We describe the smooth transition from vacuum p = −ϵ into dust, by postulating the time dependence of the γ parameter in the state equation for the scalar field driving inflation and giving masses to particles. In this sense we study the... more
We describe the smooth transition from vacuum p = −ϵ into dust, by postulating the time dependence of the γ parameter in the state equation for the scalar field driving inflation and giving masses to particles. In this sense we study the dynamics of inflation without assuming slow rolling. We proceed backwise to reconstruct the scalar field potential for the
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... from the second, using the Lorentz gauge, we get ... (2.21) de'r~ - 0, dr ... by some average of the motion of the distant astronomical objects ([26], see also [23])(2). Trying to incorporate the Mach principle into (metric)... more
... from the second, using the Lorentz gauge, we get ... (2.21) de'r~ - 0, dr ... by some average of the motion of the distant astronomical objects ([26], see also [23])(2). Trying to incorporate the Mach principle into (metric) gravity theory, Brans and Dicke have constructed a theory of gravity ...
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ABSTRACT
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Gravitational Amplification Pixel Lensing Experiment) is a new collaboration project among international partners from England, France, Germany, Italy and Switzerland that intends to perform microlensing observation by using M31 as... more
Gravitational Amplification Pixel Lensing Experiment) is a new collaboration project among international partners from England, France, Germany, Italy and Switzerland that intends to perform microlensing observation by using M31 as target. The MACHOs search is made thanks to the pixel lensing
We apply the ultrarelativistic boosting procedure to map the metric of Schwarzschild-de Sitter spacetime into a metric describing de Sitter spacetime plus a shock-wave singularity located on a null hypersurface, by exploiting the picture... more
We apply the ultrarelativistic boosting procedure to map the metric of Schwarzschild-de Sitter spacetime into a metric describing de Sitter spacetime plus a shock-wave singularity located on a null hypersurface, by exploiting the picture of the embedding of an hyperboloid in a five-dimensional Minkowski spacetime. After reverting to the usual four-dimensional formalism, we also solve the geodesic equation and evaluate the Riemann curvature tensor of the boosted Schwarzschild-de Sitter metric by means of numerical calculations, which make it possible to reach the ultrarelativistic regime gradually by letting the boost velocity approach the speed of light. Eventually, the analysis of the Kretschmann invariant (and of the geodesic equation) shows the global structure of space- time, as we demonstrate the presence of a "scalar curvature singularity" within a 3-sphere and find that it is also possible to define what we have called "boosted horizon", a sort of elastic ...
1 In our short paper we join quintessence cosmological scenarios with the duality simmetry existing in string dilaton cosmologies. Actually, we consider only the tracker potentials V = V0/ϕα and show that duality is established if α = 2.... more
1 In our short paper we join quintessence cosmological scenarios with the duality simmetry existing in string dilaton cosmologies. Actually, we consider only the tracker potentials V = V0/ϕα and show that duality is established if α = 2. PACS number(s): 98.80.Cq, 98.80.-k, 98.80.Hw, 04.20.Jb 2 Typeset using REVTEXIn this short letter we show that duality simmetry is also present in the so-called quintessence theories. Duality (see the recent review paper on this topic by Gasperini (1999) and the almost complete references therein) is an extra-simmetry existing (in the cosmological arena: we will use a standard 3+1 Friedmann-Robertson-Walker splitting of spacetime, i.e. the cosmological principle) in the minisuperspace associate with scalar-theories of gravity (see [2] and the references therein). Essentially this simmetry is such that if a(t) is a cosmological solution, then a −1 (t) is also a solution if the scalar field (dilaton) present in this approach
The ultrarelativistic boosting procedure had been applied in the literature to map the metric of Schwarzschild-de Sitter spacetime into a metric describing de Sitter spacetime plus a shock-wave singularity located on a null hypersurface.... more
The ultrarelativistic boosting procedure had been applied in the literature to map the metric of Schwarzschild-de Sitter spacetime into a metric describing de Sitter spacetime plus a shock-wave singularity located on a null hypersurface. This paper evaluates the Riemann curvature tensor of the boosted Schwarzschild-de Sitter metric by means of numerical calculations, which make it possible to reach the ultrarelativistic regime gradually by letting the boost velocity approach the speed of light. Thus, for the first time in the literature, the singular limit of curvature through Dirac's delta distribution and its derivatives is numerically evaluated for this class of spacetimes. Eventually, the analysis of the Kretschmann invariant and the geodesic equation show that the spacetime possesses a scalar curvature singularity within a 3-sphere and it is possible to define what we here call boosted horizon, a sort of elastic wall where all particles are surprisingly pushed away, as nume...
