arXiv subjects

Syed Moeez Hassan

Publications and source records attributed to Syed Moeez Hassan.

13 recordsLinked to original sources

arXiv2026

Polymer Bianchi-I with polymer matter

We analyze the effective dynamics of a polymer quantized Bianchi-I universe coupled to a polymer quantized scalar field, with a pressureless dust field acting as an internal clock. We show that for a consistent polymer quantization of the anisotropies, the volume variable has to be polymerized first, and that a choice of different polymer scales for the two leads to substantially different dynamics. We further derive an effective Friedmann equation for this model, and compute the shear scalar. We find that polymer quantizing the scalar field leads to significant differences in the evolution of various cosmological quantities as compared to standard quantization. The amount of these variations, as well as the location of the quantum bounce, depends on the matter polymer scale.

gr-qc↗

arXiv2026

An effective Friedmann equation for a bouncing anisotropic universe

We derive generalized effective Friedmann and Raychaudhuri equations for a bouncing polymer quantized Bianchi-I spacetime. We further prove that the relation numerically derived in the literature between the matter energy density and the anisotropic shear at the bounce holds exactly for any type of matter content, and derive explicit expressions for the constants that appear in that relation.

gr-qc↗

arXiv2025

Monte Carlo quantum cosmology with a modified Euclidean action

We numerically study the Euclidean quantum cosmology of a closed, homogeneous and isotropic universe with a cosmological constant. A dust field acts as a clock, and we compute the ground state wavefunction, correlation function, and mean volume of the universe by performing path integral Monte Carlo simulations. To make the path integral convergent, the argument of the exponent in the weight of the path integral is chosen to be either the absolute value of the Euclidean action, or its square. We compare our results to the standard case (with a hard cutoff at zero action), and find that the absolute value produces similar results, whereas the squared method gives substantially different results. We detail two different methods of `fixing' the squared case, and test them for the simple harmonic oscillator, and for this cosmological model. We also find that for a large positive cosmological constant, the universe undergoes multiple cycles of expansion and contraction for all three weights, and reaches much larger sizes with the squared weight.

gr-qc↗

arXiv2025

Semiclassical polymer field with a cubic potential in cosmology

We study the semiclassical dynamics of a polymer quantized scalar field with a cubic potential in cosmology. The cosmological spacetime is chosen to be homogeneous and isotropic, and we work in the polymer quantization scheme where the field operator does not exist directly. The dynamics are studied in the `e-fold' time gauge, where a choice of time gauge fixing, in terms of the scale factor, is made at the classical level. The cubic potential is interesting to study because it is unbounded from below. We find that polymer quantization regularizes this potential, and leads to periods of slow-roll as well as exactly de-Sitter inflation.

gr-qc↗

arXiv2025

Polymer cosmology with polymer matter: Effective dynamics

We study the effective dynamics of a polymer quantized scalar field living on an effective polymer quantized homogeneous and isotropic background. We use a presureless dust field as an internal clock, and work with volume variables. Our results show how the quantum bounce in the early universe is affected as the initial conditions and various physical parameters -- including the scalar field polymer scale -- are varied. We also find, generically, that there is asymmetric evolution across the bounce.

gr-qc↗

arXiv2024

From global time to local physics

Global time is a gauge or relational choice of time variable in canonical gravity. Local time is the time used in a flat patch of spacetime. We compare the dynamics of a scalar field with respect to choices of global time and Minkowski patch time in an expanding cosmology. Our main results are that evolutions starting from the same initial conditions are similar on the time scales of terrestrial experiments, and that global time leads to a mechanism for evolving coupling constants.

gr-qc↗

arXiv2020

Monte Carlo simulation of cosmologies with dust

The quantum theory of the Friedmann cosmological model with dust and cosmological constant ($Λ$) is not exactly solvable analytically. We apply Path Integral Monte Carlo (PIMC) techniques to study its quantum dynamics using the physical Hamiltonian corresponding to the dust field as a clock. We study (i) quantum fluctuations around classical paths and (ii) formulate the analogues of the no-boundary and tunnelling proposals and simulate the ground state wave functions. For $Λ< 0$ a unique ground state wave function exists. For $Λ> 0$ the physical Hamiltonian is not bounded below, but the path integral for the propagator is convergent over a range of Euclidean time $T$. We investigate the properties of the convergent propagator. The path integral can be made convergent for all values of $T$ by restricting the integral over paths with action greater than equal to zero. We explore the consequences of such a choice.

