arXiv subjects
Publications and source records attributed to Massimo Giovannini.
In this note we argue that the breaking of conformal invariance because of the coupling of a charged scalar field to gravity is not sufficient for the production of seed galactic magnetic fields during inflation.
The interplay between different ground based detectors and stochastic backgrounds of relic GW is described. A simultaneous detection of GW in the kHz and in the MHz--GHz region can point towards a cosmological nature of the signal. The sensitivity of a pair of VIRGO detectors to string cosmological models is presented. The implications of microwave cavities for stochastic GW backgrounds are discussed.
Static solutions of the higher dimensional Einstein-Hilbert gravity supplemented by quadratic curvature self-interactions are discussed in the presence of hedgehog configurations along the transverse dimensions. The quadratic part of the action is parametrized in terms of the (ghost-free) Euler-Gauss-Bonnet curvature invariant. Spherically symmetric profiles of the transverse metric admit exponentially decaying warp factors both for positive and negative bulk cosmological constants.
The sensitivity of a pair of VIRGO interferometers to gravitational waves backgrounds (GW) 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 GW are discussed.
The dynamical evolution of internal space-like dimensions breaks the invariance of the Maxwell's equations under Weyl rescaling of the (conformally flat) four-dimensional metric. Depending upon the number and upon the dynamics of internal dimensions large scale magnetic fields can be created. The requirements coming from magnetogenesis together with the other cosmological constraints are examined under the assumption that the internal dimensions either grow or shrink (in conformal time) prior to a radiation dominated epoch. If the internal dimensions are growing the magnitude of the generated magnetic fields can seed the galactic dynamo mechanism.
We present new solutions of the string cosmological effective action in the presence of a homogeneous Maxwell field with pure magnetic component. Exact solutions are derived in the case of space-independent dilaton and vanishing torsion background. In our examples the four dimensional metric is either of Bianchi-type III and VI$_{-1}$ or Kantowski-Sachs.
Physical scenarios, leading to highly energetic stochastic gravitational waves backgrounds (for frequencies ranging from the $μ$Hz up to the GHz) are examined. In some cases the typical amplitude of the logarithmic energy spectrum can be even eight orders of magnitude larger than the ordinary inflationary prediction. Scaling violations in the frequency dependence of the energy density of the produced gravitons are discussed.
We discuss the production of multi-photons squeezed states induced by the time variation of the (Abelian) gauge coupling constant in a string cosmological context. Within a fully quantum mechanical approach we solve the time evolution of the mean number of produced photons in terms of the squeezing parameters and in terms of the gauge coupling. We compute the first (amplitude interference) and second order (intensity interference) correlation functions of the magnetic part of the photon background. The photons produced thanks to the variation of the dilaton coupling are strongly bunched for the realistic case where the growth of the dilaton coupling is required to explain the presence of large scale magnetic fields and, possibly of a Faraday rotation of the Cosmic Microwave Background.
At finite hyperconductivity and finite fermionic density the flux lines of long range hypermagnetic fields may not have a topologically trivial structure. The combined evolution of the chemical potentials and of pseudoscalar fields (like the axial Higgs), possibly present for temperatures in the TeV range, can twist the hypercharge flux lines, producing, ultimately, hypermagnetic knots (HK). The dynamical features of the HK depend upon the various particle physics parameters of the model (pseudoscalar masses and couplings, strength of the electroweak phase transition, hyperconductivity of the plasma) and upon the magnitude of the primordial flux sitting in topologically trivial configurations of the hypermagnetic field. We study different cosmological scenarios where HK can be generated. We argue that the fermionic number sitting in HK can be released producing a seed for the Baryon Asymmetry of the Universe (BAU) provided the typical scale of the knot is larger than the diffusivity length scale. We derive constraints on the primordial hypermagnetic flux required by our mechanism and we provide a measure of the parity breaking by connecting the degree of knottedness of the flux lines to the BAU. We rule out the ordinary axion as a possible candidate for production (around temperatures of the order of the GeV) of {\em magnetic} knots since the produced {\em electromagnetic} helicity is negligible (for cosmological standard) if the initial amplitude of the axion oscillations is of the order of the Peccei-Quinn breaking scale.
