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Michele Maggiore

Publications and source records attributed to Michele Maggiore.

At least 91 records · Page 5Linked to original sources
arXiv2009

Path Integral Approach to non-Markovian First-Passage Time Problems

The computation of the probability of the first-passage time through a given threshold of a stochastic process is a classic problem that appears in many branches of physics. When the stochastic dynamics is markovian, the probability admits elegant analytic solutions derived from the Fokker-Planck equation with an absorbing boundary condition while, when the underlying dynamics is non-markovian, the equation for the probability becomes non-local due to the appearance of memory terms, and the problem becomes much harder to solve. We show that the computation of the probability distribution and of the first-passage time for non-Markovian processes can be mapped into the evaluation of a path-integral with boundaries, and we develop a technique for evaluating perturbatively this path integral, order by order in the non-Markovian terms.

cond-mat.stat-mech↗
arXiv2008

The physical interpretation of the spectrum of black hole quasinormal modes

When a classical black hole is perturbed, its relaxation is governed by a set of quasinormal modes with complex frequencies ω= ω_R+iω_I. We show that this behavior is the same as that of a collection of damped harmonic oscillators whose real frequencies are (ω_R^2+ω_I^2)^{1/2}, rather than simply ω_R. Since, for highly excited modes, ω_I >> ω_R, this observation changes drastically the physical understanding of the black hole spectrum, and forces a reexamination of various results in the literature. In particular, adapting a derivation by Hod, we find that the area of the horizon of a Schwarzschild black hole is quantized in units ΔA=8π\lpl^2, where \lpl is the Planck length (in contrast with the original result ΔA=4\log(3) \lpl^2). The resulting area quantization does not suffer from a number of difficulties of the original proposal; in particular, it is an intrinsic property of the black hole, independent of the spin of the perturbation.

gr-qc↗
arXiv2006

Upper limits on the gravitational mass loss of the Galaxy and the LIGO burst searches

We discuss the relevance, for the search of gravitational-wave bursts, of upper limits on the total mass loss of the Galaxy which come from various astronomical observations. For sub-millisecond bursts we obtain limits on the event rate, as a function of the GW amplitude, which are stronger than the corresponding upper limits set by LIGO in the S2 run. A detection of a burst rate saturating these limits, with the sensitivities of present and near-future runs, would imply that, with some improvement on the accuracy of astronomical observations of the Galaxy, as foreseen with the GAIA mission, it might be possible to detect gravitational waves indirectly from their effect on galactic dynamics.

gr-qc↗
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

On the possible sources of gravitational wave bursts detectable today

We discuss the possibility that galactic gravitational wave sources might give burst signals at a rate of several events per year, detectable by state-of-the-art detectors. We are stimulated by the results of the data collected by the EXPLORER and NAUTILUS bar detectors in the 2001 run, which suggest an excess of coincidences between the two detectors, when the resonant bars are orthogonal to the galactic plane. Signals due to the coalescence of galactic compact binaries fulfill the energy requirements but are problematic for lack of known candidates with the necessary merging rate. We examine the limits imposed by galactic dynamics on the mass loss of the Galaxy due to GW emission, and we use them to put constraints also on the GW radiation from exotic objects, like binaries made of primordial black holes. We discuss the possibility that the events are due to GW bursts coming repeatedly from a single or a few compact sources. We examine different possible realizations of this idea, such as accreting neutron stars, strange quark stars, and the highly magnetized neutron stars (``magnetars'') introduced to explain Soft Gamma Repeaters. Various possibilities are excluded or appear very unlikely, while others at present cannot be excluded.

gr-qc↗
arXiv2002

Perturbative and non-perturbative aspects of the two-dimensional string/Yang-Mills correspondence

It is known that YM_2 with gauge group SU(N) is equivalent to a string theory with coupling g_s=1/N, order by order in the 1/N expansion. We show how this results can be obtained from the bosonization of the fermionic formulation of YM_2, improving on results in the literature, and we examine a number of non-perturbative aspects of this string/YM correspondence. We find contributions to the YM_2 partition function of order exp{-kA/(πα' g_s)} with k an integer and A the area of the target space, which would correspond, in the string interpretation, to D1-branes. Effects which could be interpreted as D0-branes are instead stricly absent, suggesting a non-perturbative structure typical of type 0B string theories. We discuss effects from the YM side that are interpreted in terms of the stringy exclusion principle of Maldacena and Strominger. We also find numerically an interesting phase structure, with a region where YM_2 is described by a perturbative string theory separated from a region where it is described by a topological string theory.

