Dark Energy is Antigravity (and other cosmological heresies)

Shadowprophet

Shadowprophet

Truthiness
Thomas R. Morrison said:
The electromagnetic stress-energy tensor is a component of the Einstein field equations, so yes we already have a metric tensor equation for the magnitude of the gravitational field associated with electromagnetic energy. An interesting feature of the stress-energy-momentum tensor is that positive pressure generates ordinary gravity, whereas negative pressure (aka tension) contributes negatively to the tensor - tension is in effect an antigravity term (regardless of the nature of that tension).


They're not really forces (they're accelerations, which is a bit different because they're not dependent on mass, as force is). A magnetic field has positive energy so it creates a gravitational field (a very small field effect anyway). Interestingly, binding energy is a negative term - so for example, the sum of the gravitational fields for a magnet and an iron bar which are widely separated in spacetime, is actually higher than the gravitational field of the same magnet and iron bar when they're stuck together via the magnetic field. However, it appears that it's impossible to get an antigravitational field this way, because the magnetic field has positive energy, and even if you coupled the magnetic field in a bound state, you could only at best cancel out that field energy, leaving the positive rest mass of the material, which of course creates a positive gravitational field.

The prevailing theory of dark energy (as vacuum energy) gets around this because the energy of the vacuum doesn't have any rest mass, and it's uniform throughout spacetime, and with both of those characteristics an antigravitational field results.


It appears that no body of matter with a positive rest mass can undergo any combination of interactions that could reverse the polarity of its overall stress-energy tensor. I've heard this from experts in general relativity, but I haven't seen the proof myself yet. Next time I talk to one I'll ask them for the proof behind this. In the meantime I trust that they're correct, but I'd like to see the basis for that conclusion.


That's a very complicated question, it turns out. I've been making my way through that paper, and at this point it looks like they're talking about a model of quantum gravity that has a cut-off in the IR range that somehow results in a long-range antigravity field that isn't very noticeable within the galaxy clusters, but which overtakes the positive gravitational field in the long-range regime, resulting in a repulsion between the galaxy clusters (I really like this idea, because it assigns the field to material bodies, instead of the current model which assigns the field to empty space, which seems absurd to me). Their work is based on the concept of asymptotically safe gravity, which is a new concept to me, so I'll have to study that in more detail before I can figure out wtf they're talking about.


Oddly, no, the gravitational field isn't relative even though it's based on the special theory of relativity (SR). In SR, all motion is relative - when two observers in relative inertial motion pass by one another, they both see the clock of the other body ticking more slowly, which their clock seem to tick at the usual constant rate.

The gravitational field is different. An astronaut far from the Earth in a low-gravity region of space would see your clock here on the Earth ticking more slowly. And an observer on the Earth would see the astronaut's clock tricking more quickly. They both agree that the clock on the Earth is ticking more slowly, so the effect isn't relative.
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If gravity isn't reletave, Does that rule out Quantum Gravity as well?
 
Thomas R. Morrison

Thomas R. Morrison

Meh
coubob said:
Like a stone being dropped in a pool of water.
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Yesterday I read a detailed response to a question raised online by a Nobel laureate physicist who asked if the radiation coming from the galaxy clusters could explain the dark energy effect. It turns out that no, that doesn't work. The electromagnetic waves and the gravitational waves which are constantly being emitted from galaxy clusters, are much too small to explain the acceleration (and of course, the intensity of those waves drops off with distance, whereas the dark energy effect gets stronger with distance).

Shadowprophet said:
If gravity isn't reletave, Does that rule out Quantum Gravity as well?
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No. None of the other forces in the Standard Model are relative either, so that's not a problem. The problem is that gravity is an acceleration - a warp in spacetime, rather than a conventional force with conventional exchange particles. So quantum field theory and general relativity speak entirely different languages, and nobody's been able to translate one into the other yet without coming up with all kinds of infinities that break the equations.

Shadowprophet said:
It seems like there where some theories about how the universe at some point could begin compressing back instead of expanding, I can't remember if that's been disproven or not. I think the jury is still out on that.
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The cyclical universe model appears to be ruled out at this point, because the cosmological acceleration is increasing, not decreasing, and it's being doing so for about 6 billion years. I suppose that someday it could begin decreasing, and eventually reverse sign, but we see no reason to anticipate that.

