arXiv subjects

Hossein Shenavar

Publications and source records attributed to Hossein Shenavar.

6 recordsLinked to original sources

arXiv2023

Three-Body Problem in Modified Dynamics

General properties of the three-body problem in a model of modified dynamics are investigated. It is shown that the three-body problem in this model shares some characters with the similar problem in Newtonian dynamics. Moreover, the planar restricted three-body problem is solved analytically for this type of extended gravity and it is proved that under certain conditions, which generally happen at galactic and extragalactic scales, the orbits around $L_{4}$ and $L_{5}$ Lagrange points are stable. Furthermore, a code is provided to compare the behavior of orbits in the restricted three-body problem under Newtonian and modified dynamics. Orbit integrations based on this code show contrasting orbital behavior under the two dynamics and specially exhibit in a qualitative way that the rate of ejections is smaller in the modified dynamics compared to Newtonian gravity. These results could help us to search for observational signatures of extended gravities.

astro-ph.GA↗

arXiv2023

Galactic Scaling Rules in a Modified Dynamical Model

Schulz (2017) galactic scaling rules, which include baryonic Tully-Fisher relation, have been surveyed in this work within the context of a modified dynamical model. These scaling relations are derived by employing the virial theorem and applying equilibrium and stability conditions. The scaling rules are also obtained by dimensional analysis of an integral relation between surface density and circular velocity of disk galaxies. To check the validity of the scaling relations based on observational data, we have defined, based on the properties of the model, the proper equilibrium size $R_{eq}$ and equilibrium velocity $V_{eq}$ of systems. By employing these measures of length and velocity, SPARC data ( Lelli et al. 2016a ) is used to analyze the results. The viability of the scaling relations is tested and it is shown that, compared to some other measures of length and velocity, $R_{eq}$ and $V_{eq}$ provide the closest fits to the theoretical predictions. We have compared our results with prior works and have concluded that the set of baryonic Tully-Fisher relation plus mass-size relation ( or mass-velocity ) provides an appropriate description of the general characteristics of the systems. Lastly, it is shown that these scaling relations predict a certain evolution of galactic properties with redshift. This behavior provides a chance to examine the cosmic evolution of the present modified dynamical model in future works.

astro-ph.GA↗

arXiv2020

A Modified Dynamical Model of Cosmology I. Theory

Wheeler (1964) had formulated Mach's principle as the boundary condition for general relativistic field equations. Here, we use this idea and develop a modified dynamical model of cosmology based on imposing Neumann boundary condition on cosmological perturbation equations. Then, it is shown that a new term appears in the equation of motion, which leads to a modified Poisson equation. In addition, a modified Hubble parameter is derived due to the presence of the new term. Moreover, it is proved that, without a cosmological constant, such a model has a late time-accelerated expansion with an equation of state converging to $w < -1$. Also, the luminosity distance in the present model is shown to differ from that of the $ΛCDM$ model at high redshifts. Furthermore, it is found that the adiabatic sound speed squared is positive in radiation-dominated era and then converges to zero at later times. Theoretical implications of the Neumann boundary condition have been discussed, and it is shown that, by fixing the value of the conjugate momentum (under certain conditions), one could derive a similar version of modified dynamics. In a future work, we will confine the free parameters of the Neumann model based on hype Ia Supernovae, Hubble parameter data, and the age of the oldest stars.

gr-qc↗

arXiv2019

Motion of Particles in Solar and Galactic Systems by Using Neumann Boundary Condition

A new equation of motion, which is derived previously by imposing Neumann boundary condition on cosmological perturbation equations (Shenavar 2016 a), is investigated. By studying the precession of perihelion, it is shown that the new equation of motion suggests a small, though detectable, correction in orbits of solar system objects. Then a system of particles is surveyed to have a better understanding of galactic structures. Also the general form of the force law is introduced by which the rotation curve and mass discrepancy of axisymmetric disks of stars are derived. In addition, it is suggested that the mass discrepancy as a function of centripetal acceleration becomes significant near a constant acceleration $ 2c_{1}a_{0} $ where $c_{1}$ is the Neumann constant and $ a_{0} = 6.59 \times 10^{-10} $ $m/s^{2}$ is a fundamental acceleration. Furthermore, it is shown that a critical surface density equal to $ σ_{0}=a_{0}/G $, in which G is the Newton gravitational constant, has a significant role in rotation curve and mass discrepancy plots. Also, the specific form of NFW mass density profile at small radii, $ ρ\propto 1/r $, is explained too. Finally, the present model will be tested by using a sample of 39 LSB galaxies for which we will show that the rotation curve fittings are generally acceptable. The derived mass to light ratios too are found within the plausible bound except for the galaxy F571-8.

astro-ph.GA↗

arXiv2019

Pseudo-Evolution of Galaxies in LCDM Cosmology

Our knowledge about galaxy evolution comes from transforming observed galaxy properties at different redshifts to co-moving physical scales. This transformation depends on using a cosmological model. Here the effects of unintentional mixing of two different cosmological models on the size evolution of galaxies is studied. As a gedanken experiment, a galaxy of fixed proper size and luminosity is moved across different redshifts. The apparent size of this galaxy is then interpreted with a cosmological model presumed by the observer, which is different compared to the cosmology exhibited by the Universe. In such a case, a spurious size evolution of the galaxy is observed. A galaxy behaving according to the R_h=ct and Neumann's cosmology, when interpreted with the LCDM cosmological model, shows an increase in size by a factor of 1.1 and 1.3 from z=7.5 to z approx. 0, respectively. The apparent size of a galaxy in a static Euclidean cosmology, when interpreted in the LCDM model, shows a factor of 23.8 increase in size between z=7.5 to z approx. 0. This is in close agreement with the observational data with a size increase of a factor of 6.8 between z=3.2 to z approx. 0. Furthermore, using the apparent size data, it is shown that the difference between the derived proper sizes in R_h=ct, Neumann's and LCDM cosmological models are minimal.

astro-ph.GA↗

arXiv2019

Local Stability of Galactic Discs in Modified Dynamics

The local stability of stellar and fluid discs, under a new modified dynamical model, is surveyed by using WKB approximation. The exact form of the modified Toomre criterion is derived for both types of systems and it is shown that the new model is, in all situations, more locally stable than Newtonian model. In addition, it has been proved that the central surface density of the galaxies plays an important role in the local stability in the sense that LSB galaxies are more stable than HSBs. Furthermore, the growth rate in the new model is found to be lower than the Newtonian one. We found that, according to this model, the local instability is related to the ratio of surface density of the disc to a critical surface density $Σ^{crit}$. We provide observational evidence to support this result based on star formation rate in HSBs and LSBs.

astro-ph.GA↗