arXiv subjects

A. Mohammadi

Publications and source records attributed to A. Mohammadi.

At least 19 recordsLinked to original sources

arXiv2026

Vortex-Antivortex Annihilation from BPS Equations

In this work, we introduce a vortex-impurity model that admits BPS configurations containing both vortices and antivortices and allows vortex-antivortex annihilation to be described within the moduli-space approximation. For a suitable class of impurities, the BPS equations can be mapped through a coordinate transformation onto those of the impurity-free Abelian-Higgs model. In particular, we identify nonlocalized impurities that act as mirrors, generating a vortex-antivortex pair symmetrically placed with respect to the impurity. We derive the corresponding moduli-space metric and show that, within the geodesic approximation, the pair approaches the annihilation configuration asymptotically. We further extend the construction to chains and two-dimensional arrays of alternating vortices and antivortices. Finally, we introduce radial bump impurities that produce stretched and compressed vortices with nontrivial internal magnetic structures.

hep-th↗

arXiv2024

Collective coordinates method for long-range kink collisions

In this paper, we explored a class of potentials with three minima that support kink solutions exhibiting one long-range tail. We analyzed antikink-kink interactions using an effective Lagrangian based on collective coordinates and compared the results to those obtained from full dynamical simulations. To this end, we constructed the collective coordinates with the antikink-kink configuration, and also a generalized Derrick mode, choosing the kink position and the Derrick mode amplitude as the moduli. For the antikink-kink configuration, we utilized the impurity ansatz proposed in \cite{campos2024collision}. We also studied the interaction of wobbling kinks where the lowest delocalized mode is excited.

hep-th↗

arXiv2023

Additive energy of polynomial images

Given a monic polynomial $f(X)\in \mathbb{Z}_m[X]$ over a residue ring $\mathbb{Z}_m$ modulo an integer $m\ge 2$ and a discrete interval $\mathcal{I} = \{1, \ldots, H\}$ of $H \le m$ consecutive integers, considered as elements of $\mathbb{Z}_m$, we obtain a new upper bound for the additive energy of the set $f(\mathcal I)$, where $f(\mathcal I)$ denotes the image set $f(\mathcal I) = \{f(u):~u \in \mathcal I\}$. We give an application of our bounds to multiplicative character sums, improving some previous result of Shkredov and Shparlinski~(2018).

math.NT↗

arXiv2023

A quantum cosmology approach to cosmic coincidence and inflation

This work studies the quantum cosmology of a closed, spatially homogeneous, and isotropic FLRW minisuperspace model with electromagnetic radiation as a matter content. We solve the associated Wheeler-DeWitt (WDW) equation using the holographic regularization method and show that the electromagnetic zero-point energy forms the vacuum energy and provides a unified resolution to the horizon, flatness, singularity, and cosmic coincidence problems. This quantum cosmology approach composes an alternative to the usual inflationary paradigm and can be extended to more general matter contents and emergent gravity schemes.

gr-qc↗

arXiv2022

Vacuum bosonic currents induced by a compactified cosmic string in dS background

In this paper, we investigate the vacuum bosonic currents in the geometry of a compactified cosmic string in the background of the de Sitter spacetime. The currents are induced by magnetic fluxes, one running along the cosmic string and another one enclosed by the compact dimension. In order to develop this analysis, we obtain the complete set of normalized bosonic wave-functions obeying a quasiperiodicity condition. In this context, we calculate the azimuthal and axial current densities. Due to the quasiperiodicity condition, the quantum number associated with the compactification of the string along its axis becomes discrete, and we use the Abel-Plana summation formula to evaluate the non-vanishing current densities. We show that these quantities are explicitly decomposed into two contributions: one corresponds to the geometry of a straight uncompactified cosmic string and the other is induced by the compactification. We also compare the results with the literature in the case of a massive fermionic field in the same geometry.

hep-th↗

arXiv2020

Fermions in the presence of topological structures under geometric constrictions

In this work we study modifications of the spectrum of fermions interacting with kinklike structures in two-dimensional spacetime. We consider the Yukawa coupling between fermions and scalar fields that engender nontrivial internal structure and investigate how the fermion spectra change in terms of the parameters that control the kinklike configuration and the Yukawa coupling. We consider models that allow the internal structure of the kinklike solution to respond to the presence of a geometrical constriction, and show the fermion spectra may also appear directly affected by the constriction. The main results are of current interest and may be used to propose the construction of electronic devices capable of engendering new effects at the nanometric scale.

