Distinguishing standard from modified gravity in the local group and beyond
Abstract
The works in this portfolio test the hypothesis that it is not possible to extrapolate
the Newtonian inverse square law of gravity from Solar System to galaxy scales. In
particular, I look into various tests of Modified Newtonian Dynamics (MOND), which
posits a modification below a very low acceleration threshold. Although discrepancies
with Newtonian dynamics are indeed observed, they can usually be explained by
invoking an appropriate distribution of invisible mass known as dark matter (DM).
This leads to the standard cosmological paradigm, ΛCDM. I consider how it may be
distinguished from MOND using collision velocities of galaxy clusters, which should
sometimes be much faster in MOND. I focus on measuring these velocities more accurately
and conclude that this test ought to be feasible in the near future.
For the time being, I look at the much nearer and more accurately observed
Local Group (LG) of galaxies. Its main constituents the Milky Way (MW) and
Andromeda (M31) should have undergone a past close flyby in MOND but not
in ΛCDM. The fast MW-M31 relative motion around the time of their flyby would
have allowed them to gravitationally slingshot any passing LG dwarf galaxies out at
high speed. I consider whether there is any evidence for such high-velocity galaxies
(HVGs). Several candidates are found in two different ΛCDM models of the LG,
one written by a founding figure of the paradigm. The properties of these HVGs are
similar to what might be expected in MOND, especially their tendency to lie close
to a plane. Being more confident of its validity, I then used MOND to determine the
escape velocity curve of the MW over the distance range 850 kpc, finding reasonable
agreement with the latest observations. I finish by discussing possible future directions
for MOND research.
Type
Thesis, PhD Doctor of Philosophy
Collections
Description of related resources
Banik, I & Zhao, H 2015, 'Effects of lens motion and uneven magnification on image spectra', Monthly Notices of the Royal Astronomical Society, vol. 450, no. 3, pp. 3155-3168. https://doi.org/10.1093/mnras/stv802Banik, I & Zhao, H 2016, 'Dynamical history of the Local Group in ΛCDM', Monthly Notices of the Royal Astronomical Society, vol. 459, no. 2, pp. 2237-2261. https://doi.org/10.1093/mnras/stw787
Banik, I & Zhao, H 2017, 'Dynamical history of the Local Group in ΛCDM II − including external perturbers In 3D', Monthly Notices of the Royal Astronomical Society, vol. 467, no. 2, pp. 2180-2198. https://doi.org/10.1093/mnras/stx151
Banik, I & Zhao, H 2018, 'A plane of high-velocity galaxies across the Local Group', Monthly Notices of the Royal Astronomical Society, vol. 473, no. 3, pp. 4033-4054. https://doi.org/10.1093/mnras/stx2596
Banik, I & Zhao, H 2018, 'The escape velocity curve of the Milky Way in Modified Newtonian dynamics', Monthly Notices of the Royal Astronomical Society, vol. 473, no. 1, pp. 419-430. https://doi.org/10.1093/mnras/stx2350
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