Modifications in the Spectrum of Primordial Gravitational Waves Induced by Instantonic Fluctuations

Vacuum to vacuum instantonic transitions modify the power spectrum of primordial gravitational waves. We evaluate the new form of the power spectrum for ordinary gravity as well as the parity violation induced in the spectrum by a modification of General Relativity known as Holst term and we outline the possible experimental consequences.Comment: V1: 8 pages. V2: 8 pages, some points clarified, typos corrected, some references added, final result unchanged. V3: 8 pages, title changed, presentation improved, discussion of phenomenological consequences added, comments very welcome. V4: Discussion further improved, comments very very welcom

Dualism between Physical Frames and Time in Quantum Gravity

In this work we present a discussion of the existing links between the procedures of endowing the quantum gravity with a real time and of including in the theory a physical reference frame. More precisely, as first step, we develop the canonical quantum dynamics, starting from the Einstein equations in presence of a dust fluid and arrive to a Schroedinger evolution. Then, by fixing the lapse function in the path integral of gravity, we get a Schroedinger quantum dynamics, of which eigenvalues problem provides the appearance of a dust fluid in the classical limit. The main issue of our analysis is to claim that a theory, in which the time displacement invariance, on a quantum level, is broken, is indistinguishable from a theory for which this symmetry holds, but a real reference fluid is included.Comment: 9 pages, submitted to Mod. Phys. Lett. A, major replacements in section 3 and

From the Einstein-Cartan to the Ashtekar-Barbero canonical constraints, passing through the Nieh-Yan functional

The Ashtekar-Barbero constraints for General Relativity with fermions are derived from the Einstein-Cartan canonical theory rescaling the state functional of the gravity-spinor coupled system by the exponential of the Nieh-Yan functional. A one parameter quantization ambiguity naturally appears and can be associated with the Immirzi parameter.Comment: Minor changes, two references added, accepted for publication in Phys. Rev.

On the Frame Fixing in Quantum Gravity

We provide a discussion about the necessity to fix the reference frame before quantizing the gravitational field. Our presentation is based on stressing how the 3+1-slicing of the space time becomes an ambiguous procedure as referred to a quantum 4-metric.Comment: 3 pages, no figures, proceeding of the X Marcel Grossmann meeting, Rio de Janeiro, Brazil, July 20-26, 200