arXiv subjects
Publications and source records attributed to F. Briscese.
We derive the dynamical equations for a non-local gravity model in the Palatini formalism and we discuss some of the properties of this model. We have shown that, in some specific cases, the vacuum solutions of general relativity are also vacuum solutions of the non-local model, so we conclude that, at least in this case, the singularities of Einstein's gravity are not removed.
In this paper we apply the usual perturbative methodology to evaluate the one-loop effective potential in a nonlocal scalar field theory. We find that the effect induced by the nonlocaliity of the theory is always very small and we discuss the consequences of this result. In particular we argue that, looking at one-loop corrections for matter fields, it is not possible to find signals of the nonlocality of the theory in cosmological observables since, even during inflation when energies are very high, nonlocality-induced corrections are expected to be very small.
The shift in condensation temperature caused by interactions is studied up to second order in the s-wave scattering length in a Bose-Einstein condensate trapped in a temperature-dependent three-dimensional generic potential. With no assumptions other than the mean-field approach and semiclassical approximations it is shown that the inclusion of a temperature-dependent trap improves the empirical values of the numerical parameters compared to those obtained in previous reports on the temperature shift.
We show that harmonically trapped Bose-Einstein condensates can be used to constrain Planck scale physics. In particular we prove that a Planck-scale induced deformation of the Minkowski energy-momentum dispersion relation $δE \simeq ξ_1 mcp/2M_p$ produces a shift in the condensation temperature $T_c$ of about $ΔT_{c}/T_{c}^{0} \simeq 10^{-6} ξ_1 $ for typical laboratory conditions. Such a shift allows to bound the deformation parameter up to $|ξ_1| \lesssim 10^4$. Moreover we show that it is possible to enlarge $ΔT_{c}/T_{c}^{0}$ and improve the bound on $ξ_1$ lowering the frequency of the harmonic trap. Finally we compare the Planck-scale induced shift in $T_c$ with similar effects due to interboson interactions and finite size effects.
A relativistic complex scalar boson field at finite temperature $T$ is examined below its critical Bose-Einstein condensation temperature. It is shown that at the same $T$ the state with antibosons has higher entropy, lower Helmholtz free energy and higher pressure than the state without antibosons, but the same Gibbs free energy as it should. This implies that the configuration without antibosons is metastable. Results are generalized for arbitrary $d$ spatial dimensions.
The effects of a Planck-scale deformation of the Minkowski energy-momentum dispersion relation on the phenomenology of non-trapped Bose-Einstein condensates (BECs) are examined. Such a deformation is shown to cause a shift in the condensation temperature $T_{c}$ of the BEC and, for a specific functional form of deformation, this shift can be as large as the current measured precision on $T_{c}$. For a $_{37}^{85}Rb$ cold-atom BEC with a particle density $n\simeq 10^{12}cm^{-3}$ we find a fractional shift of order $10^{-4}$, but this can be much larger for even more dilute BECs. We discuss the possibility of planning specific experiments with BECs that might provide phenomenological constraints on Planck-scale physics. These corrections to $T_{c}$ are found to be extremely small for ultrarelativistic BECs implying that, in some cases, Planck-scale effects may be more important in low- rather than high-energy processes.
We unify inflation and dark matter via a single scalar field phi. One of the main difficulties for this unification is that between inflation and dark matter one needs a successful reheating process and a long lasting period of radiation. Therefore the amount of energy density in the inflaton-dark matter field phi must be fine tune after reheating to give dark matter. Here we show an alternative scheme in which the inflaton decays completely, disappearing entirely from the spectrum. However, at low energies, before matter-radiation equality, the same interaction term that leads to the inflaton decay, regenerates phi. An essential feature is that the transition between the intermediate radiation dominated to the dark matter phase is related to a quantum generation of the scalar field ϕinstead to purely classical dynamics. Thanks to this quantum transition the inflation-dark matter unification can take place naturally without fine tuning. The unification scheme presented here has three parameters, the mass of the dark matter particle m_o, the inflation parameter lambda and the coupling g for the inflaton interaction. Phenomenology fixes the value for lambda and gives a constraint between g and m_o, leaving only the mass of the dark matter particle m_o as a free parameter. These same three parameters lambda, m_o, g are present in models with inflation and a dark matter wimp particle but without unification. Therefore our unification scheme does not increase the number of parameters and it accomplishes the desired unification between inflaton and dark matter for free.
