Papers by Gianluca Allemandi
International Journal of Geometric Methods in Modern Physics, 2007
In my lectures I will deal with three seemingly unrelated problems: i) to what extent is general ... more In my lectures I will deal with three seemingly unrelated problems: i) to what extent is general relativity exceptional among metric gravity theories? ii) is it possible to define gravitational energy density applying field–theory approach to gravity? and iii) can a consistent theory of a gravitationally interacting spin–two field be developed at all? The connecting link to them is the concept of a fundamental classical spin–2 field. A linear spin–2 field introduced as a small perturbation of a Ricci–flat spacetime metric, is gauge invariant while its energy–momentum is gauge dependent. Furthermore, when coupled to gravity, the field reveals insurmountable inconsistencies in the resulting equations of motion. After discussing the inconsistencies of any coupling of the linear spin–2 field to gravity, I exhibit the origin of the fact that a gauge invariant field has the variational metric stress tensor which is gauge dependent. I give a general theorem explaining under what conditions...
Вестник Томского Государственного Педагогического Университета, 2004
The current accelerated universe could be produced by modified gravitational dynamics as it can b... more The current accelerated universe could be produced by modified gravitational dynamics as it can be seen in particular in its Palatini formulation. We analyze here a specific non-linear gravity-scalar system in the first order Palatini formalism which leads to a FRW cosmology different from the purely metric one. It is shown that the emerging FRW cosmology may lead either to an effective quintessence phase (cosmic speed-up) or to an effective phantom phase. Moreover, the already known gravity assisted dark energy dominance occurs also in the first order formalism. Finally, it is shown that a dynamical theory able to resolve the cosmological constant problem exists also in this formalism, in close parallel with the standard metric formulation.
The evidence of the acceleration of universe at present time has lead to investigate modified the... more The evidence of the acceleration of universe at present time has lead to investigate modified theories of gravity and alternative theories of gravity, which are able to explain acceleration from a theoretical viewpoint without the need of introducing dark energy. In this paper we study alternative gravitational theories defined by Lagrangians which depend on general functions of the Ricci scalar invariant in minimal interaction with matter, in view of their possible cosmological applications. Structural equations for the spacetimes described by such theories are solved and the corresponding field equations are investigated in the Palatini formalism, which prevents instability problems. Particular examples of these theories are also shown to provide, under suitable hypotheses, a coherent theoretical explanation of earlier results concerning the present acceleration of the universe and cosmological inflation. We suggest moreover a new possible Lagrangian, depending on the inverse of s...
The debate on the physical relevance of conformal transformations can be faced by taking the Pala... more The debate on the physical relevance of conformal transformations can be faced by taking the Palatini approach into account to gravitational theories. We show that conformal transformations are not only a mathematical tool to disentangle gravitational and matter degrees of freedom (passing from the Jordan frame to the Einstein frame) but they acquire a physical meaning considering the bi-metric structure of Palatini approach which allows to distinguish between spacetime structure and geodesic structure. Examples of higher-order and non-minimally coupled theories are worked out and relevant cosmological solutions in Einstein frame and Jordan frames are discussed showing that also the interpretation of cosmological observations can drastically change depending on the adopted frame.
Solar System tests give nowadays constraints on the estimated value of the cosmological constant,... more Solar System tests give nowadays constraints on the estimated value of the cosmological constant, which can be accurately derived from different experiments regarding gravitational redshift, light deflection, gravitational time-delay and geodesic precession. Assuming that each reasonable theory of gravitation should satisfy Solar System tests we use this limits on the estimated value of the cosmological constant to constrain alternative theories of Gravity, which are nowadays studied as possible theories for cosmological models and provide viable solutions to the cosmological constant problem and the explanation of the present acceleration of the Universe. We obtain that the estimated values, from Solar System tests, for the parameters appearing in the alternative theories of Gravity are orders of magnitude bigger than the values obtained in the framework of cosmologically relevant theories.
International Journal of Geometric Methods in Modern Physics, 2007
This paper is based on lectures given in Ladek-Zdroj, Poland during the 42nd edition of the Karpa... more This paper is based on lectures given in Ladek-Zdroj, Poland during the 42nd edition of the Karpac winter school on Theoretical Physics. The aim of these lectures was to give an introduction to a rigorous, mathematically well based, variational approach to alternative theories of Gravity, in view of their very interesting recent applications in the framework of Cosmology. We discuss alternative theories of Gravity both in the metric, purely affine and Palatini formalism, stressing differences and analogies both from a physical and mathematical viewpoint. Moreover we give and introduction to almost all the alternative theories of Gravity, that have been recently studied in view of their physical interest (higher order theories of Gravity, including Lovelock and Chern-Simons Lagrangians). Finally we skip to one of the most striking recent applications of alternative theories of Gravity in the framework of cosmological models: alternative theories of Gravity have been proven to provide...
