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Modified Gravity and Dark Matter at the Scale of Galaxies

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A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Gravitation".

Deadline for manuscript submissions: closed (30 September 2022) | Viewed by 6146

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Special Issue Editors

Dr. Ivan De Martino

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Guest Editor

Department of Physic, Universidad de Salamanca, 37007 Salamanca, Spain
Interests: gravitation; modified gravity; dark matter; galactic dynamics; gravitational waves
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Mahmood Roshan

E-Mail Website
Guest Editor

Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, Mashhad P.O. Box 1436, Iran
Interests: modified gravity; dark matter; galactic dynamics

Special Issue Information

Dear Colleagues,

In the 1970s, V. C. Rubin and W. K. Ford confirmed the flatness of rotation curves, leading to the conclusion that galaxies are embedded in massive dark halos that extend to large radii. In the last fifty years, dark matter has been established as one of the main components of the Universe. It is the second most important component, driving the emergence of large-scale structures. Nevertheless, its fundamental nature is still completely unknown.

Although the cold dark matter paradigm successfully explains both the emergence and evolution of cosmic structures on large scales, it faces persistent challenges on the scales of galaxies that are primarily related to dark matter distribution in the innermost regions of the halos of galaxies and to the dynamic properties of dwarf galaxies. There are three possible origins: (1) the baryonic physics that affect galaxy formation; (2) dark matter differs from the conventional cold dark matter; and (3) the theory of gravity departs from that of general relativity. Solving these discrepancies is a rapidly evolving research field.

The aim of this Special Issue is to focus on the kinematic and dynamic probes that stellar systems and galaxies offer in order to test the dark matter paradigm and the underlying theory of gravity. Contributions to this issue may be related to alternative paradigms (for instance, warm dark matter, self-interacting dark matter, fuzzy dark matter, and axions, among others), detection strategies, N-body simulations and data analysis methods, modified gravity models that modify the dynamics at the scale of galaxies, dynamical and kinematical probes of dark matter and modified gravity at the scale of stellar system and galaxies, recent discoveries confirming or questioning the standard paradigm, and reviews on the state-of-the-art of the field.

Dr. Ivan De Martino
Prof. Dr. Mahmood Roshan
Guest Editors

Manuscript Submission Information

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Keywords

dark matter
gravitation
dark matter detection strategies
small-scale CDM problems
N-body simulations

Published Papers (4 papers)

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Research

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16 pages, 806 KiB

Open AccessArticle

The Effect of a Spiral Density Wave on the Galaxy’s Rotation Curve, as Applied to the Andromeda Galaxy (M31)

by Miroslava Vukcevic

Universe 2022, 8(10), 522; https://doi.org/10.3390/universe8100522 - 08 Oct 2022

Viewed by 1233

Abstract

The rotational velocity curve, which is the circular velocity profile of the stars and gas in a spiral galaxy as a function of their distance from the galactic center, plays an important role in the kinematic and dynamic investigation of spiral galaxies. There are observations of approximately flat rotation curves (RC) at large distances that have introduced mass discrepancy between the theoretically derived RC and the observed one. In this paper, we derive a rotational velocity expression using a nonlinear spiral density wave solution for the surface mass density (SMD) within the disk. We show that the proposed nonlinear spiral solution is able to support the observed flat rotational velocity curve for large distances with no mass deficiency. The aim of the paper is to confirm the crucial importance of the mass distribution on the rotation curve profile. Although the model is limited by the fluid description of the galactic disk, it provides an improved rotational velocity expression and a rotation curve with no mass discrepancy in the outer part of the disk due to the inclusion of the spiral mass distribution. The disk mass has not been averaged within the exponential disk approximation, but it rather follows the observed spiral pattern given by the analytical solution of the nonlinear equation. The M31 galaxy has been chosen as the closest and well mapped spiral galaxy, similar in many aspects to our host galaxy, in order to apply a rotational velocity expression that accounts for nonlinear effects and derive RC. The obtained result can have a strong influence on large-scale gravity dynamics, as well. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)

