arXiv subjects

M. Giovannini

Publications and source records attributed to M. Giovannini.

At least 37 records · Page 2Linked to original sources

arXiv2002

Constraints on pre-big bang parameter space from CMBR anisotropies

The so-called curvaton mechanism --a way to convert isocurvature perturbations into adiabatic ones-- is investigated both analytically and numerically in a pre-big bang scenario where the role of the curvaton is played by a sufficiently massive Kalb--Ramond axion of superstring theory. When combined with observations of CMBR anisotropies at large and moderate angular scales, the present analysis allows us to constrain quite considerably the parameter space of the model: in particular, the initial displacement of the axion from the minimum of its potential and the rate of evolution of the compactification volume during pre-big bang inflation. The combination of theoretical and experimental constraints favours a slightly blue spectrum of scalar perturbations, and/or a value of the string scale in the vicinity of the SUSY-GUT scale.

hep-ph↗

arXiv2002

Assisting pre-big bang phenomenology through short-lived axions

We present the results of a detailed study of how isocurvature axion fluctuations are converted into adiabatic metric perturbations through axion decay, and discuss the constraints on the parameters of pre-big bang cosmology needed for consistency with present CMB-anisotropy data. The large-scale normalization of temperature fluctuations has a non-trivial dependence both on the mass and on the initial value of the axion. In the simplest, minimal models of pre-big bang inflation, consistency with the COBE normalization requires a slightly tilted (blue) spectrum, while a strictly scale-invariant spectrum requires mild modifications of the minimal backgrounds at large curvature and/or string coupling.

hep-ph↗

arXiv2002

Warped compactification on Abelian vortex in six dimensions

We consider the possibility of localizing gravity on a Nielsen-Olesen vortex in the context of the Abelian Higgs model. The vortex lives in a six-dimensional space-time with negative bulk cosmological constant. In this model we find a region of the parameter space leading, simultaneously, to warped compactification and to regular space-time geometry. A thin defect limit is studied. Regular solutions describing warped compactifications in the case of higher winding number are also presented.

hep-th↗

arXiv2001

Cosmological Magnetogenesis: what we know and what we would like to know

Problems and perspectives concerning the existence of large-scale magnetic fields are described. Heeding observations, possible origins and implications of magnetic fields in spiral galaxies and in regular clusters are scrutinized in different cosmological frameworks including scenarios where conformal invariance is broken because of the evolution of the gauge couplings.The generation of the BAU via hypermagnetic knots is reviewed. It is also argued that stochastic GW backgrounds can be generated in the LISA frequency range thanks to the presence of hypermagnetic fields.

hep-ph↗

arXiv2001

Six-dimensional Abelian vortices with quadratic curvature self-interactions

Six-dimensional Nielsen-Olesen vortices are analyzed in the context of a quadratic gravity theory containing Euler-Gauss-Bonnet self-interactions. The relations among the string tensions can be tuned in such a way that the obtained solutions lead to warped compactification on the vortex. New regular solutions are possible in comparison with the case where the gravity action only consists of the Einstein-Hilbert term. The parameter space of the model is discussed

hep-th↗

arXiv2001

A Superconductor Made by a Metal Heterostructure at the Atomic Limit Tuned at the "Shape Resonance": MgB2

We have studied the variation of Tc with charge density and lattice parameters in Mg1-xAlxB2 superconducting samples at low Al doping x<8%. We show that high Tc occurs where the chemical potential is tuned at a "superconducting shape resonance" near the energy Ec of the quantum critical point (QCP) for the dimensional transition from 2D to 3D electronic structure in a particular subband of the natural superlattice of metallic atomic boron layers. At the "shape resonance" the electrons pairs see a 2D Fermi surface at EF-w0 and a 3D Fermi surface at EF+wo, where wo is the energy cut off of the pairing interaction. The resonant amplification occurs in a narrow energy range where EF-Ec is in the range of 2wo.

cond-mat.supr-con↗

arXiv2001

Thermal conductivity of superconducting MgB2

Thermal conductivity of superconducting MgB2 was studied in both the superconducting and the normal state region. The latter is almost equally determined by the electronic - and the lattice contribution to the total thermal conductivity. In the superconducting state, however, the lattice contribution is larger. The electronic thermal conductivity below Tc was derived from the experimental data considering the Bardeen-Rickayzen-Tewordt theory together with the model of Geilikman. The analysis shows that electron scattering on static imperfections dominates.

cond-mat.supr-con↗

arXiv2000

Primordial magnetic fields from inflation?

