scholarly journals Does the Cosmological Expansion Change Local Dynamics?

Symmetry ◽  
2021 ◽  
Vol 13 (8) ◽  
pp. 1417
Author(s):  
Marcelo Schiffer
Keyword(s):  
Dark Matter ◽  
Modified Gravity ◽  
Hubble Constant ◽  
Matter Content ◽  
Matter Distribution ◽  
Present Value ◽  
Field Equations ◽  
Local Dynamics ◽  
Cosmological Expansion ◽  
Modified Gravity Theories

It is a well-known fact that the Newtonian description of dynamics within Galaxies for its known matter content is in disagreement with the observations as the acceleration approaches a0≈1.2×10−10 m/s2 (slighter larger for clusters). Both the Dark Matter scenario and Modified Gravity Theories (MGT) fail to explain the existence of such an acceleration scale. Motivated by the closeness of the acceleration scale and the Hubble constant cH0≈10−9 h m/s2, we are led to analyze whether this coincidence might have a Cosmological origin for scalar-tensor and spinor-tensor theories by performing detailed calculations for perturbations that represent the local matter distribution on the top of the cosmological background. Then, we solve the field equations for these perturbations in a power series in the present value of the Hubble constant. As we shall see, for both theories, the power expansion contains only even powers in the Hubble constant, a fact that renders the cosmological expansion irrelevant for the local dynamics.

scholarly journals Dark matter in logarithmic F(R) gravity

2019 ◽  
Vol 28 (12) ◽  
pp. 1950157 ◽  
Author(s):  
Tomohiro Inagaki ◽  
Yamato Matsuo ◽  
Hiroki Sakamoto
Keyword(s):  
General Relativity ◽  
Dark Matter ◽  
Modified Gravity ◽  
Field Method ◽  
Quantum Corrections ◽  
Prototype Model ◽  
Wide Range ◽  
Modified Gravity Theories ◽  
Primordial Inflation ◽  
Cmb Fluctuations

The logarithmic [Formula: see text]-corrected [Formula: see text] gravity is investigated as a prototype model of modified gravity theories with quantum corrections. By using the auxiliary field method, the model is described by the general relativity with a scalaron field. The scalaron field can be identified as an inflaton at the primordial inflation era. It is also one of the dark matter candidates in the dark energy (DE) era. It is found that a wide range of the parameters is consistent with the current observations of CMB fluctuations, DE and dark matter.

scholarly journals Imprint of Pressure on Characteristic Dark Matter Profiles: The Case of ESO0140040

Galaxies ◽  
2020 ◽  
Vol 8 (4) ◽  
pp. 74
Author(s):  
Kuantay Boshkayev ◽  
Talgar Konysbayev ◽  
Ergali Kurmanov ◽  
Orlando Luongo ◽  
Marco Muccino
Keyword(s):  
Dark Matter ◽  
Complex Structure ◽  
Matter Content ◽  
Model Parameters ◽  
Matter Distribution ◽  
Density Profiles ◽  
The Galaxy ◽  
Dark Matter Distribution ◽  
Observational Signature

We investigate the dark matter distribution in the spiral galaxy ESO0140040, employing the most widely used density profiles: the pseudo-isothermal, exponential sphere, Burkert, Navarro-Frenk-White, Moore and Einasto profiles. We infer the model parameters and estimate the total dark matter content from the rotation curve data. For simplicity, we assume that dark matter distribution is spherically symmetric without accounting for the complex structure of the galaxy. Our predictions are compared with previous results and the fitted parameters are statistically confronted for each profile. We thus show that although one does not include the galaxy structure it is possible to account for the same dynamics assuming that dark matter provides a non-zero pressure in the Newtonian approximation. In this respect, we solve the hydrostatic equilibrium equation and construct the dark matter pressure as a function for each profile. Consequently, we discuss the dark matter equation of state and calculate the speed of sound in dark matter. Furthermore, we interpret our results in view of our approach and we discuss the role of the refractive index as an observational signature to discriminate between our approach and the standard one.

