arXiv subjects

Angelo Tartaglia

Publications and source records attributed to Angelo Tartaglia.

At least 37 records · Page 2Linked to original sources

arXiv2012

Experimental determination of gravitomagnetic effects by means of ring lasers

A new experiment aimed to the detection of the gravito-magnetic Lense-Thirring effect at the surface of the Earth will be presented; the name of the experiment is GINGER. The proposed technique is based on the behavior of light beams in ring lasers, also known as gyrolasers. A three-dimensional array of ringlasers will be attached to a rigid monument; each ring will have a different orientation in space. Within the space-time of a rotating mass the propagation of light is indeed anisotropic; part of the anisotropy is purely kinematical (Sagnac effect), part is due to the interaction between the gravito-electric field of the source and the kinematical motion of the observer (de Sitter effect), finally there is a contribution from the gravito-magnetic component of the Earth (gravito-magnetic frame dragging or Lense-Thirring effect). In a ring laser a light beam traveling counterclockwise is superposed to another beam traveling in the opposite sense. The anisotropy in the propagation leads to standing waves with slightly different frequencies in the two directions; the final effect is a beat frequency proportional to the size of the instrument and its effective rotation rate in space, including the gravito-magnetic drag. Current laser techniques and the performances of the best existing ring lasers allow at the moment a sensitivity within one order of magnitude of the required accuracy for the detection of gravito-magnetic effects, so that the objective of GINGER is in the range of feasibility and aims to improve the sensitivity of a couple of orders of magnitude with respect to present. The experiment will be underground, probably in the Gran Sasso National Laboratories in Italy, and is based on an international collaboration among four Italian groups, the Technische Universitaet Muenchen and the University of Canterbury in Christchurch (NZ).

gr-qc↗

arXiv2012

Relativistic space-time positioning: principles and strategies

Starting from the description of space-time as a curved four-dimensional manifold, null Gaussian coordinates systems as appropriate for relativistic positioning will be discussed. Different approaches and strategies will be reviewed, implementing the null coordinates with both continuous and pulsating electromagnetic signals. In particular, methods based on purely local measurements of proper time intervals between pulses will be expounded and the various possible sources of uncertainty will be analyzed. As sources of pulses both artificial and natural emitters will be considered. The latter will concentrate on either radio- or X ray-emitting pulsars, discussing advantages and drawbacks. As for artificial emitters, various solutions will be presented, from satellites orbiting the Earth to broadcasting devices carried both by spacecrafts and celestial bodies of the solar system. In general the accuracy of the positioning is expected to be limited, besides the instabilities and drift of the sources, by the precision of the local clock, but in any case in long journeys systematic cumulated errors will tend to become dominant. The problem can be kept under control properly using a high level of redundancy in the procedure for the calculation of the coordinates of the receiver and by mixing a number of different and complementary strategies. Finally various possibilities for doing fundamental physics experiments by means of space-time topography techniques will shortly be presented and discussed.

gr-qc↗

arXiv2012

On the emergence of the Lorentz signature in an expanding universe

A mechanism producing the transition from an Euclidean to a Loretzian manifold is described. A global Robertson-Walker symmetry is assumed from the large scale data of the visible universe. Allowing for the strain of the manifold as an additional field in the Lagrangian, we interpret the symmetry as a consequence of a global texture defect. The additional term gives rise to a boundary dividing the manifold into an Euclidean plus a Lorentzian region. It is also shown that the presence in the early epoch of homogeneous matter/energy fields preserves the horizon and the signature change across it. The horizon has properties much similar to the ones of the Big Bang of the Standard Model, including the need for a phase transition of the scalar field producing particles and fields as we know them now.