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We show that the accelerated expansion of the universe can be viewed as a crossover phenomenon where the Newton constant and the cosmological constant are actually scaling operators, dynamically evolving in the attraction basin of a... more
We show that the accelerated expansion of the universe can be viewed as a crossover phenomenon where the Newton constant and the cosmological constant are actually scaling operators, dynamically evolving in the attraction basin of a non-Gaussian infrared fixed point, whose existence has been recently discussed. By linearization of the renormalized flow it is possible to evaluate the critical exponents, and it turns out that the approach to the fixed point is ruled by a marginal and a relevant direction. A smooth transition between the standard Friedmann–Lemaître–Robertson–Walker (FLRW) cosmology and the observed accelerated expansion is then obtained, so that M ≈ at late times. PACS numbers: 03.70.+k, 04.60.Ds 1.
The excess of correlation and the apparent periodic distribution of galaxies found in some recent surveys suggest the existence of oscillating behaviours in the cosmic evolution. We search for periodic behaviours in cosmological models... more
The excess of correlation and the apparent periodic distribution of galaxies found in some recent surveys suggest the existence of oscillating behaviours in the cosmic evolution. We search for periodic behaviours in cosmological models where the geometry is minimally and nonminimally coupled with a scalar field. Such solutions exist and depend upon the form of the coupling and of the potential of the field governing the dynamics but they are not always capable to provide a good fit of the data unless nonminimal coupling is invoked
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We extend the concept of quintessence to flat nonminimally coupled scalar– 1 tensor theories of gravity. By means of Noether’s symmetries for the cosmological pointlike Lagrangian L, it is possible to exhibit exact solutions for a class... more
We extend the concept of quintessence to flat nonminimally coupled scalar– 1 tensor theories of gravity. By means of Noether’s symmetries for the cosmological pointlike Lagrangian L, it is possible to exhibit exact solutions for a class of models depending on a free parameter s. This parameter comes out in the relationship existing between the coupling F(ϕ) and the potential V (ϕ) because of such a symmetry for L. When inverse power–law potentials are taken into account, a whole family of exact solutions parametrized by such an s is proposed as a class of tracker fields, and some considerations are made about them.
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We apply the ultrarelativistic boosting procedure to map the metric of Schwarzschild-de Sitter spacetime into a metric describing de Sitter spacetime plus a shock-wave singularity located on a null hypersurface, by exploiting the picture... more
We apply the ultrarelativistic boosting procedure to map the metric of Schwarzschild-de Sitter spacetime into a metric describing de Sitter spacetime plus a shock-wave singularity located on a null hypersurface, by exploiting the picture of the embedding of an hyperboloid in a five-dimensional Minkowski spacetime. After reverting to the usual four-dimensional formalism, we also solve the geodesic equation and evaluate the Riemann curvature tensor of the boosted Schwarzschild-de Sitter metric by means of numerical calculations, which make it possible to reach the ultrarelativistic regime gradually by letting the boost velocity approach the speed of light. Eventually, the analysis of the Kretschmann invariant (and of the geodesic equation) shows the global structure of space- time, as we demonstrate the presence of a "scalar curvature singularity" within a 3-sphere and find that it is also possible to define what we have called "boosted horizon", a sort of elastic ...