General Relativity and Gravitation, 2005
Alternative theories of gravity have been recently studied in connection with their cosmological ... more 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.
Bollettino Dell Unione Matematica Italiana Sezione a La Matematica Nella Societa E Nella Cultura, 2004
Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems, May 2, 2017
This paper reports an evaluation of digital support for human creativity to improve health-and-sa... more This paper reports an evaluation of digital support for human creativity to improve health-and-safety in one manufacturing plant. It reports the use of this support as part of the plant's risk management process over 66 working days. Results revealed that this use led to more complete, more useful and more novel risk resolutions, compared with the original paper process, and informed how digital creativity support can be rolled out across manufacturing plants, as well as to other domains not recognized as creative.
The Eleventh Marcel Grossmann Meeting, 2008
Solar System tests give nowadays constraints on the estimated value of the cosmological constant,... more Solar System tests give nowadays constraints on the estimated value of the cosmological constant, which can be accurately derived from different experiments regarding gravitational redshift, light deflection, gravitational time-delay and geodesic precession. Assuming that each reasonable theory of gravitation should satisfy Solar System tests we use this limits on the estimated value of the cosmological constant to constrain alternative theories of Gravity, which are nowadays studied as possible theories for cosmological models and provide viable solutions to the cosmological constant problem and the explanation of the present acceleration of the Universe. We obtain that the estimated values, from Solar System tests, for the parameters appearing in the alternative theories of Gravity are orders of magnitude bigger than the values obtained in the framework of cosmologically relevant theories.
The definition of entropy obtained for stationary black holes is extended in this paper to the ca... more The definition of entropy obtained for stationary black holes is extended in this paper to the case of non-stationary black holes. Entropy is defined as a macroscopical thermodynamical quantity which satisfies the first principle of thermodynamics. In the non-stationary case a volume term appears since the solution does not admit a Killing vector. *
International Journal of Geometric Methods in Modern Physics
We introduce a covariant and geometrical framework for entropy in relativistic theories. In this ... more We introduce a covariant and geometrical framework for entropy in relativistic theories. In this framework the entropy of gravitational systems turns out to be a geometric quantity with well-defined cohomological properties arising from the obstruction to foliating spacetime into spacelike hypersurfaces. The framework relies on general covariance within a geometric framework which was recently defined to deal with the variation of conserved quantities generated by Nöther theorem. The definition of gravitational entropy so obtained turns out to be very general: it can be generalized to causal horizons and multiple-horizon spacetimes and it can be applied to define entropy for more exotic singular solutions of Einstein field equations. The same definition is also well-suited for higher dimensions and in the case of alternative gravitational theories (e.g. Chern-Simons theories and Lovelock Gravity).
A general recipe proposed elsewhere to define, via Noether theorem, the variation of energy for a... more A general recipe proposed elsewhere to define, via Noether theorem, the variation of energy for a natural field theory is applied to Einstein-Maxwell theory. The electromagnetic field is analysed in the geometric framework of natural bundles. Einstein-Maxwell theory turns then out to be natural rather than gauge-natural. As a consequence of this assumption a correction term \a la Regge-Teitelboim is
ABSTRACT A geometrical covariant definition of the variation of conserved quantities is introduce... more ABSTRACT A geometrical covariant definition of the variation of conserved quantities is introduced for Lagrangian field theories, suitable for both metric and affine gravitational theories. When this formalism is applied to the Hilbert Lagrangian we obtain a covariant definition of the Hamiltonian (and consequently a definition of the variation of Energy) for a gravitational system. The definition of the variation of Energy depends on boundary conditions one imposes. Different boundary conditions are introduced to define different energies: the gravitational heat (corresponding to Neumann boundary conditions) and the Brown‐York quasilocal energy (corresponding to Dirichlet boundary conditions) for a gravitational system. An analogy between the behavior of a gravitational system and a macroscopical thermodynamical system naturally arises and relates control modes for the thermodynamical system with boundary conditions for the gravitational system. This geometrical and covariant framework enables one to define entropy of gravitational systems, which turns out to be a geometric quantity with well‐defined cohomological properties arising from the obstruction to foliate spacetimes into spacelike hypersurfaces. This definition of gravitational entropy is found to be very general: it can be generalized to causal horizons and multiple‐horizon spacetimes and applied to define entropy for more exotic singular solutions of the Einstein field equations. The same definition is also well‐suited in higher dimensions and in the case of alternative gravitational theories (e.g. Chern‐Simons theories, Lovelock Gravity). © 2005 American Institute of Physics
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Papers by Gianluca Allemandi