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I think the simplest explanation for dark matter is that it consists of something we already know exists and behaves exactly like dark matter: black holes. To f

I think the simplest explanation for dark matter is that it consists of something we already know exists and behaves exactly like dark matter: black holes. To fit this explanation, we would need a vast number of black holes, each relatively small, approximately the size of a tennis ball. These black holes would be large enough to have persisted since the early universe, avoiding conversion into Hawking radiation. The remaining question is how such a multitude of primordial black holes could have formed in the early universe. The most likley answer to this question is density fluctuations. During the very early moments after the Big Bang, slight variations in density could have caused regions of space to collapse under their own gravity, forming black holes. If the density contrast was large enough, these regions would collapse directly into black holes.

matthewdgreen 2y ago
Wouldn’t these black holes radiate some Hawking radiation, making them potentially detectable? And how long would they exist for? Also what keeps these black holes from interacting with normal matter and clumping in the center of galaxies?
- rthnbgrredf 2y ago
  You ask very good questions. > Wouldn’t these black holes radiate some Hawking radiation, making them potentially detectable? Yes, according to widely accepted theory, they do. However, Hawking radiation is incredibly weak. With current technology, we are not able to detect such a faint signal. > And how long would they exist for? This depends on the size of the black hole. They will last much longer than our solar system, many billions of years, until they eventually evaporate. > Also, what keeps these black holes from interacting with normal matter and clumping in the center of galaxies? They do interact with normal matter via gravitation. If primordial black holes exist, many of them would fall into the supermassive black hole at the center of our galaxy and other galaxies. However, there are also those that orbit around the center. The problem is that our telescopes currently cannot detect a tennis ball-sized black object with the mass of a few Earths, it is an incredible tiny black dot in front of a black background. Hence, they are literally dark matter.
  - matthewdgreen 2y ago
    Even if the black hole doesn’t interact electromagnetically, wouldn’t they tend to accumulate accretion disks of normal matter that would? PS I have zero knowledge in this area, it’s a serious question.
    - rthnbgrredf 2y ago
      Yes, black holes often tend to accumulate accretion disks of normal matter, which interact electromagnetically. Large black holes can capture stars and tear them apart, making these processes observable. However, if a tiny black hole with very high velocity hits a star, it will simply pierce through it like a bullet, entering on one side and exiting on the other.
irjustin 2y ago
This was covered by PBS spacetime: https://www.youtube.com/watch?v=qy8MdewY_TY&t=67s https://www.youtube.com/watch?v=qy8MdewY_TY&t=67s Still possible, but increasingly unlikely due to the narrowing size range in which these black holes are allowed to exist in.
raattgift 2y ago
> behaves exactly like dark matter Dark matter is non-collisional, doesn't decay, and doesn't strongly scatter radiation. The last is about gravitational microlensing. You address the first two in another comment, but focused on falling into central black holes of galaxies and Hawking radiation. Black holes merge (cold dark matter doesn't clump, and certainly the absorption cross-section of CDM does not grow with clumping in the event there is some "dark chemistry" in the CDM sector; black holes absorb more radiation as their masses increase, including via merger). The MACHO (massive compact halo object) models which use small black holes have to keep them very sparse and on a restricted set of orbits in order to keep them from merging in galaxy and galaxy-cluster halos where they would be detectable. It's also hard to have galaxy-cluster distributions of small black holes: the density profiles get very different as tiny black holes interact with each other. Additionally, in distributions around the central-bright-galaxy of clusters, it's hard to suppress inverse compton scattering X-rays from dust scattering around (or forming accretion structures around) tiny black holes. So the signature radiation a tiny black hole MACHO model has to worry about is less Hawking than the behaviour of normal matter moving around black-hole-infused space in clusters like Abell 2764 <https://www.esa.int/ESA_Multimedia/Images/2024/05/Euclid_s_new_view_of_galaxy_cluster_Abell_2764 https://www.esa.int/ESA_Multimedia/Images/2024/05/Euclid_s_n...> or Abell 2744 <https://esahubble.org/images/heic1506c/ https://esahubble.org/images/heic1506c/> <https://en.wikipedia.org/wiki/Abell_2744 https://en.wikipedia.org/wiki/Abell_2744> for example. It's really hard for simulators to arrive at this kind of structure using mainly MACHOs, and there seem to be lots of these structures. Lots and lots. It's somewhat easier to do with ~solar mass and bigger black holes (they can be sparser, their orbits are thus less contrived to avoid collisions -- after all star-star collisions are rare unlike dust-dust collisions), although larger-mass BHs run into limits form gravitational microlensing. In particular, BHs of 100 or more solar masses would tend to disrupt wide stellar binaries, particularly in at the outer margins of galaxies (including our own, easy enough to look for out of the plane of the disc). And there are strong limits on BHs between about 10 and 2000 solar masses from ultra-faint dwarf galaxies, where domination at that mass of BH would expand the visible part of those galaxies (especially bright star clusters like the central one in the Eridanus II dwarf satellite <https://en.wikipedia.org/wiki/Eridanus_II https://en.wikipedia.org/wiki/Eridanus_II>) via dynamical heating. The MACHO idea was certainly plausible, and projects like OGLE and MOA developed microlensing-observation techniques which are now used for finding low-mass and wide-orbit exoplanets, and "orphan" Neptune-like planets ejected from their birthplace star systems. See for example https://kmtnet.kasi.re.kr/~ulens/ https://kmtnet.kasi.re.kr/~ulens/ However, there seems to be very little hope that black holes are a large fraction of whatever the detailed description of dark matter might be. That there are plausible mechanisms for forming primordial black holes at arbitrary masses does not mean they are actually there.

