arXiv subjects

Susobhan Mandal

Publications and source records attributed to Susobhan Mandal.

29 records · Page 2Linked to original sources

arXiv2021

Low-energy effective quantum field theoretic description of excitations about soliton configurations

Solitons are the classical field configurations connecting two trivial vacua. These are also the solutions of classical field equations of motion with particle-like properties. Moreover, they are localized in space, having finite energy, and are stable against decay into radiation. The coherent state description of kink-solitons is discussed in the present article. Further, the relation between topological solitons and occupation numbers corresponding to low momentum excitations are also discussed coherently. The description of the low energy excitations about solitons in quantum field theory is the main theme of this article. Further, a few physical observables, namely some low order correlation functions, are computed up to certain integral forms. Furthermore, we have shown that it is possible to detect the presence of soliton-like classical configurations in many-particle systems from the nature of the one-point function and non-conservation of momentum feature of one-point, two-point and three-point functions in this low-energy effective field theory of these excitations.

hep-th↗

arXiv2021

Local description of S-matrix in quantum field theory in curved spacetime using Riemann-normal coordinate

The success of the S-matrix in quantum field theory in Minkowski spacetime naturally demands the extension of the construction of the S-matrix in a general curved spacetime in a covariant manner. However, it is well-known that a global description of the S-matrix may not exist in an arbitrary curved spacetime. Here, we give a local construction of S-matrix in quantum field theory in curved spacetime using Riemann-normal coordinates which mimics the methods, generally used in Minkowski spacetime. Using this construction, the scattering amplitudes and cross-sections of some scattering processes are computed in a generic curved spacetime. Further, it is also shown that these observables can be used to probe features of curved spacetime as these local observables carry curvature-dependent corrections. Moreover, the compatibility of the local construction of the S-matrix with the spacetime symmetries is also discussed in detail.

hep-th↗

arXiv2021

Equation of states in the curved spacetime of spherical degenerate stars

In the study of spherical degenerate stars such as neutron stars, general relativistic effects are incorporated by using Tolman-Oppenheimer-Volkoff equations to describe their interior spacetime. However, the equation of states employed in such studies are invariably computed in flat spacetime. We show that the equation of states computed in the curved spacetime of these stars depend explicitly on the metric function. Further, we show that ignoring such metric-dependent gravitational time dilation effect leads one to grossly underestimate the mass limits of these compact stars.

gr-qc↗

arXiv2021

Classification of quantum states based on the null energy condition

Violation of the null energy condition plays an important role both in the general theory of relativity and quantum field theory in curved spacetimes. Over the years, it has been shown that the violation of the null energy condition leads to instability and violation of causality. In quantum field theory, violation of the energy condition also depends on the quantum states apart from the geometry of curved spacetime. Hence, the quantum effects play an important role in the violation of the null energy condition. We show that the set of all the coherent states does not violate the null energy condition. Further, we also show that under certain conditions, the null energy condition is violated either by a pure state or a mixed state. Furthermore, the dynamical violation of the null energy condition by the quantum states is also discussed here.

gr-qc↗

arXiv2021

Characteristics of interaction between Gravitons and Photons

The direct detection of gravitational waves from binary mergers has been hailed as the discovery of the century. In the light of recent evidence on the existence of gravitational waves, it is now possible to know about the properties of matter under extreme conditions in compact astrophysical objects and different dynamical spacetimes. The foremost theme of the present article is to bring out the various features of the interaction between photons and gravitons that can be used in astrophysical observations. The effective action of interacting photons containing light-matter coupling and self-interaction term is constructed by eliminating the graviton degrees of freedom coupled to both matter and photons. It is shown that the equation of state of matter can be probed from the dynamics of light in this theory. The vacuum birefringence is also shown to be a generic property in this theory that arises from the non-linear nature of the self-interaction between gauge fields. Further, the non-local nature of quantum effective action with modified dispersion relation is also discussed in great detail. The above results also open an alternate way to infer the properties of gravitational waves without their direct measurement using the features of photon-graviton interaction.

gr-qc↗

arXiv2021

Solitons in a class of interacting scalar field theories without $SO(2)$ invariance

