8 ms·

Is NGC 1277 a problem for MOND?

31 points by xqcgrek2 3y ago

MattPalmer1086 3y ago
I just got into Triton station, and there's loads of interesting posts on it about MOND and dark matter. This post is really just saying errr, no there is no problem: "That sounds normal for this type of galaxy, but I guess if you disliked MOND without understanding it, you might misinterpret that title to mean there was no mass discrepancy at all, hence a problem for MOND. I guess. I’m an expert on the subject; I don’t know where non-experts get their delusions." Still fun to read though.
loxias 3y ago
As an unashamed fanboy of MOND since the early 2000s, I'm very pleased to see any sort of active discourse on the subject. Over the last 20 years it appears to have moved slightly from "complete tinfoil hat" to "likely wrong longshot, but hard to trivially dismiss". I'm not an astrophysicist, I formed the above observation by asking astrophysicist friends what they think. :)
QuadmasterXLII 3y ago
Wake me up when generally covariant mond drops
- naasking 3y ago
  Wake me up when a dark matter theory actually reproduces MONDian dynamics.
  - i_no_can_eat 3y ago
    Can you explain the precession of Mercury with MOND?
    - nathan_compton 3y ago
      It is a different limit of the gravitational theory, so I would guess MOND has nothing to say about the precession of Mercury.
      - i_no_can_eat 3y ago
        Exactly. And the well-posed gravitational theory that has MOND as a limit has not yet been found. So why is MOND considered as anything but an effective model?
        throwawaymaths 3y ago
        Why is fluid dynamics considered to be an effective model when we haven't even proven the soundness of the navier-stokes equations? For that matter, how is QM considered to be an effective model when we can't even reconcile it with gravity?
        i_no_can_eat 3y ago
        Fluid dynamics is not a fundamental theory, but an effective model, as you said. The point here is that MOND can't explain something that GR has been able to explain for 100 years (precession of Mercury). So any serious gravitational theory needs to at least that for serious consideration.
        naasking 3y ago
        If you're claiming that dark matter is superior because it is a fundamental theory, I fail to see why a fundamental theory that's empirically wrong is supposed to be better than an effective model. Rather than propping up dark matter, astrophysicists should be focusing on novel ideas that reproduce MOND, like superfluid dark matter, or at GR modifications in the MOND regime.
        i_no_can_eat 3y ago
        I never mentioned dark matter. My only point is that any modern gravitational theory needs to do at least as well as general relativity. That means (among others): explaining the precession or Mercury; describe black holes; predict gravitational waves. So if MOND is to be taken seriously, it needs to do all of that. That's all, really.
    - MattPalmer1086 3y ago
      It's a good point; MOND isn't relativistic. It is just an effective model of galactic rotation. A question for me is how dark matter theories can reproduce mondian dynamics naturally without each case requiring special tuning. Or possibly there is a deeper theory of gravity that explains both mercury's precession and galactic rotation.
      - raattgift 3y ago
        You asked a good question. Forgive me if my answer is aimed a little bit at the level of some of the other commenters in this thread (and discussion topic). I can do an ELI if someone wants. General relativity simply equates the local density of the stress-energy tensor with the Einstein tensor (and possible scalar multipliers like the cosmological constant) at every point in the whole spacetime. If we measure curvature with e.g. Einstein lensing, we know what the stress-energy tensor must be. The stress-energy tensor encodes all moving matter, including internal non-gravitational degrees of freedom (DOFs). The ~seventeen fields of the Standard Model of particle physics fill all spacetime too, and contribute to the stress-energy tensor. Notably, invisible (in the sense that they do not couple to electromagnetism) DOFs exist in those fields and how they couple to each other; these standard-model mechanisms happily generate non-negligible stress-energy but nothing that we can see at a fine-grained level in any of our telescopes. Gluons' complicated self-interactions are the paradigmatic example: their whizzing about is most of the proton invariant mass, and so most of the mass of visible matter, and so most of the stress-energy in stars (gluon-gluon