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Black holes seem to be much more abundant than previously thought. Earlier this year there was the discovery of the gravitational wave from the collision of two

Black holes seem to be much more abundant than previously thought. Earlier this year there was the discovery of the gravitational wave from the collision of two black holes with masses of about 30 times the sun, later this year another such collision was detected in the data. And all of this during an experiment that took only about a month. And now this discovery again seems to indicate that there must be many small sized black holes and that they have been very easily overlooked. Maybe it is too obvious, but I'm wondering: could dark matter be made up of all those undiscovered black holes?

T-A 10y ago
It has been suggested: http://www.extremetech.com/extreme/230715-why-gravitational-wave-detection-may-have-also-revealed-dark-matter http://www.extremetech.com/extreme/230715-why-gravitational-... arXiv preprints: https://arxiv.org/abs/1603.00464 https://arxiv.org/abs/1603.00464 https://arxiv.org/abs/1605.04023 https://arxiv.org/abs/1605.04023
jessriedel 10y ago
> Black holes seem to be much more abundant than previously thought. Not really. The event rate seen by LIGO is well within the accepted range of uncertainty prior to turning LIGO on. The uncertainty range only tightened modestly, because there's been so little data, and only went upward a bit. I'd guess it was a "surprise", given a reasonable choice of priors of the community, at something like the 1-sigma level. Which is about the chance of it raining on any given day in San Fransisco -- not that surprising. > Maybe it is too obvious, but I'm wondering: could dark matter be made up of all those undiscovered black holes? This has been considered long ago, and generally disfavored for a few reasons, most notably being the lack of observed microlensing. https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Massive_compact_halo_object The new data from LIGO is weak, and should not much change your mind about the likelihood of black hole being the dominant dark matter. (That is, the only people who should believe in BH DM are those who already did, of which there are a few.) This is an optimistic take: http://resonaances.blogspot.ca/2016/06/black-hole-dark-matter.html http://resonaances.blogspot.ca/2016/06/black-hole-dark-matte...
officialjunk 10y ago
one of the proposed physics theories on how the EM Drive works actually rids us of the need to even have dark matter/energy as a concept. of course it had yet to be proven/accepted, and may in fact be disproven at some point. http://arxiv.org/pdf/1604.03449v1.pdf http://arxiv.org/pdf/1604.03449v1.pdf
- mrfusion 10y ago
  The paper was too technical for me. How might it explain dark matter?
antognini 10y ago
As other commenters have noted, the idea that dark matter could be composed of undiscovered black holes has been proposed before. The idea is that there might be somewhat more generally "massive compact halo objects" (MACHOs), which would be at least as massive as planets, but would not emit light. A black hole would be one kind of MACHO, but neutron stars and free-floating planets would also be MACHOs as well. The issue is that these MACHOs would be detected through microlensing. If the conditions as right, as they pass in front of more distant stars, their gravity focuses the light from the background star, leading to an apparent spike in the star's brightness. By monitoring the sky for these microlensing events, astronomers have built up quite strong statistics about the presence of MACHOs in the Galaxy and have found that they are far too rare to comprise a significant component of dark matter.
- T-A 10y ago
  As pointed out in [1]: There is a window for PBHs to be DM if the BH mass is in the range 20M_sun<~ M <~ 100M_sun. Lower masses are excluded by microlensing surveys. Higher masses would disrupt wide binaries. [1] https://arxiv.org/abs/1603.00464 https://arxiv.org/abs/1603.00464
  - ISL 10y ago
    The surprising thing to me, as a practicing professional physicist, with astrophysics connections, is that the MACHO hunts stopped without closing that window. It's such a clean explanation, and so many resources have been devoted to the hunt for particle dark matter while MACHOs still had a chance. In the late 90's/early 2000's, the paradigm was "WIMPS vs MACHOs", and then the MACHOs dropped off the radar following the lensing studies. When the paper you've cited appeared, my reply to the friend that pointed it out was, "Oh, there's all those lensing studies that convincingly sunk the MACHOs more than a decade ago, can't be true." When I went back, read the lensing papers, and realized that there was still a window, I felt really let down by the astrophysics community at large. What existing physics only gravitates and is intrinsically invisible? A black hole. They may not be the dark matter, but we owe it to ourselves to be real sure that they're not.
arca_vorago 10y ago
"Could dark matter be made up of all those undiscovered black holes?" IANAP(I am not a physicist). I have a pet theory relating to this that goes something like this, and please forgive the simplification: normal matter is "seeking" to return to its original state in the other/dark universe membrane which can only be done through black holes, so dark matter wouldn't be made of black holes, but black holes are the portal through which matter becomes dark matter. In my theory Gravity is actually only a second teir side effect of normal matter seeking to return to dark matter to some degree. I would guess a way to test this would be to measure dark matter emittance near black holes. I suspect it will be highly concentrated in the area.
InclinedPlane 10y ago
Keep in mind that these were at distances of around a billion light-years. How many galaxies are there within a billion light-years of Earth? A lot. Millions in fact. Millions of galaxies all with hundreds of billions of stars in them, adding up to hundreds of quadrillions of stars in that volume.
Aelinsaar 10y ago
https://4.bp.blogspot.com/-jnY74NBc7ic/V2UcuCswL-I/AAAAAAAAB78/vWC7nOxV53ALFy5YiXQRYrYxP631PrJDgCK4B/s400/blackholedarkmatterconstraints.png https://4.bp.blogspot.com/-jnY74NBc7ic/V2UcuCswL-I/AAAAAAAAB...

