MEETING NA12, 26 e 27 Ottobre 2006, Napoli, Dipartimento di Scienze Fisiche e Sezione INFN, aula -2Ma05
Giovedi 26 Ottobre 2006:
Ore 9.15: Saluti di benvenuto (Leonardo Merola e Giancarlo D'Ambrosio)
Ore 9.30: Fernando de Felice: Time inversion and cosmic bursts
Ore 10: Monica Capone e Angelo Tartaglia: A darkless space-time
Ore 10.30-11: Coffee break nel complesso universitario di M.S.A.
Ore 11: Ester Piedipalumbo: Accelerating universe in non-minimally coupled theories of gravity
Ore 11.30: Mario La Camera: A note in braneworld cosmology
Ore 12: Luca Lusanna: Lo York map ed il ruolo dei sistemi noninerziali in Relativita` Generale
Ore 12.30-14.15: Pausa pranzo
Ore 14.15: Donato Bini: Connecting vectors and special congruences in the Kerr spacetime
Ore 14.45: Lorenzo Fatibene: A geometric interpretation of the Barbero-Immirzi connection
Ore 15.15: Giovanni Preti: Light cones and repulsive gravity
Ore 15.45-16.15: Coffee break nel complesso universitario di M.S.A.
Ore 16.15: Vincenzo F. Cardone: Effective haloes and scaling laws for spiral galaxies in f(R) theories of gravity
Ore 16.45: Fabiana De Luca: Kerr Black Hole Gravitational Lensing in the Strong Deflection Limit
Ore 17.15: Valerio Bozza: Primordial perturbations in bouncing cosmologies
Ore 17.45: Luigi Rosa: Energy-momentum tensor for a Casimir apparatus in a weak gravitational field
Ore 18.15: Sebastiano Calchi Novati: Microlensing and dark matter
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Venerdi 27 Ottobre 2006:
Ore 9.30: Matteo Luca Ruggiero: Solar System planetary orbital motions and f(R) Theories of Gravity
Ore 10 Cosimo Stornaiolo: Deformazione di metriche spaziotemporali
Ore 10.30: Giampiero Esposito: NA12 verso il 2007
Ore 10.50-11.20: Coffee break nel Complesso Universitario di M.S.A.
Ore 11.20: Christian Corda: Scalar gravitational waves from scalar-tensor gravity: production and response of interferometers
Ore 11.50-12.15: Discussione collegiale
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ABSTRACTS (IN ALPHABETICAL ORDER)
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BINI:
Title: Connecting vectors and special congruences in the Kerr spacetime
Abstract: The definition of relative accelerations and strains among a set of
comoving particles is studied in connection with the geometric
properties of the frame adapted to a fiducial observer. We find
that a relativistically complete and correct definition of strains must take
into account the transport law of the chosen spatial triad along the observer's
congruence. We use special congruences of (accelerated) test particles in some
familiar spacetimes to elucidate such a point.
The celebrated idea of Szekeres's compass of inertia, arising when
studying geodesic deviation among a set of free-falling particles, is
here generalized to the case of accelerated particles and made
observer-dependent.
This fact would be very useful when thinking about general relativistic tests on
space stations orbiting compact objects like black
holes and also in other interesting gravitational situations.
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BOZZA:
Title: Primordial perturbations in bouncing cosmologies
Abstract: Bouncing cosmologies, suggested by String/M-theory, may provide an alternative to standard inflation to account for the origin of inhomogeneities in our universe. The fundamental question regards the correct way to evolve scalar perturbations through the bounce. In this talk, we present the evolution of perturbations and the final spectrum for an arbitrary (spatially flat) bouncing cosmology, with the only assumption that the bounce is governed by a single physical scale. In particular, we find the condition for the pre-bounce growing mode of the Bardeen potential (which is scale-invariant in the Ekpyrotic scenario) to survive unaltered in the post-bounce. If some new physics acting at the bounce satisfies such a condition, bouncing cosmologies are then entitled to become a real viable alternative to inflation for the generation of the observed inhomogeneities. References: hep-th/0502047, gr-qc/0506040, hep-th/0512066.
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CALCHI NOVATI:
Title: Microlensing and dark matter
Abstract: Gravitational microlensing is a powerful tool for the study of halo
dark matter in form of MACHOs.
I report the observational status and recent results towards the LMC and M31.
In particular I discuss the LMC halo MACHO fraction as compared to the Galactic
one and the possible
detection of a MACHO signal towards M31.
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CARDONE:
Title: Effective haloes and scaling laws for spiral galaxies in f(R) theories of gravity
Abstract: Power-law fourth-order theories of gravity make it
possible to fit spiral
galaxies rotation curves without the need for a dark matter component.
After reviewing the results on the fit to the LSB galaxies, we show how
phenomenological dark halo models avaliable in literature may be recovered
as effective representations of the modified gravitational potential
induced by f(R) theories. We then demonstrate that the empirical scaling
relations in spiral galaxies (such as the Tully--Fisher and baryonic
Tully--Fisher laws) come out as a natural consequence of the modified
potential without the need to resort to dark matter haloes.