interactions is especially important in neutron stars, and neutron stars are an important diagnostic of any theory of gravitation) and molecular gas and dust; but gluons themselves are invisible and massless. Dark matter, in a nutshell, says that "empty space" (ignoring the thin, cold relic fields of the cosmological microwave background and its neutrino equivalent) has some unknown internal degrees of freedom whose action generates stress-energy. Very broadly we call the generator(s) of that stress-energy dark matter. The distribution of luminous matter (and different species thereof, and how it interacts (e.g. pressure, like ram pressure, can be important in galactic dynamics)) within a galaxy or cluster varies from galaxy to galaxy. Why shouldn't the distribution of dark matter? So, > possibly there is a deeper theory of gravity that explains both mercury's precession and galactic rotation it's just General Relativity. The difference is that we know the distribution of stress energy within our solar system much better than we know the distribution of stress energy in much more distant, or much more complicated, systems (like galaxies or clusters of thousands of galaxies). Alternatives to General Relativity broadly can take the approach that "empty space" (again ignoring the cosmic microwave background) is just that: there are no hidden non-gravitational degrees of freedom to discover there. Instead, stress-energy of the Standard Model as it is understood today generates the measured curvature seen in lensing by galaxy clusters like "El Gordo" (ACT-CL J0102-4915). Typically this is done by a redefinition of curvature to include auxiliary gravitational fields beyond the Einstein tensor, thus we get families of theories like tensor-scalar gravity, tensor-vector-scalar gravity, and so on. These fields couple with each other, so that the Newton-Einstein-like coupling in the bright parts of galaxies persists but the dynamics of that coupling generates auxiliary curvature in empty space outside the bright parts of the same galaxies. One often hears this described as introducing a new force or a "new fifth-force" produced by matter comparable to Newtonian's force of gravity; the new "force" has a different fall-off at a distance from the source matter than the 1/r^2 of Newtonian gravity or electromagnetism, and that fall-off is designed to produce Milgrom's low-acceleration relation. However (and in the spirit of a thread that started off with the words "generally covariant", MOND-compatible relativistic gravitation is still just gravitational fields being sourced by matter, but with more gravitational fields and interactions and self-interactions among and within those gravitational fields (with the weakness of gravitation proposed as cutting off these extra interactions' inducement of novel behaviours of bound systems like protons). Relativistic MOND approaches tend to be quite complicated in the gravitational sector, and of course must be no less complicated than general relativity (but for "dark matter") in the stress-energy sector. In short, the GR approach is that the apparently empty space in front of a clearly distorted smaller-angular-diameter/higher-redshift/lower-brightness galaxy (like "La Flaca" <https://webbtelescope.org/contents/media/images/2023/119/01GZY3TQ8EXHNMPPW6MKCDACMW?news=true https://webbtelescope.org/contents/media/images/2023/119/01G...>) is not really empty -- there is stress-energy there that does not interact electromagnetically (it also sticks around unlike known standard-model particles that do not interact electromagnetically: photons, relativistic neutrinos -- those fly off to infinity rather than hang around in a halo structure). Studying these systems means understanding how the non-electromagnetic stress-energy is generated, and there are many hypotheses. Conversely, the MONDian approach is that the apparently empty space around galaxies that flatten their rotation curve (and tbf there are MONDian approaches that capture some aspects of lensed objects) is really empty. Instead, the way that the standard model of particle physics generates curvature is different from General Relativity. Studying how gravity works for a particular (and more restrictive) distribution of matter is what relativistic MOND people set themselves up to do, and again there are also many hypotheses. In both these broad approaches, the distribution of visible matter varies at different scales and in different systems at the same scale. Local matter distribution drives local gravitational phenomena. MONDian approaches roughly hold that there is no wiggle