26 research outputs found
Affine Quantization and the Initial Cosmological Singularity
The Affine Coherent State Quantization procedure is applied to the case of a
FRLW universe in the presence of a cosmological constant. The quantum
corrections alter the dynamics of the system in the semiclassical regime,
providing a potential barrier term which avoids all classical singularities, as
already suggested in other models studied in the literature. Furthermore the
quantum corrections are responsible for an accelerated cosmic expansion. This
work intends to explore some of the implications of the recently proposed
"Enhanced Quantization" procedure in a simplified model of cosmology.Comment: 16 pages, 2 figures; major improvements and correction
Quantized cosmological constant in 1+1 dimensional quantum gravity with coupled scalar matter
A two dimensional matter coupled model of quantum gravity is studied in the
Dirac approach to constrained dynamics in the presence of a cosmological
constant. It is shown that after partial fixing to the conformal gauge the
requirement of a quantum realization of the conformal algebra for physical
quantum states of the fields naturally constrains the cosmological constant to
take values in a well determined and mostly discrete spectrum. Furthermore the
contribution of the quantum fluctuations of the single dynamical degree of
freedom in the gravitational sector, namely the conformal mode, to the
cosmological constant is negative, in contrast to the positive contributions of
the quantum fluctuations of the matter fields, possibly opening an avenue
towards addressing the cosmological constant problem in a more general context.Comment: 23 page
Two-dimensional quantum dilaton gravity and the quantized cosmological constant
The cosmological constant problem is one of the long-standing issues of
modern physics. While we can measure the value of the cosmological constant
with great accuracy, we are not able to calculate it in a coherent theoretical
framework. On the contrary the theoretical predictions in Quantum Field Theory
are radically different from observations. This disagreement is a hint of the
difficult conciliation of Quantum Mechanics and General Relativity in a theory
of Quantum Gravity. Current approaches to the cosmological constant problem, in
particular, do not account for the quantum nature of the gravitational
interaction and rely on perturbative calculations. In this thesis we address
the issue in the simplified framework of two-dimensional dilaton-Maxwell
gravity, coupled to scalar matter fields. In this setting we are able to
quantize our model non-perturbatively in Dirac's approach to constrained
systems. We determine that the realization of the classical symmetries at the
quantum level provides a mechanism that fixes the value of the cosmological
constant once a specific quantum state of the Universe is selected. Furthermore
Quantum Gravity introduces opposite contributions to the cosmological constant,
admitting a range of values compatible with current observations.Comment: PhD Thesis, July 2012, 127 pages, 2 figure
Duality between 1+1 dimensional Maxwell-Dilaton gravity and Liouville field theory
We present an interesting reformulation of a collection of dilaton gravity
models in two space-time dimensions into a field theory of two decoupled
Liouville fields in flat space, in the presence of a Maxwell gauge field. An
effective action is also obtained, encoding the dynamics of the dilaton field
and the single gravitational degree of freedom in a decoupled regime. This
effective action represents an interesting starting point for future work,
including the canonical quantization of these classes of non trivial models of
gravity coupled matter systems.Comment: 6 page
Fluid Entropy as Time-Variable in Canonical Quantum Gravity
The Brown-Kucha\v{r} mechanism is applied in the case of General Relativity
coupled with the Schutz' model for a perfect fluid. Using the canonical
formalism and manipulating the set of modified constraints one is able to
recover the definition of a time evolution operator, i.e. a physical
Hamiltonian, expressed as a functional of gravitational variables and the
entropy.
Comment: 5 pages, Proceedings of The 3rd Stueckelberg Workshop on Relativistic
Field Theorie
Graviton confinement inside hypermonopoles of any dimension
We show the generic existence of metastable massive gravitons in the
four-dimensional core of self-gravitating hypermonopoles in any number of
infinite-volume extra-dimensions. Confinement is observed for Higgs and gauge
bosons couplings of the order unity. Provided these resonances are light
enough, they realise the Dvali-Gabadadze-Porrati mechanism by inducing a
four-dimensional gravity law on some intermediate length scales. The effective
four-dimensional Planck mass is shown to be proportional to a negative power of
the graviton mass. As a result, requiring gravity to be four-dimensional on
cosmological length scales may solve the mass hierarchy problem.Comment: 23 pages, 6 figures, uses iopart. Misprints corrected, references
added, matches published versio