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[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

[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]

raattgift 3y ago
[part 2/2] The result discussed at the link above comes from study of a the X-ray luminous and large MACS J1149.5+2223 from the MAssive Cluster Survey (MACS, <https://iopscience.iop.org/article/10.1086/320958 https://iopscience.iop.org/article/10.1086/320958>), redshift ~ 0.544 (around 5 billion light-years distance). As with most massive clusters, platforms like Hubble have observed multiple images of background (even more distant) bright objects near the centre of the galaxy cluster. The centre is found by looking at the proper motions and other features of its constituent galaxies, and the centre is relatively transparent. These features aren't surprising in a dark matter paradigm (some DM settles to the centre of the cluster and tends to draw gas and dust into large black holes; more generally, transparent lines of sight through the cluster between its galaxies aren't vacuum: they have some density of dark matter like all shiny or background-obscuring parts of the cluster). Relativistic MOND theories struggle with this in different ways, with different possible repairs to the relativistic theory (one can also add some dark matter to most of these theories; some of these theories may allow there to be less dark matter than visible matter). Determining the mass of this particular MACS cluster is hard in any theory, and multiple images of background objects with predictable features (e.g. the "Refsdal" supernova, with a predictable light-curve, in the link at the top) help. The mass (and its distribution) and other features of the cluster can pin down the distance of multiply-lensed background objects. Theories which imply different cluster masses (e.g. no DM at all == much much less mass) should have different image multiplicities and the images appearing at different locations; this also varies the distance to the background objects. > cmb estimate Well, we can fix the evolution of the expansion of the universe to some value and compare observations to predictions of various theories of cosmology. There are parameters in \Lambda-CDM (the "concordance" model, which deliberately tracks new discoveries) that are not known to great precision; altering these constants, converting everywhere-everywhen constant parameters to scale-factor-dependent parameters, and so forth, all result in a cosmos with different features. As we discover features of our own cosmos, we'll adapt the standard/concordance model of cosmology. Not only can one depart from \Lambda-CDM without departing from General Relativity, one can also replace General Relativity in an effort to eliminate the \Lambda parameter, or Cold Dark Matter, or both. One can then see if observed features are allowed in the non-concordance theory, or whether straightforward attempts to extract such features concord with astrophysical evidence. Also, one can see if the non-concordance theory predicts features that there is no evidence for (and which the concordance model does not predict). > is this a result in favor of dark matter over mond The paper is mostly about the Hubble tension. Newtonian-limit + dark matter vs modified Newtonian dynamics + no dark matter isn't a great comparison here because the multiply-lensed-by-massive-cluster supernova simply is not a system in the Newtonian limit (and in several ways). Instead one might ask if this is something in favour of \Lambda-CDM vs a no-dark-matter (and maybe no dark energy) alternative relativistic theory of gravitation. The answer: maybe. It's easy to understand in \Lambda-CDM (see above), but perhaps that's because experience with that theory has developed intuitions (and comparable intuitions might arise if using a different set of theories). But the question of how a non-General Relativity theory of gravitation work far from the (modified) Newtonian weak-field/low-velocity limit bites in several places; there are lots of moving parts at different distances and scales (at the very largest scales visible matter looks smooth, but it gets lumpy when one zooms in and sees voids, filaments, galaxy clusters, and so forth) and getting it right at all scales is more useful than getting it "more right at least for now" at just a couple of scales but not getting it "at least as right" at others. Making concordance cosmologies based on various alternatives to General Relativity is at best a set of works-in-progress. But the general-relativistic standard cosmology is also a work-in-progress, so perhaps comparisons are more practical (what's easier to work with?) or maybe metaphysical. From a practical perspective, relativists are very used to throwing up their hands and resorting to a more complicated model of stress-energy ("matter") than they really want to calculate with. In some relativistic systems in (~stellar) astrophysics it really does matter how Yukawa coupling works, and you can't just say "oh, stiff liquid here". There are lots and lots of mechanisms which generate stress-energy, many of which were unknown even sixty years ago. Stress-energy sources the gravitational field, which is relatively simple in GR compared to alternatives like scalar-tensor/vector-tensor/scalar-vector-tensor/scalar-tensor-vector/tensor-vector-scalar (yes, these are all real theories and they're all different!), bi- and multimetric theories, non-metric theories and so on. How does moving matter vary the mathematical objects? Sometimes that's a hard question, especially if the matter is very dense, very energetic, or moving at large fractions of the speed of light. But it's the central question. Finally, if you're keen on Hubble tension, this is an excellent 2021 review: <https://iopscience.iop.org/article/10.1088/1361-6382/ac086d https://iopscience.iop.org/article/10.1088/1361-6382/ac086d> - -- [1] Hodson, Zhao (2017), Generalizing MOND to explain the missing mass in galaxy clusters, <https://arxiv.org/abs/1701.03369 https://arxiv.org/abs/1701.03369> §1, para 4,5- is a good recent overview. [2] Negrelli et al. 2020, Testing Modified Gravity theory (MOG) with Type Ia Supernovae, Cosmic Chronometers and Baryon Acoustic Oscillations <https://arxiv.org/abs/2004.13648 https://arxiv.org/abs/2004.13648>.