room -- gravitation must be generated by what we can see, and not by anything else. General Relativity is more lax -- there is no reason to expect that we can see every generator of stress-energy, or that the distribution of the generators must be uniform in space or in spacetime, and gravitational theory should not turn its back on the possibility of discovering new beyond-the-standard-model spacetime-filling matter fields. One can also go really crazy and say that matter is not the (only or primary) generator of curvature, and that geometric curvature and matter simply coincide mostly by chance. A distribution of curvature that evolved in the early universe -- a cosmic gravitational background -- is taken to be relevant to the dynamics of galaxies and clusters about as old as ours. In these theories there are also auxiliary gravitational fields, and those typically allow for a MOND but are also designed to allow for the acceleration of the expansion of the universe and other features that relativistic MOND essentially imports from general relativity (in the form of a scalar multiplier on one or more of the curvature field(s)). So, in summary I'll return to your first question: > how [can] dark matter theories reproduce mondian dynamics naturally without each case requiring special tuning They can't. There is an initial (or at least early) values surface that the laws of the theory turn into galaxies and clusters of all sorts. The initial values set up some distribution of interacting and self-interacting matter (that is initially, or evolves into, the standard model and dark matter). It also may set up primordial gravitational radiation not produced by the matter, and things like an evolving cosmological (non-)constant. Relativistic MOND is not different on this front -- there must be some initial or early values surface that the laws of the theory turn into the standard model and the various gravitational fields around MONDian galaxies, mondian wide binaries, MONDian relativistic stars (like white dwarfs and neutron stars, especially in wide binaries of those), and so forth. These approaches may be even more sensitive to the distribution of things like primordial gravitational radiation because in general gravitational radiation is considerably more complicated by the presence of fields beyond the Einstein tensor: any additional scalar, tensor, or vector field may have their own polarizations, and their polarizations may modify the polarization of the mathematical structure closest to that of the metric tensor or the Weyl tensor parts of the Einstein tensor. Thus the considerable interest in the small variations in the cosmic microwave background by people who spend any time comparing General Relativity and relativistic MONDian theories. Ultimately the "special tuning" is the "past hypothesis", which is highly similar for both approaches: the distant past of the universe was hotter, denser, and lower entropy than the universe full of galaxies. The lower entropy is the puzzle. What ordered that? Again, there are many and various thoughts, and a handful of plausible hypotheses.
- i_no_can_eat 3y ago
  Precisely. They way I see it, MOND is a useful effective theory. But it is beyond me how people still think of it as a fundamental theory when the closest that anyone ever came up of a general-relativistic version of it is a total mess. As it is, MOND fails solar system tests. So at best, it's a useful effective model for some galactic-scale phenomenology, which would need some fundamental explanation.
aeneasmackenzie 3y ago
The year is 3000AD. A 1 million cubic kilometer xenon detector cooled to 2K orbiting Neptune has not detected any WIMPs. Not to worry, the next one will be ten times bigger and at 0.5K.
mmastrac 3y ago
Headline translation: Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's law of universal gravitation to account for observed properties of galaxies. It is an alternative to the hypothesis of dark matter in terms of explaining why galaxies do not appear to obey the currently understood laws of physics. NGC1277: The galaxy is unique in that it is considered a relic of what galaxies were like in the early universe. The galaxy is composed exclusively of aging stars that were born 10 billion years ago. But unlike other galaxies in the local universe, it has not undergone any further star formation. Astronomers nickname such galaxies as "red and dead," because the stars are aging and there aren't any successive generations of younger stars.
dist-epoch 3y ago