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CORDA:
Title: Scalar gravitational waves from scalar-tensor gravity: production and response of interferometers
Abstract: Scalar-tensor gravity admits the existence of scalar modes of
gravitational waves
(SGWs). The mechanism of production and the response of interferometers to these
scalar components of gravitational waves can be studied in three different
gauges in
the massless case: the transverse-traceless (TT) gauge, the so-called “Shibata,
Nakao
and Nakamura” (SNN) gauge, and the local Lorenz gauge. The response of
interferometers
to massless SGWs is invariant in these different gauges. Our work generalizes
previous results which, in the study of the coupling between interferometers and
massless
SGWs, started from the assumption that the wavelength of the SGW is much larger
than
the distance between the test masses. Furthermore, considering situations
motivated by
string-dilaton gravity, the effect of a small mass term on the response of the
interferometer
is taken into account. In this case (massive SGW), we have a longitudinal
effect,
the response of an arm of an interferometer, which is aligned in the wave
propagation direction
is computed. The value of the longitudinal response function for
non-relativistic
massive SGW at high frequencies is very high: this fact opens the doors to the
interesting
possibility of detection of “massive” part of the signal, if advanced projects
will achieve
high sensitivities. Finally, by using previous results and the geometry of the
system,
the generalized coupling between interferometers (like VIRGO or LIGO) and
massless
SGWs is studied. The total frequency response function to massless SGWs incoming
from arbitrary directions is studied.
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DE FELICE:
Title: Time inversion and cosmic bursts
Abstract: The main feature of the cosmic burtst is the concomitance of two
properties, i.e. the high energy and the short time interval. In the
Gamma Ray Bursts, for example, we can have up to 10^54 ergs emitted in one
second!
Any model of such cosmic sources must justify these two properties. Here
we explore the possibility that the time inversion induced by a
generic naked singularity solution explains these events. The main question then
is whether naked singularities form at the end of the
gravitational collapse at least for a short but finite proper time.
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DE LUCA:
Title: Kerr Black Hole Gravitational Lensing in the Strong Deflection Limit
Abstract: I will describe the caustics, critical curves and Kerr black hole
shadow structure as derived from the lens equations solved by means of a
perturbative
approach and assuming large deflections. I will focus on the degeneracy between
the lens angular momentum and height of the
observer on the equatorial plane which cannot be disentangled, no matter which
observable is measured. Neverthless, such degeneracy can be avoided if
calculations
are pushed to higher orders in the lens angular momentum. I will then describe
some possible applications of the results.
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FATIBENE:
Title: A geometric interpretation of the Barbero-Immirzi connection
Abstract: we shall provide a global spacetime formulation of
the Barbero-Immirzi connection.
Such a connection is important in LQG where it is introduced on a space
sheet after the ADM reduction. It was believed that such SU(2)-connection on the
space sheet could not be obtained as a
restriction of a spacetime SU(2)-connection. We shall prove that this is not
true by providing an explicit construction for a global
spacetime SU(2)-connection which restricts to the standard Barbero-Immirzi
connection on space sheets.
The price to accomplish such task is to introduce a new global formulation of
General Relativity
which is equivalent to the standard frame-affine formulation (i.e.
it produces equivalent field equations)
though with objects with different transformation laws. The new formulation will
be closer to a spacetime formulation of GR
which replaces the symmetry group Spin(4) with the group SU(2) already at
spacetime
level (i.e. before projecting on a space sheet).
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LA CAMERA:
Title: A note in braneworld cosmology
Abstract: We consider brane cosmology when a 4D scalar curvature term is added to the 5D Einstein-Hilbert action. The induced brane dynamics are shown to be the usual Einstein dynamics coupled to a modified energy-momentum tensor which is well defined once the 5D Einstein equations in the bulk are solved. In order to obtain a bulk solution, which will consist of outgoing brane waves, we construct a time-dependent solution for the Randall-Sundrum model with nonvanishing spatial curvature by applying generalized boosts along the fifth dimension and patching together isometries broken by the brane. The brane equations so obtained describe a cosmological fluid obeying an equation of state with variable barotropic factor or, equivalently, an evolving scalar field with appropriate kinetic and potential energies. Finally, a class of time-dependent exact solutions is found that can describe the different periods of cosmological evolution from the inflationary epoch to the present accelerating universe.
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LUSANNA:
Title: Lo York map ed il ruolo dei sistemi noninerziali in Relativita` Generale
Abstract:
The Hamiltonian description of a family of globally hyperbolic
asymptotically flat space-times in general relativity, extended to
tetrad gravity, allows to separate the genuine tidal 2+2 degrees
of freedom of the gravitational field from the 8 (14 in tetrad
gravity) gauge variables describing inertial effects. To get
deterministic Hamilton-Dirac-Einstein equations for the tidal
variables and for matter, we have to fix completely the gauge.