gr-qc↗

arXiv2012

From the elasticity theory to cosmology and vice versa

The paper shows how a generalization of the elasticity theory to four dimensions and to space-time allows for a consistent description of the homogeneous and isotropic universe, including the accelerated expansion. The analogy is manifested by the inclusion in the traditional Lagrangian of general relativity of an additional term accounting for the strain induced in the manifold (i.e. in space-time) by the curvature, be it induced by the presence of a texture defect or by a matter/energy distribution. The additional term is sufficient to account for various observed features of the universe and to give a simple interpretation for the so called dark energy. Then, we show how the same approach can be adopted back in three dimensions to obtain the equilibrium configuration of a given solid subject to strain induced by defects or applied forces. Finally, it is shown how concepts coming from the familiar elasticity theory can inspire new approaches to cosmology and in return how methods appropriated to General Relativity can be applied back to classical problems of elastic deformations in three dimensions.

gr-qc↗

arXiv2011

A relativistic navigation system for space

We present here a method for the relativistic positioning in spacetime based on the reception of pulses from sources of electromagnetic signals whose worldline is known. The method is based on the use of a four-dimensional grid covering the whole spacetime and made of the null hypersurfaces representing the propagating pulses. In our first approach to the problem of positioning we consider radio-pulsars at infinity as primary sources of the required signals. The reason is that, besides being very good clocks, pulsars can be considered as being fixed stars for reasonably long times. The positioning is obtained linearizing the worldline of the observer for times of the order of a few periods of the signals. We present an exercise where the use of our method applied to the signals from four real pulsars permits the reconstruction of the motion of the Earth with respect to the fixed stars during three days. The uncertainties and the constraints of the method are discussed and the possibilities of using moving artificial sources carried around by celestial bodies or spacecrafts in the Solar System is also discussed.

gr-qc↗

arXiv2011

The Strained State Cosmology

The chapter expounds a theory based on the interpretation of the dark energy as a strain energy of a physical continuum. The theory is based on the analogy that exists between the properties of space-time and the properties of elastic materials, when extended to four dimensions and the Lorentzian signature. In practice the starting point is an action integral that contains an additional term built as elastic potential energy of strained materials is. Besides this vacuum energy term matter/energy fields appear as usual. The strain energy term is based on the strain tensor of empty space-time which in turn is obtained from the non trivial part of the metric tensor. The Strained State Cosmology (SSC) is deduced from this approach when a Robertson-Walker symmetry is assumed: The result accounts for the accelerated expansion of the universe and agrees with observation passing four typical cosmological tests

gr-qc↗

arXiv2011

Pulsars as celestial beacons to detect the motion of the Earth

In order to show the principle viability of a recently proposed relativistic positioning method based on the use of pulsed signals from sources at infinity, we present an application example reconstructing the world-line of an idealized Earth in the reference frame of distant pulsars. The method considers the null four-vectors built from the period of the pulses and the direction cosines of the propagation from each source. Starting from a simplified problem (a receiver at rest) we have been able to calibrate our procedure, evidencing the influence of the uncertainty on the arrival times of the pulses as measured by the receiver, and of the numerical treatment of the data. The most relevant parameter turns out to be the accuracy of the clock used by the receiver. Actually the uncertainty used in the simulations combines both the accuracy of the clock and the fluctuations in the sources. As an evocative example the method has then been applied to the case of an ideal observer moving as a point on the surface of the Earth. The input have been the simulated arrival times of the signals from four pulsars at the location of the Parkes radiotelescope in Australia. Some substantial simplifications have been made both excluding the problems of visibility due to the actual size of the planet, and the behaviour of the sources. A rough application of the method to a three days run gives a correct result with a poor accuracy. The accuracy is then enhanced to the order of a few hundred meters if a continuous set of data is assumed. The method could actually be used for navigation across the solar system and be based on artificial sources, rather than pulsars. The viability of the method, whose additional value is in the self-sufficiency, i.e. independence from any control from other operators, has been confirmed.

gr-qc↗

arXiv2010

A null frame for spacetime positioning by means of pulsating sources

We introduce an operational approach to the use of pulsating sources, located at spatial infinity, for defining a relativistic positioning and navigation system, based on the use of four-dimensional bases of null four-vectors, in flat spacetime. As a prototypical case, we show how pulsars can be used to define such a positioning system. The reception of the pulses for a set of different sources whose positions in the sky and periods are assumed to be known allows the determination of the user's coordinates and spacetime trajectory, in the reference frame where the sources are at rest. We describe our approach in flat Minkowski spacetime, and discuss the validity of this and other approximations we have considered.

