Some comments informed by Stacy McGaugh’s blog (you may know most of this already):
The rotation curves show a very tight dependence on the amount of baryonic matter alone, something which you might expect from modified gravity sourced by baryonic matter, but not so much, from ordinary gravity sourced by a mixture of baryonic matter and dark matter.
Lensing is a relativistic effect. The leading phenomenological theory of modified gravity here, MOND, is Modified Newtonian Dynamics, i.e. is defined for the nonrelativistic regime (since the rotation curves involve very small accelerations). So lensing predictions will depend on the specific relativistic extension of MOND. Incidentally, a very recent relativistic extension of MOND (“RelMOND”) is supposed to get that third CMB peak right.
McGaugh seems to regard the possibilities of structure formation in MOND as barely studied, at least when compared to Lambda CDM; and points out that the “21 cm anomaly” could be explained by there being no dark matter in the early universe.
In general, McGaugh cautions that the dark matter paradigm contains numerous parameters which keep being adjusted to match the latest data; whereas in the realm of rotation curves, MOND makes successful significant predictions; but people prefer to keep tweaking Lambda CDM, rather than trying to build on MOND’s successes.
Further comments from me:
I am agnostic about which paradigm is right—clearly they both have their merits—and a middle ground of “MOND-like DM” (e.g. Khoury’s superfluid DM, postulated to have an interaction with baryonic matter that reproduces the MOND gravitational profile) or “DM-like MOND” (e.g. Bullet Cluster lensing from flux in extra metric degrees of freedom?) is also intriguing. Either way, at the galactic scale, there seems to be a relationship between amount of baryonic matter, and strength of these dark effects, that is not explained by ordinary theories of dark matter.
I’m quite curious about whether RelMOND matches the CMB spectrum nearly as well as Lambda-CDM (which has what, 3 free parameters for dark matter and dark energy?), and how much work they had to do to get it to agree. Like, if all you care about is galaxy rotation curves, it’s easy to say that dark-matter-theorists keep changing the amount of dark matter they say is in galaxies to match observations (while, symmetrically, baryonic-matter-theorists keep changing the amount of non-visible baryonic matter they say is in galaxies to match observations). But the CMB is significantly more tightly constrained.
I found this Quanta magazine article about it which seems to indicate that it fits the CMB spectrum well but required a fair deal of fiddling with gravity to do so, but I lamentably lack the physics capabilities to evaluate the original paper.
Some comments informed by Stacy McGaugh’s blog (you may know most of this already):
The rotation curves show a very tight dependence on the amount of baryonic matter alone, something which you might expect from modified gravity sourced by baryonic matter, but not so much, from ordinary gravity sourced by a mixture of baryonic matter and dark matter.
Lensing is a relativistic effect. The leading phenomenological theory of modified gravity here, MOND, is Modified Newtonian Dynamics, i.e. is defined for the nonrelativistic regime (since the rotation curves involve very small accelerations). So lensing predictions will depend on the specific relativistic extension of MOND. Incidentally, a very recent relativistic extension of MOND (“RelMOND”) is supposed to get that third CMB peak right.
McGaugh seems to regard the possibilities of structure formation in MOND as barely studied, at least when compared to Lambda CDM; and points out that the “21 cm anomaly” could be explained by there being no dark matter in the early universe.
In general, McGaugh cautions that the dark matter paradigm contains numerous parameters which keep being adjusted to match the latest data; whereas in the realm of rotation curves, MOND makes successful significant predictions; but people prefer to keep tweaking Lambda CDM, rather than trying to build on MOND’s successes.
Further comments from me:
I am agnostic about which paradigm is right—clearly they both have their merits—and a middle ground of “MOND-like DM” (e.g. Khoury’s superfluid DM, postulated to have an interaction with baryonic matter that reproduces the MOND gravitational profile) or “DM-like MOND” (e.g. Bullet Cluster lensing from flux in extra metric degrees of freedom?) is also intriguing. Either way, at the galactic scale, there seems to be a relationship between amount of baryonic matter, and strength of these dark effects, that is not explained by ordinary theories of dark matter.
I’m quite curious about whether RelMOND matches the CMB spectrum nearly as well as Lambda-CDM (which has what, 3 free parameters for dark matter and dark energy?), and how much work they had to do to get it to agree. Like, if all you care about is galaxy rotation curves, it’s easy to say that dark-matter-theorists keep changing the amount of dark matter they say is in galaxies to match observations (while, symmetrically, baryonic-matter-theorists keep changing the amount of non-visible baryonic matter they say is in galaxies to match observations). But the CMB is significantly more tightly constrained.
I found this Quanta magazine article about it which seems to indicate that it fits the CMB spectrum well but required a fair deal of fiddling with gravity to do so, but I lamentably lack the physics capabilities to evaluate the original paper.