Archive ouverte UNIGE | last documents for author 'Stefano Foffa'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Stefano Foffa'engApparent ghosts and spurious degrees of freedom in non-local theorieshttps://archive-ouverte.unige.ch/unige:55807https://archive-ouverte.unige.ch/unige:55807Recent work has shown that non-local modifications of the Einstein equations can have interesting cosmological consequences and can provide a dynamical origin for dark energy, consistent with existing data. At first sight these theories are plagued by ghosts. We show that these apparent ghost-like instabilities do not describe actual propagating degrees of freedom, and there is no issue of ghost-induced quantum vacuum decay.Mon, 27 Apr 2015 10:31:32 +0200Cosmological dynamics and dark energy from nonlocal infrared modifications of gravityhttps://archive-ouverte.unige.ch/unige:40530https://archive-ouverte.unige.ch/unige:40530We study the cosmological dynamics of a recently proposed infrared modification of the Einstein equations, based on the introduction of a nonlocal term constructed with m2gμν□-1R, where m is a mass parameter. The theory generates automatically a dynamical dark energy component, that can reproduce the observed value of the dark energy density without introducing a cosmological constant. Fixing m so to reproduce the observed value ΩDE≃0.68, and writing wDE(a)= w0+(1-a)wa, the model provides a neat prediction for the equation of state parameters of dark energy, w0≃-1.042 and wa≃-0.020, and more generally provides a pure prediction for wDE as a function of redshift. We show that, because of some freedom in the definition of □-1, one can extend the construction so to define a more general family of nonlocal models. However, in a first approximation this turns out to be equivalent to adding an explicit cosmological constant term on top of the dynamical dark energy component. This leads to an extended model with two parameters, ΩΛ and m. Even in this extension the EOS parameter w0 is always on the phantom side, in the range -1.33 ≲w0≤-1, and there is a prediction for the relation between w0 and wa.Fri, 26 Sep 2014 09:31:08 +0200Results of the IGEC-2 search for gravitational wave bursts during 2005https://archive-ouverte.unige.ch/unige:35684https://archive-ouverte.unige.ch/unige:35684The network of resonant bar detectors of gravitational waves resumed coordinated observations within the International Gravitational Event Collaboration (IGEC-2). Four detectors are taking part in this Collaboration: ALLEGRO, AURIGA, EXPLORER and NAUTILUS. We present here the results of the search for gravitational wave bursts over 6 months during 2005, when IGEC-2 was the only gravitational wave observatory in operation. The implemented network data analysis is based on a time coincidence search among AURIGA, EXPLORER and NAUTILUS; ALLEGRO data was reserved for follow-up studies. The network amplitude sensitivity to bursts improved by a factor ≈3 over the 1997-2000 IGEC observations; the wider sensitive band also allowed the analysis to be tuned over a larger class of waveforms. Given the higher single-detector duty factors, the analysis was based on threefold coincidence, to ensure the identification of any single candidate of gravitational waves with high statistical confidence. The false detection rate was as low as 1 per century. No candidates were found.Mon, 14 Apr 2014 16:32:01 +0200The 2003 run of the EXPLORER–NAUTILUS gravitational wave experimenthttps://archive-ouverte.unige.ch/unige:35683https://archive-ouverte.unige.ch/unige:35683We report here the preliminary results of the search for gravitational wave bursts from the 2003 run of the EXPLORER–NAUTILUS experiment. The total common measuring time was 149 days. The detectors had a typical noise spectral amplitude of about 2 × 10−21 Hz−1/2, a bandwidth of the order of 10 Hz and a very good stability. We derive a new upper limit for the GW burst rate, of the order of 0.02 events/day for hRSS ≥ 2 × 10−19, and discuss the implication of this result with respect to the results obtained with the 2001 runMon, 14 Apr 2014 16:31:09 +0200Status report on the EXPLORER and NAUTILUS detectors and the present science runhttps://archive-ouverte.unige.ch/unige:35682https://archive-ouverte.unige.ch/unige:35682We report on the present scientific run (04–05) of the two detectors EXPLORER and NAUTILUS. The 04–05 run of the two detectors started in March 2004. The strain sensitivity is about 7 × 10−22 Hz−1/2 and the bandwidth is about 5 Hertz. The sensitivity for 1 ms bursts is h = 3 × 10−19Mon, 14 Apr 2014 16:30:20 +0200IGEC2: A 17-month search for gravitational wave bursts in 2005–2007https://archive-ouverte.unige.ch/unige:35673https://archive-ouverte.unige.ch/unige:35673We present here the results of a 515 day search for short bursts of