5 ms·

Or, maybe dark matter and black holes are the same thing? https://www.sciencedaily.com/releases/2021/12/211220120813.htm https://www.sciencedaily.com/releases/

by awb 5y ago

Or, maybe dark matter and black holes are the same thing? https://www.sciencedaily.com/releases/2021/12/211220120813.htm https://www.sciencedaily.com/releases/2021/12/211220120813.h... Maybe these disperse galaxies have little to no black holes?

XorNot 5y ago
The big problem with that hypothesis is that you have to explain why we see so little matter falling into these blackholes and they don't seem to: dark matter you can basically pass right through and have your trajectory disrupted a little, whereas a black hole once you hit it becomes a zone of infinite attraction - i.e. a black hole passing near a star is pretty likely to setup in orbit around it because any matter that gets close enough inevitably gets sucked in. Dark matter - having finite mass and no singularity - doesn't have this problem.
Enginerrrd 5y ago
It doesn't matter, you are assuming a dark matter theory to begin with here. You are just assuming the dark matter is made up of black holes. This is an established idea of dark matter that has been given serious attention at times, though it's never been a leading candidate that I know of.
ncmncm 5y ago
Dark matter made out of black holes would be upsetting to cosmologists, because they need for the universe to be only 4% "hadronic" matter. If the black holes are made of hadrons, then there is too much of that. But maybe the stock of primordial black holes started as Dark Matter particles. They then have the problem of explaining why so much of the Dark Matter started out in or close to a black hole. Maybe each Dark Matter particle is so dense it is a black hole all by itself? You have to be careful talking about what a black hole is made of. Stuff that falls in only has a description before it falls in. After it falls in, it can't be thought of as made out of anything, anymore. So if black holes were part of the initial universe, there is no saying which flavor of matter they represent. After things have settled down, these black holes could proceed to swallow any amount of regular matter, and any amount of "other stuff", if there is any. The assumption is generally that the "other stuff" has difficulty cooling off enough to be sucked wholesale into black holes, unlike regular matter that is happy to radiate away a big fraction of its energy as photons, leaving the rest able to collapse and form planets and stars, and thence perhaps be gathered into the nearest black hole. Meanwhile, the Dark Matter can mill around a black hole's vicinity indefinitely, maybe swooping close and getting ejected, but otherwise hardly bothered unless it blunders straight in.
- enkid 5y ago
  Where does the 4% number come from?
  - xenophonf 5y ago
    This is how (https://news.ucr.edu/articles/2020/09/28/scientists-precisely-measure-total-amount-matter-universe https://news.ucr.edu/articles/2020/09/28/scientists-precisel...): [Dr. Mohamed Abdullah] explained that one well-proven technique for determining the total amount of matter in the universe is to compare the observed number and mass of galaxy clusters per unit volume with predictions from numerical simulations. Because present-day galaxy clusters have formed from matter that has collapsed over billions of years under its own gravity, the number of clusters observed at the present time is very sensitive to cosmological conditions and, in particular, the total amount of matter. That article links to paper, co-authored by Abdullah, Anatoly Klypin, and Gillian Wilson, which concludes that "matter makes up 31.5±1.3% of the total amount of matter and energy in the universe" and "80% of matter is actually dark matter". So 20% of 31.5±1.3% is 6.3±0.26%, which is how much normal matter is in the total amount of matter and energy in the universe. I realize that's different than the 4% number given by the other commenter. This is the latest estimate I could find. Maybe the estimated ratio of dark matter has changed with newer measurements, or maybe the other commenter was working from older estimates.
    - ncmncm 5y ago
      Yes, the number -- 4%, 6%, what have you -- has bounced all over in recent years. Probably 6% is more current than 4%. One of the problems with the standard model of the early universe is that whenever an observation -- e.g. mix of light-element isotopes, expansion rate, simulations -- produces a number different from its predictions, its prediction has been easily adjusted to match.
      - xenophonf 5y ago
        One of the problems with the standard model of the early universe is that whenever an observation -- e.g. mix of light-element isotopes, expansion rate, simulations -- produces a number different from its predictions, its prediction has been easily adjusted to match. That's how the scientific method works. You come up with a hypothesis, you test it, you adjust it accordingly, and you re-test it. If you can't come up with tests for it, or if your measurements never match the hypothesis, you throw the hypothesis out. That's why string theory and MOND are getting crapped on in this thread.
andrewflnr 5y ago
Black holes as dark matter has been mostly ruled out by lensing surveys in my understanding.
- xenophonf 5y ago
  That's right. Those are called massive compact halo objects (MACHOs). From "Dark Matter: A Primer" (https://arxiv.org/abs/1006.2483 https://arxiv.org/abs/1006.2483): To hunt for these objects two collaborations, the MACHO Collaboration and the EROS-2 Survey, searched for gravitational microlensing (the changing brightness of a distant object due to the interference of a nearby object) caused by possible MACHOs in the Milky Way halo. (Other collaborations have studied this as well, such as MOA, OGLE, and SuperMACHO.) The MACHO Collaboration painstakingly observed and statistically analyzed the skies for such lensing; 11.9 million stars were studied, with only 13-17 possible lensing events detected. In April of 2007, the EROS-2 Survey reported even fewer events, observing a sample of 7 million bright stars with only one lensing candidate found. This low number of possible MACHOs can only account for a very small percentage of the non-luminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is no, and the evidence behind this claim comes from Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB). That paragraph references "The SuperMACHO Microlensing Survey," "The MACHO project: microlensing results from 5.7 years of large magellanic cloud observations," and "Limits on the MACHO content of the galactic halo from the EROS-2 survey of the magellanic clouds," if you feel like reading a bunch of physics research papers.
  - 8note 5y ago
    Machos are a certain mass of black holes, aren't they? Or does it include small but dense ones too? I suppose we'd see those on the microwave background though if hawking radiation holds true for small black holes
    - 5y ago
nwallin 5y ago
This was one of the leading theories for what dark matter is for quite some time. The term is "MACHO" or Massive Compact Halo Object. MACHO doesn't include just black holes, but also a sea of objects small enough to be very difficult to observe but massive enough so that in aggregate they can cause what we observe as dark matter. However, the evidence against MACHOs is pretty strong. Once we had a theory of what to look for, experiments were designed to detect them. The thing we looked for is microlensing: Focus a telescope at some stars, and watch for a sudden, short jump in brightness. This can be caused by a massive object passing by in front of the objects, gravitationally lensing the light. We performed these studies and the number of microlensing events that we found were not sufficient to explain dark matter. So MACHOs as an explanation for dark matter isn't the answer. It's a good suggestion though, and like I said it was a leading theory for a while until we were able to test it. https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Massive_compact_halo_object

