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Dark matter


Kevin_Hall
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I still think there are other viable possibilities besides the ones mentioned. I mentioned briefly on another thread the idea that there might be emergent dynamics at play for gravitational systems with large numbers of constituents; these wouldn’t show up in calculations unless one actually derived an exact numerical solution for a discrete system (n-body with large n) rather than a continuum simplification. Unfortunately we don’t have the computational power to do this, not even in principle.

In this picture, DM would be an artefact of our own computational limitations, and not any kind of new physics - the discrepancy that leads us to postulate DM would arise because we don’t have the means to use GR correctly for this scenario. After all, the universe is under no obligation to be computable (given current computational power), even if we do know all relevant laws of nature. There are similar problems in QCD as well.

Edited by Markus Hanke
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3 hours ago, beecee said:

(1) A possibility sure, but are we not also seeing the need for "extra matter" over smaller scales? Plus GR is our gravitational explanation/theory for the very large.

 

At smaller scales dark matter doesn't really play that much of a role. It's when you get to the region of galactic haloes when it starts to kick in. That's why you don't consider it when solving problems like, eg., the perihelion of Mercury. It's very sparse and apparently nucleates around the galaxies, but at very long distances in comparison with atomic matter. As there are no collisions, it doesn't fall anywhere near the galactic centres in any significant amount.

Einstein's equations OTOH seem to me to be formulated as some kind of compromise --a brilliant and predictive one to be sure. The Einstein tensor does not saturate all the possible geometric degrees of freedom. The other degrees of freedom are coded in the Weyl tensor. They can also be generalised in several ways, like including torsion, complexifying them... They seem so inviting for generalisation. Although this is nothing but a hunch. It also has to be said that the attempts have been many, and nobody so far seems to have been able to extend Einstein's predictions significantly.

3 hours ago, beecee said:

(3) Little BH's in the early denser universe, could grow into bigger SMBH's before they had time to evaporate.

As to primordial BHs, it's very appealing. But how big can they grow before there would be noticeable effects? I don't think you mean dark matter haloes are made up of SMBHs. We would have seen them already by their lensing effects. Plus the masses would be so out there.

I'm probably just expressing my preferences. We really need the data James Webb will provide.

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On 12/22/2021 at 6:18 PM, swansont said:

If it could interact electromagnetically it would emit thermal EM radiation. The ability to emit light has implications about how it behaves - easy dissipation of energy would allow it to “clump” more readily. And “dark” is also an acknowledgement that we don’t know what it is, as beecee has noted.

Is this why it can maintain a halo while (I'm presuming) orbiting galactic centres, and not form dark matter "planets" or major black holes?

I've often wondered why it didn't form a localized mass and eventually clear it's orbit like planets tend to.

Which would essentially remove the halo.

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3 hours ago, joigus said:

I'm probably just expressing my preferences. We really need the data James Webb will provide.

Yep, let's hope it all goes to plan....remembering at L2 (1.5 million kms, we won't be able to send astronauts to repair.

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9 hours ago, J.C.MacSwell said:

Is this why it can maintain a halo while (I'm presuming) orbiting galactic centres, and not form dark matter "planets" or major black holes?

I've often wondered why it didn't form a localized mass and eventually clear it's orbit like planets tend to.

Which would essentially remove the halo.

AFAIK that’s why. If the only channel is gravitational radiation, then the dissipation is very, very weak. All collisions would be basically elastic.

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6 minutes ago, swansont said:

AFAIK that’s why. If the only channel is gravitational radiation, then the dissipation is very, very weak. All collisions would be basically elastic.

The distribution of the DM in space now is very much non-uniform. Since it doesn't clump, was its distribution as non-uniform soon after the BB?

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1 hour ago, swansont said:

AFAIK that’s why. If the only channel is gravitational radiation, then the dissipation is very, very weak. All collisions would be basically elastic.

Similar to @Genady, I wonder why it appears clumped if it does not clump - does it mean that, at this age of universe, the gravitational radiation was enough to clump it into galactic halos, but not yet enough to form it into galactic discs?

Also, if DM is perfectly uniformly distributed, would we at all be able to detect it - could it be that there is much more of it around, but we only detect so much of it because it is not perfectly uniform?

