78 research outputs found

### Testing a quintessence model with CMBR peaks locations

We show that a model of quintessence with exponential potential,
which allows to obtain general exact solutions, can generate location of
CMBR peaks which are fully compatible with present observational data

### Arnowitt-Deser-Misner gravity with variable G and Lambda and fixed point cosmologies from the renormalization group

Models of gravity with variable G and Lambda have acquired greater relevance
after the recent evidence in favour of the Einstein theory being
nonperturbatively renormalizable in the Weinberg sense. The present paper
applies the Arnowitt-Deser-Misner (ADM) formalism to such a class of
gravitational models. Are modified action functional is then built which
reduces to the Einstein-Hilbert action when G is constant, and leads to a
power-law growth of the scale factor for pure gravity and for a massless Phi**4
theory in a Universe with Robertson-Walker symmetry, in agreement with the
recently developed fixed-point cosmology. Interestingly, the
renormalization-group flow at the fixed point is found to be compatible with a
Lagrangian description of the running quantities G and Lambda.Comment: 17 pages, Revtex-4, 2 figures. In the final version, the Hamiltonian
analysis has been amended and the presentation has been improve

### Noether symmetry approach to scalar-field-dominated cosmology with dynamically evolving G and Lambda

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.Comment: 10 pages, Revtex

### On the transition from complex to real scalar fields in modern cosmology

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.Comment: 21 pages, 6 figures. In the new version, sections I, IV and VI have
been improved, and two words have been added at the beginning of the titl

### The GRBs Hubble diagram in quintessential cosmological models

It has been recently empirically established that some of the directly
observed pa- rameters of GRBs are correlated with their important intrinsic
parameters, like the luminosity or the total radiated energy. These
correlations were derived, tested and used to standardize GRBs, i.e., to derive
their luminosity or radiated energy from one or more observables, in order to
construct an estimated fiducial Hubble diagram, assuming that radiation
propagates in the standard LambdaCDM cosmological model. We extend these
analyses by considering more general models of dark energy, and an updated data
set of high redshift GRBs. We show that the correlation parameters only weakly
depend on the cosmological model. Moreover we apply a local regression
technique to estimate, in a model independent way, the distance modulus from
the recently updated SNIa sample containing 307 SNIa (Astier et al. 2006), in
order to calibrate the GRBs 2D correlations, considering only GRBs with z <1.4.
The derived calibration parameters are used to construct a new GRBs Hubble
diagram, which we call the calibrated GRBs HD. We also compare the estimated
and calibrated GRBs HDs. It turns out that for the common GRBs they are fully
statistically consistent, thus indicating that both of them are not affected by
any systematic bias induced by the different standardizing procedures. We
finally apply our methods to calibrate 95 long GRBs with the well-known Amati
relation and construct the estimated and calibrated GRBs Hubble diagram that
extends to redshifts z ~ 8. Even in this case there is consistency between
these datasets. This means that the high redshift GRBs can be used to test
different models of dark energy. We used the calibrated GRBs HD to constrain
our quintessential cosmological model and derived the likelihood values of
Omega_m and w(0).Comment: 13 pages, 12 figures, 1 table. Accepted for publication in MNRA

### Spherically symmetric ADM gravity with variable G and Lambda(c)

This paper investigates the Arnowitt--Deser--Misner (hereafter ADM) form of
spherically symmetric gravity with variable Newton parameter G and cosmological
term Lambda(c). 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, that 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.Comment: 16 pages, 2 figures. The presentation has been improved in all
section

### On exact solutions for quintessential (inflationary) cosmological models with exponential potentials

We first study dark energy models with a minimally-coupled scalar field and
exponential potentials, admitting exact solutions for the cosmological
equations: actually, it turns out that for this class of potentials the
Einstein field equations exhibit alternative Lagrangians, and are completely
integrable and separable (i.e. it is possible to integrate the system
analytically, at least by quadratures). We analyze such solutions, especially
discussing when they are compatible with a late time quintessential expansion
of the universe. As a further issue, we discuss how such quintessential scalar
fields can be connected to the inflationary phase, building up, for this class
of potentials, a quintessential inflationary scenario: actually, it turns out
that the transition from inflation toward late-time exponential quintessential
tail admits a kination period, which is an indispensable ingredient of this
kind of theoretical models. All such considerations have also been done by
including radiation into the model.Comment: Revtex4, 10 figure

### The Accelerated expansion of the Universe as a crossover phenomenon

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--Lemaitre--Robertson--Walker (FLRW) cosmology and the
observed accelerated expansion is then obtained, so that $\Omega_M \approx
\Omega_\Lambda$ at late times.Comment: 12 pages, latex, use bibtex. In the final version, the presentation
has been improved, and new references have been adde

### Exponential Potentials for Tracker Fields

We show that a general, exact cosmological solution, where dynamics of scalar
field is assigned by an exponential potential, fulfils all the issues of dark
energy approach, both from a theoretical point of view and in comparison with
available observational data. Moreover, tracking conditions are discussed, with
a new treatment of the well known condition $\Gamma>1$. We prove that the
currently used expression for $\Gamma$ is wrong.Comment: 29 pages,12 figures; contact [email protected]; revised version, to
appear in Physical Review

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