85 research outputs found
Quintessence or Phoenix?
We show that it is impossible to determine the state equation of quintessence
models on the basis of pure observational SNIa data. An independent estimate of
is necessary. Also in this most favourable case the situation can
be problematic.Comment: 7 pages, 4 figures - [email protected]
Testing a quintessence model with CMBR peaks location
We show that a model of quintessence with exponential potential, which allows
to obtain general exact solutions, can generate locations of CMBR peaks which
are fully compatible with present observational dataComment: 7 pages, no figure
Testing an exact -gravity model at Galactic and local scales
The weak field limit for a pointlike source of a -gravity model is studied. We aim to show the viability of such a model
as a valid alternative to GR + dark matter at Galactic and local scales.
Without considering dark matter, within the weak field approximation, we find
general exact solutions for gravity with standard matter, and apply them to
some astrophysical scales, recovering the consistency of the same
-gravity model with cosmological results.}{In particular, we show that it
is possible to obtain flat rotation curves for galaxies, [and consistency with]
Solar System tests, as in the so-called "Chameleon Approach". In fact, the
peripheral velocity is shown to be expressed as , so that the Tully-Fisher relation is recovered. The results
point out the possibility of achieving alternative theories of gravity in which
exotic ingredients like dark matter and dark energy are not necessary, while
their coarse-grained astrophysical and cosmological effects can be related to a
geometric origin.Comment: 8 pages, 2 figures, accepted in Astron. & Astrop
Slow rolling, inflation and quintessence
We comment on the choice of the quintessence potential, examining the
slow-roll approximation in a minimally coupled theory of gravity. We make some
considerations on the potential behaviors, the related \gamma parameter, and
their relationships to phantom cosmology.Comment: 7 page
Noether symmetry approach in phantom quintessence cosmology
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 (2dFGRS)and from the Wide part of the VIMOS-VLT
Deep Survey (VVDS). 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.Comment: 26 pages, 13 figure
IDIOPATHIC PARTIAL EPILEPSY WITH AUDITORY FEATURES (IPEAF): A CLINICAL AND GENETIC STUDY OF 53 SPORADIC CASES
The purpose of our study was to describe the clinical characteristics of sporadic
(S) cases of partial epilepsy with auditory features (PEAF) and pinpoint
clinical, prognostic and genetic differences with respect to previously reported
familial (F) cases of autosomal dominant partial epilepsy with auditory features
(ADPEAF). We analysed 53 patients (24 females and 29 males) with PEAF diagnosed
according to the following criteria: partial epilepsy with auditory symptoms,
negative family history for epilepsy and absence of cerebral lesions on NMR
study. All patients underwent a full clinical, neuroradiological and
neurophysiological examination. Forty patients were screened for mutations in
LGI1/epitempin, which is involved in ADPEAF. Age at onset ranged from 6 to 39
years (average 19 years). Secondarily generalized seizures were the most common
type of seizures at onset (79%). Auditory auras occurred either in isolation
(53%) or associated with visual, psychic or aphasic symptoms. Low seizure
frequency at onset and good drug responsiveness were common, with 51% of patients
seizure-free. Seizures tended to recur after drug withdrawal. Clinically, no
major differences were found between S and F patients with respect to age at
onset, seizure frequency and response to therapy. Analysis of LGI1/epitempin
exons failed to disclose mutations. Our data support the existence of a peculiar
form of non-lesional temporal lobe epilepsy closely related to ADPEAF but without
a positive family history. This syndrome, here named IPEAF, has a benign course
in the majority of patients and could be diagnosed by the presence of auditory
aura. Although LGI1 mutations have been excluded, genetic factors may play an
aetiopathogenetic role in at least some of these S cases
Quintessence duality
We join quintessence cosmological scenarios with the duality simmetry
existing in string dilaton cosmologies. Actually, we consider the tracker
potential type and show that duality is only
established if .Comment: 6 LaTex Pages, submitted to Physics Letters A; completely revised
version: majior changes in the last par
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
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 at late times.Comment: 12 pages, latex, use bibtex. In the final version, the presentation
has been improved, and new references have been adde
Self-accelerating solutions of scalar-tensor gravity
Scalar-tensor gravity is the simplest and best understood modification of
general relativity, consisting of a real scalar field coupled directly to the
Ricci scalar curvature. Models of this type have self-accelerating solutions.
In an example inspired by string dilaton couplings, scalar-tensor gravity
coupled to ordinary matter exhibits a de Sitter type expansion, even in the
presence of a {\it negative} cosmological constant whose magnitude exceeds that
of the matter density. This unusual behavior does not require phantoms, ghosts
or other exotic sources. More generally, we show that any expansion history can
be interpreted as arising partly or entirely from scalar-tensor gravity. To
distinguish any quintessence or inflation model from its scalar-tensor
variants, we use the fact that scalar-tensor models imply deviations of the
post-Newtonian parameters of general relativity, and time variation of the
Newton's gravitational coupling . We emphasize that next-generation probes
of modified GR and the time variation of are an essential complement to
dark energy probes based on luminosity-distance measurements.Comment: 14 pages, 3 figure
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