15 hours ago, Markus Hanke said:

I still think there are other viable possibilities besides the ones mentioned. I mentioned briefly on another thread the idea that there might be emergent dynamics at play for gravitational systems with large numbers of constituents; these wouldn’t show up in calculations unless one actually derived an exact numerical solution for a discrete system (n-body with large n) rather than a continuum simplification. Unfortunately we don’t have the computational power to do this, not even in principle.

In this picture, DM would be an artefact of our own computational limitations, and not any kind of new physics - the discrepancy that leads us to postulate DM would arise because we don’t have the means to use GR correctly for this scenario. After all, the universe is under no obligation to be computable (given current computational power), even if we do know all relevant laws of nature. There are similar problems in QCD as well.

This is the first time I hear such argument. Puzzling. Anything more to read about it? Did pure mathematicians have an attempt at it - by examining if GR equations can at all behave in this 'unexpected' way in very complex systems?

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27 minutes ago, Danijel Gorupec said:

Also, if DM is perfectly uniformly distributed, would we at all be able to detect it - could it be that there is much more of it around, but we only detect so much of it because it is not perfectly uniform?

If there were much more of it around, it would affect the cosmological expansion. Perhaps it could be detected this way.

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42 minutes ago, Danijel Gorupec said:

Similar to @Genady, I wonder why it appears clumped if it does not clump

It appears clumped? If it was clumping easily it would be at the center.

Quote

Also, if DM is perfectly uniformly distributed, would we at all be able to detect it 

Who claimed it was perfectly uniformly distributed?

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27 minutes ago, Danijel Gorupec said:

Bad English. I wanted to ask, if there is a stuff that only interacts gravitationally, and if such stuff is distributed uniformly - are there any means to detect it?

Perhaps not. It wouldn’t cause lensing or any net gravitational attraction.

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47 minutes ago, swansont said:

How? It would all cancel. 

Each particle is attracted equally from all directions, and these will cancel indeed. But, each particle attracts all other particles and they will all accelerate toward it. This is so for all particles and thus all particles accelerate toward each other. So, the entire thing homogenously and isotropically contracts, or slows its expansion.

For a bit more precise treatment, take any particle as a center and consider particles on a sphere of radius R around it. Each particle on the sphere, according to the old Newton's theorem that holds in GR as well, is attracted to the center as if the mass of the entire ball of radius R is in the center, and effect of each larger sphere on it is 0. This holds for any point picked as a center.

The fully precise result in GR follows from increasing mass density in Friedman equation.

Edited by Genady
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5 minutes ago, Genady said:

Each particle is attracted equally from all directions, and these will cancel indeed. But, each particle attracts all other particles and they will all accelerate toward it. This is so for all particles and thus all particles accelerate toward each other. So, the entire thing homogenously and isotropically contracts, or slows its expansion.

This is contradictory. If the accelerations cancel, there is net scceleration.

5 minutes ago, Genady said:

For a bit more precise treatment, take any particle as a center and consider particles on a sphere of radius R around it.

We don’t have a sphere, and there is no center. The premise is we have mass uniformly distributed over all space.

 

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1 minute ago, swansont said:

This is contradictory. If the accelerations cancel, there is net scceleration.

We don’t have a sphere, and there is no center. The premise is we have mass uniformly distributed over all space.

 

The center is just an origin of coordinates. Of course, the premise is homogeneity and isotropy of the entire space. For any coordinate system, a particle in its origin does not move anywhere, but all other particles accelerate toward it. The same as in the Hubble expansion.

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1 hour ago, Genady said:

The center is just an origin of coordinates. Of course, the premise is homogeneity and isotropy of the entire space. For any coordinate system, a particle in its origin does not move anywhere, but all other particles accelerate toward it. The same as in the Hubble expansion.

An easy derivation, with answers to the audience's questions relevant to the above discussion, can be found here, starting about 30 minutes into the lecture: 

 

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1 hour ago, Genady said:

The center is just an origin of coordinates. Of course, the premise is homogeneity and isotropy of the entire space. For any coordinate system, a particle in its origin does not move anywhere, but all other particles accelerate toward it. The same as in the Hubble expansion.

