4 ms·

I believe the community consensus (based on surveys) is that there aren't enough microlensing events to explain the amount of dark matter we see, using MACHOs.

I believe the community consensus (based on surveys) is that there aren't enough microlensing events to explain the amount of dark matter we see, using MACHOs. You can read this even at: https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Massive_compact_halo_object I believe that I most recently heard it from Lina Necib's appearance on Sean Carroll's podcast ... that would be this episode: https://www.preposterousuniverse.com/podcast/2020/05/11/96-lina-necib-on-what-and-where-the-dark-matter-is/ https://www.preposterousuniverse.com/podcast/2020/05/11/96-l... Recommended, if you go for that sort of thing. [Edited to add: OK, turns out the consensus is unstable. There appears to be a window of BH mass range which has not been observationally excluded. See, e.g.https://en.wikipedia.org/wiki/Primordial_black_hole#Theoretical_history https://en.wikipedia.org/wiki/Primordial_black_hole#Theoreti...]

lumost 4y ago
I’d be skeptical of such a claim. We have very limited ability to detect dark black holes. The range of possible masses for said black holes is broad if primordial black holes exist.
- mturmon 4y ago
  OK, the transcript from Sean's show, from 2 years ago, is here: [https://www.preposterousuniverse.com/podcast/2020/05/11/96-lina-necib-on-what-and-where-the-dark-matter-is/ https://www.preposterousuniverse.com/podcast/2020/05/11/96-l...] And here's the relevant part. It's fair to say that Necib is not very bullish on BH's being the explanation for dark matter. (SC = Sean Carroll of Caltech, LN = Lina Necib, then at Caltech, now on faculty at MIT) SC: But it could be black holes, right? LN: Yes it could be primordial black holes. There has been a lot of work, especially recently, on trying to understand what is the possibility or what is the parameter space so if the theory of primordial black holes explaining dark matter is still possible, there is some disagreement within the community of how much of it is ruled out. The theory is not completely ruled out but quite a few chunks of it are. It depends on the measurements and there are some very small windows for example. I think 15 to 30 solar masses is still possible, something like that so it is not a completely ruled out theory. ... SC: [...] Black holes, you seem to be saying, if I can summarize it - if there is some mechanism for making black holes early on, they could be some of the dark matter but it’s hard to make them be all of it. Is that fair? LN: That’s right, yes. It’s much more difficult to make them pure, to make them all completely the dark matter. It could be a small fraction, that has not been ruled out, yes.
  - davidgrenier 4y ago
    A lot of these folks are working on the assumption that Hawking Radiation would have evaporated all small primordial black holes (the kind that could traverse the earth with little to no damage). However Hawking Radiation is a testable prediction of a theory that has yet to be tested. I dislike that physicists tend not to speak of Hawking Radiation in such terms since it is not a fact. In particular, I would argue that the Black Hole Information paradox isn't a paradox, it's an evidence against Hawking Radiation which in turn pushes back on quantum mechanics. Most of dark matter may very well be explained by small primordial black holes we're not (yet) well equipped to detect.
    - thereddaikon 4y ago
      An observation, it seems a lot of the big theories in physics are mostly math and thin on observational data. I understand the necessity. It's pretty hard to observe a black hole and run tests on one after all. It took us to the space age to start getting a lot of practical observational proof of relativity for example. Now its all confirmed relativity for now but for a lot of these other things we are sorely lacking. Hawking radiation is a good example of that as you pointed out. How do you even get observational data on hawking radiation without the starship Enterprise anyways?

lumost 4y ago

I’d be skeptical of such a claim. We have very limited ability to detect dark black holes. The range of possible masses for said black holes is broad if primordial black holes exist.

mturmon 4y ago
OK, the transcript from Sean's show, from 2 years ago, is here: [https://www.preposterousuniverse.com/podcast/2020/05/11/96-lina-necib-on-what-and-where-the-dark-matter-is/ https://www.preposterousuniverse.com/podcast/2020/05/11/96-l...] And here's the relevant part. It's fair to say that Necib is not very bullish on BH's being the explanation for dark matter. (SC = Sean Carroll of Caltech, LN = Lina Necib, then at Caltech, now on faculty at MIT) SC: But it could be black holes, right? LN: Yes it could be primordial black holes. There has been a lot of work, especially recently, on trying to understand what is the possibility or what is the parameter space so if the theory of primordial black holes explaining dark matter is still possible, there is some disagreement within the community of how much of it is ruled out. The theory is not completely ruled out but quite a few chunks of it are. It depends on the measurements and there are some very small windows for example. I think 15 to 30 solar masses is still possible, something like that so it is not a completely ruled out theory. ... SC: [...] Black holes, you seem to be saying, if I can summarize it - if there is some mechanism for making black holes early on, they could be some of the dark matter but it’s hard to make them be all of it. Is that fair? LN: That’s right, yes. It’s much more difficult to make them pure, to make them all completely the dark matter. It could be a small fraction, that has not been ruled out, yes.
- davidgrenier 4y ago
  A lot of these folks are working on the assumption that Hawking Radiation would have evaporated all small primordial black holes (the kind that could traverse the earth with little to no damage). However Hawking Radiation is a testable prediction of a theory that has yet to be tested. I dislike that physicists tend not to speak of Hawking Radiation in such terms since it is not a fact. In particular, I would argue that the Black Hole Information paradox isn't a paradox, it's an evidence against Hawking Radiation which in turn pushes back on quantum mechanics. Most of dark matter may very well be explained by small primordial black holes we're not (yet) well equipped to detect.
  - thereddaikon 4y ago
    An observation, it seems a lot of the big theories in physics are mostly math and thin on observational data. I understand the necessity. It's pretty hard to observe a black hole and run tests on one after all. It took us to the space age to start getting a lot of practical observational proof of relativity for example. Now its all confirmed relativity for now but for a lot of these other things we are sorely lacking. Hawking radiation is a good example of that as you pointed out. How do you even get observational data on hawking radiation without the starship Enterprise anyways?