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The status of advancement of the VIRGO Project is presented: the first-generation results from the Pisa seismic noise super attenuator give an upper limit to the noise transfer function of 2 × 10 −8 at 10 Hz. The upper limit to the absolute noise of the 400 kg test mass at 10 Hz has been measured to be 1.5 × 10 −13 m/√Hz. The scheme and the related problems of the VIRGO interferometer, which is supposed to work down to 10 Hz, are also presented. At the 3rd Pisa Meeting in 1986 we presented the idea of what could be a very efficient seismic noise reduction system able to give a sensitivity h ∼ 10 −25 at 10 Hz, in a 3 km interferometer for 1 year integration time. Now we have two new facts to present: the first is that the attenuation has been built, is working in Pisa, and shows remarkable characteristics. The second is the Italian-French interferometer VIRGO [1,2], a 3 km long antenna for low and high frequency (10–1000 Hz) gravitational wave (GW) detection. These two items will be presented in this article.

We propose a new time evolution law for the cosmological `constant' in a spatially flat (k = 0) Friedmann-Lemaitre-Robertson-Walker spacetime. From a thermodynamic model of the vacuum based upon work of Gibbons, Hawking and Davies, we obtain the law where r is the proper radius of the cosmological event horizon. From the field equations we can deduce a second-order differential equation for , that we solve numerically, showing that the cosmological `constant' problem could be solved phenomenologically. The decay of takes place during the deflationary era, and has the effect of creating a perfect fluid of strings. Finally, our model suggests a mechanism of destabilization of the de Sitter spacetime to explain the exit from inflation and the matter creation.

In the framework of a spatially flat FLRW model, we account for dissipative effects in a simple fluid, namely bulk viscosity and matter creation. For an isentropic evolution, we study the wavefront speeds associated with the characteristics of the fluid. With the assumption of a barotropic equation of state, we investigate the consequences on these speeds of power-law behaviour for the scale factor. Our conclusion is that, in contrast to the exponential inflation, the isentropic power-law inflation is compatible with causality, under conditions linking the generalized `adiabatic' index to the exponent m of the power law.

The total energy in a sphere containing an isotropic shear-free conducting heat fluid is studied in the frame of a spherically symmetric metric. Firstly, we examine the role played by the heat flux. Secondly, we point out the contribution to the energy by the Weyl tensor. We obtain different formulas for the total energy, and those formulas are shown to be equivalent. We derive the general solution for a conformally flat spacetime, and give an example for a nonconformally flat spacetime.

The two distinct irreversible phenomena of matter creation and bulk viscosity are considered as independent, giving cross effects in relativistic extended irreversible thermodynamics, as applied to the primordial Friedman-Lemaitre-Robertson-Walker cosmology. Some consequences are developed in the isentropic exponential inflation case.

We study the dynamical instability of a spherically symmetric anisotropic fluid which collapses adiabatically under the condition of vanishing expansion scalar. The Newtonian and post Newtonian regimes are considered in detail. It is shown that within those two approximations the adiabatic index $\Gamma_1$, measuring the fluid stiffness, does not play any role. Instead, the range of instability is determined by the anisotropy of the fluid pressures and the radial profile of the energy density, independently of its stiffness, in a way which is fully consistent with results previously obtained from the study on the Tolman mass., Comment: 12 pages Revtex. Some changes in the text to match the version accepted in Gen. Rel. Grav

We present the main results of a model, namely the event‐horizon model, based on a thermodynamical approach for the law of Λ(t).We consider a phenomenological model where we give to the effective cosmological ‘constant’ Λ the physical meaning of a ‘vacuum’ energy density, as usually admitted. In the same vein as the generalization of the black holes [1][2] and of the de Sitter event‐horizon thermodynamics [3][4] to the FLRW space‐time, we associated to this ‘0vacuum’ energy density a full thermodynamics for cosmological event horizons without further microscopic precision about the fields describing the vacuum.Without any ansatz and using thermodynamics arguments, we then deduce the law Λ∝r−2 where r is the proper radius of the event horizon in quasi‐de Sitter FLRW spatially flat spacetime.With this relation, the coincidence problem is solved by the decay of Λ(t) due to the expansion of the universe. We apply this model to the study of the recent accelerated expansion phase and to the coincidence problem....

We determine the characteristics for the system of conservation equations of a relativistic heat conducting fluid. The phenomenological thermodynamics model is based on the scalar axiom of local equilibrium yielding two equations of state for the pressure P and the temperature T in terms of energy density μ and particle density n. The wave front speeds propagating along one spatial direction nα are obtained at the first order in (q · n) (qα is the heat flow vector) and are compared to previous results.

In a recent paper a systematic study on shearing expansion-free spherically symmetric distributions was presented. As a particular case of such systems, the Skripkin model was mentioned, which corresponds to a nondissipative perfect fluid with a constant energy density. Here we show that such a model is inconsistent with junction conditions. It is shown that in general for any nondissipative fluid distribution, the expansion-free condition requires the energy density to be inhomogeneous. As an example we consider the case of dust, which allows for a complete integration., 8 pages, Latex. To appear in Phys. Rev.D. Typos corrected

Recent calculations (analytical and semi-analytical), indicating that thermal effects in the relativistic regime may enhance the stability of spherically-symmetric models, are confirmed in a new example.

International audience; We consider a cosmological horizon, named thermo-horizon, to which are associated a temperature and an entropy of Bekenstein Hawking and which obeys the first law for an energy flow calculated through the corresponding limit surface. We point out a contradiction between the first law and the definition of the total energy contained inside the horizon. This contradiction is removed when the first law is replaced by a Gibbs' equation for a vacuum-like component associated to the event horizon.