But then what of an observer some distance away? Everything must accelerate toward them, because the choice of the origin is arbitrary. Which can’t be true unless the acceleration is zero.

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1 minute ago, swansont said:

But then what of an observer some distance away? Everything must accelerate toward them, because the choice of the origin is arbitrary. Which can’t be true unless the acceleration is zero.

It is true. For observer 1, a particle P is at rest and a particle Q accelerates toward it. For observer 2, Q is at rest and P accelerates toward it.

Susskind answers exactly this question in the lecture that I've linked in the post above yours.

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8 hours ago, Danijel Gorupec said:

This is the first time I hear such argument. Puzzling. Anything more to read about it? Did pure mathematicians have an attempt at it - by examining if GR equations can at all behave in this 'unexpected' way in very complex systems?

Not that I’m aware of, though Sabine Hossenfelder has once mentioned a somewhat similar idea, albeit not from the perspective of emergence. This is something I kind of came up with myself; I call it a gravitational fluid.

As for math, I have been pondering for some time how this could be modelled, but can’t come up with anything. The GR n-body problem for large n is not computable with currently available computational resources, and I doubt it ever will be.

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On 12/23/2021 at 11:29 PM, Markus Hanke said:

I still think there are other viable possibilities besides the ones mentioned. I mentioned briefly on another thread the idea that there might be emergent dynamics at play for gravitational systems with large numbers of constituents; these wouldn’t show up in calculations unless one actually derived an exact numerical solution for a discrete system (n-body with large n) rather than a continuum simplification. Unfortunately we don’t have the computational power to do this, not even in principle.

In this picture, DM would be an artefact of our own computational limitations, and not any kind of new physics - the discrepancy that leads us to postulate DM would arise because we don’t have the means to use GR correctly for this scenario. After all, the universe is under no obligation to be computable (given current computational power), even if we do know all relevant laws of nature. There are similar problems in QCD as well.

Here it is: 

What's appealing about this line of thinking is that it proposes to combine well-known principles of physics in unexpected ways, rather than guess unexpected principles of physics in well-known ways. The first technique has always met much more success than the second.

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2 hours ago, Markus Hanke said:

Not that I’m aware of, though Sabine Hossenfelder has once mentioned a somewhat similar idea, albeit not from the perspective of emergence. This is something I kind of came up with myself; I call it a gravitational fluid.

As for math, I have been pondering for some time how this could be modelled, but can’t come up with anything. The GR n-body problem for large n is not computable with currently available computational resources, and I doubt it ever will be.

Does this idea of yours make any predictions or any potential predictions?

Can you take it any further in any way  even though  as you say this potential state is not computable even in principle?

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5 minutes ago, geordief said:

Does this idea of yours make any predictions or any potential predictions?

Not unless someone really smart finds a way to describe this mathematically in a computable way. And again, it’s just an idea, I’m making no claims - this could quite possibly be another dead end.

7 minutes ago, geordief said:

Can you take it any further in any way  even though  as you say this potential state is not computable even in principle?

I wouldn’t go as far as to say it’s not computable in principle. Basically what’s needed is a mathematical method to derive large-scale dynamics from small-scale interactions, given how the constituents interact (ie GR). I don’t know if that is possible, or how one would go about doing that. What’s certainly not possible is to solve a GR n-body problem with n~ 100 billion.

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13 hours ago, Markus Hanke said:

Not unless someone really smart finds a way to describe this mathematically in a computable way. And again, it’s just an idea, I’m making no claims - this could quite possibly be another dead end.

I wouldn’t go as far as to say it’s not computable in principle. Basically what’s needed is a mathematical method to derive large-scale dynamics from small-scale interactions, given how the constituents interact (ie GR). I don’t know if that is possible, or how one would go about doing that. What’s certainly not possible is to solve a GR n-body problem with n~ 100 billion.

I did a search on "gravitational fluid"(I do that as almost as a routine for something I don't understand )  and there  was this mini video

No idea what it is supposed to be.

BTW ,is there any possibility that DM could be clouds of mini black holes formed in the very early days after BB?

Edited by geordief
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