hep-th↗
arXiv2002

The Atick-Witten free energy, closed tachyon condensation and deformed Poincare' symmetry

The dependence of the free energy of string theory on the temperature at T much larger than the Hagedorn temperature was found long ago by Atick and Witten and is $F(T)\sim ΛT^2$, where $Λ$ diverges because of a tachyonic instability. We show that this result can be understood assuming that, above the Hagedorn transition, Poincare' symmetry is deformed into a quantum algebra. Physically this quantum algebra describes a non-commutative spatial geometry and a discrete euclidean time. We then show that in string theory this deformed Poincare' symmetry indeed emerges above the Hagedorn temperature from the condensation of vortices on the world-sheet. This result indicates that the endpoint of the condensation of closed string tachyons with non-zero winding is an infinite stack of spacelike branes with a given non-commutative world-volume geometry. On a more technical side, we also point out that $T$-duality along a circle with antiperiodic boundary conditions for spacetime fermions is broken by world-sheet vortices, and the would-be T-dual variable becomes non-compact.

hep-th↗
arXiv2001

Gravitational Waves from Electroweak Phase Transitions

Gravitational waves are generated during first-order phase transitions, either by turbolence or by bubble collisions. If the transition takes place at temperatures of the order of the electroweak scale, the frequency of these gravitational waves is today just within the band of the planned space interferometer LISA. We present a detailed analysis of the production of gravitational waves during an electroweak phase transition in different supersymmetric models where, contrary to the case of the Standard Model, the transition can be first order. We find that the stochastic background of gravitational waves generated by bubble nucleation can reach a maximum value h0^2 Omega_{gw} of order 10^{-10} - 10^{-11}, which is within the reach of the planned sensitivity of LISA, while turbolence can even produce signals at the level h0^2 Omega_{gw} \sim 10^{-9}. These values of h0^2 Omega_{gw} are obtained in the regions of the parameter space which can account for the generation of the baryon asymmetry at the electroweak scale.

gr-qc↗
arXiv2000

Stochastic backgrounds of gravitational waves

We review the motivations for the search of stochastic backgrounds of gravitational waves and we compare the experimental sensitivities that can be reached in the near future with the existing bounds and with the theoretical predictions.

gr-qc↗
arXiv2000

Confinement, asymptotic freedom and renormalons in type 0 string duals

Type 0B string theory has been proposed as the dual description of non-supersymmetric SU(N) Yang-Mills theory coupled to six scalars, in four dimensions. We study numerically and analytically the equations of motion of type 0B gravity and we find RG trajectories of the dual theory that flow from an asymptotically free UV regime to a confining IR regime. In the UV we find a one-parameter family of solutions that approach asymptotically $AdS_5\times S^5$ with a logarithmic flow of the coupling plus non-perturbative terms that correctly reproduce all UV and IR renormalon singularities. The first UV renormalon gives a contribution $\sim F_1(E)/E^2$ and we are able to predict also the form of the function $F_1(E)$, which, from the YM side, corresponds to summing all multiple-chain bubble graphs. The fact that the positions of the renormalon singularities in the Borel plane come out correctly is a non-trivial test of the conjectured duality.

hep-th↗
arXiv2000

Detection strategies for scalar gravitational waves with interferometers and resonant spheres

We compute the response and the angular pattern function of an interferometer for a scalar component of gravitational radiation in Brans-Dicke theory. We examine the problem of detecting a stochastic background of scalar GWs and compute the scalar overlap reduction function in the correlation between an interferometer and the monopole mode of a resonant sphere. While the correlation between two interferometers is maximized taking them as close as possible, the interferometer-sphere correlation is maximized at a finite value of f*d, where `f' is the resonance frequency of the sphere and `d' the distance between the detectors. This defines an optimal resonance frequency of the sphere as a function of the distance. For the correlation between the Virgo interferometer located near Pisa and a sphere located in Frascati, near Rome, we find an optimal resonance frequency f=590 Hz. We also briefly discuss the difficulties in applying this analysis to the dilaton and moduli fields predicted by string theory.

