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Register a new userA common Milgromian acceleration scale in nature
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Pavel Kroupa
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A central problem of contemporary physics is whether the law of gravity is non-Newtonian on galaxy scales. Rodrigues et al. argue that Milgromian gravitation, which solves the flat rotation curve problem without the need for dark matter particles, is ruled out at > 10{sigma} significance. To a large extent, this conclusion relies on galaxies with very uncertain distances and/or nearly edge-on orientations, where dust obscuration often becomes significant. Applying appropriate quality cuts to the data leaves only a handful of outliers to the predictions of Milgromian gravitation according to the analysis of Rodrigues et al., but even these outliers can be explained with Milgromian gravitation.
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Dark matter phenomena in rotationally supported galaxies exhibit a characteristic acceleration scale of $g_dagger approx 1.2times 10^{-10}$ m s$^{-2}$. Whether this acceleration is a manifestation of a universal scale, or merely an emergent property with an intrinsic scatter, has been debated in the literature. Here we investigate whether a universal acceleration scale exists in dispersion-supported galaxies using two uniform sets of integral field spectroscopy (IFS) data from SDSS-IV MaNGA and ATLAS$^{rm 3D}$. We apply the spherical Jeans equation to 15 MaNGA and 4 ATLAS$^{rm 3D}$ slow-rotator E0 (i.e., nearly spherical) galaxies. Velocity dispersion profiles for these galaxies are well determined with observational errors under control. Bayesian inference indicates that all 19 galaxies are consistent with a universal acceleration of $g_dagger=1.5_{-0.6}^{+0.9}times 10^{-10}$ m s$^{-2}$. Moreover, all 387 data points from the radial bins of the velocity dispersion profiles are consistent with a universal relation between the radial acceleration traced by dynamics and that predicted by the observed distribution of baryons. This universality remains if we include 12 additional non-E0 slow-rotator elliptical galaxies from ATLAS$^{rm 3D}$. Finally, the universal acceleration from MaNGA and ATLAS$^{rm 3D}$ is consistent with that for rotationally supported galaxies, so our results support the view that dark matter phenomenology in galaxies involves a universal acceleration scale.
Previous studies of globular cluster (GC) systems show that there appears to be a universal specific GC formation efficiency $eta$ which relates the total mass of GCs to the virial mass of host dark matter halos, $M_{vir}$ (Georgiev et al 2010, Spitler & Forbes2009). In this paper, the specific frequency, $S_N$, and specific GC formation efficiency, $eta$, are derived as functions of $M_{vir}$ in Milgromian dynamics, i.e., in modified Newtonian dynamics (MOND). In Milgromian dynamics, for the galaxies with GCs, the mass of the GC system, $M_{GC}$, is a two-component function of $M_{vir}$ instead of a simple linear relation. An observer in a Milgromian universe, who interprets this universe as being Newtonian/Einsteinian, will incorrectly infer a universal constant fraction between the mass of the GC system and a (false) dark matter halo of the baryonic galaxy. In contrast to a universal constant of $eta$, in a Milgromian universe, for galaxies with $M_{vir} <= 10^{12}msun$, $eta$ decreases with the increase of $M_{vir}$, while for massive galaxies with $M_{vir}>10^{12}msun$, $eta$ increases with the increase of $M_{vir}$.
We study the radial acceleration relation (RAR) between the total ($a_{rm tot}$) and baryonic ($a_{rm bary}$) centripetal acceleration profiles of central galaxies in the cold dark matter (CDM) paradigm. We analytically show that the RAR is intimately connected with the physics of the quasi-adiabatic relaxation of dark matter in the presence of baryons in deep potential wells. This cleanly demonstrates how the mean RAR and its scatter emerge in the low-acceleration regime ($10^{-12},{rm m,s}^{-2}lesssim a_{rm bary}lesssim10^{-10},{rm m,s}^{-2}$) from an interplay between baryonic feedback processes and the distribution of CDM in dark halos. Our framework allows us to go further and study both higher and lower accelerations in detail, using analytical approximations and a realistic mock catalog of $sim342,000$ low-redshift central galaxies with $M_rleq-19$. We show that, while the RAR in the baryon-dominated, high-acceleration regime ($a_{rm bary}gtrsim10^{-10},{rm m,s}^{-2}$) is very sensitive to details of the relaxation physics, a simple `baryonification prescription matching the relaxation results of hydrodynamical CDM simulations is remarkably successful in reproducing the observed RAR without any tuning. And in the (currently unobserved) ultra-low-acceleration regime ($a_{rm bary}lesssim 10^{-12},{rm m,s}^{-2}$), the RAR is sensitive to the abundance of diffuse gas in the halo outskirts, with our default model predicting a distinctive break from a simple power-law-like relation for HI-deficient, diffuse gas-rich centrals. Our mocks also show that the RAR provides more robust, testable predictions of the $Lambda$CDM paradigm at galactic scales, with implications for alternative gravity theories, than the baryonic Tully-Fisher relation.
Since its publication 1983, Milgromian dynamics (aka MOND) has been very successful in modeling the gravitational potential of galaxies from baryonic matter alone. However, the dynamical modeling has long been an unsolved issue. In particular, the setup of a stable galaxy for Milgromian N-body calculations has been a major challenge. Here, we show a way to set up disc galaxies in MOND for calculations in the PHANTOM OF RAMSES (PoR) code by Lughausen (2015) and Teyssier (2002). The method is done by solving the QUMOND Poisson equations based on a baryonic and a phantom dark matter component. The resulting galaxy models are stable after a brief settling period for a large mass and size range. Simulations of single galaxies as well as colliding galaxies are shown.
The KBC void is a local underdensity with the observed relative density contrast $delta equiv 1 - rho/rho_{0} = 0.46 pm 0.06$ between 40 and 300 Mpc around the Local Group. If mass is conserved in the Universe, such a void could explain the $5.3sigma$ Hubble tension. However, the MXXL simulation shows that the KBC void causes $6.04sigma$ tension with standard cosmology ($Lambda$CDM). Combined with the Hubble tension, $Lambda$CDM is ruled out at $7.09sigma$ confidence. Consequently, the density and velocity distribution on Gpc scales suggest a long-range modification to gravity. In this context, we consider a cosmological MOND model supplemented with $11 , rm{eV}/c^{2}$ sterile neutrinos. We explain why this $ u$HDM model has a nearly standard expansion history, primordial abundances of light elements, and cosmic microwave background (CMB) anisotropies. In MOND, structure growth is self-regulated by external fields from surrounding structures. We constrain our model parameters with the KBC void density profile, the local Hubble and deceleration parameters derived jointly from supernovae at redshifts $0.023 - 0.15$, time delays in strong lensing systems, and the Local Group velocity relative to the CMB. Our best-fitting model simultaneously explains these observables at the $1.14%$ confidence level (${2.53 sigma}$ tension) if the void is embedded in a time-independent external field of ${0.055 , a_{_0}}$. Thus, we show for the first time that the KBC void can naturally resolve the Hubble tension in Milgromian dynamics. Given the many successful a priori MOND predictions on galaxy scales that are difficult to reconcile with $Lambda$CDM, Milgromian dynamics supplemented by $11 , rm{eV}/c^{2}$ sterile neutrinos may provide a more holistic explanation for astronomical observations across all scales.
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