scholarly journals Testing modified gravity with wide binaries in Gaia DR2

2019 ◽  
Vol 488 (4) ◽  
pp. 4740-4752 ◽  
Author(s):  
Charalambos Pittordis ◽  
Will Sutherland
Keyword(s):  
Dark Matter ◽  
Binary Stars ◽  
Modified Gravity ◽  
Simulated Data ◽  
Peak Shift ◽  
Wide Binary ◽  
A Value ◽  
Modified Gravity Theories ◽  
Clear Peak ◽  
Gaia Dr2

ABSTRACT Several recent studies have shown that very wide binary stars can potentially provide an interesting test for modified-gravity theories which attempt to emulate dark matter; these systems should be almost Newtonian according to standard dark-matter theories, while the predictions for MOND-like theories are distinctly different, if the various observational issues can be overcome. Here we explore an observational application of the test from the recent Gaia DR2 data release: we select a large sample of ∼24 000 candidate wide binary stars with distance $\lt 200 \, {\rm pc}$ and magnitudes G < 16 from Gaia DR2, and estimated component masses using a main-sequence mass–luminosity relation. We then compare the frequency distribution of pairwise relative projected velocity (relative to circular-orbit value) as a function of projected separation; these distributions show a clear peak at a value close to Newtonian expectations, along with a long ‘tail’ which extends to much larger velocity ratios; the ‘tail’ is considerably more numerous than in control samples constructed from DR2 with randomized positions, so its origin is unclear. Comparing the velocity histograms with simulated data, we conclude that MOND-like theories without an external field effect (ExFE) are strongly inconsistent with the observed data since they predict a peak-shift in clear disagreement with the data; testing MOND-like theories with an ExFE is not decisive at present, but has good prospects to become decisive in future with improved modelling or understanding of the high-velocity tail, and additional spectroscopic data.

scholarly journals Blackhole in nonlocal gravity: comparing metric from Newmann–Janis algorithm with slowly rotating solution

2020 ◽  
Vol 80 (7) ◽  
Author(s):  
Utkarsh Kumar ◽  
Sukanta Panda ◽  
Avani Patel
Keyword(s):  
General Relativity ◽  
Gravitational Field ◽  
Modified Gravity ◽  
Kerr Metric ◽  
Slow Rotation ◽  
Field Equations ◽  
External Region ◽  
Modified Gravity Theories ◽  
Spacetime Metric ◽  
Gravity Theories

Abstract The strong gravitational field near massive blackhole is an interesting regime to test General Relativity (GR) and modified gravity theories. The knowledge of spacetime metric around a blackhole is a primary step for such tests. Solving field equations for rotating blackhole is extremely challenging task for the most modified gravity theories. Though the derivation of Kerr metric of GR is also demanding job, the magical Newmann–Janis algorithm does it without actually solving Einstein equation for rotating blackhole. Due to this notable success of Newmann–Janis algorithm in the case of Kerr metric, it has been being used to obtain rotating blackhole solution in modified gravity theories. In this work, we derive the spacetime metric for the external region of a rotating blackhole in a nonlocal gravity theory using Newmann–Janis algorithm. We also derive metric for a slowly rotating blackhole by perturbatively solving field equations of the theory. We discuss the applicability of Newmann–Janis algorithm to nonlocal gravity by comparing slow rotation limit of the metric obtained through Newmann–Janis algorithm with slowly rotating solution of the field equation.

scholarly journals The Train Wreck Cluster Abell 520 and the Bullet Cluster 1E0657-558 in a Generalized Theory of Gravitation

Galaxies ◽  
2018 ◽  
Vol 6 (2) ◽  
pp. 41 ◽  
Author(s):  
Norman Israel ◽  
John Moffat
Keyword(s):  
Dark Matter ◽  
Gravitational Field ◽  
Modified Gravity ◽  
Gravitational Theory ◽  
Field Limit ◽  
X Ray ◽  
Weak Gravitational Field ◽  
Modified Gravity Theories ◽  
Theory Of Gravitation ◽  
Map Data