raattgift 3y ago

[part 2/2] The result discussed at the link above comes from study of a the X-ray luminous and large MACS J1149.5+2223 from the MAssive Cluster Survey (MACS, <https://iopscience.iop.org/article/10.1086/320958 https://iopscience.iop.org/article/10.1086/320958>), redshift ~ 0.544 (around 5 billion light-years distance). As with most massive clusters, platforms like Hubble have observed multiple images of background (even more distant) bright objects near the centre of the galaxy cluster. The centre is found by looking at the proper motions and other features of its constituent galaxies, and the centre is relatively transparent. These features aren't surprising in a dark matter paradigm (some DM settles to the centre of the cluster and tends to draw gas and dust into large black holes; more generally, transparent lines of sight through the cluster between its galaxies aren't vacuum: they have some density of dark matter like all shiny or background-obscuring parts of the cluster). Relativistic MOND theories struggle with this in different ways, with different possible repairs to the relativistic theory (one can also add some dark matter to most of these theories; some of these theories may allow there to be less dark matter than visible matter). Determining the mass of this particular MACS cluster is hard in any theory, and multiple images of background objects with predictable features (e.g. the "Refsdal" supernova, with a predictable light-curve, in the link at the top) help. The mass (and its distribution) and other features of the cluster can pin down the distance of multiply-lensed background objects. Theories which imply different cluster masses (e.g. no DM at all == much much less mass) should have different image multiplicities and the images appearing at different locations; this also varies the distance to the background objects. > cmb estimate Well, we can fix the evolution of the expansion of the universe to some value and compare observations to predictions of various theories of cosmology. There are parameters in \Lambda-CDM (the "concordance" model, which deliberately tracks new discoveries) that are not known to great precision; altering these constants, converting everywhere-everywhen constant parameters to scale-factor-dependent parameters, and so forth, all result in a cosmos with different features. As we discover features of our own cosmos, we'll adapt the standard/concordance model of cosmology. Not only can one depart from \Lambda-CDM without departing from General Relativity, one can also replace General Relativity in an effort to eliminate the \Lambda parameter, or Cold Dark Matter, or both. One can then see if observed features are allowed in the non-concordance theory, or whether straightforward attempts to extract such features concord with astrophysical evidence. Also, one can see if the non-concordance theory predicts features that there is no evidence for (and which the concordance model does not predict). > is this a result in favor of dark matter over mond The paper is mostly about the Hubble tension. Newtonian-limit + dark matter vs modified Newtonian dynamics + no dark matter isn't a great comparison here because the multiply-lensed-by-massive-cluster supernova simply is not a system in the Newtonian limit (and in several ways). Instead one might ask if this is something in favour of \Lambda-CDM vs a no-dark-matter (and maybe no dark energy) alternative relativistic theory of gravitation. The answer: maybe. It's easy to understand in \Lambda-CDM (see above), but perhaps that's because experience with that theory has developed intuitions (and comparable intuitions might arise if using a different set of theories). But the question of how a non-General Relativity theory of gravitation work far from the (modified) Newtonian weak-field/low-velocity limit bites in several places; there are lots of moving parts at different distances and scales (at the very largest scales visible matter looks smooth, but it gets lumpy when one zooms in and sees voids, filaments, galaxy clusters, and so forth) and getting it right at all scales is more useful than getting it "more right at least for now" at just a couple of scales but not getting it "at least as right" at others. Making concordance cosmologies based on various alternatives to General Relativity is at best a set of works-in-progress. But the general-relativistic standard cosmology is also a work-in-progress, so perhaps comparisons are more practical (what's easier to work with?) or maybe metaphysical. From a practical perspective, relativists are very used to throwing up their hands and resorting to a more complicated model of stress-energy ("matter") than they really want to calculate with. In some relativistic systems in (~stellar) astrophysics it really does matter how Yukawa coupling works, and you can't just say "oh, stiff liquid here". There are lots and lots of mechanisms which generate stress-energy, many of which were unknown even sixty years ago. Stress-energy sources the gravitational field, which is relatively simple in GR compared to alternatives like scalar-tensor/vector-tensor/scalar-vector-tensor/scalar-tensor-vector/tensor-vector-scalar (yes, these are all real theories and they're all different!), bi- and multimetric theories, non-metric theories and so on. How does moving matter vary the mathematical objects? Sometimes that's a hard question, especially if the matter is very dense, very energetic, or moving at large fractions of the speed of light. But it's the central question. Finally, if you're keen on Hubble tension, this is an excellent 2021 review: <https://iopscience.iop.org/article/10.1088/1361-6382/ac086d https://iopscience.iop.org/article/10.1088/1361-6382/ac086d> - -- [1] Hodson, Zhao (2017), Generalizing MOND to explain the missing mass in galaxy clusters, <https://arxiv.org/abs/1701.03369 https://arxiv.org/abs/1701.03369> §1, para 4,5- is a good recent overview. [2] Negrelli et al. 2020, Testing Modified Gravity theory (MOG) with Type Ia Supernovae, Cosmic Chronometers and Baryon Acoustic Oscillations <https://arxiv.org/abs/2004.13648 https://arxiv.org/abs/2004.13648>.