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How do you explaining gravitational lensing with no visible mass, without dark matter? In other words situations where we see twin images of a very distant gal

How do you explaining gravitational lensing with no visible mass, without dark matter? In other words situations where we see twin images of a very distant galaxy because light curves around intervening mass, without sufficient mass observable to explain the deflection? I was under the impression that we could map dark matter localized to a precision that tended to rule out measurement error or a general divergence from Einstein's laws, unless it had a peculiar non-uniform distribution. Edit: maybe somebody can explain these maps - http://www.sciencemag.org/news/2017/03/scientists-unveil-most-detailed-map-dark-matter-date http://www.sciencemag.org/news/2017/03/scientists-unveil-mos...

dinglebert1234 9y ago
Because our calculations are made with poor assumptions.
deleted 9y ago
[deleted]
vivekd 9y ago
>How do you explaining gravitational lensing with no visible mass, without dark matter? Black holes? The physicist says: >What struck me was some regularity in the anomaly. The rotational velocities were not just larger than expected, they became constant with radius. Why? Sure, if there was dark matter, the speed of stars would be greater, but the rotation curves, meaning the rotational speed drawn as a function of the radius, could still go up and down depending on its distribution. But they didn’t. That seems pretty damning for any theory that relies on dark matter.
- _ph_ 9y ago
  >>How do you explaining gravitational lensing with no visible mass, without dark matter? >Black holes? Well, black holes are not really black, you can see their accretion disk. Also, a centralized huge mass would create a different velocity profile from what we are seeing. So black holes are pretty much out.
  - vivekd 9y ago
    I am not certain about the mechanics of it, but one of the most prominent theories right now is that dark matter is actually primordial black holes 1. 1. https://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter https://www.nasa.gov/feature/goddard/2016/nasa-scientist-sug...
    - jrvidal 9y ago
      Sorry, but I don't understand your argument. When asked 'how do you explain lensing without dark matter?', your answer is 'with this particular flavor of dark matter'?
      - vivekd 9y ago
        Well we're seeing a bunch of stuff happening out there and calling it dark matter. 1. The big problem is not enough stuff. Galaxies don't have enough stuff in them to let gravity hold them together. 2. A second problem is that we have stuff out there that our instruments can't detect but seems to be putting out gravity and bending the light. The orthodoxy of dark matter says 1 and 2 are connected. That dark matter is bending light and causing extra stuff in galaxies to allow them to form and keep them from flying apart. This physicist says no, just gravity doesn't work quite the way we think it does, because the stars in galaxy are moving at a constant speed relative to their distance from the galaxies center - but if there were a whole bunch of extra stuff in there that we can't see creating extra gravity all over the place, it should mess with the speed and cause it to be different depending on what parts had more dark matter in it. My parent was saying, how do you explain the invisible stuff putting out gravity and bending light, and suggested that it could not be explained by black holes. I was pointing out that one of the main explanations for it is black holes, but that still doesn't solve the first big problem of not enough stuff.
        jrvidal 9y ago
        I see now, makes sense.
- hwillis 9y ago
  Can't be black holes. For one thing they would start becoming visible through Hawking radiation if they were small enough to be homogenous like dark matter. Secondly baryon acoustic oscillations set a limit on how many black holes there can be. For your second part, the constant velocity is very well explained. The dark matter is just very broadly distributed. That's a natural consequence of not interacting with anything. It clumps up much less.
  - mark-r 9y ago
    How does this dark matter have enough gravitational effect to change the structure of the universe, yet not clump together? It doesn't make sense.
    - michaelcampbell 9y ago
      Unless it doesn't exhibit gravitational properties with itself? Just a thought; I have no training or experience here.
nightcracker 9y ago
In the scenario you're describing a black hole would qualify, however IIRC there are other aspects of dark matter that black holes can't explain.
- RockyMcNuts 9y ago
  If you assume a black hole, only certain masses and distances work, and they lens other objects. Picture I had was, if you had multiple objects affected, you could map the distribution of the gravitational anomaly, but it's over my head in terms of what the math and the assumptions are.
- antognini 9y ago
  Yes, black holes and other compact objects have pretty much been ruled out as candidates for dark matter. The issue is that they would produce gravitational microlensing signals which are not observed. In brief, when the black hole passes in front of a star, its gravitational field magnifies the light from the star in a characteristic way. By analyzing the shape of the light curve, you can figure out pretty much everything about the system like the masses and the distances. Systematic surveys for these microlensing events like OGLE have not observed black hole microlensing events at the rate that would be needed for black holes to constitute most of the dark matter out there. They do observe quite a few planetary microlensing events, though.
signa11 9y ago
> How do you explaining gravitational lensing with no visible mass, without dark matter? black hole would produce the same result right ?
- hwillis 9y ago
  No. Dark matter is spread relatively homogeneously so you would need very small back holes, and those would be visible or wouldn't have existed this long. Baryon acoustic oscillations also limit how much normal matter we can have, and they say there has to be a great deal of non-baryonic matter.
  - Sir_Cmpwn 9y ago
    I think you only grok half the story here. You're correct - black holes are not a popular candidate for dark matter, for reasons you described. But this article is not about candidates for dark matter, it's about removing dark matter from the equation entirely. If I understand correctly, black holes here are given as an explanation for isolated examples of strange gravitational lensing effects.
    - hwillis 9y ago
      There aren't isolated examples of strange gravitational lensing. The lensing being discussed occurs at the scales of galaxies and clusters of galaxies, so you need an incredible number of tiny black holes, brown stars, or other MACHOs.
      - Sir_Cmpwn 9y ago
        You're right, gravitational lensing anomolies explained by dark matter are more commonplace than I had thought.
maratd 9y ago
When we're confronted with something we can't explain using our existing equations and theories, the conclusion is that there must be some mysterious thing there that fixes our current equations and makes them work again? A thing we can't detect or see or have any other evidence of ... other than our equations and theories need that thing there for them to work? > In other words situations where we see twin images of a very distant galaxy It's entirely possible our equations do not function well on those scales.
- joeberon 9y ago
  Firstly, they function well enough to predict that gravitational waves exist, the ones which we measured being produced by an extreme gravitational event. Secondly, many other areas of physics have worked this out. The prime example of Quantum Field Theory and the Standard Model, which has been incredibly successful in predicting the existence of new types of particles from purely mathematical equations and theories (and trust me that stuff is pretty damn abstract, even compared to General Relativity) It is of course a possibility, although you make it sound somewhat obvious, as if physicists just haven't bothered to question that the widely-regarded theories might be wrong. I assure you this is not the case; there are no politics involved in this kind of research which is stopping people presenting questions such as "are we sure our current models are correct?" In cases like this, it is safe to assume that researchers have already explored any ideas you could come up with, especially the particularly simple ones...
  - maratd 9y ago
    > Firstly, they function well enough to predict that gravitational waves exist, the ones which we measured being produced by an extreme gravitational event. I don't think that aspect of the theories is in dispute? Newtonian physics well enough at certain scales, but breaks down at others. The question is if there's something better out there and if we adopt something better, would it remove the need for all this "dark matter". > Secondly, many other areas of physics have worked this out. The prime example of Quantum Field Theory and the Standard Model Wait, what? Quantum Field Theory suggests the existence of dark matter?

