arXiv subjects

Stefano Foffa

Publications and source records attributed to Stefano Foffa.

65 records · Page 4Linked to original sources

arXiv2004

Experimental signatures of gravitational wave bursters

Gravitational wave bursters are sources which emit repeatedly bursts of gravitational waves, and have been recently suggested as potentially interesting candidates for gravitational wave (GW) detectors. Mechanisms that could give rise to a GW burster can be found for instance in highly magnetized neutron stars (the magnetars which explain the phenomenon of soft gamma repeaters), in accreting neutron stars and in hybrid stars with a quark core. We point out that these sources have very distinctive experimental signatures. In particular, as already observed in the gamma-ray bursts from soft gamma repeaters, the energy spectrum of the events is a power-law, dN\sim E^{-γ}dE with γ\simeq 1.6, and they have a distribution of waiting times (the times between one outburst and the next) significantly different from the distribution of uncorrelated events. We discuss possible detection strategies that could be used to search for these events in existing gravitational wave detectors.

gr-qc↗

arXiv2004

A braneworld puzzle about entropy bounds and a maximal temperature

Entropy bounds applied to a system of N species of light quantum fields in thermal equilibrium at temperature T are saturated in four dimensions at a maximal temperature T_max=M_Planck/N^1/2. We show that the correct setup for understanding the reason for the saturation is a cosmological setup, and that a possible explanation is the copious production of black holes at this maximal temperature. The proposed explanation implies, if correct, that N light fields cannot be in thermal equilibrium at temperatures T above T_max. However, we have been unable to identify a concrete mechanism that is efficient and quick enough to prevent the universe from exceeding this limiting temperature. The same issues can be studied in the framework of AdS/CFT by using a brane moving in a five dimensional AdS-Schwarzschild space to model a radiation dominated universe. In this case we show that T_max is the temperature at which the brane just reaches the horizon of the black hole, and that entropy bounds and the generalized second law of thermodynamics seem to be violated when the brane continues to fall into the black hole. We find, again, that the known physical mechanisms, including black hole production, are not efficient enough to prevent the brane from falling into the black hole. We propose several possible explanations for the apparent violation of entropy bounds, but none is a conclusive one.

hep-th↗

arXiv2003

Bouncing pre-big bang on the brane

A regular bouncing universe is obtained in the context of a dilaton-gravity brane world scenario. The scale factor starts in a contracting inflationary phase both in the Einstein and in the string frame, it then undergoes a bounce (due to interaction with the bulk Weyl tensor), and subsequently enters into a decelerated expanding era. This graceful exit is obtained at low curvature and low coupling, and without violating the Null Energy Condition.

hep-th↗

arXiv2002

Pre-big bang on the brane

The equations of motion and junction conditions for a gravi-dilaton brane world scenario are studied in the string frame. It is shown that they allow Kasner-like solutions on the brane, which makes the dynamics of the brane very similar to the low curvature phase of pre-big bang cosmology. Analogies and differences of this scenario with the Randall-Sundrum one and with the standard bulk pre-big bang dynamics are also discussed.

hep-th↗

arXiv2002

The Shortest Scale of Quantum Field Theory

It is suggested that the Minkowski vacuum of quantum field theories of a large number of fields N would be gravitationally unstable due to strong vacuum energy fluctuations unless an N dependent sub-Planckian ultraviolet momentum cutoff is introduced. We estimate this implied cutoff using an effective quantum theory of massless fields that couple to semi-classical gravity and find it (assuming that the cosmological constant vanishes) to be bounded by $M_Planck/N^1/4$. Our bound can be made consistent with entropy bounds and holography, but does not seem to be equivalent to either, and it relaxes but does not eliminate the implied bound on N inherent in entropy bounds.

hep-th↗

arXiv2002

Probing the Planck Scale with Neutrino Oscillations

Quantum gravity "foam", among its various generic Lorentz non-invariant effects, would cause neutrino mixing. It is shown here that, if the foam is manifested as a nonrenormalizable effect at scale M, the oscillation length generically decreases with energy $E$ as (E/M)^(-2). Neutrino observatories and long-baseline experiments should have therefore already observed foam-induced oscillations, even if M is as high as the Planck energy scale. The null results, which can be further strengthened by better analysis of current data and future experiments, can be taken as experimental evidence that Lorentz invariance is fully preserved at the Planck scale, as is the case in critical string theory.

hep-ph↗

arXiv2001

Causal Entropy Bound for Non-Singular Cosmologies

The conditions for validity of the Causal Entropy Bound (CEB) are verified in the context of non-singular cosmologies with classical sources. It is shown that they are the same conditions that were previously found to guarantee validity of the CEB: the energy density of each dynamical component of the cosmic fluid needs to be sub-Planckian and not too negative, and its equation of state needs to be causal. In the examples we consider, the CEB is able to discriminate cosmologies which suffer from potential physical problems more reliably than the energy conditions appearing in singularity theorems.

hep-th↗

arXiv1999

Generalized Second Law in String Cosmology

A generalized second law in string cosmology accounts for geometric and quantum entropy in addition to ordinary sources of entropy. The proposed generalized second law forbids singular string cosmologies, under certain conditions, and forces a graceful exit transition from dilaton-driven inflation by bounding curvature and dilaton kinetic energy.

hep-th↗

arXiv1999

Loop corrections and graceful exit in string cosmology

We examine the effect of perturbative string loops on the cosmological pre-big-bang evolution. We study loop corrections derived from heterotic string theory compactified on a $Z_N$ orbifold and we consider the effect of the all-order loop corrections to the Kahler potential and of the corrections to gravitational couplings, including both threshold corrections and corrections due to the mixed Kahler-gravitational anomaly. We find that string loops can drive the evolution into the region of the parameter space where a graceful exit is in principle possible, and we find solutions that, in the string frame, connect smoothly the superinflationary pre-big-bang evolution to a phase where the curvature and the derivative of the dilaton are decreasing. We also find that at a critical coupling the loop corrections to the Kahler potential induce a ghost-like instability, i.e. the kinetic term of the dilaton vanishes. This is similar to what happens in Seiberg-Witten theory and signals the transition to a new regime where the light modes in the effective action are different and are related to the original ones by S-duality. In a string context, this means that we enter a D-brane dominated phase.

hep-th↗

arXiv1998

Supersymmetric Vacuum Configurations in String Cosmology

We examine in a cosmological context the conditions for unbroken supersymmetry in N=1 supergravity in D=10 dimensions. We show that the cosmological solutions of the equations of motion obtained considering only the bosonic sector correspond to vacuum states with spontaneous supersymmetry breaking. With a non vanishing gravitino-dilatino condensate we find a solution of the equations of motion that satisfies necessary conditions for unbroken supersymmetry and that smoothly interpolates between Minkowski space and DeSitter space with a linearly growing dilaton, thus providing a possible example of a supersymmetric and non-singular pre-big-bang cosmology.

gr-qc↗

arXiv1997

Anisotropic String Cosmology at Large Curvatures

We study the effect of the antisymmetric tensor field $B_{μν}$ on the large curvature phase of string cosmology. It is well-known that a non-vanishing value of $H=dB$ leads to an anisotropic expansion of the spatial dimensions. Correspondingly, in the string phase of the model, including $α'$ corrections, we find anisotropic fixed points of the evolution, which act as regularizing attractors of the lowest order solutions. The attraction basin can also include isotropic initial conditions for the scale factors. We present explicit examples at order $α'$ for different values of the number of spatial dimensions and for different ansätze for $H$.

gr-qc↗