Marco Valerio Battisti

The Big-Bang singularity in the framework of a Generalized Uncertainty Principle

Abstract We analyze the quantum dynamics of the Friedmann–Robertson–Walker Universe in the context of a Generalized Uncertainty Principle. Since the isotropic Universe dynamics resembles that of a one-dimensional particle, we quantize it with the commutation relations associated to an extended formulation of the Heisenberg algebra. The evolution of the system is described in terms of a massless scalar field taken as a relational time. We construct suitable wave packets and analyze their dynamics from a quasi-classical region to the initial singularity. The appearance of a non-singular dynamics comes out as far as the behavior of the probability density is investigated. Furthermore, reliable indications arise about the absence of a big-bounce, as predicted in recent issues of loop quantum cosmology.

Scalar field theory on noncommutative Snyder spacetime

We construct a scalar field theory on the Snyder noncommutative space-time. The symmetry underlying the Snyder geometry is deformed at the co-algebraic level only, while its Poincare algebra is undeformed. The Lorentz sector is undeformed at both the algebraic and co-algebraic level, but the coproduct for momenta (defining the star product) is non-coassociative. The Snyder-deformed Poincare group is described by a non-coassociative Hopf algebra. The definition of the interacting theory in terms of a nonassociative star product is thus questionable. We avoid the nonassociativity by the use of a space-time picture based on the concept of the realization of a noncommutative geometry. The two main results we obtain are (i) the generic (namely, for any realization) construction of the co-algebraic sector underlying the Snyder geometry and (ii) the definition of a nonambiguous self-interacting scalar field theory on this space-time. The first-order correction terms of the corresponding Lagrangian are explicitly computed. The possibility to derive Noether charges for the Snyder space-time is also discussed.

Classical and Quantum Features of the Mixmaster Singularity

This review paper is devoted to analyzing the main properties of the cosmological singularity associated with the homogeneous and inhomogeneous Mixmaster model. After the introduction of the main tools required to treat the cosmological issue, we review in detail the main results obtained over the last forty years on the Mixmaster topic. We first assess the classical picture of the homogeneous chaotic cosmologies and, after a presentation of the canonical method for the quantization, we develop the quantum Mixmaster behavior. Finally, we extend both the classical and the quantum features to the fully inhomogeneous case. Our survey analyzes the fundamental framework of the Mixmaster picture and completes it by accounting for recent and peculiar outstanding results.

Quantum dynamics of the Taub universe in a generalized uncertainty principle framework

The implications of a generalized Uncertainty principle on the Taub cosmological model are investigated. The model is studied in the Arnowitt-Deser-Misner reduction of the dynamics and therefore a time variable is ruled out. Such a variable is quantized in a canonical way and the only physical degree of freedom of the system (related to the universe anisotropy) is quantized by means of a modified Heisenberg algebra. The analysis is performed at both the classical and quantum level. In particular, at quantum level, the motion of wave packets is investigated. The two main results obtained are as follows: (i) The classical singularity is probabilistically suppressed. The universe exhibits a stationary behavior and the probability amplitude is peaked in a determinate region. (ii) The generalized uncertainty principle wave packets provide the right behavior in the establishment of a quasi-isotropic configuration for the universe.

Modification of Heisenberg uncertainty relations in noncommutative Snyder space-time geometry

We show that the Euclidean Snyder noncommutative space implies infinitely many different physical predictions. The distinct frameworks are specified by generalized uncertainty relations underlying deformed Heisenberg algebras. Considering the one-dimensional case in the minisuperspace arena, the bouncing Universe dynamics of loop quantum cosmology can be recovered.

Big bounce in dipole cosmology

We derive the cosmological big bounce scenario from the dipole approximation of loop quantum gravity. We show that a nonsingular evolution takes place for any matter field and that, by considering a massless scalar field as a relational clock for the dynamics, the semiclassical proprieties of a large class of initial states are preserved on the other side of the bounce. This model thus enhances the relation between loop quantum cosmology and the full theory.

The Mixmaster Universe in a generalized uncertainty principle framework

Abstract The Bianchi IX cosmological model is analyzed in a generalized uncertainty principle framework. The Arnowitt–Deser–Misner reduction of the dynamics is performed and a time-coordinate, namely the volume of the Universe, naturally arises. Such a variable is treated in the ordinary way while the anisotropies (the physical degrees of freedom) are described by a deformed Heisenberg algebra. The analysis of the model (passing through Bianchi I and II) is performed at classical level by studying the modifications induced on the symplectic geometry by the deformed algebra. We show that the Universe cannot isotropize because of the deformed Kasner dynamics, the triangular allowed domain is asymptotically stationary with respect to the particle (Universe) and its bounces against the walls are not interrupted by the deformed effects. Furthermore, no reflection law can be in general obtained since the Bianchi II model is no longer analytically integrable. This way, the deformed Mixmaster Universe can be still considered as a chaotic system.

Evolutionary quantum dynamics of a generic universe

Abstract The implications of an evolutionary quantum gravity are addressed in view of formulating a new dark matter candidate. We consider a Schrodinger dynamics for the gravitational field associated to a generic cosmological model and then we solve the corresponding eigenvalue problem, inferring its phenomenological issue for the actual universe. The spectrum of the super-Hamiltonian is determined including a free inflaton field, the ultrarelativistic thermal bath and a perfect gas into the dynamics. We show that, when a Planckian cut-off is imposed in the theory and the classical limit of the ground state is taken, then a dark matter contribution cannot arise because its critical parameter Ω dm is negligible today when the appropriate cosmological implementation of the model is provided. Thus, we show that, from a phenomenological point of view, an evolutionary quantum cosmology overlaps the Wheeler–DeWitt approach and therefore it can be inferred as appropriate to describe early stages of the universe without significant traces on the later evolution. Finally, we provide indications that the horizon paradox can be solved in the Planck era by the morphology of the universe wave function.

QUANTUM COSMOLOGY WITH A MINIMAL LENGTH

Quantum cosmology in the presence of a fundamental minimal length is analyzed in the context of the flat isotropic and the Taub cosmological models. Such minimal scale comes out from a generalized uncertainty principle and the quantization is performed in the minisuperspace representation. Both the quantum Universes are singularity-free and (i) in the isotropic model no evidences for a Big-Bounce appear; (ii) in the Taub one a quasi-isotropic configuration for the Universe is predicted by the model.

Polymer quantum dynamics of the Taub universe

Within the framework of nonstandard (Weyl) representations of the canonical commutation relations, we investigate the polymer quantization of the Taub cosmological model. The Taub model is analyzed within the Arnowitt-Deser-Misner reduction of its dynamics, by which a time variable arises. While the energy variable and its conjugate momentum are treated as ordinary Heisenberg operators, the anisotropy variable and its conjugate momentum are represented by the polymer technique. The model is analyzed at both classical and quantum level. As a result, classical trajectories flatten with respect to the potential wall, and the cosmological singularity is not probabilistically removed. In fact, the dynamics of the wave packets is characterized by an interference phenomenon, which, however, is not able to stop the evolution towards the classical singularity.