gr-qc↗

arXiv2018

Gravity, time and varying constants

There are theories which implement the idea that the constants of nature may be "time dependent." These introduce new fields representing "evolving constants," in addition to physical fields. We argue that dynamical matter coupling constants can arise naturally in non-perturbative matter-gravity theories, after a choice of global time is made. We illustrate the idea in scalar field cosmology with spatial volume as a global clock, and compute the time dependence of the scalar mass and self-interaction coupling constants.

gr-qc↗

arXiv2018

The Universe as an Oscillator

We apply the idea of using a matter time gauge in quantum gravity to quantum cosmology. For the Friedmann-Lemaitre-Robertson-Walker (FLRW) Universe with dust and cosmological constant $Λ$, we show that the dynamics maps exactly to the simple harmonic oscillator in the dust-time gauge. For $Λ>0$ the oscillator frequency is imaginary, for $Λ<0$ it is real, and for $Λ=0$ the Universe is a free particle. This result provides (i) a simple demonstration of non-perturbative singularity avoidance in quantum gravity for all $Λ$, (ii) an exact Lorentzian Hartle-Hawking wave function, and (iii) gives the present age of the Universe as the characteristic decay time of the propagator.

gr-qc↗

arXiv2018

Quantum gravitational collapse as a Dirac particle on the half-line

We show that the quantum dynamics of a thin spherical shell in general relativity is equivalent to the Coulomb-Dirac equation on the half line. The Hamiltonian has a one-parameter family of self-adjoint extensions with a discrete energy spectrum $|E| < m$, and a continuum of scattering states for $|E|>m$, where $m$ is the rest mass of the shell and $E$ is the Arnowitt-Deser-Misner mass. For sufficiently large $m$, the ground state energy level is negative. This suggests that classical positivity of energy does not survive quantization. The scattering states provide a realization of singularity avoidance. We speculate on the consequences of these results for black hole radiation.

gr-qc↗

arXiv2018

A non-perturbative Hamiltonian approach to the cosmological constant problem

It was recently suggested that the cosmological constant problem as viewed in a non-perturbative framework is intimately connected to the choice of time and a physical Hamiltonian. We develop this idea further by calculating the non-perturbative vacuum energy density as a function of the cosmological constant with multiple choices of time. We also include a spatial curvature of the universe and generalize this calculation beyond cosmology at a classical level. We show that vacuum energy density depends on the choice of time, and in almost all time gauges, is a non-linear function of the cosmological constant. This non-linear relation is a calculation for the vacuum energy density given some arbitrary value of the cosmological constant. Hence, in this non-perturbative framework, the cosmological constant problem does not arise. We also discuss why the conventional cosmological constant problem is not well-posed, and formulate and answer the question: "Does vacuum gravitate?" Finally, we give a derivation of reduction to quantum mechanics on a flat background from a non-perturbative gravity-matter theory.

gr-qc↗

arXiv2017

Semiclassical cosmology with polymer matter

In loop quantum cosmology, polymer quantization is applied to gravity and Schrodinger quantization to matter. This approach misses interesting cosmological dynamics coming from the polymer quantization of matter. We demonstrate this in semiclassical cosmology with a scalar field and pressureless dust: gravity is kept classical, dust is used to fix the time gauge, and polymer quantization effects are isolated in the scalar field. The resulting dynamics shows a period of inflation, both with and without a scalar potential, and the emergence of a classical universe at late times. Since gravity is not quantized, the cosmological singularity is not resolved, but our results suggest that polymer quantization of both gravity and matter are important for a complete picture.

gr-qc↗

arXiv2015

Polymer inflation

We consider the semi-classical dynamics of a free massive scalar field in a homogeneous and isotropic cosmological spacetime. The scalar field is quantized using the polymer quantization method assuming that it is described by a gaussian coherent state. For quadratic potentials, the semi-classical equations of motion yield a universe that has an early "polymer inflation" phase which is generic and almost exactly de Sitter, followed by a epoch of slow-roll inflation. We compute polymer corrections to the slow roll formalism, and discuss the probability of inflation in this model using a physical Hamiltonian arising from time gauge fixing. We also show how in this model, it is possible to obtain a significant amount of slow-roll inflation from sub-Planckain initial data, hence circumventing some of the criticisms of standard scenarios. These results show the extent to which a quantum gravity motivated quantization method affects early universe dynamics.

astro-ph.CO↗