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I was reflecting on the large amount of energy released in the black hole merger GW150914. The black holes were about 36 and 29 solar masses, with the resulting black hole being 62 solar masses and 3 solar masses radiated as gravitational waves. This made me wonder, can the generation of gravitational waves violate conservation of baryon number? (It seems to be the case here, assuming the black hole energy is/was baryonic in origin, though I know that might not have anything to do with the gravitational waves, maybe it's the no-hair theorem saying black holes don't count baryons.) I understand that this is supposed to be an approximate conservation law (though I don't know its Noether corresponding symmetry) so this would not be crazy, but I wasn't aware of any baryon nonconservation otherwise.

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    $\begingroup$ +1 for a good question. When I consider two large bodies merging, intuition says the energy for the gravitational waves comes from the relative kinetic energies and gravitational energy of the bodies (in the same way quadrapole radiation from a set of charges doesn't violate baryon number. That is just intuition however, with nothing to back it up. $\endgroup$ – R. Rankin Jun 26 '17 at 4:28
  • $\begingroup$ @R.Rankin I wondered the same thing, especially since the result is supposed to be a Kerr black hole with significant angular momentum. But I don't really know! $\endgroup$ – Charles Jun 26 '17 at 6:55
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    $\begingroup$ I disagree that the duplicate answers the baryon number question. @johnrennie $\endgroup$ – anna v Jun 27 '17 at 4:16
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Baryon non-conservation is the basic unsolved problem in the standard cosmological model where particle physics is not providing an answer. Our universe is primarily made out of baryons, and the standard model of particle physics used after the inflation period at the quark gluon etc stage, before the formation of protons, predicts almost equal numbers of particles and antiparticles. It is an open research question. See the answer in this link to a similar question

In the standard model of particle physics there is measured a CP violation . But it is not enough to explain why our universe is made mostly out of baryons.

Now as far as black holes merging and energy appearing in gravitational waves, the problem of baryon number is not relevant. Gravitational waves consist of gravitons (in a quantized theory of gravity) which just carry spin and energy momentum and couple gravitationally, similar to photons. An incandescent lamp does not lose baryons when it gives off light which consists of photons. In an analogous way gravitational waves are emitted by the merger, and the merged black hole will have the sum of baryon number of the two incoming ones ( conditional on the solution of the conservation of baryonic number problem described above).

As the quantization of gravity is in the research stage, the question of conservation of laws at the singularities is still an open question. Certainly the final theory should contain the explanation of the baryon asymmetry observed in our current universe, but it is a different story than gravitational waves.

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    $\begingroup$ Can you provide a reference? I don't see any evidence supporting this claim "the merged black hole will have the sum of baryon number of the two incoming ones" in your answer. $\endgroup$ – Prof. Legolasov Jun 26 '17 at 7:09
  • $\begingroup$ @SolenodonParadoxus it is an extrapolation of the standard model which depends on baryon number conservation. The same as assuming a charged black hole will have the sum of the charge of the masses that fell in. Now when the problem of baryon number conservation is solved one will be able to look further into what happens to the baryon numbers carried by the infalling stuff, depending on the mechanism. $\endgroup$ – anna v Jun 26 '17 at 10:39
  • $\begingroup$ This is great, it also makes a concrete prediction: the energy of the gravitational wave is bounded above by the kinetic, etc. energy of the merging objects. Thanks! $\endgroup$ – Charles Jun 26 '17 at 20:18
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    $\begingroup$ Black holes do not have a baryon number hair equivalent to the charge. The information of baryon number is lost as particles go into a BH. There is no baryon number for a BH. All contingent on solving the information loss problem of BHs, which requires a quantum theory of gravity. $\endgroup$ – Bob Bee Jun 27 '17 at 0:40
  • $\begingroup$ @BobBee baryon number is a quantum number so of course all I say above depends on a quantized gravity. A concrete quantization model would have to deal with this. arxiv.org/abs/1605.00543 for example refutes the no hair for baryons $\endgroup$ – anna v Jun 27 '17 at 4:04

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