6 ms·

I wonder what happens if instead of dark matter halos, mond is used for galaxies that do the lensing. If the cmb estimate gives the correct hubble constant, and

I wonder what happens if instead of dark matter halos, mond is used for galaxies that do the lensing. If the cmb estimate gives the correct hubble constant, and mond doesn't do enough lensing for this result to make sense (less mass, less lensing?), is this result a point in favor of dark matter over mond?

scrubs 3y ago
I'll have to find the link but mond is net minus. There are related cosmological issues of gravity mond seems less good or just bad for
- scrubs 3y ago
  "MOND holds that for accelerations smaller than an a0 value of roughly 1.2 × 10−10 m/s2, accelerations increasingly depart from the standard M · G / r 2 Newtonian relationship of mass and distance, wherein gravitational strength is proportional to mass and the inverse square of distance. Specifically, the theory holds that when gravity is well below the a0 value, its rate of change—including the curvature of spacetime—increases with the square root of mass (rather than linearly as per Newtonian law) and decreases linearly with distance (rather than distance squared)." [1] I'll bet that in the 20 years a space based experiment can give some data to determine better if MOND is right. [1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics See also: https://bigthink.com/starts-with-a-bang/modifying-gravity/ https://bigthink.com/starts-with-a-bang/modifying-gravity/ https://physics.aps.org/articles/v14/143 https://physics.aps.org/articles/v14/143
SirIsaac 3y ago
The problem with MOND is that it's just math. It doesn't explain anything in terms of cause and effect because it doesn't have a physical basis. The problem with the cold dark matter model is that it posits a massive, slow moving particle. A slow massive particle would be attracted to other bodies and to other dark matter particles and form clumps in orbit. This is not observed. There's a better hypothesis that not only explains the cause of gravity, but also posits that dark matter consists of positive and negative electric photons that continually radiates from all massive particles. Lots of it. They can't be detected electrically because their net effect is neutral but they still cause gravity. https://medium.com/@RebelScience/demystifying-gravity-and-dark-matter-f89f40601a8f https://medium.com/@RebelScience/demystifying-gravity-and-da...
raattgift 3y ago
[part 1/2] > I wonder what happens if ... mond is used for galaxies that do the lensing tl;dr: massive galaxy cluster with multiple images of a cosmologically-distant background supernova is a relativistic system, so neither Newtonian limit of GR nor MOND is suitable and thus defy useful comparison. Instead compare relativistic theories that reliably produce Newtonian limit (e.g. General Relativity + cold dark matter) or MOND (long list of relativistic theories...) at the limbs of galaxies. The article at the top corresponds with the arXiv preprint <https://arxiv.org/abs/2305.06367 https://arxiv.org/abs/2305.06367>. The foreground is a MAssive Cluster Survey (MACS, <https://iopscience.iop.org/article/10.1086/518603 https://iopscience.iop.org/article/10.1086/518603>) MACS J1149+2223, a large group of very X-ray luminous galaxies redshift z ~ 0.543 (about 5 billion light-years distance). As is typical for massive clusters, there are many multiple images of background events, concentrated around the centre of the cluster. > mond First, for convenience, let's just accept the empirical MOND relationship for all galaxies (i.e., we don't look at possible/apparent exceptions). Blobs of hydrogen gas at the tips of edge-on spiral-type galaxies have too large a red/blue shift for Newton+visible matter alone; and blobs of hydrogen gas scattered across the face of elliptical-type galaxies have too many outlying red/blue shifts (they move radially, rising through and sinking back below the bright surface). These features, wrapped up in "rotation curve", are compatible with a modified Newton (+ visible matter), where the modification in this limit is a simple function. However, the combination of ~optical background sources and the centre of massive cluster demands a relativistic theory (see [1], which goes to your question in detail), and there have been many attempts to produce a relativistic MOND. Most of these involve adding an additional mathematical structure (e.g. a vector or scalar field) or more to the Einstein Field Equations to which all matter couples, driving these new gravitational fields' potentials, gradients, and dynamics. The goal, very broadly, is to have all the relevant gravitational dynamics produced only by the matter known from the Standard Model, interacting only with the mechanisms allowed in that model of matter. In these theories, the empirical MOND relationship is supposed to be inevitable in the weak-field / nonrelativistic limit. By comparison, General Relativity adds a new species of matter to the Einstein Field Equations. The existing curvature tensor (and its decomposition fraction, the metric tensor) is the sole universally-coupled field. Hypothetical new matter (e.g. axions, sterile neutrinos, ...) would drive the metric tensor just like old matter; such new matter may or may not interact with other matter in non-gravitational ways. Unmodified Newtonian dynamics is inevitable in the weak field / nonrelativistic limit in the absence of this new matter. The empirical MOND relationship appears only if extra matter is added. The problem is that we don't see that extra matter, and do not know how (or if) it interacts non-gravitationally with the Standard Model. (There are additional ideas, usually based in string theory/M-theory, wherein a relativistic theory of gravitation -- General Relativity, or one of the many flavours of relativistic MOND -- emerges at relatively low energies.) Quoting [1] (§2), The MOND paradigm has always struggled with rectifying the mass discrepancy in galaxy clusters without including an extra mass component such as neutrinos or non-luminous baryonic matter. In its usual form MOND is unable to boost the gravitational acceleration in clusters sufficiently whilst this framework is still able to match observational constraints provided by galaxy dynamics. The reason for MOND’s inability to accomplish this is that the gravitational acceleration in galaxy clusters is relatively high, in general x>1, so the MOND boost to gravity is weak. However, there is an observed mass deficit in galaxy clusters that would require the cluster to be in the deep-MOND regime (x<<1) to rectify with MOND alone. Put more simply, the mass profile of the centre of massive clusters makes constituent galaxies whirl around within the cluster too much for MOND alone. There are a variety of ideas about how this could be fixed, mostly by building a more complicated relativistic theory for which MOND (modified Newton + no "new" matter) continues to be the weak-field limit. The General Relativity answer is that the (luminous-)mass deficit implies the presence of dark matter in regions of the cluster far outside any of its galaxies. [continued in part 2/2]

