Are black holes (like those detected by LIGO, for instance) part of baryonic content of universe or part of dark matter? Why?

  • $\begingroup$ Related: astronomy.stackexchange.com/questions/11960/… $\endgroup$
    – jng224
    Sep 26, 2020 at 19:25
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    $\begingroup$ Thanks. An answer contains this: "However, once matter has accreted into a BH, the information about its origins (dark matter or baryonic) is lost." This is my opinion, And this is why I am not sure if the mass of BHS is included in 5% of baryonic content of universe of in dark matter bulk, $\endgroup$
    – Leos Ondra
    Sep 26, 2020 at 19:59

1 Answer 1


Black holes formed after the epoch of primordial nucleoynthesis would be part of the baryonic matter, since they would be mostly formed of baryons. It is possible or even probable that every black hole must contain some non-baryonic matter too, since there is nothing to prevent it being accreted, in small amounts, directly into black holes. However, the cross-section for that is very small because dark matter falling towards a black hole has no means to lose angular momentum. Unless there is almost a direct hit within the photon sphere then it won't be accreted. Normal matter can form an accretion disc and lose energy and angular momentum through viscosity, radiation and other processes.

Since the fraction of dark matter that is in black holes formed since the big bang is negligible, then the approximation that their mass consists only of baryons is hardly a big deal when it comes to doing the mass budget for the universe.

Primordial black holes on the other hand, that were already present at the epoch of cosmological nucleoynthesis, would not be counted as baryonic matter and it would be possible that they could contribute significantly to the required non-baryonic component, if they exist.

Could there be dark matter black holes? is essential reading.

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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
    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
    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
    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
    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
    Sep 27, 2020 at 15:27

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