Search results for "GRAVITY"

Probing Models of Extended Gravity using Gravity Probe B and LARES experiments

2014

We consider models of Extended Gravity and in particular, generic models containing scalar-tensor and higher-order curvature terms, as well as a model derived from noncommutative spectral geometry. Studying, in the weak-field approximation, the geodesic and Lense-Thirring processions, we impose constraints on the free parameters of such models by using the recent experimental results of the Gravity Probe B and LARES satellites.

The Crossing Symmetric Bethe-Salpeter Equation

1972

As you may recall from the lectures of Prof. Sand-has [1], in non-relativistic quantum theory,the scattering amplitude satisfies the Lippmann-Schwinger equation, $$T = V + V{G_o}T$$ (1) It can be explicitly shown that if V=V+, T satisfies the elastic unitarity relation, Im T=TT+.

Waves Propagation in Turbulent Superfluid Helium in Presence of Combined Rotation and Counterflow

2010

A complete study of the propagation of waves (namely longitudinal density and temperature waves, longitudinal and transversal velocity waves and heat waves) in turbulent superfluid helium is made in three situations: a rotating frame, a thermal counterflow, and the simultaneous combination of thermal counterflow and rotation. Our analysis aims to obtain as much as possible information on the tangle of quantized vortices from the wave speed and attenuation factor of these different waves, depending on their relative direction of propagation with respect to the rotation vector.

Minimum main sequence mass in quadratic Palatini f(R) gravity

2019

General relativity yields an analytical prediction of a minimum required mass of roughly $\ensuremath{\sim}0.08--0.09\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ for a star to stably burn sufficient hydrogen to fully compensate photospheric losses and, therefore, to belong to the main sequence. Those objects below this threshold (brown dwarfs) eventually cool down without any chance to stabilize their internal temperature. In this work we consider quadratic Palatini $f(\mathcal{R})$ gravity and show that the corresponding Newtonian hydrostatic equilibrium equation contains a new term whose effect is to introduce a weakening/strengthening of the gravitational interaction inside astrophysical…

Asymptotic Safety, Fractals, and Cosmology

2013

These lecture notes introduce the basic ideas of the asymptotic safety approach to quantum Einstein gravity (QEG). In particular they provide the background for recent work on the possibly multi-fractal structure of the QEG space-times. Implications of asymptotic safety for the cosmology of the early Universe are also discussed.

Hyperboloidal slicing approach to quasinormal mode expansions: The Reissner-Nordström case

2018

We study quasi-normal modes of black holes, with a focus on resonant (or quasi-normal mode) expansions, in a geometric frame based on the use of conformal compactifications together with hyperboloidal foliations of spacetime. Specifically, this work extends the previous study of Schwarzschild in this geometric approach to spherically symmetric asymptotically flat black hole spacetimes, in particular Reissner-Nordstr\"om. The discussion involves, first, the non-trivial technical developments needed to address the choice of appropriate hyperboloidal slices in the extended setting as well as the generalization of the algorithm determining the coefficients in the expansion of the solution in te…

Boson stars in Palatini f(R) gravity

2021

Thermal conduction and modeling of static stellar coronal loops

1993

We have modeled stellar coronal loops in static conditions for a wide range of loop length, plasma pressure at the base of the loop and stellar surface gravity, so as to describe physical conditions that can occur in coronae of stars ranging from low mass dwarfs to giants as well as on a significant fraction of the Main-Sequence stars.

U(N) invariant dynamics for a simplified loop quantum gravity model

2011

The implementation of the dynamics in Loop Quantum Gravity (LQG) is still an open problem. Here, we discuss a tentative dynamics for the simplest class of graphs in LQG: Two vertices linked with an arbitrary number of edges. We use the recently introduced U(N) framework in order to construct SU(2) invariant operators and define a global U(N) symmetry that will select the homogeneous/isotropic states. Finally, we propose a Hamiltonian operator invariant under area-preserving deformations of the boundary surface and we identify possible connections of this model with Loop Quantum Cosmology.

Hydrodynamic Modeling of Accretion Shock on CTTSs

2009

High resolution (R ~ 600) X-ray observations of some classical T Tauri stars (CTTSs) (TW Hya, BP Tau, V4046 Sgr, MP Mus and RU Lupi) have shown the presence of X-ray plasma at T ~ 2–3 × 106 K and denser than n e ~ 1011 cm-3 [1, 2, 3, 4, 5], which suggests an origin different from the coronal one (n e ~ 1010 cm_3). Stationary models demonstrated that X-ray emission from CTTSs could also be produced by the accreting material [6]. We address this issue with the aid of a time-dependent hydrodynamic numerical model describing the impact of an accretion stream onto the chromosphere of a CTTS (see [7] for more details). Our simulations include the effects of gravity, radiative losses from opticall…