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ABSTRACT The cosmological constant has attained a leading role in recent researches in cosmology. We extend the cosmic no-hair theorem to non-minimally coupled theories of gravity where ordinary matter is also present in the form of a... more
ABSTRACT The cosmological constant has attained a leading role in recent researches in cosmology. We extend the cosmic no-hair theorem to non-minimally coupled theories of gravity where ordinary matter is also present in the form of a perfect fluid. To achieve this goal we give a set of conditions for obtaining the asymptotic de Sitter expansion independently of any initial data (no fine-tuning problem), that is, we introduce a time-dependent (effective) cosmological constant. Finally, we apply the results to some specific models.
We present the first results of the analysis of data collected during the 1998-99 observational campaign at the 1.3 meter McGraw-Hill Telescope, towards the Andromeda galaxy (M 31), aimed to detect gravitational microlensing effects as a... more
We present the first results of the analysis of data collected during the 1998-99 observational campaign at the 1.3 meter McGraw-Hill Telescope, towards the Andromeda galaxy (M 31), aimed to detect gravitational microlensing effects as a probe for the presence of dark matter in our Galaxy and in the M 31 halo. The analysis is performed using the pixel lensing technique, which consists of the study of flux variations of unresolved sources and has been proposed and implemented by the AGAPE collaboration. We carry out a shape ...
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The supernovae Ia data are used to analyze two general exact solutions for quintessence models. The best fit values for $\Omega_{m0}$ are smaller than in the $\Lambda $-term model, but still acceptable. With present-day data, it is not... more
The supernovae Ia data are used to analyze two general exact solutions for quintessence models. The best fit values for $\Omega_{m0}$ are smaller than in the $\Lambda $-term model, but still acceptable. With present-day data, it is not possible to discriminate among the various situations.
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Summary We study the conditions for an inflationary universe in the presence of both matter and scalar field, making some considerations which are particularly useful when exact solutions are given. An interesting application is also... more
Summary We study the conditions for an inflationary universe in the presence of both matter and scalar field, making some considerations which are particularly useful when exact solutions are given. An interesting application is also analysed.
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The problem of deriving a shock-wave geometry with cosmological constant by boosting a Schwarzschild-de Sitter (or anti-de Sitter) black hole is re-examined. Unlike previous work in the literature, we deal with the exact Schwarzschild-de... more
The problem of deriving a shock-wave geometry with cosmological constant by boosting a Schwarzschild-de Sitter (or anti-de Sitter) black hole is re-examined. Unlike previous work in the literature, we deal with the exact Schwarzschild-de Sitter (or anti-de Sitter) metric. By virtue of peculiar cancellations in this exact calculation, where the metric does not depend linearly on the mass parameter, we find a singularity of distributional nature on a null hypersurface, which corresponds to a shock-wave geometry derived in a fully non-perturbative way. The result agrees with previous calculations, where the metric had been linearized in the mass parameter.
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We extend the concept of quintessence to flat nonminimally coupled scalar-tensor theories of gravity. By means of Noether's symmetries for the cosmological pointlike Lagrangian L, it is possible to exhibit exact solutions for a class of... more
We extend the concept of quintessence to flat nonminimally coupled scalar-tensor theories of gravity. By means of Noether's symmetries for the cosmological pointlike Lagrangian L, it is possible to exhibit exact solutions for a class of models depending on a free parameter s. This parameter comes out in the relationship existing between the coupling F(φ) and the potential V(φ) because of such a symmetry for L. When inverse power-law potentials are taken into account, a whole family of exact solutions parametrized by such an s is proposed as a class of tracker fields, and some considerations are made about them.
