The radial acceleration relation and a magnetostatic analogy in quasilinear MOND

Brown, Katherine; Abraham, Roshan Mammen; Kell, Leo; Mathur, Harsh

doi:10.1088/1367-2630/aaca23

Cited by 9 publications

(7 citation statements)

References 38 publications

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“…Instead, it requires the weaker condition that departures of g from spherical symmetry are accurately captured by applying the MOND ν function to g N , which is itself not spherically symmetric. This may explain why Jones-Smith et al (2018) found that QUMOND gravitational fields in disk galaxies could be estimated rather well using the algebraic MOND approximation, justifying our use of Equation 7. We discuss its accuracy in Section 9.3.1, finding it should work well in the Solar neighbourhood where the WBT would be conducted.…”

Section: Governing Equationssupporting

confidence: 55%

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Testing gravity with wide binary stars like α Centauri

Banik

Zhao

2018

Monthly Notices of the Royal Astronomical Society

116

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We consider the feasibility of testing Newtonian gravity at low accelerations using wide binary (WB) stars separated by 3 kAU. These systems probe the accelerations at which galaxy rotation curves unexpectedly flatline, possibly due to Modified Newtonian Dynamics (MOND). We conduct Newtonian and MOND simulations of WBs covering a grid of model parameters in the system mass, semi-major axis, eccentricity and orbital plane. We self-consistently include the external field (EF) from the rest of the Galaxy on the Solar neighbourhood using an axisymmetric algorithm. For a given projected separation, WB relative velocities reach larger values in MOND. The excess is ≈ 20% adopting its simple interpolating function, as works best with a range of Galactic and extragalactic observations. This causes noticeable MOND effects in accurate observations of ≈ 500 WBs, even without radial velocity measurements.We show that the proposed Theia mission may be able to directly measure the orbital acceleration of Proxima Centauri towards the 13 kAU-distant α Centauri. This requires an astrometric accuracy of ≈ 1 µas over 5 years. We also consider the long-term orbital stability of WBs with different orbital planes. As each system rotates around the Galaxy, it experiences a time-varying EF because this is directed towards the Galactic Centre. We demonstrate approximate conservation of the angular momentum component along this direction, a consequence of the WB orbit adiabatically adjusting to the much slower Galactic orbit. WBs with very little angular momentum in this direction are less stable over Gyr periods. This novel direction-dependent effect might allow for further tests of MOND.

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Section: Governing Equationssupporting

confidence: 55%

“…This is known to work rather well in disk galaxies, at least once the vertical Newtonian gravity g N,z is taken into account when determining ν so as to more rigorously implement the algebraic MOND approximation (Jones-Smith et al 2018). For a thin disk, g N,z may be estimated as g N,z = − 2πGΣ sign (z)…”

Section: The Effect Of the Galactic Diskmentioning

confidence: 99%

Testing gravity with wide binary stars like α Centauri

Banik

Zhao

2018

Monthly Notices of the Royal Astronomical Society

116

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“…This approximation is exactly correct in spherical symmetry and works rather well in axisymmetric problems (Angus et al 2012;Jones-Smith et al 2018). It is expected to work particularly well just outside the disc ).…”

Section: Initializing a Milgromian Discmentioning

confidence: 76%

Barred spiral galaxies in modified gravity theories

Roshan

Banik

Ghafourian

et al. 2021

Monthly Notices of the Royal Astronomical Society

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When bars form within galaxy formation simulations in the standard cosmological context, dynamical friction with dark matter (DM) causes them to rotate rather slowly. However, almost all observed galactic bars are fast in terms of the ratio between corotation radius and bar length. Here, we explicitly display an 8σ tension between the observed distribution of this ratio and that in the EAGLE simulation at redshift 0. We also compare the evolution of Newtonian galactic discs embedded in DM haloes to their evolution in three extended gravity theories: Milgromian Dynamics (MOND), a model of non-local gravity, and a scalar-tensor-vector gravity theory (MOG). Although our models start with the same initial baryonic distribution and rotation curve, the long-term evolution is different. The bar instability happens more violently in MOND compared to the other models. There are some common features between the extended gravity models, in particular the negligible role played by dynamical friction − which plays a key role in the DM model. Partly for this reason, all extended gravity models predict weaker bars and faster bar pattern speeds compared to the DM case. Although the absence of strong bars in our idealized, isolated extended gravity simulations is in tension with observations, they reproduce the strong observational preference for ‘fast’ bar pattern speeds, which we could not do with DM. We confirm previous findings that apparently ‘ultrafast’ bars can be due to bar-spiral arm alignment leading to an overestimated bar length, especially in extended gravity scenarios where the bar is already fast.

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“…As a general comment, we note that it is impractical, in general, to solve a many-body system in full, but as is usually realized, there are problems where we can apply a mean-field approximation, where we consider the motion of a single body (e.g., a star in a galaxy) in the combined mean field of all the others assumed time independent. 9 In some important applications, the motion of the constituents is described, to a good approximation, as the combination of several trajectories, with distinct frequencies, or of bunches of frequencies (which we lump into one for simplicity),…”

Section: Examplesmentioning

confidence: 99%

“…Among second-tier predictions, one may count some fine details of the rotation curves (e.g., Refs. [8][9][10][11]), the exact value of η in the M − σ relation, and its dependence on dimensionless attributes of the system; the exact dependence of the effective two-body force on the masses; the exact nature and strength of the external-field effect, etc.…”

Section: Introductionmentioning

confidence: 99%

Models of modified-inertia formulation of MOND

Milgrom¹

2022

Preprint

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Models of 'modified-inertia' formulation of MOND are described and applied to nonrelativistic many-body systems. Whereas the inter-body forces are Newtonian, the expression for their inertia is modified from the Newtonian ma to comply with the basic tenets of MOND. This results in time-nonlocal equations of motion. Momentum, angular momentum, and energy are (nonlocally) defined for bodies, and the total values are conserved for isolated many-body systems. The models make all the salient MOND predictions. Yet, they differ in important ways from existing 'modifiedgravity' formulations in their second-tier predictions. Indeed, the heuristic value of the model is in limelighting such possible differences. The models describe correctly the motion of a composite body in a low-acceleration field even when the internal accelerations of its constituents are high (e.g., a star in a galaxy). They exhibit a MOND external field effect (EFE) that shows some important differences from what we have come to expect from modified-gravity versions: In one, simple example of the models, what determines the EFE, in the case of a dominant external field, is µ(θ aex /a0), where µ(x) is the MOND 'interpolating function' that describes rotation curves, compared with µ(aex/a0) for presently-known 'modified-gravity' formulations. The two main differences are that while aex is the momentary value of the external acceleration, aex is a certain time average of it, and that θ > 1 is an extra factor that depends on the frequency ratio of the external-and internal-field variations. Only ratios of frequencies enter, and a0 remains the only new dimensioned constant. For example, for a system on a circular orbit in a galaxy (such as the vertical dynamics in a disc galaxy), the first difference disappears, since aex = aex. But the θ factor can appreciably enhance the EFE in quenching MOND effects, over what is deduced in modified gravity. This θ enhancement is important in most applications of the EFE. Some exact solutions are also described, such as for rotation curves, for an harmonic force, and the general, two-body problem, which in the deep-MOND regime reduces to a single-body problem.

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The radial acceleration relation and a magnetostatic analogy in quasilinear MOND

Cited by 9 publications

References 38 publications

Testing gravity with wide binary stars like α Centauri

Testing gravity with wide binary stars like α Centauri

Barred spiral galaxies in modified gravity theories

Models of modified-inertia formulation of MOND

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