by awb 5y ago

XorNot 5y ago

The big problem with that hypothesis is that you have to explain why we see so little matter falling into these blackholes and they don't seem to: dark matter you can basically pass right through and have your trajectory disrupted a little, whereas a black hole once you hit it becomes a zone of infinite attraction - i.e. a black hole passing near a star is pretty likely to setup in orbit around it because any matter that gets close enough inevitably gets sucked in. Dark matter - having finite mass and no singularity - doesn't have this problem.

Enginerrrd 5y ago

It doesn't matter, you are assuming a dark matter theory to begin with here. You are just assuming the dark matter is made up of black holes. This is an established idea of dark matter that has been given serious attention at times, though it's never been a leading candidate that I know of.

ncmncm 5y ago

Dark matter made out of black holes would be upsetting to cosmologists, because they need for the universe to be only 4% "hadronic" matter. If the black holes are made of hadrons, then there is too much of that. But maybe the stock of primordial black holes started as Dark Matter particles. They then have the problem of explaining why so much of the Dark Matter started out in or close to a black hole. Maybe each Dark Matter particle is so dense it is a black hole all by itself? You have to be careful talking about what a black hole is made of. Stuff that falls in only has a description before it falls in. After it falls in, it can't be thought of as made out of anything, anymore. So if black holes were part of the initial universe, there is no saying which flavor of matter they represent. After things have settled down, these black holes could proceed to swallow any amount of regular matter, and any amount of "other stuff", if there is any. The assumption is generally that the "other stuff" has difficulty cooling off enough to be sucked wholesale into black holes, unlike regular matter that is happy to radiate away a big fraction of its energy as photons, leaving the rest able to collapse and form planets and stars, and thence perhaps be gathered into the nearest black hole. Meanwhile, the Dark Matter can mill around a black hole's vicinity indefinitely, maybe swooping close and getting ejected, but otherwise hardly bothered unless it blunders straight in.