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The explanation of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. If the acceleration is driven by some form of dark energy, one can try to constrain the parameters... more
The explanation of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. If the acceleration is driven by some form of dark energy, one can try to constrain the parameters using a cosmographic approach. Our high-redshift analysis allows us to put constraints on the cosmographic expansion up to the fifth order. It is based on the Union2 Type Ia Supernovae (SNIa) data set, the Hubble diagram constructed from some Gamma Ray Bursts luminosity distance indicators, and gaussian priors on the distance from the Baryon Acoustic Oscillations (BAO), and the Hubble constant h (these priors have been included in order to help break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the cosmographic parameters we use the Markov Chain Monte Carlo Method (MCMC). We finally investigate implications of our results for the dark energy, in particular, we focus on the parametrization of the dark energy equation of state (EOS). Actually, a possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state, w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here we investigate different models of dark energy, described through several parametrizations of the equation of state, by comparing the cosmographic and the EOS series.
The authors analyse Einstein equations with a scalar field by means of a computer algebra program, testing for the existence of alternative Lagrangians. After proposing an algebraic test, they apply it to both the cases of minimal and... more
The authors analyse Einstein equations with a scalar field by means of a computer algebra program, testing for the existence of alternative Lagrangians. After proposing an algebraic test, they apply it to both the cases of minimal and nonminimal coupling between gravity and the scalar field.
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This paper investigates the Arnowitt Deser Misner (hereafter ADM) form of spherically symmetric gravity with variable Newton parameter G and cosmological term Λc. The Newton parameter is here treated as a dynamical variable, rather than... more
This paper investigates the Arnowitt Deser Misner (hereafter ADM) form of spherically symmetric gravity with variable Newton parameter G and cosmological term Λc. The Newton parameter is here treated as a dynamical variable, rather than being merely an external parameter as in previous work on closely related topics. The resulting Hamilton equations are obtained; interestingly, a static solution exists, which reduces to Schwarzschild geometry in the limit of constant G, describing a Newton parameter ruled by a nonlinear differential equation in the radial variable r. A remarkable limiting case is the one for which the Newton parameter obeys an almost linear growth law at large r. An exact solution for G as a function of r is also obtained in the case of vanishing cosmological constant. Some observational implications of these solutions are obtained and briefly discussed.
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We study some problems arising from the introduction of a complex scalar field in cosmology, modelling its possible behaviors in both the inflationary and dark energy stages of the universe. Such examples contribute to show that, while... more
We study some problems arising from the introduction of a complex scalar field in cosmology, modelling its possible behaviors in both the inflationary and dark energy stages of the universe. Such examples contribute to show that, while the complex nature of the scalar field can be indeed important during inflation, it loses its meaning in the later dark-energy dominated era of cosmology, when the phase of the complex field is practically constant, and there is indeed a transition from complex to real scalar field. In our considerations, the Noether symmetry approach turns out to be a useful tool once again. We arrive eventually at a potential containing the sixth and fourth powers of the scalar field, and the resulting semiclassical quantum cosmology is studied to gain a better understanding of the inflationary stage.
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We present general exact solutions for two classes of exponential potentials in scalar field models for quintessence. The coupling is minimal and we consider only dust and scalar field. To some extent, it is possible to reproduce... more
We present general exact solutions for two classes of exponential potentials in scalar field models for quintessence. The coupling is minimal and we consider only dust and scalar field. To some extent, it is possible to reproduce experimental results from supernovae.