gr-qc↗

arXiv2009

Massive gravitational waves from the Cosmic Defect theory

The Cosmic Defect theory (CD), which is presented elsewhere in this conference, introduces in the standard Einstein-Hilbert Lagrangian an elastic term accounting for the strain of space-time viewed as a four-dimensional physical continuum. In this framework the Ricci scalar acts as the kinetical term of the strain field whose potential is represented by the additional terms. Here we are presenting the linearised version of the theory in order to analyze its implications in the weak field limit. First we discuss the recovery of the Newtonian limit. We find that the typical static weak field limit imposes a constraint on the values of the two parameters (Lame coefficients) of the theory. Once the constraint has been implemented, the typical gravitational potential turns out to be Yukawa-like. The value for the Yukawa parameter is consistent with the constraints coming from the experimental data at the Solar system and galactic scales. We then come to the propagating solutions of the linearised Einstein equations in vacuo, i.e. to gravitational waves. Here, analogously with other alternative or extended theories of gravity, the presence of the strain field produces massive waves, where massive (in this completely classical context) means subluminal. Furthermore longitudinal polarization modes are allowed too, thus lending, in principle, a way for discriminating these waves from the plane GR ones.

gr-qc↗

arXiv2009

From Elastic Continua to Space-time

Since the early days of the theory of electromagnetism and of gravity the idea of space, then space-time, as a sort of physical continuum hovered the scientific community. Actually general relativity shows the strong similarity that exists between the geometrical properties of space-time and the ones of a strained elastic continuum. The bridge between geometry and the elastic potential, as well in three as in three plus one dimensions, is the strain tensor, read as the non-trivial part of the metric tensor. On the basis of this remark and exploiting appropriate multidimensional embeddings, it is possible to build a full theory of space-time, allowing to account for the accelerated expansion of the universe. How this can be obtained is the content of the paper. The theory fits the cosmic accelerated expansion data from type Ia supernovae better than the Lambda-CDM model.

gr-qc↗

arXiv2009

A tensor theory of space-time as a strained material continuum

The classical theory of strain in material continua is reviewed and generalized to space-time. Strain is attributed to "external" (matter/energy fields) and intrinsic sources fixing the global symmetry of the universe (defects in the continuum). A Lagrangian for space-time is worked out, adding to the usual Hilbert term an "elastic" contribution from intrinsic strain. This approach is equivalent to a peculiar tensor field, which is indeed part of the metric tensor. The theory gives a configuration of space-time accounting both for the initial inflation and for the late acceleration. Considering also the contribution from matter the theory is used to fit the luminosity data of type Ia supernovae, giving satisfactory results.

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arXiv2008

Emission vs Fermi coordinates: applications to relativistic positioning systems

A 4-dimensional relativistic positioning system for a general spacetime is constructed by using the so called "emission coordinates". The results apply in a small region around the world line of an accelerated observer carrying a Fermi triad, as described by the Fermi metric. In the case of a Schwarzschild spacetime modeling the gravitational field around the Earth and an observer at rest at a fixed spacetime point, these coordinates realize a relativistic positioning system alternative to the current GPS system. The latter is indeed essentially conceived as Newtonian, so that it necessarily needs taking into account at least the most important relativistic effects through Post-Newtonian corrections to work properly. Previous results concerning emission coordinates in flat spacetime are thus extended to this more general situation. Furthermore, the mapping between spacetime coordinates and emission coordinates is completely determined by means of the world function, which in the case of a Fermi metric can be explicitly obtained.

gr-qc↗

arXiv2007

Mapping Cartesian Coordinates into Emission Coordinates: some Toy Models

After briefly reviewing the relativistic approach to positioning systems based on the introduction of the emission coordinates, we show how explicit maps can be obtained between the Cartesian coordinates and the emission coordinates, for suitably chosen set of emitters, whose world-lines are supposed to be known by the users. We consider Minkowski space-time and the space-time where a small inhomogeineity is introduced (i.e. a small "gravitational" field), both in 1+1 and 1+3 dimensions.