On a recent Sean Carroll podcast he said the galaxy/dark matter problem is the easy one to solve. There is another one, much harder - acoustic oscillation immediately after the Big Bang, visible in the microwave background, which MOND can't account for. Basically normal matter goes both ways, back-and-forth, compresses and expands, but dark matter goes only one-way, only expands. And this can be seen in the microwave background.
- throwawaymaths 3y ago
  Explaining the baryon acoustic oscillation with LCDM presumes that the phenomenon must have the same causal explanation as galaxy rotation curves. That's not necessarily the case. It should be considered "a nice feature" that LCDM explains this but when you have a mass distribution that can be arbitrarily assigned in space, it feels like there are a lot of "nice things you could explain". For example, they tried to explain the precession of mercury with a hunk of dm orbiting the sun called "Vulcan" (we now know you don't need dark matter to explain this). I believe the Vulcan steps in SF are named after this hypothetical dark matter planet. It's in the same neighborhood as mars, Venus, and Saturn and built around the same time
  - raattgift 3y ago
    > when you have a mass distribution that can be arbitrarily assigned in space, it feels like there are a lot of "nice things you could explain" Like an eight-planet star system, or a star system with a hot Jupiter, or a multi-star system with no planets, all arising within the same giant molecular cloud (GMC)? Even though the cloud-elements behave sanely, even small arbitrary changes in initial values can produce dramatically different later arrangements. Replace the galactic GMC with a dark matter filament in the cosmic web, and why be surprised that all sorts of galaxies form within? I don't think there is reason to fear arbitrary mass distributions so long as they obey constraints (if you're interested in the maths, the Einstein constraint equations are a good starting point you'll find in textbooks -- Robert Wald's chapter 10 is good and mathy, Alcubierre's or Baumgarte and Shapiro's numerical relativity textbooks are good alternatives for people who think computationally -- and lecture materials). We know empirical constraints and lots of solutions at many scales in the known universe already, and that constraint equations can be found for many other classical field theories; Dirac and successors (Regge et al., Rothe & Rothe et al.) are good resources wrt quantum field theories. Roughly the slogan is to lay down some arbitrary but plausibly generic configuration and see how it evolves. Do it often enough and you should get familiar structures, or you're back to the drawing board. Large cosmological dark matter simulations that do exactly this are new hat. Standard cosmology solutions often get compared in the same project with alternative theories, sometimes following the spirit of the Standard Model Extension in parametrizing everything possible and plugging in different coefficient values, sometimes simply trying to trace cosmological implementations of an IV formalism on some published alternative to general relativity. For example, Vogelsberger et al., <https://www.nature.com/articles/s42254-019-0127-2 https://www.nature.com/articles/s42254-019-0127-2>, <https://arxiv.org/abs/1909.07976v6 https://arxiv.org/abs/1909.07976v6> in which §5.3 has a brief literature review.
throwawaymaths 3y ago
TL/DR: compact galaxies rotate fast enough that their centripetal acceleration limit for MOND. Therefore it should look Newtonian, therefore You expect to "not see dark matter" in them. FTA: Eyeballing/back of the envelope the NGC 1277 data suggests it falls into that category (me: probably a more careful calculation is warranted). Usually galaxies that fit this are dense elliptical; 1277 is ultramassive lenticular, though to be fair lenticular ~elliptical.
- MattPalmer1086 3y ago
  I don't believe that's entirely accurate (I am not a cosmologist). It says that because of the acceleration, MOND predicts Newtonian motion. It does not say that you expect not to see dark matter, just that dark matter is not needed to explain that motion. However, this is a problem for dark matter theories precisely because we expect galaxies to form in concentrations of dark matter: "The infall of baryons acts to further concentrate the central dark matter. So the nominal expectation is that there should be plenty of dark matter near the centers of galaxies rather than none at all. That’s not what we see here, so nominally NGC 1277 presents more of a challenge for the dark matter paradigm than it does for MOND"
  - throwawaymaths 3y ago
    We're saying the same thing.