gravitational waves by the IGEC2 observatory. This network included 4 cryogenic resonant-bar detectors: AURIGA, EXPLORER, and NAUTILUS in Europe, and ALLEGRO in America. These results cover the time period from November 6th 2005 until April 15th 2007, partly overlapping the first long term observations by the LIGO interferometric detectors. The observatory operated with high duty cycle, namely, 57% for fourfold coincident observations, and 94% for threefold observations. The sensitivity was the best ever obtained by a bar network: we could detect, with an efficiency >50%, impulsive events with a burst strain amplitude hrss≲1×10−19 Hz−1/2. The network data analysis was based on time coincidence searches over at least three detectors, used a blind search technique, and was tuned to achieve a false alarm rate of 1/century. When the blinding was removed, no gravitational wave candidate was found.Mon, 14 Apr 2014 16:00:42 +0200Extracting the three- and four-graviton vertices from binary pulsars and coalescing binarieshttps://archive-ouverte.unige.ch/unige:35672https://archive-ouverte.unige.ch/unige:35672Using a formulation of the post-Newtonian expansion in terms of Feynman graphs, we discuss how various tests of general relativity (GR) can be translated into measurement of the three- and four-graviton vertices. In problems involving only the conservative dynamics of a system, a deviation of the three-graviton vertex from the GR prediction is equivalent, to lowest order, to the introduction of the parameter βPPN in the parametrized post-Newtonian formalism, and its strongest bound comes from lunar laser ranging, which measures it at the 0.02% level. Deviation of the three-graviton vertex from the GR prediction, however, also affects the radiative sector of the theory. We show that the timing of the Hulse-Taylor binary pulsar provides a bound on the deviation of the three-graviton vertex from the GR prediction at the 0.1% level. For coalescing binaries at interferometers we find that, because of degeneracies with other parameters in the template such as mass and spin, the effects of modified three- and four-graviton vertices is just to induce an error in the determination of these parameters and, at least in the restricted PN approximation, it is not possible to use coalescing binaries for constraining deviations of the vertices from the GR predictionMon, 14 Apr 2014 15:59:55 +0200EXPLORER and NAUTILUS gravitational wave detectors: a status reporthttps://archive-ouverte.unige.ch/unige:35669https://archive-ouverte.unige.ch/unige:35669The two cryogenic resonant bar detectors of the ROG Collaboration, EXPLORER and NAUTILUS, have been taking data continuously with a high duty cycle for several years. We report here on the status of recent analysis of the data and in particular on the results of the burst searches in the year 2004Mon, 14 Apr 2014 15:57:13 +0200All-sky incoherent search for periodic signals with Explorer 2005 datahttps://archive-ouverte.unige.ch/unige:35666https://archive-ouverte.unige.ch/unige:35666The data collected during 2005 by the resonant bar Explorer are divided into segments and incoherently summed in order to perform an all-sky search for periodic gravitational wave signals. The parameter space of the search spanned about 40 Hz in frequency, over 23 927 positions in the sky. Neither source orbital corrections nor spindown parameters have been included, with the result that the search was sensitive to isolated neutron stars with a frequency drift less than 6 × 10−11 Hz s−1. No gravitational wave candidates have been found by means of the present analysis, which led to a best upper limit of 3.1 × 10−23 for the dimensionless strain amplitude.Mon, 14 Apr 2014 15:55:20 +0200Multichannel matched filtering for spherical gravitational wave antennashttps://archive-ouverte.unige.ch/unige:2218https://archive-ouverte.unige.ch/unige:2218We study the performance of a multidimensional matched filter as a follow-up module of the coherent method recently developed by two of us for the detection of gravitational wave bursts by spherical resonant detectors. We have tested this strategy on the same set of injections used for the coherent method and found that the matched filter sensibly improves the determination of relevant parameters as the arrival time, amplitude, central frequency and arrival direction of the signal. The matched filter also improves the false alarm rate, reducing it roughly by a factor of 3. The hierarchical structure of the whole analysis pipeline allows to obtain these results without a significant increase of the computation time.Fri, 10 Jul 2009 15:15:12 +0200Supersymmetric vacuum configurations in string cosmologyhttps://archive-ouverte.unige.ch/unige:2216https://archive-ouverte.unige.ch/unige:2216We 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.Thu, 