UA-34446393-1
  • For an up-to-date list including  the most recent preprints see arXiv.
  • For bibliometrics and citation data see Google Scholar, Scopus.

Journals

Books

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Starting from the weak field limit, we discuss astrophysical applications of Extended Theories of Gravity where higher order curvature invariants and scalar fields are considered by generalizing the Hilbert-Einstein action linear in the Ricci curvature scalar R. Results are compared to General Relativity in the hypothesis that Dark Matter contributions to the dynamics can be neglected thanks to modified gravity. In particular, we consider stellar hydrostatic equilibrium, galactic rotation curves, and gravitational lensing. Finally, we discuss the weak field limit in the Jordan and Einstein frames pointing out how effective quantities, as gravitational potentials, transform from one frame to the other and the interpretation of results can completely change accordingly.


Fourth Order Theories of Gravity have recently attracted a lot of interest as candidates to explain the observed cosmic acceleration, the flatness of the rotation curves of spiral galaxies, the large scale structure and other relevant astrophysical phenomena. This means that the "Dark Side" issue of the Universe could be completely reversed considering dark matter and dark energy as "shortcomings" of General Relativity in its simplest formulation (a linear theory in the Ricci scalar R, minimally coupled to the standard perfect fluid matter) and claiming for the "correct" theory of Gravity as that derived by matching the largest number of observational data, without imposing any theory a priori. As a working hypothesis, accelerating behavior of cosmic fluid, large scale structure, potential of galaxy clusters, rotation curves of spiral galaxies could be reproduced by means of extending General Relativity to generic actions containing higher order and non-minimally coupled terms in curvature invariants. In other words, gravity could act in different ways at different scales and the above "shortcomings" could be due to the incorrect extrapolations of the Einstein theory, actually tested at short scales and low energy regimes. Very likely, what we call "dark matter and "dark energy" could be nothing else but signals of the breakdown of General Relativity at large scales. Then, it is a crucial point testing these Extended Theories in the weak field limit. In this sense, comparing these theories to General Relativity could be a fundamental step to retain or rule out them. In this review paper, after a survey of what is intended for Extended Theories of Gravity in the so called metric approach, we extensively discuss their Newtonian and the post-Newtonian limits pointing out, in details, their resemblances and differences with respect to General Relativity. Particular emphasis is placed on the exact solutions and methods used to obtain them. Finally, it is clearly shown that General Relativity results, in the Solar System context, are easily recovered since Einstein theory is a particular case of this extended approach. This is a crucial point against several wrong results in literature stating that these theories (e.g. f(R)-gravity) are not viable at local scale.

Proceedings

  • The Quadratic Coefficient of the Electron Cloud Mapping - S. Petracca, A. Stabile, T. Demma - Proceedings of the ECLOUD'12: Joint INFN-CERN-EuCARD-AccNet Workshop on Electron Cloud Effects (2012) - ISBN 978-92-9083-386-4, ISSN 0007-8328 DOI 10.5170/CERN-2013-002
  • Numerical Analysis of Galactic Rotation Curves - A. Stabile. G. Scelza - Mathematica Italia User Group Meeting 2011 – Atti del Convegno - ISBN 978-88-96810-02-6
  • An Analytical Formula of the Electron Cloud Linear Map Coefficient in a Strong Dipole – S. Petracca, A. Stabile, T. Demma, G. Rumolo - Beam Dynamics and Electromagnetic Fileds D05 Instabilities, Processes, Impedances, Countermeasures - 2nd International Particle Physics 2011 (IPAC 2011) - MOPS056 ISBN 978-92-9083-366-6
  • The Quadratic Coefficient of the Electron CLoud Mapping – T. Demma, S. Petracca, A. Stabile – Proceedings of the XVIII RiNEm EDAP2 - ISBN 978-88-905261-1-4 (printed edition) ISBN 978-88-905261-0-7 (Electronic edition)
  • Open Cell Metal Foams for Beam Linears? – R. P. Croce, S. Petracca, A. Stabile – Proceedings of the XVIII RiNEm POST6 - ISBN 978-88-905261-1-4 (Printed edition) ISBN 978-88-905261-0-7 (Electronic edition)
  • Weak Field Approach in f(R)-Gravity – A. Stabile, S. Capozziello – 54th Meeting of the Italian Astronomical Society - Italian astronomy: perspectives for the next decade (SAIt 2010) – Mem. S.A.It. Suppl. vol 19, 63, 2012 ISSN 1824-0178
  • E-cloud Map Formalism: an Analytical Expression for Quadratic Coefficient – T. Demma, S. Petracca, A. Stabile – Beam Dynamics and Electromagnetic Fields D05 Instabilities, Processes, Impedances, Countermeasures - International Particle Accelerator Conference 2010 (IPAC 2010) - TUPD037 ISBN 978-92-9083-352-9
  • Maps for Electron Clouds: Application to LHC Conditioning - T. Demma, R. Cimino, A. Drago, S. Petracca, A. Stabile - Beam Dynamics and Electromagnetic Fields D04 Processes, Impedances, Countermeasures – Particle Accelerator Conference 2009 - PAC09 FR5RFP071

