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> For instance, we think black holes can consume matter, and then convert it to energy in the form of Hawking radiation as they slowly decay, without having to

> For instance, we think black holes can consume matter, and then convert it to energy in the form of Hawking radiation as they slowly decay, without having to bother with eating an equal quantity of antimatter. > we're just guessing Black holes (BHs) are not a very realistic candidate for solving baryon asymmetry. Where's the antimatter outside the horizon of a modern (as in after structure formation) astrophysical BH? If it's not there, it can't fall in. This is really hard to work around even for early direct-collapse super-massive BHs; hierarchical growth is already essentially ruled out. Worse, how do you keep signatures of annihilations out of the region near the BHs, including the accretion material and any jets? Or are you expecting primordial BHs to couple differently to baryons and their antis? How do you suppress that difference in the weak field limit, or more generally after first light? (And in either case, how do you make sure that virtually all of the antimatter is locked up in BHs?) Essentially you keep coming back to having the stress-energy already significantly (really, almost entirely) segregated into particles and their antis, around the time of gravitational collapse, or you depart dramatically from General Relativity in a regime in which it is already supported by evidence. Finally, where are you hiding all these black holes, whenever they formed? If only BHs break baryon symmetry, the contribution to \Omega implies a lot of lensing. (Speculating in the direction of a dust of tiny remnants or the like is also hard work, and usually involves beyond-the-standard-model new physics anyway, although there is a small literature that involves operators like \partial_{\mu}F(R)J^{\mu}, where J^{\mu} is the baryon or lepton current, and R is the curvature scalar or the Riemann tensor (R_{\mu\nu\rho\sigma}R^{\mu\nu\rho\sigma}) or a more complex term, and afaik none of these model-builders take backreaction into account yet.)

ajross 8y ago
> Black holes (BHs) are not a very realistic candidate for solving baryon asymmetry. I'm reasonably certain grandparent wasn't proposing this as a theory, just using it as a simple gedankenexperiment to show that gravity isn't inherently respectful of charge conservation.
- MrEldritch 8y ago
  Bingo. Well, not quite - black holes would be respective of charge conservation! A black hole only has three properties, in our current understanding of general relativity - but "electrical charge" is one of those properties. But there's nothing that stops it, say, eating protons and spitting out positrons later.
  - raattgift 8y ago
    > But there's nothing that stops it, say, eating protons and spitting out positrons later Oh, I see what you mean, but it's not clearly because of symmetries breaking inside the horizon, rather than high-energy pairs taking energy from the background. Semiclassically: collapse an isolated star (here we depart from Hawking's formalism), and observe nothing but photons (with wavelengths comparable to the curvature radius) forming an atmosphere densest around ~ 3m \lt r \lessapprox 4m until m is very small, at which point you'll observe pair production in the atmosphere densest around ~ 4m. \lessapprox 4m is the back reaction mess of the inner atmosphere on chaotic and mostly plunging trajectories, and the highly dynamical part of the spacetime. [cf. Unruh https://link.aps.org/doi/10.1103/PhysRevD.15.365 https://link.aps.org/doi/10.1103/PhysRevD.15.365 nb 3rd paragraph, "It must be remembered that talk about particles is a very crude and metaphorical way of talking about the physics near the horizon of the black hole", and Giddings https://arxiv.org/abs/1511.08221 https://arxiv.org/abs/1511.08221 ] In the Hawking formalism the background is static, and that forces the use of an infalling negative energy; that's not a real symmetry being broken gravitationally, and so I'm wary about the idea of breaking baryon (and lepton) symmetry with black holes. So really this is mostly a wordy objection to "spitting out".
  - raattgift 8y ago
    > A black hole only has three properties, in our current understanding of general relativity - but "electrical charge" is one of those properties An isolated black hole, at a suitable coordinate time, has mass, electric charge, angular momentum (three components), linear momentum (three components) and spatial position (three components). Holding the BH at the spatial origin drops the last six. Also, a slight wrinkle: this state is asymptotic -- at timescales less than light-crossing there can be substantial additional hair. At longer timescales, some configurations can persist on much longer than light-crossing scales -- one example is the magnetic field at the newly formed horizon of a isolated collapsed rotating magnetar. "isolated" here can get tricky in practice as well. But in the usual case, no, you can't look at a black hole and tell whether someone much earlier threw (classical picture) in one shell of matter of mass M vs two concentric shells of matter at 1/2 M each or three concentric shells of matter of 1/4 M, 1/2 M and 1/4 M (or 1/2 M, 1/4 M and 1/4 M, etc.). But what's this shells picture for non-negligible charge? (Switching to a dust doesn't help, fwiw). More formally, the no-hair theorem says that in a stationary electrovacuum, a black hole solution takes on a specific form. That mostly means that we should be able to perturb a Kerr-Newman BH solution and get the right results for an astrophysical BH.