09 Jul 2009 10:52:52 +0200Symmetry breaking aspects of the effective Lagrangian for quantum black holeshttps://archive-ouverte.unige.ch/unige:2215https://archive-ouverte.unige.ch/unige:2215The physical excitations entering the effective Lagrangian for quantum black holes are related to a Goldstone boson which is present in the Rindler limit and is due to the spontaneous breaking of the translation symmetry of the underlying Minkowski space. This physical interpretation, which closely parallels similar well-known results for the effective stringlike description of flux tubes in QCD, gives a physical insight into the problem of describing the quantum degrees of freedom of black holes. It also suggests that the recently suggested concept of 'black hole complementarity' emerges at the effective Lagrangian level rather than at the fundamental level.Thu, 09 Jul 2009 10:16:30 +0200Anisotropic string cosmology at large curvatureshttps://archive-ouverte.unige.ch/unige:2213https://archive-ouverte.unige.ch/unige:2213We study the effect of the antisymmetric tensor field B_{mu u} 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.Thu, 09 Jul 2009 10:13:39 +0200Loop corrections and graceful exit in string cosmologyhttps://archive-ouverte.unige.ch/unige:2212https://archive-ouverte.unige.ch/unige:2212We 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.Thu, 09 Jul 2009 10:11:57 +0200Generalized second law in string cosmologyhttps://archive-ouverte.unige.ch/unige:2211https://archive-ouverte.unige.ch/unige:2211A 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.Thu, 09 Jul 2009 10:09:49 +0200The shortest scale of quantum field theoryhttps://archive-ouverte.unige.ch/unige:2209https://archive-ouverte.unige.ch/unige:2209It 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 semiclassical gravity and find it (assuming that the cosmological constant vanishes) to be bounded by MPlanck/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.Tue, 07 Jul 2009 14:12:47 +0200CFT, holography, and causal entropy boundhttps://archive-ouverte.unige.ch/unige:2208https://archive-ouverte.unige.ch/unige:2208The causal entropy bound (CEB) is confronted with recent explicit entropy calculations in weakly and strongly coupled conformal field theories (CFTs) in arbitrary dimension $D$. For CFT's with a large number of fields, $N$, the CEB is found to be valid for temperatures not exceeding a value of order $M_P/N^{{1over D-2}}$, in agreement with large $N$ bounds in generic cut-off theories of gravity, and with the generalized second law. It is also shown that for a large class of models including high-temperature weakly coupled CFT's and strongly coupled CFT's with AdS duals, the CEB, despite the fact that it relates extensive quantities, is equivalent to (a generalization of) a purely holographic entropy bound proposed by E. Verlinde.Tue, 07 Jul 2009 14:07:06 +0200Probing the Planck Scale with neutrino oscillationshttps://archive-ouverte.unige.ch/unige:2207https://archive-ouverte.unige.ch/unige:2207Quantum 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.Tue, 07 Jul 2009 14:02:44 +0200Causal entropy bound for non-singular cosmologieshttps://archive-ouverte.unige.ch/unige:2206https://archive-ouverte.unige.ch/unige:2206The conditions for the validity of the causal entropy bound ~CEB! are verified in the context of nonsingular cosmologies with classical sources. It is shown that they are the same conditions that were previously found to guarantee the 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.Tue, 07 Jul 2009 14:00:53 +0200Pre-big bang on the branehttps://archive-ouverte.unige.ch/unige:2205https://archive-ouverte.unige.ch/unige:2205The 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.Tue, 07 Jul 2009 13:58:03 +0200Bouncing pre- big bang on the branehttps://archive-ouverte.unige.ch/unige:2204https://archive-ouverte.unige.ch/unige:2204A 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 bulkWeyl 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.Tue, 07 Jul 2009 13:48:57 +0200A braneworld puzzle about entropy bounds and a maximal temperaturehttps://archive-ouverte.unige.ch/unige:2203https://archive-ouverte.unige.ch/unige:2203Entropy 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 Tmax = MPlanck/√N. 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 Tmax. 