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20 pages, 382 KiB

Open AccessEditor’s ChoiceArticle

Nonlocal Gravity: Modification of Newtonian Gravitational Force in the Solar System

by Mahmood Roshan and Bahram Mashhoon

Universe 2022, 8(9), 470; https://doi.org/10.3390/universe8090470 - 08 Sep 2022

Cited by 5 | Viewed by 1612

Abstract

Nonlocal gravity (NLG) is a classical nonlocal generalization of Einstein’s theory of gravitation developed in close analogy with the nonlocal electrodynamics of media. It appears that the nonlocal aspect of the universal gravitational interaction could simulate dark matter. Within the Newtonian regime of NLG, we investigate the deviation of the gravitational force from the Newtonian inverse square law as a consequence of the existence of the effective dark matter. In particular, we work out the magnitude of this deviation in the solar system out to 100 astronomical units. Moreover, we give an improved lower limit for the short-range parameter of the reciprocal kernel of NLG. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)

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Figure A1

14 pages, 800 KiB

Open AccessArticle

Splashback Radius in a Spherical Collapse Model

by Antonino Del Popolo and Morgan Le Delliou

Universe 2022, 8(9), 462; https://doi.org/10.3390/universe8090462 - 06 Sep 2022

Viewed by 1010

Abstract

It was shown several years ago that dark matter halo outskirts are characterized by very steep density profiles in a very small radial range. This feature has been interpreted as a pile-up of different particle orbits at a similar location, namely, splashback material at half an orbit after collapse. Adhikari et al. (2014) obtained the location of the splashback radius through a very simple model by calculating a dark matter shell trajectory in the secondary infall model while it crosses a growing NFW profile-shaped dark matter halo. Because they imposed a halo profile instead of calculating it from the trajectories of the shells of dark matter, they were not able to find the dark matter profile around the splashback radius. In the present paper, we use an improved spherical infall model taking into account shell crossing as well as several physical effects such as ordered and random angular momentum, dynamical friction, adiabatic contraction, etc. This allows us to determine the density profile from the inner to the outer region and to study the behavior of the outer density profile. We compare the density profiles and their logarithmic slope of with the simulation results of Diemer and Kravtsov (2014), finding a good agreement between the prediction of the model and the simulations. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)

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Review

Jump to: Research

30 pages, 3392 KiB

Open AccessReview

Dark Coincidences: Small-Scale Solutions with Refracted Gravity and MOND

by Valentina Cesare

Universe 2023, 9(1), 56; https://doi.org/10.3390/universe9010056 - 16 Jan 2023

Cited by 2 | Viewed by 1598

Abstract

General relativity and its Newtonian weak field limit are not sufficient to explain the observed phenomenology in the Universe, from the formation of large-scale structures to the dynamics of galaxies, with the only presence of baryonic matter. The most investigated cosmological model, the [...] Read more.

Λ

CDM, accounts for the majority of observations by introducing two dark components, dark energy and dark matter, which represent ∼95% of the mass-energy budget of the Universe. Nevertheless, the

Λ

CDM model faces important challenges on the scale of galaxies. For example, some very tight relations between the properties of dark and baryonic matters in disk galaxies, such as the baryonic Tully–Fisher relation (BTFR), the mass discrepancy–acceleration relation (MDAR), and the radial acceleration relation (RAR), which see the emergence of the acceleration scale

a_{0} ≃ 1.2 \times 10^{- 10}

m s

^{- 2}

, cannot be intuitively explained by the CDM paradigm, where cosmic structures form through a stochastic merging process. An even more outstanding coincidence is due to the fact that the acceleration scale

a_{0}

, emerging from galaxy dynamics, also seems to be related to the cosmological constant

Λ

. Another challenge is provided by dwarf galaxies, which are darker than what is expected in their innermost regions. These pieces of evidence can be more naturally explained, or sometimes even predicted, by modified theories of gravity, that do not introduce any dark fluid. I illustrate possible solutions to these problems with the modified theory of gravity MOND, which departs from Newtonian gravity for accelerations smaller than

a_{0}

, and with Refracted Gravity, a novel classical theory of gravity introduced in 2016, where the modification of the law of gravity is instead regulated by a density scale. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)

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