This means to choose a global non-inertial frame, centered on a
time-like observer, with the associated radar 4-coordinates and
relativistic inertial effects. In the York canonical basis the
shape of the simultaneity (and Cauchy) 3-surfaces, namely of the
convention for the synchronization of clocks identifying the
instantaneous 3-space, is determined by the arbitrary trace of the
extrinsic curvature. This relativistic inertial effect has no
Newtonian counterpart, since Newton 3-space is absolute. Every
solution of Einstein equations in a completely fixed gauge
dynamically identifies a convention of clock synchronization, i.e. a
notion of instantaneous 3-space, in terms of the tidal variables
and the matter. The functional freedom in fixing the trace of the
extrinsic curvature can be used to eliminate dark matter as an
independent quantity by reducing it to a relativistic inertial
effect: this freedom replaces the freedom in the modification of
Newton's second law in the MOND model in the absolute Newton
3-space.
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PIEDIPALUMBO:
Title: Accelerating universe in non-minimally coupled theories of gravity
Abstract: We consider scalar tensor theories of gravity in an accelerating
universe, studying a
family of power law solutions, for which exact solutions of the Einstein
equations are known. We also compare theoretical
predictions of our models with observations. We concentrate on the following
data: the publicly available data on type Ia
supernovae, the parameters of large scale structure determined by the 2-degree
Field Galaxy Redshift Survey (2dFGRS), and the
the measurements of cosmological distance with the Sunyaev-Zel'dovich effect. It
turns out that these models
have a very interesting feature of producing in a natural way an epoch of
accelerated expansion.
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PRETI:
Title: Light cones and repulsive gravity
Abstract: The black hole and naked singularity solutions of Einstein equations
corresponding to Schwarzschild, Reissner-Nordstrom, Kerr and Kerr-Newman
spacetimes are considered, and the imprints that the repulsive gravity regions
of these spacetimes leave on the behaviour of light cones are analysed.
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ROSA:
Title: Energy-momentum tensor for a Casimir apparatus in a weak gravitational field
Abstract:
We study the influence of the gravity acceleration on the
regularized energy-momentum
tensor of the quantized electromagnetic field between two plane parallel
conducting
plates. We work in the weak-field approximation assuming that the gravity
acceleration is
constant. A perturbative expansion, to first order in this parameter, of the
Green
functions involved and of the energy-momentum tensor, is derived by means of the
covariant
geodesic point splitting procedure. A more systematic derivation is therefore
obtained of the
theoretical prediction according to which the Casimir device in a weak
gravitational
field will experience a tiny repulsive force.
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RUGGIERO:
Titlle: Solar System planetary orbital motions and f(R) Theories of Gravity
Abstract: We study the effects of f(R) Theories of Gravity on
Solar System gravitational tests. In particular, starting
from an exact solution of the field equations in vacuum,
in the Palatini formalism, we work out the effects that
the modification to the Newtonian potential would induce
on the Keplerian orbital elements of the Solar System
planets, and compare them with the latest results in
planetary orbit determination from the EPM2004
ephemerides. It turns out that the longitudes of perihelia and the mean longitudes are affected by secular
precessions. We obtain the resulting best estimate of the
parameter k which measures the non linearity of the
gravitational theory. We use our results to constrain the
cosmological constant and show how f(R) functions can be
constrained, in principle.
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STORNAIOLO:
Title: Deformazione di metriche spaziotemporali
Abstract: Introduco la possibilita` di ottenere nuove metriche a partire dalla
deformazione di una metrica esatta. Tali deformazioni sono ottenute
attraverso trasformazioni non olonome di campi di tetrade e sono definite
tramite una matrice di funzioni scalari. In tal modo si ottiene un modo
naturale e covariante per definire le perturbazioni di qualsiasi spaziotempo, le
simmetrie approssimate. Verranno discusse anche le altre
possibili applicazioni di questo approccio.
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TARTAGLIA e CAPONE:
Title: A darkless space time
Abstract:
In cosmology it has become usual to introduce new entities as dark matter
and dark energy in order to explain otherwise unexplained observational
facts. Here, a different approach is described treating space time as a
continuum endowed with properties similar to the ones of ordinary material
continua, such as internal viscosity and strain distributions originated by
defects in the texture. A Lagrangian modelled on the one valid for simple
dissipative phenomena in fluids is built and used for empty space time. The
internal "viscosity" is shown to correspond to a four-vector field. Using
the known symmetry of the universe, assuming the vector field to be
divergence-less and solving the Euler-Lagrange equation we obtain directly
inflation and a phase of accelerated expansion of space time. The vector
field is shown to be connected with the displacement vector field induced
by a point defect in a four-dimensional continuum. The only parameter in
the theory is the "strength" of the defect. It is possible to fix it in
such a way to quantitatively reproduce the acceleration of the universe.
With the unique value of the parameter that reproduces the estimated age of
the universe we obtain the duration of the accelerated phase and show that
ordinary matter with the known values for its present energy density still
give rise to the same behaviour as for empty spacetime.