arXiv subjects

Nick Kwidzinski

Publications and source records attributed to Nick Kwidzinski.

7 recordsLinked to original sources

arXiv2020

Hamiltonian formulation of dust cloud collapse

We consider the gravitational collapse of self-gravitating spherical dust cloud in the Hamiltonian formalism. We address both homogeneous and inhomogeneous cases. Our novel derivation of the Hamiltonian of the system is based on the improved variational principle that was proposed in \cite{KMM}. The present derivation differs from usual treatments due to the presence of an extra boundary term added to the Hilbert action. As expected, the standard equations of motion are retrieved. However, differently from other treatments, the total Hamiltonian obtained with our procedure in the Schwarzschild time-gauge is identical to the total mass of the system as measured from infinity, as it would be expected. Implications for the quantization of the system are suggested.

gr-qc↗

arXiv2019

Singularity avoidance in Bianchi I quantum cosmology

We extend recent discussions of singularity avoidance in quantum gravity from isotropic to anisotropic cosmological models. The investigation is done in the framework of quantum geometrodynamics (Wheeler-DeWitt equation). We formulate criteria of singularity avoidance for general Bianchi class A models and give explicit and detailed results for Bianchi I models with and without matter. We find that the classical singularities can generally be avoided in these models.

gr-qc↗

arXiv2019

Curvature invariants for the Bianchi IX spacetime filled with tilted dust

We present an analysis of the Kretschmann and Weyl squared scalars for the general Bianchi IX model filled with tilted dust. Particular attention is given to the asymptotic regime close to the singularity for which we provide heuristic considerations supported by numerical simulations. The present paper is an extension of our earlier publication [1].

gr-qc↗

arXiv2018

On the dynamics of the general Bianchi IX spacetime near the singularity

We show that the complex dynamics of the general Bianchi IX universe in the vicinity of the spacelike singularity can be approximated by a simplified system of equations. Our analysis is mainly based on numerical simulations. The properties of the solution space can be studied by using this simplified dynamics. Our results will be useful for the quantization of the general Bianchi IX model.

gr-qc↗

arXiv2016

The classical harmonic chain: solution via Laplace transforms and continued fractions

The harmonic chain is a classical many-particle system which can be solved exactly for arbitrary number of particles (at least in simple cases, such as equal masses and spring constants). A nice feature of the harmonic chain is that the final result for the displacements of the individual particles can be easily understood -- therefore, this example fits well into a course of classical mechanics for undergraduates. Here we show how to calculate the displacements by solving equations of motion for the Laplace transforms $\mathcal{L}\left\{q_n\right\}(s)$ of the displacements $q_n(t)$. This leads to a continued fraction representation of the Laplace transforms which can be evaluated analytically. The inverse Laplace transform of $\mathcal{L}\left\{q_n\right\}(s)$ finally gives the displacements which generically have the form of Bessel functions. We also comment on the similarities between this approach and the Green function method for quantum many-particle systems.

physics.class-ph↗

arXiv2016

Classical and quantum cosmology of Born-Infeld type models

We discuss Born-Infeld type fields (tachyon fields) in classical and quantum cosmology. We first partly review and partly extend the discussion of the classical solutions and focus in particular on the occurrence of singularities. For quantization, we employ geometrodynamics. In the case of constant potential, we discuss both Wheeler-DeWitt quantization and reduced quantization. We are able to give various solutions and discuss their asymptotics. For the case of general potential, we transform the Wheeler-DeWitt equation to a form where it leads to a difference equation. Such a difference equation was previously found in the quantization of black holes. We give explicit results for the cases of constant potential and inverse squared potential and point out special features possessed by solutions of the difference equation.

gr-qc↗

arXiv2014

Comparing the dynamics of diagonal and general Bianchi IX spacetime

We make comparison of the dynamics of the diagonal and nondiagonal Bianchi IX models in the evolution towards the cosmological singularity. Apart from the original variables, we use the Hubble normalized ones commonly applied in the examination of the dynamics of homogeneous models. Applying the dynamical systems method leads to the result that in both cases the continuous space of critical points is higher dimensional and they are of the nonhyperbolic type. This is a generic feature of the dynamics of both cases and seems to be independent on the choice of phase space variables. The topologies of the corresponding critical spaces are quite different. We conjecture that the nondiagonal case may carry a new type of chaos different from the one specific to the usually examined diagonal one.

gr-qc↗