Spontaneous violation of the energy conditions

A decade ago, it was shown that a wide class of scalar-tensor theories can pass very restrictive weak field tests of gravity and yet exhibit non-perturbative strong field deviations away from General Relativity. This phenomenon was called `Spontaneous Scalarization' and causes the (Einstein frame) scalar field inside a neutron star to rapidly become inhomogeneous once the star's mass increases above some critical value. For a star whose mass is below the threshold, the field is instead nearly uniform (a state which minimises the star's energy) and the configuration is similar to the General Relativity one. Here, we show that the spontaneous scalarization phenomenon is linked to another strong field effect: a spontaneous violation of the weak energy condition.Comment: 10 pages, 1 figure, accepted for publication in The Astrophysical Journal Letter

Spontaneous Scalarization and Boson Stars

We study spontaneous scalarization in Scalar-Tensor boson stars. We find that scalarization does not occur in stars whose bosons have no self-interaction. We introduce a quartic self-interaction term into the boson Lagrangian and show that when this term is large, scalarization does occur. Strong self-interaction leads to a large value of the compactness (or sensitivity) of the boson star, a necessary condition for scalarization to occur, and we derive an analytical expression for computing the sensitivity of a boson star in Brans-Dicke theory from its mass and particle number. Next we comment on how one can use the sensitivity of a star in any Scalar-Tensor theory to determine how its mass changes when it undergoes gravitational evolution. Finally, in the Appendix, we derive the most general form of the boson wavefunction that minimises the energy of the star when the bosons carry a U(1) charge.Comment: 23 pages, 5 postscript figures. Typing errors corrected. Includes some new text that relates the paper to several previous results. Accepted for publication in PR

Tensor mass and particle number peak at the same location in the scalar-tensor gravity boson star models - an analytical proof

Recently in boson star models in framework of Brans-Dicke theory, three possible definitions of mass have been identified, all identical in general relativity, but different in scalar-tensor theories of gravity.It has been conjectured that it's the tensor mass which peaks, as a function of the central density, at the same location where the particle number takes its maximum.This is a very important property which is crucial for stability analysis via catastrophe theory. This conjecture has received some numerical support. Here we give an analytical proof of the conjecture in framework of the generalized scalar-tensor theory of gravity, confirming in this way the numerical calculations.Comment: 9 pages, latex, no figers, some typos corrected, reference adde

Charged scalar-tensor boson stars: Equilibrium, stability, and evolution

We study charged boson stars in scalar-tensor (ST) gravitational theories. We analyze the weak field limit of the solutions and analytically show that there is a maximum charge to mass ratio for the bosons above which the weak field solutions are not stable. This charge limit can be greater than the GR limit for a wide class of ST theories. We numerically investigate strong field solutions in both the Brans-Dicke and power law ST theories. We find that the charge limit decreases with increasing central boson density. We discuss the gravitational evolution of charged and uncharged boson stars in a cosmological setting and show how, at any point in its evolution, the physical properties of the star may be calculated by a rescaling of a solution whose asymptotic value of the scalar field is equal to its initial asymptotic value. We focus on evolution in which the particle number of the star is conserved and we find that the energy and central density of the star decrease as the cosmological time increases. We also analyze the appearance of the scalarization phenomenon recently discovered for neutron stars configurations and, finally, we give a short discussion on how making the correct choice of mass influences the argument over which conformal frame, the Einstein frame or the Jordan frame, is physical.Facultad de Ciencias Exacta

Neutron Stars in a Varying Speed of Light Theory

We study neutron stars in a varying speed of light (VSL) theory of gravity in which the local speed of light depends upon the value of a scalar field $\phi$. We find that the masses and radii of the stars are strongly dependent on the strength of the coupling between $\phi$ and the matter field and that for certain choices of coupling parameters, the maximum neutron star mass can be arbitrarily small. We also discuss the phenomenon of cosmological evolution of VSL stars (analogous to the gravitational evolution in scalar-tensor theories) and we derive a relation showing how the fractional change in the energy of a star is related to the change in the cosmological value of the scalar field.Comment: 15 pages, 2 figures. Added solutions with a more realistic equation of state. To be published in PR