matthewdgreen 2y ago

Wouldn’t these black holes radiate some Hawking radiation, making them potentially detectable? And how long would they exist for? Also what keeps these black holes from interacting with normal matter and clumping in the center of galaxies?

rthnbgrredf 2y ago
You ask very good questions. > Wouldn’t these black holes radiate some Hawking radiation, making them potentially detectable? Yes, according to widely accepted theory, they do. However, Hawking radiation is incredibly weak. With current technology, we are not able to detect such a faint signal. > And how long would they exist for? This depends on the size of the black hole. They will last much longer than our solar system, many billions of years, until they eventually evaporate. > Also, what keeps these black holes from interacting with normal matter and clumping in the center of galaxies? They do interact with normal matter via gravitation. If primordial black holes exist, many of them would fall into the supermassive black hole at the center of our galaxy and other galaxies. However, there are also those that orbit around the center. The problem is that our telescopes currently cannot detect a tennis ball-sized black object with the mass of a few Earths, it is an incredible tiny black dot in front of a black background. Hence, they are literally dark matter.
- matthewdgreen 2y ago
  Even if the black hole doesn’t interact electromagnetically, wouldn’t they tend to accumulate accretion disks of normal matter that would? PS I have zero knowledge in this area, it’s a serious question.
  - rthnbgrredf 2y ago
    Yes, black holes often tend to accumulate accretion disks of normal matter, which interact electromagnetically. Large black holes can capture stars and tear them apart, making these processes observable. However, if a tiny black hole with very high velocity hits a star, it will simply pierce through it like a bullet, entering on one side and exiting on the other.

irjustin 2y ago

This was covered by PBS spacetime: https://www.youtube.com/watch?v=qy8MdewY_TY&t=67s https://www.youtube.com/watch?v=qy8MdewY_TY&t=67s Still possible, but increasingly unlikely due to the narrowing size range in which these black holes are allowed to exist in.

raattgift 2y ago

> behaves exactly like dark matter Dark matter is non-collisional, doesn't decay, and doesn't strongly scatter radiation. The last is about gravitational microlensing. You address the first two in another comment, but focused on falling into central black holes of galaxies and Hawking radiation. Black holes merge (cold dark matter doesn't clump, and certainly the absorption cross-section of CDM does not grow with clumping in the event there is some "dark chemistry" in the CDM sector; black holes absorb more radiation as their masses increase, including via merger). The MACHO (massive compact halo object) models which use small black holes have to keep them very sparse and on a restricted set of orbits in order to keep them from merging in galaxy and galaxy-cluster halos where they would be detectable. It's also hard to have galaxy-cluster distributions of small black holes: the density profiles get very different as tiny black holes interact with each other. Additionally, in distributions around the central-bright-galaxy of clusters, it's hard to suppress inverse compton scattering X-rays from dust scattering around (or forming accretion structures around) tiny black holes. So the signature radiation a tiny black hole MACHO model has to worry about is less Hawking than the behaviour of normal matter moving around black-hole-infused space in clusters like Abell 2764 <https://www.esa.int/ESA_Multimedia/Images/2024/05/Euclid_s_new_view_of_galaxy_cluster_Abell_2764 https://www.esa.int/ESA_Multimedia/Images/2024/05/Euclid_s_n...> or Abell 2744 <https://esahubble.org/images/heic1506c/ https://esahubble.org/images/heic1506c/> <https://en.wikipedia.org/wiki/Abell_2744 https://en.wikipedia.org/wiki/Abell_2744> for example. It's really hard for simulators to arrive at this kind of structure using mainly MACHOs, and there seem to be lots of these structures. Lots and lots. It's somewhat easier to do with ~solar mass and bigger black holes (they can be sparser, their orbits are thus less contrived to avoid collisions -- after all star-star collisions are rare unlike dust-dust collisions), although larger-mass BHs run into limits form gravitational microlensing. In particular, BHs of 100 or more solar masses would tend to disrupt wide stellar binaries, particularly in at the outer margins of galaxies (including our own, easy enough to look for out of the plane of the disc). And there are strong limits on BHs between about 10 and 2000 solar masses from ultra-faint dwarf galaxies, where domination at that mass of BH would expand the visible part of those galaxies (especially bright star clusters like the central one in the Eridanus II dwarf satellite <https://en.wikipedia.org/wiki/Eridanus_II https://en.wikipedia.org/wiki/Eridanus_II>) via dynamical heating. The MACHO idea was certainly plausible, and projects like OGLE and MOA developed microlensing-observation techniques which are now used for finding low-mass and wide-orbit exoplanets, and "orphan" Neptune-like planets ejected from their birthplace star systems. See for example https://kmtnet.kasi.re.kr/~ulens/ https://kmtnet.kasi.re.kr/~ulens/ However, there seems to be very little hope that black holes are a large fraction of whatever the detailed description of dark matter might be. That there are plausible mechanisms for forming primordial black holes at arbitrary masses does not mean they are actually there.