It appears now that the universe will keep expanding until each galaxy cluster is alone and all of the others have moved beyond the cosmic horizon. Eventually all the stars will die out, radioactive decay will get slower and slower, and everything will be consumed by a single remaining black hole. And if Hawking is correct, that black hole will slowly evaporate as it emits photons, until quadrillions of years later it shrinks to a very small size and explodes in a blast of electromagnetic radiation, and that will be the end of matter for all eternity. Pretty bleak.
 
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Shadowprophet

Shadowprophet

Truthiness
Thomas R. Morrison said:
Yesterday I read a detailed response to a question raised online by a Nobel laureate physicist who asked if the radiation coming from the galaxy clusters could explain the dark energy effect. It turns out that no, that doesn't work. The electromagnetic waves and the gravitational waves which are constantly being emitted from galaxy clusters, are much too small to explain the acceleration (and of course, the intensity of those waves drops off with distance, whereas the dark energy effect gets stronger with distance).


No. None of the other forces in the Standard Model are relative either, so that's not a problem. The problem is that gravity is an acceleration - a warp in spacetime, rather than a conventional force with conventional exchange particles. So quantum field theory and general relativity speak entirely different languages, and nobody's been able to translate one into the other yet without coming up with all kinds of infinities that break the equations.
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It makes me wonder, To someone that loves Science as I do, I look at Einstein with great esteem, He was a great man and still is, But, I wonder sometimes if we are now living in a time that we are actually seeing some of his theories disproven?
 
Thomas R. Morrison

Thomas R. Morrison

Meh
Shadowprophet said:
It makes me wonder, To someone that loves Science as I do, I look at Einstein with great esteem, He was a great man and still is, But, I wonder sometimes if we are now living in a time that we are actually seeing some of his theories disproven?
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It's very tempting to think that dark energy and dark matter indicate a flaw in the general theory of relativity. But in every observational test that's been conducted, it still passes with flying colors. I saw a new test of GR using gravitational lensing, and that worked out to within observational error too.

All we really know for sure is that both the standard model and general relativity are incomplete, and even Einstein knew that, which is why he spent the rest of his life looking for a unified field theory. The universe works as a single system, so there must be a single equation to explain it all. It's maddening that we haven't figured it out yet. And it's bizarre that all of the predictions of both theories have been proven - that's why nobody knows the best path forward.

Dark energy and dark matter are the only two phenomena that don't fit into either theory. Damned peculiar, is what it is.

Shadowprophet said:
Okay, I'm going to do it, I'm going to jump off the deep end for a moment, Please if this is absurd please, Let me know, Because this is just a thought, I'm not ready to make it a theory or defend it or anything,

What can both push and pull at the same time. First I thought of a sphere, imagine a gravitational sphere cosmic in scale. As the sphere expands it has its own density it's own gravitational attractions within, But outside the bubble or at some sort of cosmic scale event horizon, There could be a dense gravitational-like wave or ripple that is pulling or pushing as the case may be at an inverse or opposite force because it's affecting the inside of the sphere , But, Not because it's an opposite force at all, It's the exact same kind of gravity we experience here, It's just an intense event horizon or the shell of the bubble has a lot of gravitational force of its own and counteracts?

If this sounds insane, I need to hear that said. Because People don't learn unless learning is hard and has lessons in it.
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No that's not insane at all - in fact Manu Paranjape and his grad students published a couple of papers about this kind of thing recently. In theory, it appears to be possible to create a bubble with a high enough surface tension (aka negative pressure) that from the outside, it appears to have an effective negative mass and a negative gravitational field, while inside it still has positive gravity: sort of an inside-out gravitational dipole. Here's the paper about it:

"Negative mass bubbles in de Sitter space-time," Mbarek and Paranjape, Physical Review D, 2014
 
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humanoidlord

humanoidlord

ce3 researcher
now i might be misremenbering stuff but din't daniel fry predict the dark energy effect years before it was discovered? that always made me scratch my head
 
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