scrubs 3y ago

I'll have to find the link but mond is net minus. There are related cosmological issues of gravity mond seems less good or just bad for

scrubs 3y ago
"MOND holds that for accelerations smaller than an a0 value of roughly 1.2 × 10−10 m/s2, accelerations increasingly depart from the standard M · G / r 2 Newtonian relationship of mass and distance, wherein gravitational strength is proportional to mass and the inverse square of distance. Specifically, the theory holds that when gravity is well below the a0 value, its rate of change—including the curvature of spacetime—increases with the square root of mass (rather than linearly as per Newtonian law) and decreases linearly with distance (rather than distance squared)." [1] I'll bet that in the 20 years a space based experiment can give some data to determine better if MOND is right. [1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics See also: https://bigthink.com/starts-with-a-bang/modifying-gravity/ https://bigthink.com/starts-with-a-bang/modifying-gravity/ https://physics.aps.org/articles/v14/143 https://physics.aps.org/articles/v14/143

SirIsaac 3y ago

The problem with MOND is that it's just math. It doesn't explain anything in terms of cause and effect because it doesn't have a physical basis. The problem with the cold dark matter model is that it posits a massive, slow moving particle. A slow massive particle would be attracted to other bodies and to other dark matter particles and form clumps in orbit. This is not observed. There's a better hypothesis that not only explains the cause of gravity, but also posits that dark matter consists of positive and negative electric photons that continually radiates from all massive particles. Lots of it. They can't be detected electrically because their net effect is neutral but they still cause gravity. https://medium.com/@RebelScience/demystifying-gravity-and-dark-matter-f89f40601a8f https://medium.com/@RebelScience/demystifying-gravity-and-da...