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The cosmological problem can be approached in several ways. The issue is essentially connected with the fact that no ultimate, theory still exists. The standard cosmological model, which describes the observed Universe sufficiently well,... more
The cosmological problem can be approached in several ways. The issue is essentially connected with the fact that no ultimate, theory still exists. The standard cosmological model, which describes the observed Universe sufficiently well, fails in the very early stages of evolution since it cannot coherently solve some shortcomings. They essentially are the flatness, the horizon, the isotropy problems and, mainly, the singularity problem, that is the failure of general relativity at primordial eras. In that framework, it is not possible to explain the large-scale structure starting from some theory of primordial perturbations. Such problems can be solved, through the inflationary paradigm, which essentially means to change matter and/or gravitational sectors in GR and to obtain, by these changes, a superluminal expansion of the cosmological background. Inflation is realized in several ways. It becomes extremely important to handle cosmological models coming from many kinds of theories. Nöther symmetries seems to address this issue since it allows to solve exactly the dynamics of several theories. The interest in such an approach becomes greater since the solutions found are physically interesting. The solutions the authors have found are Friedmann, power law, pole-like and de-Sitter-like, that is they cover the whole range of usually required cosmological behaviours. From the observational point of view such solutions can be interpreted as the background on which to compare the observational data from large-scale structure or on which to formulate a theory of cosmological perturbations.
Perfect-fluid matter, satisfying the equation of state p=(γ-1)ρ, is considered in cosmologies where the geometry is nonminimally coupled with a scalar field φ and the potential of φ is λφ4+Λ. Exact solutions are found when γ is a constant... more
Perfect-fluid matter, satisfying the equation of state p=(γ-1)ρ, is considered in cosmologies where the geometry is nonminimally coupled with a scalar field φ and the potential of φ is λφ4+Λ. Exact solutions are found when γ is a constant describing the ordinary forms of matter (γ=1, dust, γ=4/3, radiation, γ=2, stiff matter and γ=0, scalar field matter) and a discussion is done in order to recover Einstein gravity and the Newton constant observed today. The various solutions can be classified according to the different values of γ, λ and Λ.
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We perform a systematic analysis of nonminimally coupled cosmologies in (n+1)-dimensional homogeneous and isotropic spacetimes, searching for Nöther's symmetries and generalizing the results of our previous works. We obtain (i) the... more
We perform a systematic analysis of nonminimally coupled cosmologies in (n+1)-dimensional homogeneous and isotropic spacetimes, searching for Nöther's symmetries and generalizing the results of our previous works. We obtain (i) the absence of symmetries when the spatial curvature constant k is nonzero and n=2, 3, but their existence for all the other n; (ii) the existence of such symmetries for every number of spatial dimensions (except n=1) when k=0. In this latter case, we are able to find a general transformation through which we recover the string-dilaton effective action in (n+1) dimensions and the major peculiarity of string cosmology: the scale factor duality. Furthermore, the symmetry fixes a relation among the coupling F(varphi), the potential V(varphi) of the scalar field varphi, the number of spatial dimensions and the spatial curvature constant. When this is the case, it is possible to find a constant of motion and then get the general solution of the dynamics. Finally, in the framework of the so-called Induced Gravity Theories, we are able to obtain the Newton constant at the present time (t-->∞) depending on the number of spatial dimensions and directly related to the constant of motion existing in such a model.
This paper studies the cosmological equations for a scalar field Phi in the framework of a quantum gravity modified Einstein--Hilbert Lagrangian where G and Lambda are dynamical variables. It is possible to show that there exists a... more
This paper studies the cosmological equations for a scalar field Phi in the framework of a quantum gravity modified Einstein--Hilbert Lagrangian where G and Lambda are dynamical variables. It is possible to show that there exists a Noether symmetry for the point Lagrangian describing this scheme in a FRW universe. Our main result is that the Noether Symmetry Approach fixes both Lambda = Lambda(G) and the potential V = V(Phi) of the scalar field. The method does not lead, however, to easily solvable equations, by virtue of the higher dimensionality of the reduced configuration space involved, the additional variable being the running Newton coupling.
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The authors study non-flat Friedmann-Robertson-Walker (FRW) cosmologies searching for Noether symmetries in the pointlike Lagrangians derived from a general non-minimally coupled gravitational action. The presence of a quadratic coupling... more
The authors study non-flat Friedmann-Robertson-Walker (FRW) cosmologies searching for Noether symmetries in the pointlike Lagrangians derived from a general non-minimally coupled gravitational action. The presence of a quadratic coupling makes the dynamics degenerate if the spatial-curvature constant is different from zero, while exact solutions, through the existence of the symmetry, are found in the minimally coupled case.