enkid 5y ago
Where does the 4% number come from?
- xenophonf 5y ago
  This is how (https://news.ucr.edu/articles/2020/09/28/scientists-precisely-measure-total-amount-matter-universe https://news.ucr.edu/articles/2020/09/28/scientists-precisel...): [Dr. Mohamed Abdullah] explained that one well-proven technique for determining the total amount of matter in the universe is to compare the observed number and mass of galaxy clusters per unit volume with predictions from numerical simulations. Because present-day galaxy clusters have formed from matter that has collapsed over billions of years under its own gravity, the number of clusters observed at the present time is very sensitive to cosmological conditions and, in particular, the total amount of matter. That article links to paper, co-authored by Abdullah, Anatoly Klypin, and Gillian Wilson, which concludes that "matter makes up 31.5±1.3% of the total amount of matter and energy in the universe" and "80% of matter is actually dark matter". So 20% of 31.5±1.3% is 6.3±0.26%, which is how much normal matter is in the total amount of matter and energy in the universe. I realize that's different than the 4% number given by the other commenter. This is the latest estimate I could find. Maybe the estimated ratio of dark matter has changed with newer measurements, or maybe the other commenter was working from older estimates.
  - ncmncm 5y ago
    Yes, the number -- 4%, 6%, what have you -- has bounced all over in recent years. Probably 6% is more current than 4%. One of the problems with the standard model of the early universe is that whenever an observation -- e.g. mix of light-element isotopes, expansion rate, simulations -- produces a number different from its predictions, its prediction has been easily adjusted to match.
    - xenophonf 5y ago
      One of the problems with the standard model of the early universe is that whenever an observation -- e.g. mix of light-element isotopes, expansion rate, simulations -- produces a number different from its predictions, its prediction has been easily adjusted to match. That's how the scientific method works. You come up with a hypothesis, you test it, you adjust it accordingly, and you re-test it. If you can't come up with tests for it, or if your measurements never match the hypothesis, you throw the hypothesis out. That's why string theory and MOND are getting crapped on in this thread.

andrewflnr 5y ago

Black holes as dark matter has been mostly ruled out by lensing surveys in my understanding.

xenophonf 5y ago
That's right. Those are called massive compact halo objects (MACHOs). From "Dark Matter: A Primer" (https://arxiv.org/abs/1006.2483 https://arxiv.org/abs/1006.2483): To hunt for these objects two collaborations, the MACHO Collaboration and the EROS-2 Survey, searched for gravitational microlensing (the changing brightness of a distant object due to the interference of a nearby object) caused by possible MACHOs in the Milky Way halo. (Other collaborations have studied this as well, such as MOA, OGLE, and SuperMACHO.) The MACHO Collaboration painstakingly observed and statistically analyzed the skies for such lensing; 11.9 million stars were studied, with only 13-17 possible lensing events detected. In April of 2007, the EROS-2 Survey reported even fewer events, observing a sample of 7 million bright stars with only one lensing candidate found. This low number of possible MACHOs can only account for a very small percentage of the non-luminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is no, and the evidence behind this claim comes from Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB). That paragraph references "The SuperMACHO Microlensing Survey," "The MACHO project: microlensing results from 5.7 years of large magellanic cloud observations," and "Limits on the MACHO content of the galactic halo from the EROS-2 survey of the magellanic clouds," if you feel like reading a bunch of physics research papers.
- 8note 5y ago
  Machos are a certain mass of black holes, aren't they? Or does it include small but dense ones too? I suppose we'd see those on the microwave background though if hawking radiation holds true for small black holes
  - 5y ago

nwallin 5y ago

This was one of the leading theories for what dark matter is for quite some time. The term is "MACHO" or Massive Compact Halo Object. MACHO doesn't include just black holes, but also a sea of objects small enough to be very difficult to observe but massive enough so that in aggregate they can cause what we observe as dark matter. However, the evidence against MACHOs is pretty strong. Once we had a theory of what to look for, experiments were designed to detect them. The thing we looked for is microlensing: Focus a telescope at some stars, and watch for a sudden, short jump in brightness. This can be caused by a massive object passing by in front of the objects, gravitationally lensing the light. We performed these studies and the number of microlensing events that we found were not sufficient to explain dark matter. So MACHOs as an explanation for dark matter isn't the answer. It's a good suggestion though, and like I said it was a leading theory for a while until we were able to test it. https://en.wikipedia.org/wiki/Massive_compact_halo_object https://en.wikipedia.org/wiki/Massive_compact_halo_object