There are no reasons why the energy spectra of the relic gravitons, amplified by the pumping action of the background geometry, should not increase at high frequencies. A typical example of this behavior are quintessential inflationary models where the slopes of the energy spectra can be either blue or mildly violet. In comparing the predictions of scenarios leading to blue and violet graviton spectra we face the problem of correctly deriving the sensitivities of the interferometric detectors. Indeed, the expression of the signal-to-noise ratio not only depends upon the noise power spectra of the detectors but also upon the spectral form of the signal and, therefore, one can reasonably expect that models with different spectral behaviors will produce different signal-to-noise ratios. By assuming monotonic (blue) spectra of relic gravitons we will give general expressions for the signal-to-noise ratio in this class of models. As an example we studied the case of quintessential gravitons. The minimum achievable sensitivity to $h^2_{0} Ω_{GW}$ of different pairs of detectors is computed, and compared with the theoretical expectations.
Topologically non-trivial configurations of the hypermagnetic flux lines lead to the formation of hypermagnetic knots (HK) whose decay might seed the Baryon Asymmetry of the Universe (BAU).HK can be dynamically generated provided a topologically trivial (i.e. stochastic) distribution of flux lines is already present in the symmetric phase of the electroweak (EW) theory. In spite of the mechanism generating the HK, their typical size must exceed the diffusivity length scale. In the minimal standard model (MSM) (but not necessarily in its supersymmetric extension) HK are washed out. A classical hypermagnetic background in the symmetric phase of the EW theory can produce interesting amounts of gravitational radiation.
If the energy momentum tensor contains bulk viscous stresses violating the dominant energy condition (DOC) the energy spectra of the relic gravitons (produced at the time of the DOC's violation) increase in frequency in a calculable way. In a general relativistic context we give examples where the DOC is only violated for a limited amount of time after which the ordinary (radiation dominated) evolution takes place. We connect our discussion to some recent remarks of Grishchuk concerning the detectability of the stochastic gravitational wave background by the forthcoming interferometric detectors.
The energy spectra of gravitational waves (GW) produced in quintessential inflationary models increase in frequency and exhibit a sharp spike around 170 GHz where the associated fraction of critical energy density today stored in relic gravitons is of the order of $10^{-6}$. We contrast our findings with the spectra of ordinary inflationary models and we comment about possible detetction strategies of the spike.
A large class of quintessential inflationary models, recently proposed by Peebles and Vilenkin, leads to post-inflationary phases whose effective equation of state is stiffer than radiation. The expected gravitational waves logarithmic energy spectra are tilted towards high frequencies and characterized by two parameters: the inflationary curvature scale at which the transition to the stiff phase occurs and the number of (non conformally coupled) scalar degrees of freedom whose decay into fermions triggers the onset of a gravitational reheating of the Universe. Depending upon the parameters of the model and upon the different inflationary dynamics (prior to the onset of the stiff evolution) the relic gravitons energy density can be much more sizeable than in standard inflationary models, for frequencies larger than 1 Hz. We estimate the required sensitivity for detection of the predicted spectral amplitude and show that the allowed region of our parameter space leads to a signal smaller (by one 1.5 orders of magnitude) than the advanced LIGO sensitivity at a frequency of 0.1 KHz. The maximal signal, in our context, is expected in the GHz region where the energy density of relic gravitons in critical units (i.e. $h_0^2 Ω_{GW}$) is of the order of $10^{-6}$, roughly eight orders of magnitude larger than in ordinary inflationary models. Smaller detectors (not necessarily interferometers) can be relevant for detection purposes in the GHz frequency window. We suggest/speculate that future measurements through microwave cavities can offer interesting perspectives.
