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Astronomy

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    $\begingroup$ This is an interesting perspective, since (as I understand it) the “no-hair theorem” says that baryon number is not necessarily conserved during black hole evolution. $\endgroup$
    – rob
    Commented Sep 27, 2020 at 0:03
  • $\begingroup$ @rob I guess I'not following what "catch" or problem you think there is. Black holes formed since the big bang are almost entirely made of baryons. Therefore they count as baryonic mass in cosmology. Black holes that were present prior to nucleoynthesis were unavailable to form nuclei and are counted as non-baryonic dark matter. $\endgroup$
    – ProfRob
    Commented Sep 27, 2020 at 6:55
  • $\begingroup$ Is there a simple explanation why the cross section for accreting dark matter should be much smaller than for ordinary matter? $\endgroup$
    – TimRias
    Commented Sep 27, 2020 at 11:00
  • $\begingroup$ I had imagined black hole formation as a way that matter is transferred from the baryonic sector to the non-baryonic sector, just like black hole evaporation is a transfer to the radiation sector. (A sentence which wants more caveats than I can fit in a comment.) I see why your distinction is useful for cosmology, but it surprised me. $\endgroup$
    – rob
    Commented Sep 27, 2020 at 15:14
  • $\begingroup$ @rob Your definition isn't useful. Black holes that are not primordial are counted as baryonic mass because they are made of baryons. Non-primordial black holes don't evaporate on any interesting timescale. $\endgroup$
    – ProfRob
    Commented Sep 27, 2020 at 15:27