raattgift 3y ago

[part 1/2] > I wonder what happens if ... mond is used for galaxies that do the lensing tl;dr: massive galaxy cluster with multiple images of a cosmologically-distant background supernova is a relativistic system, so neither Newtonian limit of GR nor MOND is suitable and thus defy useful comparison. Instead compare relativistic theories that reliably produce Newtonian limit (e.g. General Relativity + cold dark matter) or MOND (long list of relativistic theories...) at the limbs of galaxies. The article at the top corresponds with the arXiv preprint <https://arxiv.org/abs/2305.06367 https://arxiv.org/abs/2305.06367>. The foreground is a MAssive Cluster Survey (MACS, <https://iopscience.iop.org/article/10.1086/518603 https://iopscience.iop.org/article/10.1086/518603>) MACS J1149+2223, a large group of very X-ray luminous galaxies redshift z ~ 0.543 (about 5 billion light-years distance). As is typical for massive clusters, there are many multiple images of background events, concentrated around the centre of the cluster. > mond First, for convenience, let's just accept the empirical MOND relationship for all galaxies (i.e., we don't look at possible/apparent exceptions). Blobs of hydrogen gas at the tips of edge-on spiral-type galaxies have too large a red/blue shift for Newton+visible matter alone; and blobs of hydrogen gas scattered across the face of elliptical-type galaxies have too many outlying red/blue shifts (they move radially, rising through and sinking back below the bright surface). These features, wrapped up in "rotation curve", are compatible with a modified Newton (+ visible matter), where the modification in this limit is a simple function. However, the combination of ~optical background sources and the centre of massive cluster demands a relativistic theory (see [1], which goes to your question in detail), and there have been many attempts to produce a relativistic MOND. Most of these involve adding an additional mathematical structure (e.g. a vector or scalar field) or more to the Einstein Field Equations to which all matter couples, driving these new gravitational fields' potentials, gradients, and dynamics. The goal, very broadly, is to have all the relevant gravitational dynamics produced only by the matter known from the Standard Model, interacting only with the mechanisms allowed in that model of matter. In these theories, the empirical MOND relationship is supposed to be inevitable in the weak-field / nonrelativistic limit. By comparison, General Relativity adds a new species of matter to the Einstein Field Equations. The existing curvature tensor (and its decomposition fraction, the metric tensor) is the sole universally-coupled field. Hypothetical new matter (e.g. axions, sterile neutrinos, ...) would drive the metric tensor just like old matter; such new matter may or may not interact with other matter in non-gravitational ways. Unmodified Newtonian dynamics is inevitable in the weak field / nonrelativistic limit in the absence of this new matter. The empirical MOND relationship appears only if extra matter is added. The problem is that we don't see that extra matter, and do not know how (or if) it interacts non-gravitationally with the Standard Model. (There are additional ideas, usually based in string theory/M-theory, wherein a relativistic theory of gravitation -- General Relativity, or one of the many flavours of relativistic MOND -- emerges at relatively low energies.) Quoting [1] (§2), The MOND paradigm has always struggled with rectifying the mass discrepancy in galaxy clusters without including an extra mass component such as neutrinos or non-luminous baryonic matter. In its usual form MOND is unable to boost the gravitational acceleration in clusters sufficiently whilst this framework is still able to match observational constraints provided by galaxy dynamics. The reason for MOND’s inability to accomplish this is that the gravitational acceleration in galaxy clusters is relatively high, in general x>1, so the MOND boost to gravity is weak. However, there is an observed mass deficit in galaxy clusters that would require the cluster to be in the deep-MOND regime (x<<1) to rectify with MOND alone. Put more simply, the mass profile of the centre of massive clusters makes constituent galaxies whirl around within the cluster too much for MOND alone. There are a variety of ideas about how this could be fixed, mostly by building a more complicated relativistic theory for which MOND (modified Newton + no "new" matter) continues to be the weak-field limit. The General Relativity answer is that the (luminous-)mass deficit implies the presence of dark matter in regions of the cluster far outside any of its galaxies. [continued in part 2/2]