We present a class of exact cosmological solutions of the low energy string effective action in the presence of a homogeneous magnetic fields. We discuss the physical properties of the obtained (fully anisotropic) cosmologies paying particular attention to their vacuum limit and to the possible isotropization mechanisms. We argue that quadratic curvature corrections are able to isotropize fully anisotropic solutions whose scale factors describe accelerated expansion. Moreover, the degree of isotropization grows with the duration of the string phase. We follow the fate of the shear parameter in a decelerated phase where, dilaton, magnetic fields and radiation fluid are simultaneously present. In the absence of any magnetic field a long string phase immediately followed by radiation is able to erase large anisotropies. Conversely, if a short string phase is followed by a long dilaton dominated phase the anisotropies can be present, in principle, also at later times. The presence of magnetic seeds after the end of the string phase can induce further anisotropies which can be studied within the formalism reported in this paper.
The inhomogeneities associated with massless Kalb-Ramond axions can be amplified not only in isotropic (four-dimensional) string cosmological models but also in the fully anisotropic case. If the background geometry is isotropic, the axions (which are not part of the homogeneous background) develop, outside the horizon, growing modes leading, ultimately, to logarithmic energy spectra which are "red" in frequency and increase at large distance scales. We show that this conclusion can be evaded not only in the case of higher dimensional backgrounds with contracting internal dimensions but also in the case of string cosmological scenarios which are completely anisotropic in four dimensions. In this case the logarithmic energy spectra turn out to be "blue" in frequency and, consequently, decreasing at large distance scales. We elaborate on anisotropic dilaton-driven models and we argue that, incidentally, the background models leading to (or flat) logarithmic energy spectra for axionic fluctuations are likely to be isotropized by the effect of string tension corrections.
The dynamical symmetries of hot and electrically neutral plasmas in a highly conducting medium suggest that, after the epoch of the electron-positron annihilation, magnetohydrodynamical configurations carrying a net magnetic helicity can be present. The simultaneous conservation of the magnetic flux and helicity implies that the (divergence free) field lines will possess inhomogeneous knot structures acting as source seeds in the evolution equations of the scalar, vector and tensor fluctuations of the background geometry. We give explicit examples of magnetic knot configurations with finite energy and we compute the induced metric fluctuations. Since magnetic knots are (conformally) coupled to gravity via the vertex dictated by the equivalence principle, they can imprint spikes in the gravitational wave spectrum for frequencies compatible with the typical scale of the knot corresponding, in our examples, to a present frequency range of $10^{-11}$--$10^{-12}$ Hertz. At lower frequencies the spectrum is power-suppressed and well below the COBE limit. For smaller length scales (i.e. for larger frequencies) the spectrum is exponentially suppressed and then irrelevant for the pulsar bounds. Depending upon the number of knots of the configuration, the typical amplitude of the gravitational wave logarithmic energy spectrum (in critical units) can be even four orders of magnitude larger than the usual flat (inflationary) energy spectrum generated thanks to the parametric amplification of the vacuum fluctuations.
There are no reasons why the singularity in the growth of the dilaton coupling should not be regularised, in a string cosmological context, by the presence of classical inhomogeneities. We discuss a class of inhomogeneous dilaton-driven models whose curvature invariants are all bounded and regular in time and space. We prove that the non-space-like geodesics of these models are all complete in the sense that none of them reaches infinity for a finite value of the affine parameter. We conclude that our examples represent truly singularity-free solutions of the low energy beta functions. We discuss some symmetries of the obtained solutions and we clarify their physical interpretation. We also give examples of solutions with spherical symmetry. In our scenario each physical quantity is everywhere defined in time and space, the big-bang singularity is replaced by a maximal curvature phase where the dilaton kinetic energy reaches its maximum. The maximal curvature is always smaller than one (in string units) and the coupling constant is also smaller than one and it grows between two regimes of constant dilaton, implying, together with the symmetries of the solutions, that higher genus and higher curvature corrections are negligible. We argue that our examples describe, in a string cosmological context, the occurrence of ``little bangs''(i.e. high curvature phases which never develop physical singularities). They also suggest the possibility of an unexplored ``pre-little-bang'' phase.