1998 (IPP)

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Conformal transformations in classical gravitational theories and in cosmology
(2015-03-11) Faraoni, Valerio; Gunzing , Edgard; Nardone, Pasquale
In recent years, the use of conformal transformation techniques has become widespread in the literature on gravitational theories alternative to general relativity, on cosmology, and on nonminimally coupled scalar fields. Tipically, the transformation to the Einstein frame is generated by a fundamental scalar field already present in the theory. In this context, the problem of which conformal frame is the physical one has to be dealt with and, in the general case, it has been clarified only recently; the formulation of a theory in the "new" conformal frame leads to departures from canonical Einstein gravity. In this article, we review the literature on conformal transformations in classical gravitational theories and in cosmology, seen both as purely mathematical tools and as maps with physically relevant aspects. It appears particularly urgent to refer the analysis of experimental tests of Brans-Dicke and scalar-tensor theories of gravity, as well as the predictions of cosmological inflationary scenarios, to the physical conformal frame, in order to have a meaningful comparison with the observations.
Does the non-minimal coupling of the scalar field improve or destroy inflation ?
(2015-03-11) Faraoni, Valerio
Influence of the cosmological expansion on local systems
(2015-03-11) Cooperstock, F.I.; Faraoni, Valerio; Vollick, D.N.
Following renewed interest, the problem of whether the cosmological expansion affects the dynamics of local systems is reconsidered. The cosmological correction to the equations of motion in the locally inertial Fermi normal frame (the relevant frame for astronomical observations) is computed. The evolution equations for the cosmological perturbation of the two-body problem are solved in this frame. The effect on the orbit is insignificant as are the effects on the galactic and galactic cluster scales.
Lensing by gravitational waves in scalar – tensor gravity: Einstein frame analysis
(2015-03-11) Faraoni, Valerio; Gunzing , Edgard
The amplification of a light beam due to intervening gravitational waves is studied. The previous Jordan frame result according to which the amplification is many orders of magnitude larger in scalar-tensor gravity than in general relativity does not hold in the Einstein conformal frame. Lensing by gravitational waves is discussed in relation to the ongoing and proposed VLBI observations aimed at detecting the scintillation effect.
Tales of tails in cosmology
(2015-03-11) Faraoni, Valerio; Gunzing , Edgard
Late time mild inflation (LTMI) proposes to solve the age of the universe problem and the discrepancy between locally and globally measured values of the Hubble parameter. However, the mechanism proposed to achieve LTMI is found to be physically pathological by applying the theory of tails for the solutions of wave equations in curved spaces. Alternative mechanisms for LTMI are discussed, and the relevance of scalar wave tails for cosmology is emphasized.
When a mass term does not represent a mass
(2015-03-11) Faraoni, Valerio; Cooperstock, F.I.
The definition of mass of a scalar field in a curved space has often been generalized by grouping coupling terms between the field and the Ricci curvature with non-curvature-related mass terms. In a broader point of view, one sees that a common misunderstanding resulting from such an identification leads one, in the case of the spin 2 field, to regard the cosmological constant as a non-vanishing mass of cosmological origin for the graviton. Similarly, there are inconsistencies for the spin 1 field. Instead, the intrinsic mass of a field should be regarded as being independent of the background curvature.