The hot plasma above the electroweak scale contains (hyper) charged scalar particles which are coupled to Abelian gauge fields. Scalars may interact with gravity in a non-conformally invariant way and thus their fluctuations can be amplified during inflation. These fluctuations lead to creation of electric currents and produce inhomogeneous distribution of charge density, resulting in the generation of cosmological magnetic fields. We address the question whether these fields can be coherent at large scales so that they may seed the galactic magnetic fields. Depending upon the mass of the charged scalar and upon various cosmological (critical fraction of energy density in matter, Hubble constant) and particle physics parameters we found that the magnetic fields generated in this way are much larger than vacuum fluctuations. However, their amplitude on cosmological distances is found to be too small for seeding the galactic magnetic fields.

hep-ph↗

arXiv1999

Maximal overlap and sensitivity of a VIRGO pair to graviton backgrounds

The sensitivity of a pair of VIRGO interferometers to gravitational waves backgrounds of cosmological origin is analyzed for the cases of maximal and minimal overlap of the two detectors. The improvements in the detectability prospects of scale-invariant and non-scale-invariant logarithmic energy spectra of relic gravitons are discussed.

astro-ph↗

arXiv1999

Upgraded VIRGO detector(s) and stochastic gravitational waves backgrounds

The sensitivity achievable by a pair of VIRGO detectors to stochastic and isotropic gravitational wave backgrounds of cosmological origin is discussed in view of the development of a second VIRGO interferometer. We describe a semi-analytical technique allowing to compute the signal-to-noise ratio for (monotonic or non-monotonic) logarithmic energy spectra of relic gravitons of arbitrary slope. We apply our results to the case of two correlated and coaligned VIRGO detectors and we compute their achievable sensitivities. The maximization of the overlap reduction function is discussed. We focus our attention on a class of models whose expected sensitivity is more promising, namely the case of string cosmological gravitons. We perform our calculations both for the case of minimal string cosmological scenario and in the case of a non-minimal scenario where a long dilaton dominated phase is present prior to the onset of the ordinary radiation dominated phase. In this framework, we study possible improvements of the achievable sensitivities by selective reduction of the thermal contributions (pendulum and pendulum's internal modes) to the noise power spectra of the detectors. Since a reduction of the shot noise does not increase significantly the expected sensitivity of a VIRGO pair (in spite of the relative spatial location of the two detectors) our findings support the experimental efforts directed towards a substantial reduction of thermal noise.

gr-qc↗

arXiv1997

Primordial Hypermagnetic Fields and Triangle Anomaly

The high-temperature plasma above the electroweak scale $\sim 100$ GeV may have contained a primordial hypercharge magnetic field whose anomalous coupling to the fermions induces a transformation of the hypermagnetic energy density into fermionic number. In order to describe this process, we generalize the ordinary magnetohydrodynamical equations to the anomalous case. We show that a not completely homogeneous hypermagnetic background induces fermion number fluctuations, which can be expressed in terms of a generic hypermagnetic field configuration. We argue that, depending upon the various particle physics parameters involved in our estimate (electron Yukawa coupling, strength of the electroweak phase transition) and upon the hypermagnetic energy spectrum, sizeable matter-antimatter fluctuations can be generated in the plasma. These fluctuations may modify the predictions of the standard Big Bang nucleosynthesis (BBN). We derive constraints on the magnetic fields from the requirement that the homogeneous BBN is not changed. We analyse the influence of primordial magnetic fields on the electroweak phase transition and show that some specific configurations of the magnetic field may be converted into net baryon number at the electroweak scale.