We present a type of dark energy models where the particles of dark energy phi are dynamically produced via a quantum transition at very low energies. The scale where the transition takes places depends on the strength g of the interaction between phi and a relativistic field varphi. We show that a g \simeq 10^{-12} gives a generation scale E_gen simeq eV with a cross section sigma simeq 1 pb close to the WIMPs cross section sigma_w simeq pb at decoupling. The number density n_phi of the ϕparticles are a source term in the eq. of motion of phi and it generates the scalar potential v(phi) responsible for the late time acceleration of our universe. Since the appearance of phi may be at very low scales, close to present time, the cosmological coincidence problem can be explained simply due to the size of the coupling constant. We unify dark energy with inflation in terms of a single scalar field phi, and we use the same potential v for inflation and dark energy. However, after inflation phi decays completely and reheats the universe at a scale E_RH propto h^2 m_Pl, where h is the coupling between the SM particles and varphi. The field phi disappears from the spectrum during, from reheating until its re-generation, and therefore it does not interfere with the standard decelerating radiation/matter cosmological model allowing for a successful unification scheme. The same interaction term that gives rise to the inflaton decay accounts for the late time re-generation of phi. We present a simple model where the strength of the g and h couplings are set by the inflation scale E_I with g=h^2 propto E_I/m_Pl giving a reheating scale E_RH propto E_I=100TeV and phi-generation scale E_gen propto E_I^2/m_pl=eV << E_RH.
We study a scalar phi field that unifies inflation and dark energy with a long period of a hot decelerating universe in between these two stages of inflation. A key feature is that the transition between the intermediate decelerated phase to the dark energy phase is related to a quantum regeneration of the scalar field phi instead to purely classical dynamics. The interaction V_int(phi,varphi) between phi and a second scalar field varphi allows not only for phi to decay at a high energy E_I, even though phi does not oscillate around the minimum of its potential, but it also regenerates phi at an energy E_{BD} \simeq E_I^2 close to present time. It predicts extra relativistic energy and an interacting dark energy which may account for a w<-1. In the model presented here we have E_I \simeq 100 TeV and E_{BD} \simeq 1 eV. The low value of the back decay energy E_{BD} gives an explanation of the coincidence problem.
An analysis of some modified gravity models, based on the study of pure Schwarzschild and of Schwarzschild-de Sitter black holes, and involving the use of the Noether charge method, is carried out. Corrections to the classical Einsteinian black hole entropy appear. It is shown explicitly how the condition of positive entropy can be used in order to constrain the viability of modified gravity theories.
We develop here a mechanism that, without making use of a cosmological constant, reproduces an accelerating universe. This is done by taking into account Casimir vacuum energy density, assuming that the underlying theory allows a maximum momentum, that turns out to be the leading contribution term to Einstein equations in a large expanding FRW universe. As stated in numerous quantum gravity studies, we postulate that maximum momentum is related to the existence of the Planck length as a fundamental length. This insight, together with the assumption of a Planck scale correction to the energy/momentum dispersion-relation on a FRW background, is used here to calculate Casimir vacuum energy. We show that, under these hypothesis, an accelerated universe expansion is obtained. As last step we analyze the compatibility of the resulting model with experimental data, writing down the equation of state for Casimir energy and pressure and observing that this equation of state belongs to a class of models that naturally fits cosmological observations. We emphasize that our result relies, once a fundamental length is introduced in Casimir effect, just on general arguments thus it is independent on an explicit form of the energy-momentum dispersion relation.
It is shown that phantom scalar models can be mapped into a mathematically equivalent, modified $F(R)$ gravity, which turns out to be complex, in general. Only for even scalar potentials is the ensuing modified gravity real. It is also demonstrated that, even in this case, modified gravity becomes complex at the region where the original phantom dark energy theory develops a Big Rip singularity. A number of explicit examples are presented which show that these two theories are not completely equivalent, from the physical viewpoint. This basically owes to the fact that the physical metric in both theories differ in a time-dependent conformal factor. As a result, an FRW accelerating solution, or FRW instanton, in the scalar-tensor theory may look as a deccelerating FRW solution, or a non-instantonic one, in the corresponding modified gravity theory.