is something that gets produced in particle accelerators all of the time, and people use anti-matter routinely for brain and heart scans (google for positron emission tomography).

No one has figured out how to calculate the true force generated by vacuum energy.

They have actually, it's not a hard calculation

http://en.wikipedia.org/wiki/Casimir_effect

[twofish-quant]

I think General Relativity should be modified and the current theory is merely an approximation.

So do a lot of other people. The astrophysics database search for modified gravity has about 10000 hits

http://adsabs.harvard.edu/cgi-bin/basic_connect?qsearch=modified+gravity

The problem is not just saying "let's modify GR." The hard part is to say "if you modified GR in this way, then you will get observations that do or don't match what we see."

Also one way that physicists thinks instead of talking about *one* possible modification to GR, what you do is to try to classify all possible modifications to GR in several groups and then try to knock them over.

[bapowell]

No one has figured out how to calculate the true force generated by vacuum energy.
Why do you say this? Also, why do you think your suggestion better explains the expansion of space than the Friedmann solution? Have you worked out the relevant quantities in your theory: expansion rate, redshift relations, age of the universe, etc? Can you fit supernova, CMB, and large scale structure data with your idea?