A major hurdle for modified gravity theories is to explain the dynamics of galaxy clusters. A case is made for a generalized gravitational theory called Scalar-Tensor-Vector-Gravity (STVG) or MOG (Modified Gravity) to explain merging cluster dynamics. The paper presents the results of a re-analysis of the Bullet Cluster, as well as an analysis of the Train Wreck Cluster in the weak gravitational field limit without dark matter. The King- β model is used to fit the X-ray data of both clusters, and the κ -maps are computed using the parameters of this fit. The amount of galaxies in the clusters is estimated by subtracting the predicted κ -map from the κ -map data. The estimate for the Bullet Cluster is that 14.1 % of the cluster is composed of galaxies. For the Train Wreck Cluster, if the Jee et al. data are used, 25.7 % of the cluster is composed of galaxies. The baryon matter in the galaxies and the enhanced strength of gravitation in MOG shift the lensing peaks, making them offset from the gas. The work demonstrates that this generalized gravitational theory can explain merging cluster dynamics without dark matter.

scholarly journals SOME ASPECTS OF GENERALIZED MODIFIED GRAVITY MODELS

2013 ◽  
Vol 22 (08) ◽  
pp. 1330017 ◽  
Author(s):  
R. MYRZAKULOV ◽  
L. SEBASTIANI ◽  
S. ZERBINI
Keyword(s):  
Modified Gravity ◽  
Correction Term ◽  
High Redshift ◽  
Early Time ◽  
Gravity Models ◽  
High Frequency Oscillation ◽  
Field Equations ◽  
Modified Gravity Theories ◽  
Gravity Theories ◽  
General Aspects

In this paper, we review some general aspects of modified gravity theories, investigating mathematical and physical properties and, more specifically, the feature of viable and realistic models able to reproduce the dark energy (DE) epoch and the early-time inflation. We will discuss the black hole (BH) solutions in generalized theories of gravity: it is of fundamental interest to understand how properties and laws of BHs in General Relativity (GR) can be addressed in the framework of modified theories. In particular, we will discuss the energy issue and the possibility to derive the First Law of thermodynamics from the field equations. Then, in the analysis of cosmological solutions, we will pay particular attention to the occurrence of finite-time future singularities and to the possibility to avoid them in [Formula: see text]-gravity. Furthermore, realistic models of F(R)-gravity will be analyzed in detail. A general feature occurring in matter era will be shown, namely, the high derivatives of Hubble parameter may be influenced by the high frequency oscillation of the DE and some correction term may be required in order to stabilize the theory at high redshift. The inflationary scenario is also carefully analyzed and a unified description of the universe is evolved. In the final part of the work, we will look at the last developments in modified gravity, namely, we will investigate cosmological and BH solutions in a covariant field theory of gravity and we will introduce the extended "teleparallel" F(T)-gravity theories. A nice application to the dark matter (DM) problem will be presented.

scholarly journals The radial acceleration relation and its emergent nature

2019 ◽  
Vol 15 (S359) ◽  
pp. 457-459
Author(s):  
Davi C. Rodrigues ◽  
Valerio Marra
Keyword(s):  
Dark Matter ◽  
Modified Gravity ◽  
Recent Analysis ◽  
Radial Acceleration ◽  
Modified Gravity Theories ◽  
Bayesian Test ◽  
Gravity Theories

AbstractWe review some of our recent results about the Radial Acceleration Relation (RAR) and its interpretation as either a fundamental or an emergent law. The former interpretation is in agreement with a class of modified gravity theories that dismiss the need for dark matter in galaxies (MOND in particular). Our most recent analysis, which includes refinements on the priors and the Bayesian test for compatibility between the posteriors, confirms that the hypothesis of a fundamental RAR is rejected at more than 5σ from the very same data that was used to infer the RAR.

