Declining rotation curves of galaxies as a test of gravitational theory

Haghi, Hosein; Bazkiaei, Amir E.; Zonoozi, Akram Hasani; Kroupa, Pavel

doi:10.1093/mnras/stw573

Cited by 98 publications

(69 citation statements)

References 116 publications

(160 reference statements)

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“…Using the principle of continuity, the rotation curve of a galaxy must be slightly affected even if the EF on it is much weaker than its internal gravity. Applying this idea, Haghi et al (2016) analyzed whether MOND achieves a better fit to the rotation curves of a sample of 18 disk galaxies once the EFE is considered. Their work relied on a plausible analytic estimate of how the EFE would weaken the internal gravity of these galaxies.…”

Section: Mond Without An External Field Effectmentioning

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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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: Mond Without An External Field Effectmentioning

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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“…µ=0,α=0 (Newtonian) µ=0.01,α=10 µ=0.01,α=100 µ=10,α=10 µ=10,α=100 µ=100,α=10 µ=100,α=100 external field effect (EFE) must be taken into account when dealing with objects embedded in the external tidal field of a more massive system (Angus 2008;Haghi et al 2009;Malekjani, Rahvar & Haghi 2009;Haghi, Baumgardt & Kroupa 2011;Haghi et al 2016). This is because of the intrinsic non-linearity of the Poisson equation that is related to the violation of the strong equivalence principle in MOND.…”

Section: R [Pc]mentioning

confidence: 99%

Testing modified gravity with dwarf spheroidal galaxies

Haghi

Amiri

2016

Mon. Not. R. Astron. Soc.

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The observed velocity dispersion of the classical dwarf spheroidal (dSph) galaxies of the Milky Way (MW) requires the Newtonian stellar mass-to-light (M * /L) ratios in the range of about 10 to more than 100 solar units that are well outside the acceptable limit predicted by stellar population synthesis models. Using Jeans analysis, we calculate the line-of-sight velocity dispersion (σ los ) of stars in eight MW dSphs in the context of the modified gravity (MOG) theory of Moffat, assuming a constant M * /L ratio without invoking the exotic cold dark matter. First, we use the weak field approximation of MOG and assume the two parameters α and µ of the theory to be constant as has already been inferred from fitting to the observed rotational data of The HI Nearby Galaxy Survey catalogue of galaxies. We find that the derived M * /L ratios for almost all dSphs are too large to be explained by the stellar population values. In order to fit the line-of-sight velocity dispersions of the dSph with reasonable M * /L values, we must vary α and µ on a case by case basis. A common pair of values cannot be found for all dSphs. Comparing with the values found from rotation curve fitting, it appears that µ correlates strongly with galaxy luminosity, shedding doubt on it as a universal constant.

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“…Some observations pointing to the EFE in action in galactic systems are described, e.g., in Refs. [48][49][50][51][52][53] One dramatic manifestation of the effect is that an external acceleration ≫ a 0 annihilates MOND effects within a system, even if the internal accelerations are ≪ a 0 [1]. In other words, standard dynamics is restored within a system subject to a high external acceleration field.…”

Section: The Mond External-field Effectmentioning

confidence: 99%

Noncovariance at low accelerations as a route to MOND

Milgrom

2019

Phys. Rev. D

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MOND has limelighted the fact that Newtonian dynamics and general relativity (GR) have not been verified to any accuracy at very low accelerations -at or below the MOND acceleration a0: Without invoking made-to-measure "dark matter", Newtonian dynamics (and hence general relativity) fail in accounting for galactic dynamics at such low accelerations. In particular, we do not have evidence that all the cherished, underlying principles of Newtonian dynamics or GR, such as locality or Lorentz invariance, still apply in the MOND limit. I discuss the possibility that the principle of general covariance might not apply in this limit. This would be in line with suggestions that general covariance, where it does hold, is only an emergent, hence approximate, property of relativistic dynamics. This idea also resonates well with MOND, which hinges on accelerations, for example because the existence of an effective absolute inertial frame is natural in MOND. Relaxing general covariance affords more freedom in constructing candidate MOND theories. For example, it may permit constructing pure-metric, local MOND theories, which is thought impossible with general covariance. I exemplify this with a MOND-oriented, oversimplified, noncovariant theory, where the gravitational Lagrangian isνγ )/2 is the (nonscalar) first-derivative part of the Ricci scalar R. Γ γ µν is the Levi-Civita connection of a metric, gµν, which couples minimally to matter, and ℓM = c 2 /a0 is the MOND length, which is of cosmological magnitude, being, e.g., of the order of the de Sitter radius of our Universe. This LM gives a covariant theory in the high-acceleration limit by requiring that F(z) → z + ζ, for z ≫ 1, which gives GR with a cosmological constant ζc −4 a 2 0 . In the MOND limit F ′ (z ≪ 1) ∝ z 1/2 . In the nonrelativistic limit the metric has a solution of the form gµν ≈ ηµν − 2φδµν, as in GR, but the potential φ solves a MOND, nonlinear Poisson analog. This form of gµν also produces gravitational lensing as in GR, only with the MOND potential. I show that this theory is a fixed-gauge expression of bimetric MOND (BIMOND), with the auxiliary metric constrained to be flat. The latter theory is thus a covariantized version of the formerá-la Stückelberg. This theory is also a special case of so-called f (Q) theories -aquadratic generalizations of "symmetric, teleparallel GR", which are, in turn, also equivalent to constrained BIMOND-type theories. PACS numbers:where a H ≡ cH is the acceleration associated with the cosmological expansion rate, H (the Hubble-Lemaitre constant), and a H (0) is its present-day value, and ℓ Λ = (Λ/3) −1/2 is the radius associated with Λ -the observed equivalent of a cosmological constant. Defining the MOND length asgalactic dynamics and cosmology tell us that ℓ M ∼ ℓ Λ ∼ ℓ H , where ℓ H ≡ c/H 0 is the Hubble distance today. This numerical "coincidence", if fundamental, may have far-reaching ramifications for MOND, and for gravity in general (e.g., Ref.[10]). Unless one invokes large quantities of "dark matter" in galactic ...

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Declining rotation curves of galaxies as a test of gravitational theory

Cited by 98 publications

References 116 publications

Testing gravity with wide binary stars like α Centauri

Testing gravity with wide binary stars like α Centauri

Testing modified gravity with dwarf spheroidal galaxies

Noncovariance at low accelerations as a route to MOND

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