T-A 10y ago

It has been suggested: http://www.extremetech.com/extreme/230715-why-gravitational-wave-detection-may-have-also-revealed-dark-matter http://www.extremetech.com/extreme/230715-why-gravitational-... arXiv preprints: https://arxiv.org/abs/1603.00464 https://arxiv.org/abs/1603.00464 https://arxiv.org/abs/1605.04023 https://arxiv.org/abs/1605.04023

jessriedel 10y ago

> Black holes seem to be much more abundant than previously thought. Not really. The event rate seen by LIGO is well within the accepted range of uncertainty prior to turning LIGO on. The uncertainty range only tightened modestly, because there's been so little data, and only went upward a bit. I'd guess it was a "surprise", given a reasonable choice of priors of the community, at something like the 1-sigma level. Which is about the chance of it raining on any given day in San Fransisco -- not that surprising. > Maybe it is too obvious, but I'm wondering: could dark matter be made up of all those undiscovered black holes? This has been considered long ago, and generally disfavored for a few reasons, most notably being the lack of observed microlensing. https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Massive_compact_halo_object The new data from LIGO is weak, and should not much change your mind about the likelihood of black hole being the dominant dark matter. (That is, the only people who should believe in BH DM are those who already did, of which there are a few.) This is an optimistic take: http://resonaances.blogspot.ca/2016/06/black-hole-dark-matter.html http://resonaances.blogspot.ca/2016/06/black-hole-dark-matte...

officialjunk 10y ago

one of the proposed physics theories on how the EM Drive works actually rids us of the need to even have dark matter/energy as a concept. of course it had yet to be proven/accepted, and may in fact be disproven at some point. http://arxiv.org/pdf/1604.03449v1.pdf http://arxiv.org/pdf/1604.03449v1.pdf

mrfusion 10y ago
The paper was too technical for me. How might it explain dark matter?

antognini 10y ago

As other commenters have noted, the idea that dark matter could be composed of undiscovered black holes has been proposed before. The idea is that there might be somewhat more generally "massive compact halo objects" (MACHOs), which would be at least as massive as planets, but would not emit light. A black hole would be one kind of MACHO, but neutron stars and free-floating planets would also be MACHOs as well. The issue is that these MACHOs would be detected through microlensing. If the conditions as right, as they pass in front of more distant stars, their gravity focuses the light from the background star, leading to an apparent spike in the star's brightness. By monitoring the sky for these microlensing events, astronomers have built up quite strong statistics about the presence of MACHOs in the Galaxy and have found that they are far too rare to comprise a significant component of dark matter.

T-A 10y ago
As pointed out in [1]: There is a window for PBHs to be DM if the BH mass is in the range 20M_sun<~ M <~ 100M_sun. Lower masses are excluded by microlensing surveys. Higher masses would disrupt wide binaries. [1] https://arxiv.org/abs/1603.00464 https://arxiv.org/abs/1603.00464
- ISL 10y ago
  The surprising thing to me, as a practicing professional physicist, with astrophysics connections, is that the MACHO hunts stopped without closing that window. It's such a clean explanation, and so many resources have been devoted to the hunt for particle dark matter while MACHOs still had a chance. In the late 90's/early 2000's, the paradigm was "WIMPS vs MACHOs", and then the MACHOs dropped off the radar following the lensing studies. When the paper you've cited appeared, my reply to the friend that pointed it out was, "Oh, there's all those lensing studies that convincingly sunk the MACHOs more than a decade ago, can't be true." When I went back, read the lensing papers, and realized that there was still a window, I felt really let down by the astrophysics community at large. What existing physics only gravitates and is intrinsically invisible? A black hole. They may not be the dark matter, but we owe it to ourselves to be real sure that they're not.

arca_vorago 10y ago

"Could dark matter be made up of all those undiscovered black holes?" IANAP(I am not a physicist). I have a pet theory relating to this that goes something like this, and please forgive the simplification: normal matter is "seeking" to return to its original state in the other/dark universe membrane which can only be done through black holes, so dark matter wouldn't be made of black holes, but black holes are the portal through which matter becomes dark matter. In my theory Gravity is actually only a second teir side effect of normal matter seeking to return to dark matter to some degree. I would guess a way to test this would be to measure dark matter emittance near black holes. I suspect it will be highly concentrated in the area.

InclinedPlane 10y ago

Keep in mind that these were at distances of around a billion light-years. How many galaxies are there within a billion light-years of Earth? A lot. Millions in fact. Millions of galaxies all with hundreds of billions of stars in them, adding up to hundreds of quadrillions of stars in that volume.

Aelinsaar 10y ago

https://4.bp.blogspot.com/-jnY74NBc7ic/V2UcuCswL-I/AAAAAAAAB78/vWC7nOxV53ALFy5YiXQRYrYxP631PrJDgCK4B/s400/blackholedarkmatterconstraints.png https://4.bp.blogspot.com/-jnY74NBc7ic/V2UcuCswL-I/AAAAAAAAB...