Effects of non-vanishing dark matter pressure in the Milky Way Galaxy

2021 ◽  
Vol 508 (1) ◽  
pp. 1543-1554
Author(s):  
K Boshkayev ◽  
T Konysbayev ◽  
E Kurmanov ◽  
O Luongo ◽  
D Malafarina ◽  
...  
Keyword(s):  
Black Hole ◽  
Dark Matter ◽  
Gravitational Lensing ◽  
Matter Content ◽  
Dark Matter Halo ◽  
Matter Distribution ◽  
Milky Way Galaxy ◽  
The Galaxy ◽  
Dark Matter Distribution ◽  
Inner Parts

ABSTRACT We consider the possibility that the Milky Way’s dark matter halo possesses a non-vanishing equation of state. Consequently, we evaluate the contribution due to the speed of sound, assuming that the dark matter content of the galaxy behaves like a fluid with pressure. In particular, we model the dark matter distribution via an exponential sphere profile in the galactic core, and inner parts of the galaxy whereas we compare the exponential sphere with three widely used profiles for the halo, i.e. the Einasto, Burkert and Isothermal profile. For the galactic core, we also compare the effects due to a dark matter distribution without black hole with the case of a supermassive black hole in vacuum and show that present observations are unable to distinguish them. Finally we investigate the expected experimental signature provided by gravitational lensing due to the presence of dark matter in the core.

scholarly journals KERR–NEWMAN BLACK HOLES IN f(R) THEORIES

2013 ◽  
Vol 11 (01) ◽  
pp. 1450001 ◽  
Author(s):  
J. A. R. CEMBRANOS ◽  
A. DE LA CRUZ-DOMBRIZ ◽  
P. JIMENO ROMERO
Keyword(s):  
Black Holes ◽  
Scalar Curvature ◽  
Modified Gravity ◽  
Constant Scalar Curvature ◽  
Field Equations ◽  
Metric Tensor ◽  
Modified Gravity Theories ◽  
Astrophysical Objects ◽  
Newman Black Hole ◽  
The Rich

In the context of f(R) modified gravity theories, we study the Kerr–Newman black hole solutions. We study nonzero constant scalar curvature solutions and discuss the metric tensor that satisfies the modified field equations. We conclude that, in the absence of a cosmological constant, the black holes (BHs) existence is determined by the sign of a parameter h dependent of the mass, the charge, the spin and the scalar curvature. Different values of this parameter lead to diverse astrophysical objects, such as extremal and marginal extremal BHs. Thermodynamics of BHs are then studied, as well as their local and global stability. We analyze these features in a large variety of f(R) models. We remark the main differences with respect to general relativity and show the rich thermodynamical phenomenology that characterizes this framework.

Spherically symmetric traversable wormholes in f(R2,T) gravity

2019 ◽  
Vol 16 (10) ◽  
pp. 1950147 ◽  
Author(s):  
M. Zubair ◽  
Quratulien Muneer ◽  
Saira Waheed
Keyword(s):  
Modified Gravity ◽  
Arbitrary Constant ◽  
Energy Momentum Tensor ◽  
Energy Condition ◽  
Momentum Tensor ◽  
Anisotropic Fluid ◽  
Spherically Symmetric ◽  
Matter Distribution ◽  
Field Equations ◽  
Traversable Wormholes

In this paper, we explore the possibility of wormhole solutions existence exhibiting spherical symmetry in an interesting modified gravity based on Ricci scalar term and trace of energy–momentum tensor. For this reason, we assume the matter distribution as anisotropic fluid and a specific viable form of the generic function given by [Formula: see text] involving [Formula: see text] and [Formula: see text], two arbitrary constant parameters. For having a simplified form of the resulting field equations, we assume three different forms of EoS of the assumed matter contents. In each case, we find the numerical wormhole solutions and analyze their properties for the wormhole existence graphically. The graphical behavior of the energy condition bounds is also investigated in each case. It is found that a realistic wormhole solutions satisfying all the properties can be obtained in each case.

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