31 points by xqcgrek2 3y ago

I just got into Triton station, and there's loads of interesting posts on it about MOND and dark matter. This post is really just saying errr, no there is no problem: "That sounds normal for this type of galaxy, but I guess if you disliked MOND without understanding it, you might misinterpret that title to mean there was no mass discrepancy at all, hence a problem for MOND. I guess. I’m an expert on the subject; I don’t know where non-experts get their delusions." Still fun to read though.

loxias 3y ago

As an unashamed fanboy of MOND since the early 2000s, I'm very pleased to see any sort of active discourse on the subject. Over the last 20 years it appears to have moved slightly from "complete tinfoil hat" to "likely wrong longshot, but hard to trivially dismiss". I'm not an astrophysicist, I formed the above observation by asking astrophysicist friends what they think. :)

QuadmasterXLII 3y ago

Wake me up when generally covariant mond drops

naasking 3y ago
Wake me up when a dark matter theory actually reproduces MONDian dynamics.
- i_no_can_eat 3y ago
  Can you explain the precession of Mercury with MOND?
  - nathan_compton 3y ago
    It is a different limit of the gravitational theory, so I would guess MOND has nothing to say about the precession of Mercury.
    - i_no_can_eat 3y ago
      Exactly. And the well-posed gravitational theory that has MOND as a limit has not yet been found. So why is MOND considered as anything but an effective model?
      - throwawaymaths 3y ago
        Why is fluid dynamics considered to be an effective model when we haven't even proven the soundness of the navier-stokes equations? For that matter, how is QM considered to be an effective model when we can't even reconcile it with gravity?
        i_no_can_eat 3y ago
        Fluid dynamics is not a fundamental theory, but an effective model, as you said. The point here is that MOND can't explain something that GR has been able to explain for 100 years (precession of Mercury). So any serious gravitational theory needs to at least that for serious consideration.
        naasking 3y ago
        If you're claiming that dark matter is superior because it is a fundamental theory, I fail to see why a fundamental theory that's empirically wrong is supposed to be better than an effective model. Rather than propping up dark matter, astrophysicists should be focusing on novel ideas that reproduce MOND, like superfluid dark matter, or at GR modifications in the MOND regime.
        i_no_can_eat 3y ago
        I never mentioned dark matter. My only point is that any modern gravitational theory needs to do at least as well as general relativity. That means (among others): explaining the precession or Mercury; describe black holes; predict gravitational waves. So if MOND is to be taken seriously, it needs to do all of that. That's all, really.
  - MattPalmer1086 3y ago
    It's a good point; MOND isn't relativistic. It is just an effective model of galactic rotation. A question for me is how dark matter theories can reproduce mondian dynamics naturally without each case requiring special tuning. Or possibly there is a deeper theory of gravity that explains both mercury's precession and galactic rotation.
    - raattgift 3y ago
      You asked a good question. Forgive me if my answer is aimed a little bit at the level of some of the other commenters in this thread (and discussion topic). I can do an ELI if someone wants. General relativity simply equates the local density of the stress-energy tensor with the Einstein tensor (and possible scalar multipliers like the cosmological constant) at every point in the whole spacetime. If we measure curvature with e.g. Einstein lensing, we know what the stress-energy tensor must be. The stress-energy tensor encodes all moving matter, including internal non-gravitational degrees of freedom (DOFs). The ~seventeen fields of the Standard Model of particle physics fill all spacetime too, and contribute to the stress-energy tensor. Notably, invisible (in the sense that they do not couple to electromagnetism) DOFs exist in those fields and how they couple to each other; these standard-model mechanisms happily generate non-negligible stress-energy but nothing that we can see at a fine-grained level in any of our telescopes. Gluons' complicated self-interactions are the paradigmatic example: their whizzing about is most of the proton invariant mass, and so most of the mass of visible matter, and so most of the stress-energy in stars (gluon-gluon interactions is especially important in neutron stars, and neutron stars are an important diagnostic of any theory of gravitation) and molecular gas and dust; but gluons themselves are invisible and massless. Dark matter, in a nutshell, says that "empty space" (ignoring the thin, cold relic fields of the cosmological microwave background and its neutrino equivalent) has some unknown internal degrees of freedom whose action generates stress-energy. Very broadly we call the generator(s) of that stress-energy dark matter. The distribution of luminous matter (and different species thereof, and how it interacts (e.g. pressure, like ram pressure, can be important in galactic dynamics)) within a galaxy or cluster varies from galaxy to galaxy. Why shouldn't the distribution of dark matter? So, > possibly there is a deeper theory of gravity that explains both mercury's precession and galactic rotation it's just General Relativity. The difference is that we know the distribution of stress energy within our solar system much better than we know the distribution of stress energy in much more distant, or much more complicated, systems (like galaxies or clusters of thousands of galaxies). Alternatives to General Relativity broadly can take the approach that "empty space" (again ignoring the cosmic microwave background) is just that: there are no hidden non-gravitational degrees of freedom to discover there. Instead, stress-energy of the Standard Model as it is understood today generates the measured curvature seen in lensing by galaxy clusters like "El Gordo" (ACT-CL J0102-4915). Typically this is done by a redefinition of curvature to include auxiliary gravitational fields beyond the Einstein tensor, thus we get families of theories like tensor-scalar gravity, tensor-vector-scalar gravity, and so on. These