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 Tmax 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.Tue, 07 Jul 2009 13:44:43 +0200Sensitivity of a small matter-wave interferometer to gravitational waveshttps://archive-ouverte.unige.ch/unige:2202https://archive-ouverte.unige.ch/unige:2202We study the possibility of using matter wave interferometry techniques to build a gravitational wave detector. We derive the response function and find that it contains a term proportional to the derivative of the gravitational wave, a point which has been disputed recently. We then study in detail the sensitivity that can be reached by such a detector and find that, if it is operated near resonance, it can reach potentially interesting values in the high frequency regime. The correlation between two or more of such devices can further improve the sensitivity for a stochastic signal.Tue, 07 Jul 2009 13:41:47 +0200Experimental signatures of gravitational wave burstershttps://archive-ouverte.unige.ch/unige:2201https://archive-ouverte.unige.ch/unige:2201Gravitational 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 -ray bursts from soft gamma repeaters, the energy spectrum of the events is a power-law, dN ∼ E− dE with ≃ 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.Tue, 07 Jul 2009 13:38:40 +0200Event trigger generator for resonant spherical detectors of gravitational waveshttps://archive-ouverte.unige.ch/unige:2200https://archive-ouverte.unige.ch/unige:2200We have set up and tested a pipeline for processing the data from a spherical gravitational wave detector with six transducers. The algorithm exploits the multichannel capability of the system and provides a list of candidate events with their arrival direction. The analysis starts with the conversion of the six detector outputs into the scalar and the five quadrupolar modes of the sphere, which are proportional to the corresponding gravitational wave spherical components. Event triggers are then generated by an adaptation of the WaveBurst algorithm. Event validation and direction reconstruction are made by cross-checking two methods of different inspiration: geometrical (lowest eigenvalue) and probabilistic (maximum likelihood). The combination of the two methods is able to keep substantially unaltered the efficiency and can reduce drastically the detections of fake events (to less than ten per cent). We show a quantitative test of these ideas by simulating the operation of the resonant spherical detector miniGRAIL, whose planned sensitivity in its frequency band (few hundred Hertz's around 3kHz) is comparable with the present LIGO one.Tue, 07 Jul 2009 13:33:43 +0200All-sky search of NAUTILUS datahttps://archive-ouverte.unige.ch/unige:2059https://archive-ouverte.unige.ch/unige:2059A search for periodic gravitational-wave signals from isolated neutron stars in the NAUTILUS detector data is presented. We have analyzed half a year of data over the frequency band lang922.2; 923.2rang Hz, the spindown range lang − 1.463 × 10−8; 0rang Hz/s and over the entire sky. We have divided the data into two day stretches and we have analyzed each stretch coherently using matched filtering. We have imposed a low threshold for the optimal detection statistic to obtain a set of candidates that are further examined for coincidences among various data stretches. For some candidates we have also investigated the change of the signal-to-noise ratio when we increase the observation time from 2 to 4 days. Our analysis has not revealed any gravitational-wave signals. Therefore we have imposed upper limits on the dimensionless gravitational-wave amplitude over the parameter space that we have searched. Depending on frequency, our upper limit ranges from 3.4 × 10−23 to 1.3 × 10−22. We have attempted a statistical verification of the hypotheses leading to our conclusions. We estimate that our upper limit is accurate to within 18%.Tue, 16 Jun 2009 16:13:51 +0200Coherent detection method of gravitational wave bursts for spherical antennashttps://archive-ouverte.unige.ch/unige:2058https://archive-ouverte.unige.ch/unige:2058We provide a comprehensive theoretical framework and a quantitative test of the method we recently proposed for processing data from a spherical detector with five or six transducers. Our algorithm is a trigger event generator performing a coherent analysis of the sphere channels. In order to test our pipeline we first built a detailed numerical model of the detector, including deviations from the ideal case such as quadrupole modes splitting, and non-identical transducer readout chains. This model, coupled with a Gaussian noise generator, has then been used to produce six time series, corresponding to the outputs of the six transducers attached to the sphere. We finally injected gravitational wave burst signals into the data stream, as well as bursts of non-gravitational origin in order to mimic the presence of non-Gaussian noise, and then processed the mock data. We report quantitative results for the detection efficiency versus the false alarm rate and for the affordability of the reconstruction of the direction of arrival. In particular, the combination of the two direction reconstruction methods can reduce by a factor of 10 the number of false alarms due to the non-Gaussian noise.Tue, 16 Jun 2009 15:34:55 +0200