dinglebert1234 9y ago

Because our calculations are made with poor assumptions.

deleted 9y ago

[deleted]

vivekd 9y ago

>How do you explaining gravitational lensing with no visible mass, without dark matter? Black holes? The physicist says: >What struck me was some regularity in the anomaly. The rotational velocities were not just larger than expected, they became constant with radius. Why? Sure, if there was dark matter, the speed of stars would be greater, but the rotation curves, meaning the rotational speed drawn as a function of the radius, could still go up and down depending on its distribution. But they didn’t. That seems pretty damning for any theory that relies on dark matter.

_ph_ 9y ago
>>How do you explaining gravitational lensing with no visible mass, without dark matter? >Black holes? Well, black holes are not really black, you can see their accretion disk. Also, a centralized huge mass would create a different velocity profile from what we are seeing. So black holes are pretty much out.
- vivekd 9y ago
  I am not certain about the mechanics of it, but one of the most prominent theories right now is that dark matter is actually primordial black holes 1. 1. https://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter https://www.nasa.gov/feature/goddard/2016/nasa-scientist-sug...
  - jrvidal 9y ago
    Sorry, but I don't understand your argument. When asked 'how do you explain lensing without dark matter?', your answer is 'with this particular flavor of dark matter'?
    - vivekd 9y ago
      Well we're seeing a bunch of stuff happening out there and calling it dark matter. 1. The big problem is not enough stuff. Galaxies don't have enough stuff in them to let gravity hold them together. 2. A second problem is that we have stuff out there that our instruments can't detect but seems to be putting out gravity and bending the light. The orthodoxy of dark matter says 1 and 2 are connected. That dark matter is bending light and causing extra stuff in galaxies to allow them to form and keep them from flying apart. This physicist says no, just gravity doesn't work quite the way we think it does, because the stars in galaxy are moving at a constant speed relative to their distance from the galaxies center - but if there were a whole bunch of extra stuff in there that we can't see creating extra gravity all over the place, it should mess with the speed and cause it to be different depending on what parts had more dark matter in it. My parent was saying, how do you explain the invisible stuff putting out gravity and bending light, and suggested that it could not be explained by black holes. I was pointing out that one of the main explanations for it is black holes, but that still doesn't solve the first big problem of not enough stuff.
      - jrvidal 9y ago
        I see now, makes sense.
hwillis 9y ago
Can't be black holes. For one thing they would start becoming visible through Hawking radiation if they were small enough to be homogenous like dark matter. Secondly baryon acoustic oscillations set a limit on how many black holes there can be. For your second part, the constant velocity is very well explained. The dark matter is just very broadly distributed. That's a natural consequence of not interacting with anything. It clumps up much less.
- mark-r 9y ago
  How does this dark matter have enough gravitational effect to change the structure of the universe, yet not clump together? It doesn't make sense.
  - michaelcampbell 9y ago
    Unless it doesn't exhibit gravitational properties with itself? Just a thought; I have no training or experience here.