gr-qc↗
arXiv2000

Gravitational Wave Experiments and Early Universe Cosmology

Gravitational-wave experiments with interferometers and with resonant masses can search for stochastic backgrounds of gravitational waves of cosmological origin. We review both experimental and theoretical aspects of the search for these backgrounds. We give a pedagogical derivation of the various relations that characterize the response of a detector to a stochastic background. We discuss the sensitivities of the large interferometers under constructions (LIGO, VIRGO, GEO600, TAMA300, AIGO) or planned (Avdanced LIGO, LISA) and of the presently operating resonant bars, and we give the sensitivities for various two-detectors correlations. We examine the existing limits on the energy density in gravitational waves from nucleosynthesis, COBE and pulsars, and their effects on theoretical predictions. We discuss general theoretical principles for order-of-magnitude estimates of cosmological production mechanisms, and then we turn to specific theoretical predictions from inflation, string cosmology, phase transitions, cosmic strings and other mechanisms. We finally compare with the stochastic backgrounds of astrophysical origin.

gr-qc↗
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

D-branes and Cosmology

D-branes, topological defects in string theory on which string endpoints can live, may give new insight into the understanding of the cosmological evolution of the Universe at early epochs. We analyze the dynamics of D-branes in curved backgrounds and discuss the parameter space of M-theory as a function of the coupling constant and of the curvature of the Universe. We show that D-branes may be efficiently produced by gravitational effects. Furthermore, in curved spacetimes the transverse fluctuations of the D-branes develop a tachyonic mode and when the fluctuations grow larger than the horizon the branes become tensionless and break up. This signals a transition to a new regime. We discuss the implications of our findings for the singularity problem present in string cosmology, suggesting the existence of a limiting value for the curvature which is in agreement with the value suggested by the cosmological version of the holography principle. We also comment on possible implications for the so-called brane world scenario, where the Standard Model gauge and matter fields live inside some branes while gravitons live in the bulk.

hep-th↗
arXiv1998

Massive string modes and non-singular pre-big-bang cosmology

Perturbative $α'$ corrections to the low energy string effective action have been recently found to have a potentially regularizing effect on the singularity of the lowest order pre-big-bang solutions. Whether they actually regularize it, however, cannot be determined working at any finite order in a perturbative expansion in powers of the string constant $α'$, because of scheme dependence ambiguities. Physically, these corrections are dominated by the integration over the first few massive string states. Very massive string modes, instead, can have a regularizing effect which is non-perturbative in $α'$ and which basically comes from the fact that in a gravitational field with Hubble constant $H$ they are produced with an effective Hawking temperature $T=H/(2π)$, and an infinite production rate would occur if this temperature exceeded the Hagedorn temperature. We discuss technical and conceptual difficulties of this non-perturbative regularization mechanism.

gr-qc↗
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↗
arXiv1998

High-Energy Physics with Gravitational-Wave Experiments

We discuss the possible relevance of gravitational-wave (GW) experiments for physics at very high energy. We examine whether, from the experience gained with the computations of various specific relic GW backgrounds, we can extract statements and order of magnitude estimates that are as much as possible model-independent, and we try to distinguish between general conclusions and results related to specific cosmological mechanisms. We examine the statement that the Virgo/LIGO experiments probe the Universe at temperatures $T\sim 10^{7}-10^{10}$ GeV (or timescales $t\sim 10^{-20}-10^{-26}$ sec) and we consider the possibility that they could actually probe the Universe at much higher energy scales, including the typical scales of grand unification, string theory and quantum gravity. We consider possible scenarios, depending on how the inflationary paradigm is implemented. We discuss the prospects for detection with present and planned experiments. In particular, a second Virgo interferometer correlated with the planned one, and located within a few tens of kilometers from the first, could reach an interesting sensitivity for stochastic GWs of cosmological origin.

gr-qc↗