hep-th↗

arXiv2020

Quark-hadron phase transition in DGP including BD brane

A DGP brane-world model with a perfect fluid brane matter including a Brans-Dicke (BD) scalar field on brane has been utilized to investigate the problem of the quark-hadron phase (QHP) transition in early times of the Universe evolution. The presence of the BD scalar field comes up with some modification terms in the Friedmann equation. Since the behavior of phase transition strongly depends on the basic evolution equations, even a small change in these relations might come to interesting results about the time of transition. The phase transition is investigated using two scenarios of the first-order phase transition and smooth crossover phase transition. For first-order scenario, which is used for intermediate temperature regime, the evolution of the physical quantities, such as temperature and scale factor, are investigated before, during and after the phase transition. The results show that the transition occurs in about micro-second. In the next part, the phenomenon is studied by assuming a smooth crossover transition where the lattice QCD data is utilized to obtain a realistic equation of state for the matter. The investigation for this part is performed in two regimes of high and low-temperature. Using trace anomaly in the high-temperature regime specifies a simple equation of state which states that the quark-gluon behaves like radiation. However, in the low-temperature regime, the trace anomaly is affected by discretization effects, and the hadron resonance gas model is utilized instead. Using this model, a more realistic equation of state could be found in the low-temperature regime. The crossover phase transition in both regimes is considered. The results determine that the transition occurs at the time around a few micro-second. Also, it is realized that the transition in the low-temperature regime occurs after the transition in the high-temperature regime.

gr-qc↗

arXiv2020

Induced Fermionic vacuum polarization in dS spacetime with a compactified cosmic string

We study the fermionic condensate (FC) and the vacuum expectation value (VEV) of the energy-momentum tensor for a massive spinor field in the de Sitter (dS) spacetime including an ideal cosmic string. In addition, spatial dimension along the string is compactified to a circle of length $L$. The fermionic field is assumed to obey quasi-periodic condition along the $z$-axis. There are also magnetic fluxes running along the cosmic string and enclosed by the compact dimension. Both, the FC and the VEV of the energy-momentum tensor, are decomposed into two parts: one induced by the cosmic string in dS spacetime considering the absence of the compactification, and another one induced by the compactification. In particular, we show that the FC vanishes for a massless fermionic field.

hep-th↗

arXiv2019

Bayesian Structure Learning in Sparse Gaussian Graphical Models

Decoding complex relationships among large numbers of variables with relatively few observations is one of the crucial issues in science. One approach to this problem is Gaussian graphical modeling, which describes conditional independence of variables through the presence or absence of edges in the underlying graph. In this paper, we introduce a novel and efficient Bayesian framework for Gaussian graphical model determination which is a trans-dimensional Markov Chain Monte Carlo (MCMC) approach based on a continuous-time birth-death process. We cover the theory and computational details of the method. It is easy to implement and computationally feasible for high-dimensional graphs. We show our method outperforms alternative Bayesian approaches in terms of convergence, mixing in the graph space and computing time. Unlike frequentist approaches, it gives a principled and, in practice, sensible approach for structure learning. We illustrate the efficiency of the method on a broad range of simulated data. We then apply the method on large-scale real applications from human and mammary gland gene expression studies to show its empirical usefulness. In addition, we implemented the method in the R package BDgraph which is freely available at http://CRAN.R-project.org/package=BDgraph

stat.ME↗

arXiv2019

Diagonal actions in positive characteristic

We prove positive characteristic analogues of certain measure rigidity theorems in characteristic zero. More specifically we give a classification result for positive entropy measures on quotients of $\operatorname{SL}_d$ and a classification of joinings for higher rank actions on simply connected absolutely almost simple groups.

math.DS↗

arXiv2018

Lorentz-violating dimension-five operator contribution to the black body radiation

We investigate the thermodynamics of a photon gas in an effective field theory model that describes Lorentz violations through dimension-five operators and Horava-Lifshitz theory. We explore the electrodynamics of the model which includes higher order derivatives in the Lagrangian that can modify the dispersion relation for the propagation of the photons. We shall focus on the deformed black body radiation spectrum and modified Stefan-Boltzmann law to address the allowed bounds on the Lorentz-violating parameter.