Brans-Dicke Boson Stars: Configurations and Stability through Cosmic History

We make a detailed study of boson star configurations in Jordan--Brans--Dicke theory, studying both equilibrium properties and stability, and considering boson stars existing at different cosmic epochs. We show that boson stars can be stable at any time of cosmic history and that equilibrium stars are denser in the past. We analyze three different proposed mass functions for boson star systems, and obtain results independently of the definition adopted. We study how the configurations depend on the value of the Jordan--Brans--Dicke coupling constant, and the properties of the stars under extreme values of the gravitational asymptotic constant. This last point allows us to extract conclusions about the stability behaviour concerning the scalar field. Finally, other dynamical variables of interest, like the radius, are also calculated. In this regard, it is shown that the radius corresponding to the maximal boson star mass remains roughly the same during cosmological evolution.Comment: 9 pages RevTeX file with nine figures incorporated (uses RevTeX and epsf

Charged Scalar-Tensor Boson Stars: Equilibrium, Stability and Evolution

We study charged boson stars in scalar-tensor (ST) gravitational theories. We analyse the weak field limit of the solutions and analytically show that there is a maximum charge to mass ratio for the bosons above which the weak field solutions are not stable. This charge limit can be greater than the GR limit for a wide class of ST theories. We numerically investigate strong field solutions in both the Brans Dicke and power law ST theories. We find that the charge limit decreases with increasing central boson density. We discuss the gravitational evolution of charged and uncharged boson stars in a cosmological setting and show how, at any point in its evolution, the physical properties of the star may be calculated by a rescaling of a solution whose asymptotic value of the scalar field is equal to its initial asymptotic value. We focus on evolution in which the particle number of the star is conserved and we find that the energy and central density of the star decreases as the cosmological time increases. We also analyse the appearance of the scalarization phenomenon recently discovered for neutron stars configurations and, finally, we give a short discussion on how making the correct choice of mass influences the argument over which conformal frame, the Einstein frame or the Jordan frame, is physical.Comment: RevTeX, 27 pages, 9 postscript figures. Minor revisions and updated references. Accepted for publication in Phys. Rev.

Neutron star in presence of torsion-dilaton field

We develop the general theory of stars in Saa's model of gravity with propagating torsion and study the basic stationary state of neutron star. Our numerical results show that the torsion force decreases the role of the gravity in the star configuration leading to significant changes in the neutron star masses depending on the equation of state of star matter. The inconsistency of the Saa's model with Roll-Krotkov-Dicke and Braginsky-Panov experiments is discussed.Comment: 29 pages, latex, 24 figures, final version. Added: 1)comments on different possible mass definitions; 2)new sections: a)the inconsistency of the Saa's model with Roll-Krotkov-Dicke and Braginsky-Panov experiments; b)stability analysis via catastrophe theory; 3)new figers added and some figures replaced. 4)new reference

Quantification of Electromechanical Coupling to Prevent Inappropriate Implantable Cardioverter-Defibrillator Shocks

Objective To test specialised processing of laser Doppler signals for discriminating ventricular fibrillation(VF) from common causes of inappropriate therapies. Background Inappropriate ICD therapies remain a clinically important problem associated with morbidity and mortality. Tissue perfusion biomarkers, to assist automated diagnosis of VF, suffer the vulnerability of sometimes mistaking artefact and random noise for perfusion, which could lead to shocks being inappropriately withheld. Methods We developed a novel processing algorithm that combines electrogram data and laser Doppler perfusion monitoring, as a method for assessing circulatory status. We recruited 50 patients undergoing VF induction during ICD implantation. We recorded non-invasive laser Doppler and continuous electrograms, during both sinus-rhythm and VF. For each patient we simulated two additional scenarios that may lead to inappropriate shocks: ventricular-lead fracture and T-wave oversensing. We analysed the laser Doppler using three methods for reducing noise: (i)Running Mean, (ii)Oscillatory Height, (iii)a novel quantification of Electro-Mechanical coupling which gates laser Doppler against electrograms. We additionally tested the algorithm during exercise induced sinus tachycardia. Results Only the Electro-mechanical coupling algorithm found a clear perfusion cut-off between sinus rhythm and VF (sensitivity and specificity 100%). Sensitivity and specificity remained 100% during simulated lead fracture and electrogram oversensing. (AUC: Running Mean 0.91, Oscillatory Height 0.86, Electro-Mechanical Coupling 1.00). Sinus tachycardia did not cause false positives. Conclusions Quantifying the coupling between electrical and perfusion signals increases reliability of discrimination between VF and artefacts that ICDs may interpret as VF. Incorporating such methods into future ICDs may safely permit reductions of inappropriate shocks