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Qasem Exirifard

Publications and source records attributed to Qasem Exirifard.

27 records · Page 2Linked to original sources
arXiv2010

Generalized Lovelock gravity

In the Riemann geometry, the metric's equation of motion for an arbitrary Lagrangian is succinctly expressed in term of the first variation of the action with respect to the Riemann tensor if the Riemann tensor were independent of the metric. Let this variation be called the E-tensor. Noting that the E-tensor and equations of the motion for a general Lovelock gravity have the same differential degree, we define generalized Lovelock gravity as polynomial scalar densities constructed out from the Riemann tensor and its arbitrary covariant derivatives such that they lead to the same differential degree for the E-tensor and the metric's equation of motion. We consider Lagrangian densities which are functional of the metric and the first covariant derivative of the Riemann tensor. We then present the first non-trivial examples of the generalized Lovelock gravity terms.

gr-qc↗
arXiv2010

Phenomenological covariant approach to gravity

We covariantly modify the Einstein-Hilbert action such that the modified action perturbatively resolves the flat rotational velocity curve of the spiral galaxies and gives rise to the Tully-Fisher relation, and dynamically generates the cosmological constant. This modification requires introducing just a single new universal parameter.

gr-qc↗
arXiv2009

Visiting Horava-Lifshitz gravity in extra dimensions

The Horava-Lifshitz gravity, having broken the symmetry of space and time, includes three objects: the spatial metric $g_{ij}$, the lapse variable $N$, and the shift variable $N_{i}$. Each of these objects have their own scaling dimensions. The action of the theory is required to be invariant under this scaling, and spatial diffeomorphism, and the temporal foliation symmetry. Noting that action can non-trivially depend on the lapse variable, we suggest to consider the Horva's approach to quantum gravity in higher dimensions such that a set of extra spacial coordinates possess a scaling dimension different from that of rest. In so so doing, we propose a new power counting renormalizable theory for quantum gravity in its UV point. We show that the IR point and UV point of the proposal possess the same number of degrees of freedom in 3, 8 and 27 extra-dimensional space-time geometry.

hep-th↗
arXiv2009

Solar system tests do not rule out 1/R gravity

We argue that Solar system tests do not rule out 1/R gravity at least due to the reason addressed in Phys. Rev. D 74 (2006) 121501 [astro-ph/0610483] (ref. [1]) and subsequent published papers. Ref. [1] has not only modified the Einstein-Hilbert action but also has changed the boundary conditions since they altered the equations of motion. In Einstein-Hilbert action equations are second order, so the fall off of the fields suffices to single out a unique solution. In 1/R gravity the equations are fourth order, so we should impose additional boundary conditions. Perhaps the boundary condition we must impose is that the abrupt change in the second derivative of the metric near the surface of the Sun remains intact by adding `1/R' corrections to the equations of motion. The solution of 1/R gravity with this boundary condition remains consistent with the solar system tests. Ref. [1] assumes that as soon as they perturbatively modified the equations then the Ricci scalar becomes smooth on the surface of the Sun. This assumption is simply wrong because the boundary conditions and equations of motions are two different entities.

astro-ph↗
arXiv2009

Lower bound on the mass of a black hole

We consider gravity coupled to a massive field whose Compton's wavelength is far larger than the Planck's length. In the low energy effective action for gravity, thus, it is the perturbation in the Compton's wavelength that breaks first as the sub-sub-leading quantum perturbation grows stronger. When this break occurs, we can not trust the perturbative information about the form of the low energy effective action. We translate this break into the lowest limit on the mass of a classical black hole. In D=4, using the electron's mass, this requires the black hole to be heavier than 10^{14} kg.

gr-qc↗
arXiv2008

Effectively Emergent Quantum Mechanics

We consider non minimal coupling between matters and gravity in modified theories of gravity. In contrary to the current common sense, we report that quantum mechanics can effectively emerge when the space-time geometry is sufficiently flat. In other words, quantum mechanics might play no role when and where the space-time geometry is highly curved. We study the first two simple models of Effectively Emergent Quantum Mechanics(EEQM): R-dependent EEQM and G-dependent EEQM where R is the Ricci scalar and G is the Gauss-Bonnet Lagrangian density. We discuss that these EEQM theories might be fine tuned to remain consistent with all the implemented experiments and performed observations. In particular, we observe that G-dependent EEQM softens the problem of quantum gravity.

gr-qc↗
arXiv2008

Constraints on $f(R_{ijkl}R^{ijkl})$ gravity: An evidence against the covariant resolution of the Pioneer anomaly

We consider corrections in the form of $ΔL(R_{ijkl}R^{ijkl})$ to the Einstein-Hilbert Lagrangian. Then we compute the corrections to the Schwarszchild geometry due to the inclusion of this general term to the Lagrangian. We show that $ΔL_3=α_{1/3}(R_{ijkl}R^{ijkl})^{1/3}$ gives rise to a constant anomalous acceleration for objects orbiting the Sun onward the Sun. This leads to the conclusion that $α_{1/3}=(13.91\pm 2.11) \times 10^{-26}(\frac{1}{\text{meters}})^{2/3}$ would have covariantly resolved the Pioneer anomaly if this value of $α_{1/3}$ had not contradicted other observations. We notice that the experimental bounds on $ΔL_3$ grows stronger in case we examine the deformation of the space-time geometry around objects lighter than the Sun. We therefore use the high precision measurements around the Earth (LAGEOS and LLR) and obtain a very strong constraint on the corrections in the form of $ΔL(R_{ijkl}R^{ijkl})$ and in particular $ΔL=α_n(R_{ijkl}R^{ijkl})^n$. This bound requires $α_{1/3}\leq6.12\times 10^{-29}(\frac{1}{\text{meters}})^{2/3}$. Therefore it refutes the covariant resolution of the Pioneer anomaly.

gr-qc↗
arXiv2007

The world-sheet corrections to dyons in the Heterotic theory

All the linear alpha-prime corrections, however excluding the gravitational Chern-Simons correction, are studied in the toroidally compactified critical Heterotic string theory. These corrections are computed to the entropy for a BPS static spherical four dimensional dyonic black hole which represents a wrapped fundamental string carrying arbitrary winding and momentum charges along one cycle in the presence of KK-monopole and H-monopole charges associated to another cycle. It is verified that after the inclusion of the gravitational Chern-Simons corrections [hep-th/0608182], all the linear alpha-prime corrections to the entropy for the supersymmetric dyon can be reproduced by the inclusion of only the Gauss-Bonnet Lagrangian to the supergravity approximation of the induced Lagrangian.

hep-th↗