fields couple with each other, so that the Newton-Einstein-like coupling in the bright parts of galaxies persists but the dynamics of that coupling generates auxiliary curvature in empty space outside the bright parts of the same galaxies. One often hears this described as introducing a new force or a "new fifth-force" produced by matter comparable to Newtonian's force of gravity; the new "force" has a different fall-off at a distance from the source matter than the 1/r^2 of Newtonian gravity or electromagnetism, and that fall-off is designed to produce Milgrom's low-acceleration relation. However (and in the spirit of a thread that started off with the words "generally covariant", MOND-compatible relativistic gravitation is still just gravitational fields being sourced by matter, but with more gravitational fields and interactions and self-interactions among and within those gravitational fields (with the weakness of gravitation proposed as cutting off these extra interactions' inducement of novel behaviours of bound systems like protons). Relativistic MOND approaches tend to be quite complicated in the gravitational sector, and of course must be no less complicated than general relativity (but for "dark matter") in the stress-energy sector. In short, the GR approach is that the apparently empty space in front of a clearly distorted smaller-angular-diameter/higher-redshift/lower-brightness galaxy (like "La Flaca" <https://webbtelescope.org/contents/media/images/2023/119/01GZY3TQ8EXHNMPPW6MKCDACMW?news=true https://webbtelescope.org/contents/media/images/2023/119/01G...>) is not really empty -- there is stress-energy there that does not interact electromagnetically (it also sticks around unlike known standard-model particles that do not interact electromagnetically: photons, relativistic neutrinos -- those fly off to infinity rather than hang around in a halo structure). Studying these systems means understanding how the non-electromagnetic stress-energy is generated, and there are many hypotheses. Conversely, the MONDian approach is that the apparently empty space around galaxies that flatten their rotation curve (and tbf there are MONDian approaches that capture some aspects of lensed objects) is really empty. Instead, the way that the standard model of particle physics generates curvature is different from General Relativity. Studying how gravity works for a particular (and more restrictive) distribution of matter is what relativistic MOND people set themselves up to do, and again there are also many hypotheses. In both these broad approaches, the distribution of visible matter varies at different scales and in different systems at the same scale. Local matter distribution drives local gravitational phenomena. MONDian approaches roughly hold that there is no wiggle room -- gravitation must be generated by what we can see, and not by anything else. General Relativity is more lax -- there is no reason to expect that we can see every generator of stress-energy, or that the distribution of the generators must be uniform in space or in spacetime, and gravitational theory should not turn its back on the possibility of discovering new beyond-the-standard-model spacetime-filling matter fields. One can also go really crazy and say that matter is not the (only or primary) generator of curvature, and that geometric curvature and matter simply coincide mostly by chance. A distribution of curvature that evolved in the early universe -- a cosmic gravitational background -- is taken to be relevant to the dynamics of galaxies and clusters about as old as ours. In these theories there are also auxiliary gravitational fields, and those typically allow for a MOND but are also designed to allow for the acceleration of the expansion of the universe and other features that relativistic MOND essentially imports from general relativity (in the form of a scalar multiplier on one or more of the curvature field(s)). So, in summary I'll return to your first question: > how [can] dark matter theories reproduce mondian dynamics naturally without each case requiring special tuning They can't. There is an initial (or at least early) values surface that the laws of the theory turn into galaxies and clusters of all sorts. The initial values set up some distribution of interacting and self-interacting matter (that is initially, or evolves into, the standard model and dark matter). It also may set up primordial gravitational radiation not produced by the matter, and things like an evolving cosmological (non-)constant. Relativistic MOND is not different on this front -- there must be some initial or early values surface that the laws of the theory turn into the standard model and the various gravitational fields around MONDian galaxies, mondian wide binaries, MONDian relativistic stars (like white dwarfs and neutron stars, especially in wide binaries of those), and so forth. These approaches may be even more sensitive to the distribution of things like primordial gravitational radiation because in general gravitational radiation is considerably more complicated by the presence of fields beyond the Einstein tensor: any additional scalar, tensor, or vector field may have their own polarizations, and their polarizations may modify the polarization of the mathematical structure closest to that of the metric tensor or the Weyl tensor parts of the Einstein tensor. Thus the considerable interest in the small variations in the cosmic microwave background by people who spend any time comparing General Relativity and relativistic MONDian theories. Ultimately the "special tuning" is the "past hypothesis", which is highly similar for both approaches: the distant past of the universe was hotter, denser, and lower entropy than the universe full of galaxies. The lower entropy is the puzzle. What ordered that? Again, there are many and various thoughts, and a handful of plausible hypotheses.
i_no_can_eat 3y ago
Precisely. They way I see it, MOND is a useful effective theory. But it is beyond me how people still think of it as a fundamental theory when the closest that anyone ever came up of a general-relativistic version of it is a total mess. As it is, MOND fails solar system tests. So at best, it's a useful effective model for some galactic-scale phenomenology, which would need some fundamental explanation.

