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FTA: Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with partic

FTA: Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.

pavel_lishin 5y ago
Does "friction" refer to something different here? I thought that Dark Matter didn't even interact with itself in any meaningful sense, and therefore had no friction.
- Keysh 5y ago
  Friction in this context refers to "dynamical friction", where massive objects -- e.g., dense cluster cores -- moving through a medium of many small masses -- e.g., DM particles -- will experience gravitational drag and slow down. https://en.wikipedia.org/wiki/Dynamical_friction https://en.wikipedia.org/wiki/Dynamical_friction
SAI_Peregrinus 5y ago
Except that the core thing that makes dark matter dark is that it doesn't interact via the electromagnetic force, and friction is a result of the electromagnetic interaction. So that statement is nonsense.
- dnautics 5y ago
  No that's not right. Dark matter itself causes an entirely gravitational "friction". If you have a noninteracting particle over time a bunch of them get slingshotted to very high velocity on accident which bleeds the net momentum of the rest of the system. If you've ever written n-body simulators you would see this.
  - gus_massa 5y ago
    I think you are correct, but I like freebees, ... Do you have an online demo? Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter? For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
    - dnautics 5y ago
      > Do they escape or they just form a cloud that gets hotter? They escape. yeah, having done this before, I am 100% sure that the simulation will be inexact due to numerics, and that the inexactness will be worse around these phenomena, but we are probably talking, gut feeling say, under 20% in most scenarios where you see an escape (I think the observable error is effectively unbounded because you could in theory get two particles within one ULP or even two particles that collide and cause a NaN error)... But in general I don't think that changes the qualitative nature of the phenomenon. There's probably a reasonably easily derived "starting from three particles at rest" where you can see one of them escape from the other two; if not 3 then four.
    - dnautics 5y ago
      I did once joke to a physicist that maybe dark matter is floating point errors, and for a second his face went white.
Keysh 5y ago
This is rather overstated (especially the "incredibly unlikely" bit). The best attempt I've seen to summarize this is in this paper https://arxiv.org/abs/1412.7719 https://arxiv.org/abs/1412.7719 which argues that there's roughly a 10% chance of getting something as extreme as the Bullet Cluster under Lambda-CDM. So, mildly unlikely, but not even approaching the (problematic) P < 0.05 threshold that's emblematic of the "replication crisis" in fields like experimental psychology.
XorNot 5y ago
Yes and I'm straight up not buying that argument because it being improbable that the bullet cluster might arise doesn't explain why it did: remove particle dark matter and now you're left with gravitational effects floating around in space totally divorced from the shape and location of regular matter which should be producing them.

pavel_lishin 5y ago

Does "friction" refer to something different here? I thought that Dark Matter didn't even interact with itself in any meaningful sense, and therefore had no friction.

Keysh 5y ago
Friction in this context refers to "dynamical friction", where massive objects -- e.g., dense cluster cores -- moving through a medium of many small masses -- e.g., DM particles -- will experience gravitational drag and slow down. https://en.wikipedia.org/wiki/Dynamical_friction https://en.wikipedia.org/wiki/Dynamical_friction

SAI_Peregrinus 5y ago

Except that the core thing that makes dark matter dark is that it doesn't interact via the electromagnetic force, and friction is a result of the electromagnetic interaction. So that statement is nonsense.

dnautics 5y ago
No that's not right. Dark matter itself causes an entirely gravitational "friction". If you have a noninteracting particle over time a bunch of them get slingshotted to very high velocity on accident which bleeds the net momentum of the rest of the system. If you've ever written n-body simulators you would see this.
- gus_massa 5y ago
  I think you are correct, but I like freebees, ... Do you have an online demo? Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter? For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
  - dnautics 5y ago
    > Do they escape or they just form a cloud that gets hotter? They escape. yeah, having done this before, I am 100% sure that the simulation will be inexact due to numerics, and that the inexactness will be worse around these phenomena, but we are probably talking, gut feeling say, under 20% in most scenarios where you see an escape (I think the observable error is effectively unbounded because you could in theory get two particles within one ULP or even two particles that collide and cause a NaN error)... But in general I don't think that changes the qualitative nature of the phenomenon. There's probably a reasonably easily derived "starting from three particles at rest" where you can see one of them escape from the other two; if not 3 then four.
  - dnautics 5y ago
    I did once joke to a physicist that maybe dark matter is floating point errors, and for a second his face went white.

Keysh 5y ago

This is rather overstated (especially the "incredibly unlikely" bit). The best attempt I've seen to summarize this is in this paper https://arxiv.org/abs/1412.7719 https://arxiv.org/abs/1412.7719 which argues that there's roughly a 10% chance of getting something as extreme as the Bullet Cluster under Lambda-CDM. So, mildly unlikely, but not even approaching the (problematic) P < 0.05 threshold that's emblematic of the "replication crisis" in fields like experimental psychology.

XorNot 5y ago

Yes and I'm straight up not buying that argument because it being improbable that the bullet cluster might arise doesn't explain why it did: remove particle dark matter and now you're left with gravitational effects floating around in space totally divorced from the shape and location of regular matter which should be producing them.