<p>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 <em>some</em> 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.</p>
<p>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.</p>
<p>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.</p>
<p><a href="https://astronomy.stackexchange.com/questions/11960/could-there-be-dark-matter-black-holes">Could there be dark matter black holes?</a> is essential reading.</p>

CC BY-SA 4.0
2020-09-27T00:03:05.617Z
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.
rob
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2020-09-27T06:55:15.617Z
@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.
ProfRob
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2020-09-27T11:00:29.147Z
Is there a simple explanation why the cross section for accreting dark matter should be much smaller than for ordinary matter?
TimRias
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2020-09-27T15:14:45.443Z
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.
rob
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2020-09-27T15:27:31.537Z
@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.
ProfRob
CC BY-SA 4.0
2020-09-27T15:30:50.093Z
@mmeent Because dark matter is non-dissipative and doesn't form accretion discs around black holes. There needs to be an almost direct hit for a dark matter particle to be trapped by a black hole.
ProfRob
CC BY-SA 4.0
2020-09-29T07:20:45.823Z
Thank you for the detailed answer. In the meantime I had a look at some review papers about cosmology (for example this one: https://www.pnas.org/content/112/40/12243).
Leos Ondra
CC BY-SA 4.0
2020-09-26T22:54:05.667Z
2020-09-27T15:39:44.420Z
2020-09-27T15:39:44.420Z
ProfRob
<p>Are black holes (like those detected by LIGO, for instance) part of baryonic content of universe or part of dark matter? Why?</p>

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2020-09-26T19:25:06.510Z
Related: https://astronomy.stackexchange.com/questions/11960/could-there-be-dark-matter-black-holes
jng224
CC BY-SA 4.0
2020-09-26T19:59:35.763Z
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,
Leos Ondra
<black-hole><cosmology><dark-matter>
Are black holes considered part of baryonic content of universe?
Leos Ondra
# Are black holes considered part of baryonic content of universe?

(Asked by: username_0 on 2020-09-26)

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

**Tags:** black-hole, cosmology, dark-matter

> **Comments:**
> **username_1**: Related: https://astronomy.stackexchange.com/questions/11960/could-there-be-dark-matter-black-holes  
> **username_0**: 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,  

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## 1 answers

### Answer by username_2 on 2020-09-26. Score: 10 | Accepted answer

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?](https://astronomy.stackexchange.com/questions/11960/could-there-be-dark-matter-black-holes) is essential reading.

> **Comments:**
> **username_3**: 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.  
> **username_2**: @username_3 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.  
> **username_4**: Is there a simple explanation why the cross section for accreting dark matter should be much smaller than for ordinary matter?  
> **username_3**: 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.  
> **username_2**: @username_3 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.  
> **username_2**: @mmeent Because dark matter is non-dissipative and doesn't form accretion discs around black holes. There needs to be an almost direct hit for a dark matter particle to be trapped by a black hole.  
> **username_0**: Thank you for the detailed answer. In the meantime I had a look at some review papers about cosmology (for example this one: https://www.pnas.org/content/112/40/12243).  

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