hep-ph↗

arXiv1997

Primordial magnetic fields, anomalous isocurvature fluctuations and Big Bang nucleosynthesis

We show that the presence of primordial stochastic (hypercharge) magnetic fields before the electroweak (EW) phase transition induces isocurvature fluctuations (baryon number inhomogeneities). Depending on the details of the magnetic field spectrum and on the particle physics parameters (such as the strength of the EW phase transition and electron Yukawa couplings) these fluctuations may survive until the Big Bang nucleosynthesis (BBN). Their lenghtscale may exceed the neutron diffusion length at that time, while their magnitude can be so large that sizable antimatter domains are present. This provides the possibility of a new type of initial conditions for non-homogeneous BBN or, from a more conservative point of view, stringent bounds on primordial magnetic fields.

hep-ph↗

arXiv1997

Normal modes for metric fluctuations in a class of higher-dimensional backgrounds

We discuss a gauge invariant approach to the theory of cosmological perturbations in a higher-dimensonal background. We find the normal modes which diagonalize the perturbed action, for a scalar field minimally coupled to gravity, in a higher-dimensional manifold M of the Bianchi-type I, under the assumption that the translations along an isotropic spatial subsection of M are isometries of the full, perturbed background. We show that, in the absence of scalar field potential, the canonical variables for scalar and tensor metric perturbations satisfy exactly the same evolution equation, and we discuss the possible dependence of the spectrum on the number of internal dimensions.

gr-qc↗

arXiv1995

Gravitational Radiation from String Cosmology

A spectrum of relic stochastic gravitational radiation, strongly tilted towards high frequencies, and characterized by two basic parameters is shown to emerge in a class of string theory models. We estimate the required sensitivity for detection of the predicted gravitational radiation and show that a region of our parameter space is within reach for some of the plannedgravitational-wave detectors.

hep-th↗

arXiv1995

Relic Gravitational Waves from String Cosmology

A large class of string-cosmology backgrounds leads to a spectrum of relic stochastic gravitational waves, strongly tilted towards high frequencies, and characterized by two basic parameters of the cosmological model. We estimate the required sensitivity for detection of the predicted gravitational radiation and show that a region of our parameter space is within reach for some of the planned gravitational-wave detectors.

hep-th↗

arXiv1995

Electromagnetic Origin of the CMB Anisotropy in String Cosmology

In the inflationary scenarios suggested by string theory, the vacuum fluctuations of the electromagnetic field can be amplified by the time-evolution of the dilaton background, and can grow large enough to explain both the origin of the cosmic magnetic fields and of the observed CMB anisotropy. The normalization of the perturbation spectrum is fixed, and implies a relation between the perturbation amplitude at the COBE scale and the spectral index $n$. Working within a generic two-parameter family of backgrounds, a large scale anisotropy $ΔT/T\simeq 10^{-5}$ is found to correspond to a spectral index in the range $n\simeq 1.11 - 1.17$.

astro-ph↗

arXiv1995

Primordial Magnetic Fields From String Cosmology

Sufficiently large seeds for generating the observed (inter)galactic magnetic fields emerge naturally in string cosmology from the amplification of electromagnetic vacuum fluctuations due to a dynamical dilaton background. The success of the mechanism depends crucially on two features of the so-called pre-big-bang scenario, an early epoch of dilaton-driven inflation at very small coupling, and a sufficiently long intermediate stringy era preceding the standard radiation-dominated evolution.

hep-th↗

arXiv1995

EVOLUTION OF A STRING NETWORK IN BACKGROUNDS WITH ROLLING HORIZONS

We discuss the temporal variation of the equation of state of a classical string network, evolving in a background in which the Hubble radius 1/H shrinks to a minimum and then re-expands to infinity. We also present a method to look for self-consistent non-vacuum string backgrounds, correponding to the simultaneous solution of the gravi-dilaton backgrounds equations and of the string equations of motion.

hep-th↗