In this article, we study kink soliton configurations in interacting scalar field theories containing two fields without $SO(2)$ invariance. We study a class of such theories, the well-known Montonen-Sarker-Trullinger-Bishop model is one of them. These models are interesting since the $U(1)$ current is not conserved in them due to the presence of explicit symmetry breaking terms in the action. The existence of kink soliton configurations is shown in terms of a system of first-order ordinary differential equations. Although $U(1)$ current in these models are non-conserved, our approach is general enough to study soliton configurations in a generic two interacting scalar field theory. We also discuss other benefits of this approach.

hep-th↗

arXiv2020

Existence of conserved quantities and their algebra in curved spacetime

In General Relativity, finding out the geodesics of a given spacetime manifold is an important task because it determines which classical processes are dynamically forbidden. Conserved quantities play an important role in solving geodesic equations of a general spacetime manifold. Furthermore, knowing all possible conserved quantities of a system tells about the hidden symmetries of that system since, conserved quantities are deeply connected with the symmetries of the system, which are very important in their own right. Conserved quantities are also useful to capture certain features of spacetime manifold for an asymptotic observer. In this article, we show the existence of these conserved charges and their algebra for a class of dynamical systems in a generic curved spacetime.

gr-qc↗

arXiv2019

Characteristics of Z(2) multi-kink soliton configurations

In this article, we first briefly review the solution of Z(2) kink solitons. Then we construct some multi-kink soliton configurations which are static and show their few features which are actually important to characterize their stability conditions. Not only that this show also the particle characteristics of these kink configurations in these solitonic configurations. Then we will talk about dynamical kinks and show the affect of dynamics in the expression of force exerted by the neighbouring kink and anti-kink on each other in the multi-kink configurations. We have also defined an algebra through which we can write down equivalent ways of writing down multi-kink configuration mathematically.

hep-th↗

arXiv2019

Reduced Chandrasekhar mass limit due to the fine-structure constant

The electromagnetic interaction alters the Chandrasekhar mass limit by a factor which depends, as computed in the literature, on the atomic number of the positively charged nuclei present within the degenerate matter. Unfortunately, the methods employed for such computations break Lorentz invariance ab initio. By employing the methods of finite temperature relativistic quantum field theory, we show that in the leading order, the effect of electromagnetic interaction reduces the Chandrasekhar mass limit for non-general-relativistic, spherically symmetric white dwarfs by a universal factor of $(1-3α/4π)$, $α$ being the fine-structure constant.

astro-ph.SR↗

arXiv2019

A unified theory for both massless, massive vector fields and non-triviality of physical Hilbert space of this theory

In this article, I talk about a model which is known and often used to unify both massless and massive vector free-field theory, discuss the importance of auxiliary scalar field introduced in the Lagrangian density that we consider in this theory which turns out to be unphysical in massive vector field case which I also show. But for massless theory in 3+1-dimensional case, I show that this auxiliary field makes the physical Hilbert space non-trivial in terms of introducing an extra physical degree of freedom or an extra non-trivial kind of polarized state which is absent in Maxwell's electrodynamics theory (free-massless vector field theory). And this state is also shown to be present in 1+1-dimensional theory whereas there is no on-shell photon in Maxwell's theory of electromagnetism, which is a massless vector field theory.

physics.gen-ph↗

arXiv2018

U(1) symmetry breaking under canonical transformation in real scalar field theory

In this article, I have considered a real scalar field theory and able to show that under Bogoliubov transformation in infinite volume limit or thermodynamic limit the transformed Hamiltonian no longer invariant under U(1) action defined appropriately as it was before doing transformation. We also have checked this fact by looking at the correlation functions under the action of U(1) group. We suitably defined field operators that are associated with particle production phenomena then we can also show that correlation functions of such field operators also don't follow U(1) invariance, shown in this article. This is a consequence of non-invariance of transformed Hamiltonian under U(1) action. Since, we know Bogoliubov transformation in curved spacetime is equivalent to doing a coordinate transformation, therefore this result directly shows the phenomena of particle production under the affect of gravity since changing coordinate is equivalent to turn on gravity according to Einstein's equivalence principle in GR. I also show that particle production does not take place out of vacuum state but it can happen out of other many-particle states and vacuum state is not an eigenvector of Hamltonian operator in transformed Fock space and vacuum state does not remain vacuum state under time evolution.

physics.gen-ph↗