We present a complete set of the equations and matching conditions required for the description of physically meaningful charged, dissipative, spherically symmetric gravitational collapse with shear. Dissipation is described with both free-streaming and diffusion approximations. The effects of viscosity are also taken into account. The roles of different terms in the dynamical equation are analyzed in detail. The dynamical equation is coupled to a causal transport equation in the context of Israel-Stewart theory. The decrease of the inertial mass density of the fluid, by a factor which depends on its internal thermodynamic state, is reobtained, with the viscosity terms included. In accordance with the equivalence principle, the same decrease factor is obtained for the gravitational force term. The effect of the electric charge on the relation between the Weyl tensor and the inhomogeneity of energy density is discussed., Comment: 23 pages, Latex. To appear in Phys. Rev. D. Some references corrected

We apply the thermodynamical model of the cosmological event horizon of the spatially flat FLRW metrics to the study of the recent accelerated expansion phase and to the coincidence problem. This model, called “ehT model” hereafter, led to a dark energy (DE) density Λ varying as r −2 , where r is the proper radius of the event horizon. Recently, another model motivated by the holographic principle gave an independent justification of the same relation between Λ and r. We probe the theoretical results of the ehT model with respect to the SnIa observations and we compare it to the model deduced from the holographic principle, which we call “LHG model” in the following. Our results are in excellent agreement with the observations for H 0 = 64 km s −1 Mpc −1 , and Ω Λ 0 = 0.63 −0.01 +0.1 , which leads to q 0 = − 0.445 and z T ≃ 0.965 .

Here we study some general properties of spherical shear-free collapse. Its general solution when imposing conformal flatness is reobtained [1,2] and matched to the outgoing Vaidya spacetime. We propose a simple model satisfying these conditions and study its physical consequences. Special attention deserve, the role played by relaxational processes and the conspicuous link betweeen dissipation and density inhomogeneity., 13 pages Latex. Some misprints in eqs.(17), (30) and (35) have been corrected

We present the whole set of equations with regularity and matching conditions required for the description of physically meaningful static cylindrically symmmetric distributions of matter, smoothly matched to Levi-Civita vacuum spacetime. It is shown that the conformally flat solution with equal principal stresses represents an incompressible fluid. It is also proved that any conformally flat cylindrically symmetric static source cannot be matched through Darmois conditions to the Levi-Civita spacetime. Further evidence is given that when the Newtonian mass per unit length reaches 1/2 the spacetime has plane symmetry., Comment: 13 pages, Latex

We compare Israel's relativistic causal theory of transient effects with the relativistic extended irreversible thermodynamics of Pav\'on, Jou, and Casas-Vasqu\`ez, applying both to the case of cosmic fluid with bulk viscosity only. They are completely equivalent up to the first order in the parameter characterizing the departure from equilibrium. We show, if certain relations between the main variables of the two theories hold, that they are also equivalent up to the second order in this parameter. We also extend up to any order Israel's theory and give the relations permitting again the equivalence between the two theories in this general case. Some cosmological consequences are discussed.

We consider the evolution of cavities within spherically symmetric relativistic fluids, under the assumption that proper radial distance between neighboring fluid elements remains constant during their evolution (purely areal evolution condition). The general formalism is deployed and solutions are presented. Some of them satisfy Darmois conditions whereas others present shells and must satisfy Israel conditions, on either one or both boundary surfaces. Prospective applications of these results to some astrophysical scenarios is suggested., Comment: 10 pages Revtex. To appear in Class. Quantum Grav.

The present study of the space location of supernovae in their parent galaxies suggests a new method to obtain indicative values of the SN-galaxy center distances. This method is based on the use of the notions of ‘luminosity distance’ and ‘angular diameter distance’. On this account, it appears that one could avoid the corrections for the projection effect in no faced-on galaxies. In Section 2, a sample of 25 type I supernovae is treated at once by means of two distinct procedures: the one by Barbon, Capaccioli, and Ciatti, the one defined here. The good agreement of the results is shown, specially about the surface density of supernovae. In the last part, we try to test the influence of the internal obscuration of the SNs by their own galaxies, on the basis of a correction suggested by Nottale. The frequency distribution and the surface density of the objects of our sample support the consistency of such a correction.

Etude de l'instabilite dynamique d'une distribution de fluide a symetrie spherique lors de l'effondrement non adiabatique et subissant une dissipation sous la forme d'un ecoulement radial de chaleur. Les limites newtoniennes et post-newtoniennes sont considerees et l'etendue pour l'instabilite dynamique pour le rapport des chaleurs specifiques est donnee

Under general assumptions we estimate the number of gravitational pulses per day detected by a given array. This allows us to define a figure of merit, I , for any array and to put forward the conditions for the birth of gravitational astronomy. Typical values and orders of magnitude are discussed.

A recent result has shown that the fourth-order theories of gravity with the action S=F \ensuremath{\surd}-g (2\ensuremath{\Lambda}+R+(1/2)${\mathrm{aR}}^{2}$)${d}^{4}$x are conformally equivalent to general relativity with a scalar field. This result may enlighten the validity of timelike and null convergence conditions in spatially homogeneous cosmological models with a diagonal metric. This approach points out that, under particular conditions, the timelike and null geodesics may be complete.

IN an important series of papers for the determination of the Hubble constant, Sandage and Tammann1–6 have concluded that the Universe is expanding in a remarkably uniform and isotropic way, with Hubble constant (H) 55±5 km s−1 Mpc−1. If true, this result is evidently crucial for all future cosmological models. As a consequence it should be tested and analysed in all possible ways. Their low H value is evidently based on their recalibration of the absolute magnitudes of the various luminosity classes of Sc-type galaxies based on two samples of nearby, and one sample of distant galaxies (Tables 3 and 4, ref. 5, and the Table, ref. 6).