ajross 8y ago

> Black holes (BHs) are not a very realistic candidate for solving baryon asymmetry. I'm reasonably certain grandparent wasn't proposing this as a theory, just using it as a simple gedankenexperiment to show that gravity isn't inherently respectful of charge conservation.

MrEldritch 8y ago
Bingo. Well, not quite - black holes would be respective of charge conservation! A black hole only has three properties, in our current understanding of general relativity - but "electrical charge" is one of those properties. But there's nothing that stops it, say, eating protons and spitting out positrons later.
- raattgift 8y ago
  > But there's nothing that stops it, say, eating protons and spitting out positrons later Oh, I see what you mean, but it's not clearly because of symmetries breaking inside the horizon, rather than high-energy pairs taking energy from the background. Semiclassically: collapse an isolated star (here we depart from Hawking's formalism), and observe nothing but photons (with wavelengths comparable to the curvature radius) forming an atmosphere densest around ~ 3m \lt r \lessapprox 4m until m is very small, at which point you'll observe pair production in the atmosphere densest around ~ 4m. \lessapprox 4m is the back reaction mess of the inner atmosphere on chaotic and mostly plunging trajectories, and the highly dynamical part of the spacetime. [cf. Unruh https://link.aps.org/doi/10.1103/PhysRevD.15.365 https://link.aps.org/doi/10.1103/PhysRevD.15.365 nb 3rd paragraph, "It must be remembered that talk about particles is a very crude and metaphorical way of talking about the physics near the horizon of the black hole", and Giddings https://arxiv.org/abs/1511.08221 https://arxiv.org/abs/1511.08221 ] In the Hawking formalism the background is static, and that forces the use of an infalling negative energy; that's not a real symmetry being broken gravitationally, and so I'm wary about the idea of breaking baryon (and lepton) symmetry with black holes. So really this is mostly a wordy objection to "spitting out".
- raattgift 8y ago
  > A black hole only has three properties, in our current understanding of general relativity - but "electrical charge" is one of those properties An isolated black hole, at a suitable coordinate time, has mass, electric charge, angular momentum (three components), linear momentum (three components) and spatial position (three components). Holding the BH at the spatial origin drops the last six. Also, a slight wrinkle: this state is asymptotic -- at timescales less than light-crossing there can be substantial additional hair. At longer timescales, some configurations can persist on much longer than light-crossing scales -- one example is the magnetic field at the newly formed horizon of a isolated collapsed rotating magnetar. "isolated" here can get tricky in practice as well. But in the usual case, no, you can't look at a black hole and tell whether someone much earlier threw (classical picture) in one shell of matter of mass M vs two concentric shells of matter at 1/2 M each or three concentric shells of matter of 1/4 M, 1/2 M and 1/4 M (or 1/2 M, 1/4 M and 1/4 M, etc.). But what's this shells picture for non-negligible charge? (Switching to a dust doesn't help, fwiw). More formally, the no-hair theorem says that in a stationary electrovacuum, a black hole solution takes on a specific form. That mostly means that we should be able to perturb a Kerr-Newman BH solution and get the right results for an astrophysical BH.