hep-th↗

arXiv2018

Fermion bound states in geometrically deformed backgrounds

This work deals with the behavior of fermions in the background of kinklike structures in the two-dimensional spacetime. The kinklike structures appear from bosonic scalar field models that engender distinct profiles and interact with the fermion fields via the standard Yukawa coupling. We first consider two models that engender parity symmetry, one leading to the exclusion of fermion bound states, and the other to the inclusion of bound states, when the parameter that controls the bosonic structure varies from zero to unity. We then go on and investigate another model where the kinklike solution explicitly breaks parity symmetry, leading to fermion bound states that are spatially asymmetric.

hep-th↗

arXiv2018

Tachyon constant-roll inflation

The constant-roll inflation is studied where the inflaton is taken as a tachyon field. Since in this approach the second slow-roll parameter is assumed to be of order one instead of being small, then the perturbation parameters will be considered again. The results are compared with observational data, and it is confirmed that the model could stand as a proper candidate for inflation.

hep-ph↗

arXiv2017

Viscous Warm Inflation: Hamilton-Jacobi formalism

Using Hamilton-Jacobi formalism, The scenario of warm inflation with viscous pressure is considered. The formalism gives a way of computing the slow-rolling parameters without extra approximation, and it is well-known as a powerful method in cold inflation. The model is studied in detail for three different cases of dissipation and bulk viscous pressure coefficients. In the first case where both coefficients are taken as a constant, it is shown that the case could not portray warm inflationary scenario compatible with observational data even it is possible to restrict the model parameters. For other cases, the results shows that the model could properly predicts the perturbation parameters in which they stay in perfect agreement with Planck data. As a further argument, $r-n_s$ and $α_s-n_s$ are drown that show the required result could stand in acceptable area expressing a compatibility with observational data.

astro-ph.CO↗

arXiv2017

Inflationary universe in the presence of a minimal measurable length

In this paper, we will study the effect of having a minimum measurable length on inflationary cosmology. We will analyze the inflationary cosmology in the Jacobson approach. In this approach, gravity is viewed as an emergent thermodynamical phenomenon. We will demonstrate that the existence of a minimum measurable length will modify the Friedmann equations in the Jacobson approach. We will use this modified Friedmann equation to analyze the effect of minimum measurable length scale on inflationary cosmology. This analysis will be performed using the Hamiltonian-Jacobi approach. We compare our results to recent data and find that our model may agree with the recent data.

gr-qc↗

arXiv2017

Intermediate inflation driven by DBI scalar field

Picking out DBI scalar field as inflation, the slow-rolling inflationary scenario is studied by attributing an exponential time function to scale factor; known as intermediate inflation. The perturbation parameters of the model are estimated numerically for two different cases and the final result is compared with Planck data. The diagram of tensor-to-scalar ratio $r$ versus scalar spectra index $n_s$ is illustrated, and it is found out that they are in acceptable range, as suggested by Planck. In addition, the acquired values for amplitude of scalar perturbation reveals the ability of the model for depicting a good picture of the universe in one of the earliest stage. As a further argument, the non-Gaussianity is investigated displaying that the model prediction stands in $68\%$ CL regime; according to latest Planck data.

gr-qc↗

arXiv2017

Light of Planck-2015 on Non-Canonical Inflation

Slow-roll inflationary scenario is considered in non-canonical scalar field model supposing a power-law function for kinetic term, and using two formalisms. In the first approach, the potential is considered as a power-law function, that is the most common approach in studying inflation. Hamilton-Jacobi approach is selected as the second formalism, so that the Hubble parameter is introduced as a function of scalar field instead of the potential. Employing the last observational data, the free parameters of the model are constrained, and the predicted form of the potential and attractor behavior of the model are considered in detail.

gr-qc↗

arXiv2017

Hamilton-Jacobi formalism to warm inflationary scenario

Hamilton-Jacobi formalism as a powerful method is being utilized to reconsider warm inflationary scenario, where the scalar field as the main component deriving inflation interacts with other field. Separating the context to strong and weak dissipative regimes, the goal is followed for two popular function of dissipation coefficient. Applying slow-rolling approximations, the required perturbation parameters are extracted and by comparison to the latest Planck data, the free parameters are restricted. Possibility of producing an acceptable inflation is studied where the result shows that for all cases the model could successfully suggests amplitude of scalar perturbation, scalar spectral index , its running, and the tensor-to-scalar ratio.

gr-qc↗