arXiv subjects

Nafiseh Rahmanpour

Publications and source records attributed to Nafiseh Rahmanpour.

6 recordsLinked to original sources

arXiv2023

A spherically symmetric gravitational solution of nearly conformally flat metric~measure~space

In this manuscript, we study the nearly flat approximation of a conformally invariant gravitational theory in metric measure space (MMS). In addition, we investigate the transformation of the energy-momentum tensor in this context and obtain the vacuum solution of MMS and its weak field limit in the spherically symmetric coordinates. We show that while it is already a vacuum solution, it can simulate dark matter when restricted to the framework of general relativity, i.e., a symmetry-broken conformal frame. This is done by employing a density function which is an essential part of MMS. We derive an equation for the density function for a general profile of a rotation curve obtained from observations. Specifically, the density function corresponding to two well-known profiles PSS and NFW are provided.

gr-qc↗

arXiv2019

Hairy Waves: Gravitational Waves in the presence of Screening Mechanism

We consider a binary system in the context of screened modified gravity models and investigate its emitted gravitational wave which has been shown that is a hairy wave. We derive power of the gravitational wave and the binding energy of the binary system and then trace back the effect of the additional scalar field, present at screening models, in the frequency of GW emitted from the binary. The binary is considered as two astrophysical black holes in a spherically symmetric galactic ambient/halo with constant density. Effectively in the ISCO region the screening force is in charge due to the sharp transition in the density of matter field. It is shown that, in the presence of screening, when one companion of the binary system is between $R_{thinshell}$ and $R_{ISCO}$, the frequency of the gravitational emission have decreased, compared to GR without screening.

gr-qc↗

arXiv2018

SO(4; 2) and derivatively coupled dRGT massive gravity

In this paper we study the possibility of assigning a geometric structure to the Lie groups. It is shown the Poincaré and Weyl groups have geometrical structure of the Riemann-Cartan and Weyl space-time respectively. The geometric approach to these groups can be carried out by considering the most general (non)metricity conditions, or equivalently, tetrad postulates which we show that can be written in terms of the group's gauge fields. By focusing on the conformal group we apply this procedure to show that a nontrivial 3-metrics geometry may be extracted from the group's Maurer-Cartan structure equations. We systematically obtain the general characteristics of this geometry, i.e. its most general nonmetricity conditions, tetrad postulates and its connections. We then deal with the gravitational theory associated to the conformal group's geometry. By proposing an Einstein-Hilbert type action, we conclude that the resulting gravity theory has the form of quintessence where the scalar field derivatively coupled to massive gravity building blocks.

gr-qc↗

arXiv2016

Metric Measure Space as a Framework for Gravitation

In this manuscript, we show how conformal invariance can be incorporated in a classical theory of gravitation, in the context of metric measure space. Metric measure space involves a geometrical scalar $f$, dubbed as density function, which here appears as a conformal degree of freedom. In this framework, we present conformally invariant field equations, the relevant identities and geodesic equations. In metric measure space, the volume element and accordingly the operators with integral based definitions are modified. For instance, the divergence operator in this space differs from the Riemannian one. As a result, a gravitational theory formulated in this space has a generalized second Bianchi identity and a generalized conservation of energy-momentum tensor. It is shown how, by using the generalized identity for conservation of energy-momentum tensor, one can obtain a conformally invariant geodesic equation. By comparison of the geodesic equations in metric measure space with the Bohmian trajectories, in both relativistic and non-relativistic regimes, a relation between density function $f$ and the quantum potential is proposed. This suggests metric measure space to be considered as a suitable framework for geometric description of Bohm's quantum mechanics. On the other hand, as it is known, Weyl geometry is one of the main approaches to construct conformally invariant gravitational models. Regarding the fact that the connection in the integrable Weyl space is modified and in metric measure space remains the same as it is in the Riemann space, the mathematical analogy between these two spaces is also discussed.

gr-qc↗

arXiv2015

A Conformally Invariant Theory of Gravitation in Metric Measure Space

In this manuscript, a conformally invariant theory of gravitation in the context of metric measure space is studied. The proposed action is invariant under both diffeomorphism and conformal transformations. Using the variational method, a generalization of the Einstein equation is obtained, wherein the conventional tensors are replaced by their conformally invariant counterparts, living in metric measure space. The invariance of the geometrical part of the action under a diffeomorphism leads to a generalized contracted second Bianchi identity. In metric measure space, the covariant derivative is the same as it is in the Riemannian space. Hence, in contrast to the Weyl space, the metricity and integrability are maintained. However, it is worth noting that in metric measure space the divergence of a tensor is not simply the contraction of the covariant derivative operator with the tensor that it acts on. Despite the fact that metric measure space and integrable Weyl space, are constructed based on different assumptions, it is shown that some relations in these spaces, such as the contracted second Bianchi identity, are completely similar.

gr-qc↗

arXiv2012

Multi-Metric Gravity via Massive Gravity

A generalization to the theory of massive gravity is presented which includes three dynamical metrics. It is shown that at the linear level, the theory predicts a massless spin-2 field which is decoupled from the other two gravitons which are massive and interacting. In this regime the matter naturally should be coupled to massless graviton which introduces a preferred metric that is the average of the primary metrics. The cosmological solution of the theory shows the de-Sitter behavior with a function of mass as its cosmological constant. Surprisingly, it lacks any non-trivial solution when one of the metrics is taken to be Minkowskian and seems to enhance the predictions which suggests that there is no homogeneous, isotropic and flat solution for the standard massive cosmology.

hep-th↗