nightcracker 9y ago

In the scenario you're describing a black hole would qualify, however IIRC there are other aspects of dark matter that black holes can't explain.

RockyMcNuts 9y ago
If you assume a black hole, only certain masses and distances work, and they lens other objects. Picture I had was, if you had multiple objects affected, you could map the distribution of the gravitational anomaly, but it's over my head in terms of what the math and the assumptions are.
antognini 9y ago
Yes, black holes and other compact objects have pretty much been ruled out as candidates for dark matter. The issue is that they would produce gravitational microlensing signals which are not observed. In brief, when the black hole passes in front of a star, its gravitational field magnifies the light from the star in a characteristic way. By analyzing the shape of the light curve, you can figure out pretty much everything about the system like the masses and the distances. Systematic surveys for these microlensing events like OGLE have not observed black hole microlensing events at the rate that would be needed for black holes to constitute most of the dark matter out there. They do observe quite a few planetary microlensing events, though.

signa11 9y ago

> How do you explaining gravitational lensing with no visible mass, without dark matter? black hole would produce the same result right ?

hwillis 9y ago
No. Dark matter is spread relatively homogeneously so you would need very small back holes, and those would be visible or wouldn't have existed this long. Baryon acoustic oscillations also limit how much normal matter we can have, and they say there has to be a great deal of non-baryonic matter.
- Sir_Cmpwn 9y ago
  I think you only grok half the story here. You're correct - black holes are not a popular candidate for dark matter, for reasons you described. But this article is not about candidates for dark matter, it's about removing dark matter from the equation entirely. If I understand correctly, black holes here are given as an explanation for isolated examples of strange gravitational lensing effects.
  - hwillis 9y ago
    There aren't isolated examples of strange gravitational lensing. The lensing being discussed occurs at the scales of galaxies and clusters of galaxies, so you need an incredible number of tiny black holes, brown stars, or other MACHOs.
    - Sir_Cmpwn 9y ago
      You're right, gravitational lensing anomolies explained by dark matter are more commonplace than I had thought.

maratd 9y ago

When we're confronted with something we can't explain using our existing equations and theories, the conclusion is that there must be some mysterious thing there that fixes our current equations and makes them work again? A thing we can't detect or see or have any other evidence of ... other than our equations and theories need that thing there for them to work? > In other words situations where we see twin images of a very distant galaxy It's entirely possible our equations do not function well on those scales.

joeberon 9y ago
Firstly, they function well enough to predict that gravitational waves exist, the ones which we measured being produced by an extreme gravitational event. Secondly, many other areas of physics have worked this out. The prime example of Quantum Field Theory and the Standard Model, which has been incredibly successful in predicting the existence of new types of particles from purely mathematical equations and theories (and trust me that stuff is pretty damn abstract, even compared to General Relativity) It is of course a possibility, although you make it sound somewhat obvious, as if physicists just haven't bothered to question that the widely-regarded theories might be wrong. I assure you this is not the case; there are no politics involved in this kind of research which is stopping people presenting questions such as "are we sure our current models are correct?" In cases like this, it is safe to assume that researchers have already explored any ideas you could come up with, especially the particularly simple ones...
- maratd 9y ago
  > Firstly, they function well enough to predict that gravitational waves exist, the ones which we measured being produced by an extreme gravitational event. I don't think that aspect of the theories is in dispute? Newtonian physics well enough at certain scales, but breaks down at others. The question is if there's something better out there and if we adopt something better, would it remove the need for all this "dark matter". > Secondly, many other areas of physics have worked this out. The prime example of Quantum Field Theory and the Standard Model Wait, what? Quantum Field Theory suggests the existence of dark matter?