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We study the consequences due to time varying G and Λ in scalar-tensor theories of gravity for cosmology, inspired by the modifications introduced by the Renormalization Group (RG) equations in the Quantum Einstein Gravity. We assume a... more
We study the consequences due to time varying G and Λ in scalar-tensor theories of gravity for cosmology, inspired by the modifications introduced by the Renormalization Group (RG) equations in the Quantum Einstein Gravity. We assume a power-law scale factor in presence contemporarily of both the scalar field and the matter components of the cosmic fluid, and analyze a special case and its generalization, also showing the possibility of a phantom cosmology. In both such situations we find a negative kinetic term for the scalar field Q and, possibly, an equation-of-state parameter wQ < -1. A violation of dominant energy condition (DEC) for Q is also possible in both of them; but, while in the first special case the Q-energy density then remains positive, in the second one we find it negative.
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We construct minisuperspace models for a class of theories of gravity nonminimally coupled with a scalar field. We show that when a Nöther symmetry exists, it is always possible to integrate the Wheeler-DeWitt equation and recover the... more
We construct minisuperspace models for a class of theories of gravity nonminimally coupled with a scalar field. We show that when a Nöther symmetry exists, it is always possible to integrate the Wheeler-DeWitt equation and recover the semiclassical regime for the wave function of the universe. In this sense, we can interpret the Nöther symmetries as a selection rule in the philosophy of the so called Hartle criterion: when they exist, it is possible to select classical universes.
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In the framework of renormalization-group improved cosmologies, we analyze both theoretically and observationally the exact and general solution of the matter-dominated cosmological equations, using the expression of \Lambda = \Lambda(G)... more
In the framework of renormalization-group improved cosmologies, we analyze both theoretically and observationally the exact and general solution of the matter-dominated cosmological equations, using the expression of \Lambda = \Lambda(G) already determined by the integration method employed in a previous paper. A rough comparison between such a model and the concordance \LambdaCDM model as to the magnitude-redshift relationship has been already done, without showing any appreciable differences. We here perform a more refined study of how astrophysical data (Union2 set) on type-I supernovae, gamma ray bursts (in a sample calibrated in a model independent way with the SneIa dataset), and gas fraction in galaxy clusters (using a sample of Chandra measurements of the X-ray gas mass fraction) affect the model and constrain its parameters. We also apply a cosmographic approach to our cosmological model and estimate the cosmographic parameters by fitting both the supernovae and the gamma ray bursts datasets. We show that this matter-dominated cosmological model with variable Newton parameter and variable cosmological term is indeed compatible with the observations above (on type Ia supernovae, the gamma ray bursts Hubble diagram, and the gas mass fraction in X-ray luminous galaxy clusters). The cosmographic approach adopted confirms such conclusions. Finally, it seems possible to include radiation into the model, since numerical integration of the equations derived by the presence of both radiation and matter shows that, after inflation, the total density parameter is initially dominated by the radiation contribution and later by the matter one.
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In the framework of phantom quintessence cosmology, we use the Noether Symmetry Approach to obtain general exact solutions for the cosmological equations. This result is achieved by the quintessential (phantom) potential determined by the... more
In the framework of phantom quintessence cosmology, we use the Noether Symmetry Approach to obtain general exact solutions for the cosmological equations. This result is achieved by the quintessential (phantom) potential determined by the existence of the symmetry itself. A comparison between the theoretical model and observations is worked out. In particular, we use type Ia supernovae and large-scale structure parameters determined from the 2-degree Field Galaxy Redshift Survey and from the Wide part of the VIMOS-VLT Deep Survey ). It turns out that the model is compatible with the presently available observational data. Moreover we extend the approach to include radiation. We show that it is compatible with data derived from recombination and it seems that quintessence do not affect nucleosynthesis results.