Alternative theories of gravity and neutron stars
From Firenze University Press Journal: Il Colle di Galileo
Jacopo Soldateschi, Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze
Niccolò Bucciantini, Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze
Gravity, the weakest of the four fundamental interactions known to exist in nature, is currently explained by the theory of general relativity, conceived by Albert Einstein at the start of the twentieth century. This theory has so far proved to be greatly successful, managing to ex-plain phenomena as different as the coalescence of black holes in other galaxies or the more familiar physics of falling bodies. However, as time went by, some problems arose that general elativity does not seem to be able to solve. From the purely theoretical and conceptual point of view, it does not seem suitable to form the basis of quantum gravity, that is, the theory that should unify the domains of gravity and quantum mechanics. From the experimental point of view, the rotation curve of galaxies as well as the evolution of the universe itself require the existence of a completely unknown form of matter-energy, dark matter and dark energy.
A possible solution to these problems, which also represents an alternative to the introduction of dark matter and dark energy, is that of “alternative theories of gravity”: theories which extend general relativity in such a way as to resolve these issues. Among the many proposed alternative theories of gravity, the most promising ones seem those belonging to the “scalar-tensor” category. In these theories, the gravitational interaction is mediated both by a particle known as the graviton, which is called “tensorial” because of its physical properties, and by a hypothetical “scalar” particle.In our work, we have studied how the most extreme material objects in the known universe, neutron stars, behave in some scalar-tensor theories. In fact, a particular class of these theories contains a physical phenomenon called “spontaneous scalarization”, according to which the effect of the scalar field becomes more important as the surrounding matter becomes more compact. Since neutron stars are the most compact material objects in the known universe, their importance in testing alternative theories of gravity becomes clear. Moreover, this phenomenon allows neutron stars to display evident — that is, potentially observable — modifications with re-spect to their counterparts in general relativity, while allowing this class of theories to remain viable according to the most stringent constraints coming from astronomical observations.
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