aeneasmackenzie 3y ago

The year is 3000AD. A 1 million cubic kilometer xenon detector cooled to 2K orbiting Neptune has not detected any WIMPs. Not to worry, the next one will be ten times bigger and at 0.5K.

mmastrac 3y ago

Headline translation: Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's law of universal gravitation to account for observed properties of galaxies. It is an alternative to the hypothesis of dark matter in terms of explaining why galaxies do not appear to obey the currently understood laws of physics. NGC1277: The galaxy is unique in that it is considered a relic of what galaxies were like in the early universe. The galaxy is composed exclusively of aging stars that were born 10 billion years ago. But unlike other galaxies in the local universe, it has not undergone any further star formation. Astronomers nickname such galaxies as "red and dead," because the stars are aging and there aren't any successive generations of younger stars.

dist-epoch 3y ago

On a recent Sean Carroll podcast he said the galaxy/dark matter problem is the easy one to solve. There is another one, much harder - acoustic oscillation immediately after the Big Bang, visible in the microwave background, which MOND can't account for. Basically normal matter goes both ways, back-and-forth, compresses and expands, but dark matter goes only one-way, only expands. And this can be seen in the microwave background.

throwawaymaths 3y ago
Explaining the baryon acoustic oscillation with LCDM presumes that the phenomenon must have the same causal explanation as galaxy rotation curves. That's not necessarily the case. It should be considered "a nice feature" that LCDM explains this but when you have a mass distribution that can be arbitrarily assigned in space, it feels like there are a lot of "nice things you could explain". For example, they tried to explain the precession of mercury with a hunk of dm orbiting the sun called "Vulcan" (we now know you don't need dark matter to explain this). I believe the Vulcan steps in SF are named after this hypothetical dark matter planet. It's in the same neighborhood as mars, Venus, and Saturn and built around the same time
- raattgift 3y ago
  > when you have a mass distribution that can be arbitrarily assigned in space, it feels like there are a lot of "nice things you could explain" Like an eight-planet star system, or a star system with a hot Jupiter, or a multi-star system with no planets, all arising within the same giant molecular cloud (GMC)? Even though the cloud-elements behave sanely, even small arbitrary changes in initial values can produce dramatically different later arrangements. Replace the galactic GMC with a dark matter filament in the cosmic web, and why be surprised that all sorts of galaxies form within? I don't think there is reason to fear arbitrary mass distributions so long as they obey constraints (if you're interested in the maths, the Einstein constraint equations are a good starting point you'll find in textbooks -- Robert Wald's chapter 10 is good and mathy, Alcubierre's or Baumgarte and Shapiro's numerical relativity textbooks are good alternatives for people who think computationally -- and lecture materials). We know empirical constraints and lots of solutions at many scales in the known universe already, and that constraint equations can be found for many other classical field theories; Dirac and successors (Regge et al., Rothe & Rothe et al.) are good resources wrt quantum field theories. Roughly the slogan is to lay down some arbitrary but plausibly generic configuration and see how it evolves. Do it often enough and you should get familiar structures, or you're back to the drawing board. Large cosmological dark matter simulations that do exactly this are new hat. Standard cosmology solutions often get compared in the same project with alternative theories, sometimes following the spirit of the Standard Model Extension in parametrizing everything possible and plugging in different coefficient values, sometimes simply trying to trace cosmological implementations of an IV formalism on some published alternative to general relativity. For example, Vogelsberger et al., <https://www.nature.com/articles/s42254-019-0127-2 https://www.nature.com/articles/s42254-019-0127-2>, <https://arxiv.org/abs/1909.07976v6 https://arxiv.org/abs/1909.07976v6> in which §5.3 has a brief literature review.

throwawaymaths 3y ago

TL/DR: compact galaxies rotate fast enough that their centripetal acceleration limit for MOND. Therefore it should look Newtonian, therefore You expect to "not see dark matter" in them. FTA: Eyeballing/back of the envelope the NGC 1277 data suggests it falls into that category (me: probably a more careful calculation is warranted). Usually galaxies that fit this are dense elliptical; 1277 is ultramassive lenticular, though to be fair lenticular ~elliptical.

MattPalmer1086 3y ago
I don't believe that's entirely accurate (I am not a cosmologist). It says that because of the acceleration, MOND predicts Newtonian motion. It does not say that you expect not to see dark matter, just that dark matter is not needed to explain that motion. However, this is a problem for dark matter theories precisely because we expect galaxies to form in concentrations of dark matter: "The infall of baryons acts to further concentrate the central dark matter. So the nominal expectation is that there should be plenty of dark matter near the centers of galaxies rather than none at all. That’s not what we see here, so nominally NGC 1277 presents more of a challenge for the dark matter paradigm than it does for MOND"
- throwawaymaths 3y ago
  We're saying the same thing.