gr-qc↗

arXiv2007

Binary black hole merger in the extreme mass ratio limit

We discuss the transition from quasi-circular inspiral to plunge of a system of two nonrotating black holes of masses $m_1$ and $m_2$ in the extreme mass ratio limit $m_1m_2\ll (m_1+m_2)^2$. In the spirit of the Effective One Body (EOB) approach to the general relativistic dynamics of binary systems, the dynamics of the two black hole system is represented in terms of an effective particle of mass $μ\equiv m_1m_2/(m_1+m_2)$ moving in a (quasi-)Schwarzschild background of mass $M\equiv m_1+m_2$ and submitted to an ${\cal O}(μ)$ radiation reaction force defined by Padé resumming high-order Post-Newtonian results. We then complete this approach by numerically computing, à la Regge-Wheeler-Zerilli, the gravitational radiation emitted by such a particle. Several tests of the numerical procedure are presented. We focus on gravitational waveforms and the related energy and angular momentum losses. We view this work as a contribution to the matching between analytical and numerical methods within an EOB-type framework.

gr-qc↗

arXiv2006

Gravitational Faraday Rotation in Binary Pulsar Systems

We study the gravitational Faraday rotation, on linearly polarized light rays emitted by a pulsar, orbiting another compact object. We relate the rotation angle to the orbital phase of the emitting pulsar, as well as to other parameters describing its orbit and the orientation of the angular momentum of the binary companion. We give numerical estimates of the effect for the double-pulsar system PSR J0737-3039, and we note that the expected magnitude is exceedingly small, making the effect unlikely to be observed with present technology. It is however interesting per se, since in this phenomenon, gravito-magnetism plays a leading role, unlike what happens, for instance, when studying light bending or gravitational time delay, where it appears as a correction to the gravito-electric contribution. Also, we envisage the possibility that this effect could be relevant, at least in principle, for a pulsar orbiting a non charged black-hole.

astro-ph↗

arXiv2006

Doppler Effects from Bending of Light Rays in Curved Space-Times

We study Doppler effects in curved space-time, i.e. the frequency shifts induced on electromagnetic signals propagating in the gravitational field. In particular, we focus on the frequency shift due to the bending of light rays in weak gravitational fields. We consider, using the PPN formalism, the gravitational field of an axially symmetric distribution of mass. The zeroth order, i.e. the sphere, is studied then passing to the contribution of the quadrupole moment, and finally to the case of a rotating source. We give numerical estimates for situations of physical interest, and by a very preliminary analysis, we argue that analyzing the Doppler effect could lead, in principle, in the foreseeable future, to the measurement of the quadrupole moment of the giant planets of the Solar System.

gr-qc↗

arXiv2005

A post-Keplerian parameter to test gravito-magnetic effects in binary pulsar systems

We study the pulsar timing, focusing on the time delay induced by the gravitational field of the binary systems. In particular, we study the gravito-magnetic correction to the Shapiro time delay in terms of Keplerian and post-Keplerian parameters, and we introduce a new post-Keplerian parameter which is related to the intrinsic angular momentum of the stars. Furthermore, we evaluate the magnitude of these effects for the binary pulsar systems known so far. The expected magnitude is indeed small, but the effect is important per se.

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arXiv2005

Post-Newtonian Parameters from Alternative Theories of Gravity

Alternative theories of gravity have been recently studied in connection with their cosmological applications, both in the Palatini and in the metric formalism. The aim of this paper is to propose a theoretical framework (in the Palatini formalism) to test these theories at the solar system level and possibly at the galactic scales. We exactly solve field equations in vacuum and find the corresponding corrections to the standard general relativistic gravitational field. On the other hand, approximate solutions are found in matter cases starting from a Lagrangian which depends on a phenomenological parameter. Both in the vacuum case and in the matter case the deviations from General Relativity are controlled by parameters that provide the Post-Newtonian corrections which prove to be in good agreement with solar system experiments.

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