Alternative Gravity Rotation Curves for the LITTLE THINGS Survey

Abstract

Galactic rotation curves have proven to be the testing ground for dark matter bounds in spiral galaxies of all morphologies. Dwarf galaxies serve as an increasingly interesting case of rotation curve dynamics due to their typically rising rotation curve as opposed to the flattening curve of large spirals. Dwarf galaxies usually vary in galactic structure and mostly terminate at small radial distances. This, coupled with the fact that Cold Dark Matter theories struggle with the universality of galactic rotation curves, allow for exclusive features of alternative gravitational models to be analyzed. Recently, THINGS (The HI Nearby Galactic Survey) has been extended to include a sample of 25 dwarf galaxies now known as the LITTLE THINGS Survey. Here, we show an application of alternative gravitational models to the LITTLE THINGS survey, specifically focusing on conformal gravity and Modified Newtonian Dynamics. In this work, we provide an analysis and discussion of the rotation curve predictions of each theory to the sample. Further, we show how these two alternative gravitational models account for the recently observed universal trends in centripetal accelerations in spiral galaxies. This work highlights the similarities and differences of the predictions of the two theories in dwarf galaxies. The sample is not large or diverse enough to strongly favor a single theory, but we posit that both conformal gravity and MOND can provide an accurate description of the galactic dynamics in the LITTLE THINGS sample without the need for dark matter.

Asymmetry between galaxies with different spin patterns: A comparison between COSMOS, SDSS, and Pan-STARRS

“…Since substantial part of the mass of galaxies is not luminous, galaxy rotation is also a tool to measure galaxy mass (Sofue 2016(Sofue , 2017. Alternative models to the physics of galaxy rotation such as Conformal Gravity and Modified Newtonian Dynamics (MOND) have also been proposed, and show good agreement with observations (O'Brien et al 2017;Wojnar et al 2018).…”

Conformal gravity does not predict flat galaxy rotation curves

“…We have revisited the most celebrated phenomenological consequence of CG, namely that the MK vacuum solution for a static, spherically-symmetric system predicts flat galaxy rotation curves, without the need for dark matter [4,[50][51][52][53][54]. This prediction is based on the assumption that massive (test) particles have fixed rest masses and follow timelike geodesics in the MK metric.…”

“…The above conformal equivalence immediately raises some concerns about the use of the MK line-element (12)(13) in modelling galaxy rotation curves without invoking dark matter [50][51][52][53][54]. A key assumption in such analyses is that a matter test particle follows a timelike geodesic in the spacetime geometry defined by the MK line-element.…”

“…CG is most celebrated phenomenologically, however, for its fitting of flat galaxy rotation curves without the need for dark matter [50][51][52][53][54]. This is based on the trajectories of massive particles in the MK metric, with parameter values that are consistent with solar-system tests, although the requirement for matching the MK metric onto a static, spherically-symmetric matter source suggests that it may not be possible to set the parameters in a consistent manner [55][56][57].…”

“…This analysis was criticised in [64], however, who pointed out that the MK metric is conformally equivalent to the SdS metric, without the need to introduce a scalar field [1,14,65,66], and that the scalar field in the special solution investigated in [63] has vanishing energy-momentum, which was therefore considered to be 'trivial' because it has no effect on the geometry. It is unclear from [64], however, whether these criticisms are considered to validate previous analyses [50][51][52][53][54] using the MK metric to fit flat galaxy rotation curves.…”