PhD thesis

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The Higher Order Theories of Gravity f(R, R_{\alpha\beta}R^{\alpha\beta}) - theory, where R is the Ricci scalar, R_{\alpha\beta} is the Ricci tensor and f is any analytic function - have recently attracted a lot of interest as alternative candidates to explain the observed cosmic acceleration, the flatness of the rotation curves of spiral galaxies and other relevant astrophysical phenomena. It is a crucial point testing these alternative theories in the so called weak field and newtonian limit of a f(R, R_{\alpha\beta}R^{\alpha\beta}) - theory. With this "perturbation technique" it is possible to find spherically symmetric solutions and compare them with the ones of General Relativity. On both approaches we found a modification of General Relativity: the behaviour of gravitational potential presents a modification Yukawa - like in the newtonian case and a massive propagation in the weak field case. When the modification of the theory is removed (i.e. f(R, R_{\alpha\beta}R^{\alpha\beta})\,=\,R, Hilbert - Einstein lagrangian) we find the usual outcomes of General Relativity. Also the Noether symmetries technique has been investigated to find some time independent spherically symmetric solutions.
The rotation curve for M33 continues to rise well beyond the optical disk ... as measured by the HI. The lower curve on the plot (dashed line) shows what the rotational velocity of objects in the M33 galaxy is expected to be based on the luminous matter in the galaxy. Clearly, the experimental points do not match the dashed line: the rotational velocity of objects outside the galaxy is far faster than the prediction.

MD thesis

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  • Analysis of Compatible Models with respect to the Physics of the Center of the Galaxy

In this thesis we explore the possibility of self-gravitating non-baryonic systems (in particular boson stars and / or neutrinos star) may be present at the center of galaxies, with special attention to our galaxy: The Milky Way. From a dynamical point of view, we analyze the information contained in the observations, concluding that they are compatible with a single massive object (the so-called Sgr A *), without necessarily requiring that this is an extremely massive black hole. The hypothesis of a "dark" stellar cluster (white dwarfs, neutron stars) or a cluster of holes blacks with the size Sun-like is analyzed through the gravothermal catastrophe theory, and is excluded because the average life of the cluster is incompatible with the age of the Galaxy. Moreover these structures reproduce satisfactorily the dynamic demands. In the different models reproducing the observed central mass the required parameter are derived, and further the possible existence of boson or neutrinos stars is analyzed. In addition, the models are able to explain the nature of Sgr A * without invoking the presence of singularities. We analyze the growth factor around the star, trying to explain the absence of black hole. In fact by using the phenomenon of fragmentation in the case of boson star we give an answer to the problem. In the case of neutrinos star unlike we have only a some suggestions for the solution. Finally, we suggest the possible observations for the proposed models. In conclusion, we can say that no model fully satisfies the physics of the galactic center and the hypothesis of the black hole is far from conclusive discussion.
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Starting from a large scale radio image, let us zoom in on Sgr A*. We start with a view about 2 parsecs across; if we put the Sun at the center, the nearest star would be near the edge, but in the Galactic Center the frame is filled with millions of stars (none of which can be seen in the radio image). We zoom in by a factor of about 1000 to see the Sgr A* radio source just barely resolved by our highest resolution radio measurements.