IRIS Università degli Studi di Bolognahttps://cris.unibo.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Sat, 24 Jul 2021 13:52:05 GMT2021-07-24T13:52:05Z101141Complex Lagrangians and phantom cosmologyhttp://hdl.handle.net/11585/34538Titolo: Complex Lagrangians and phantom cosmology
Abstract: Motivated by the generalization of quantum theory for the case of non-Hermitian Hamiltonians with PT symmetry, we show how a classical cosmological model describes a smooth transition from ordinary dark energy to the phantom one. The model is based on a classical complex Lagrangian of a scalar field. Specific symmetry properties analogous to PT in non-Hermitian quantum mechanics lead to purely real equation of motion.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/11585/345382006-01-01T00:00:00ZAsymptotic Freedom in Inflationary Cosmology with a non-minimally coupled Higgs fieldhttp://hdl.handle.net/11585/115254Titolo: Asymptotic Freedom in Inflationary Cosmology with a non-minimally coupled Higgs field
Abstract: The new results in the inflation model based on the non-minimally coupled Higgs field are presented.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/11585/1152542012-01-01T00:00:00ZScalar field cosmological models with finite scale factor singularities.http://hdl.handle.net/11585/67577.1Titolo: Scalar field cosmological models with finite scale factor singularities.
Abstract: We construct a scalar field based cosmological model, possessing a cosmological singularity characterized by a finite value of the cosmological radius and an infinite scalar curvature. Using the methods of the qualitative theory of differential equations, we give a complete description of the cosmological evolutions in the model under consideration. There are four classes of evolutions, two of which have finite lifetimes, while the other two undergo an infinite expansion. We discuss also the relation between the dynamics of the model and the intermediate inflation phenomenology.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11585/67577.12009-01-01T00:00:00ZDo supernovae favor tachyonic Big Brake instead de Sitter?http://hdl.handle.net/11585/90569Titolo: Do supernovae favor tachyonic Big Brake instead de Sitter?
Abstract: We investigate whether a tachyonic scalar field, encompassing both dark energy and dark matter-like features will drive our universe towards a Big Brake singularity or a de Sitter expansion. In doing this it is crucial to establish the parameter domain of the model, which is compatible with type Ia supernovae data. We find the 1-sigma contours and evolve the tachyonic sytem into the future. We conclude, that both future evolutions are allowed by observations, Big Brake becoming increasingly likely with the increase of the positive model parameter k.
Fri, 01 Jan 2010 00:00:00 GMThttp://hdl.handle.net/11585/905692010-01-01T00:00:00ZCosmology of non-Hermitian (C)PT-invariant scalar matterhttp://hdl.handle.net/11585/76661Titolo: Cosmology of non-Hermitian (C)PT-invariant scalar matter
Abstract: We use the generalization of quantum theory for the case of non-Hermitian Hamiltonians with (C)PT symmetry to show how a classical cosmological model describes a smooth transition from ordinary dark energy to the phantom one. We consider the PT symmetric flat Friedmann model of two scalar fields with positive kinetic terms. The solution for the normal field is real while the solution for the second field is purely imaginary, realizing classically the 'phantom' behavior. The energy density and pressure happen to be real and the corresponding geometry is well-defined. The Lagrangian for the linear perturbations leads to positive energy densities for both the fields, so that the problem of stability does not arise. The phantom phase in the cosmological evolution appears to be transient and the Big Rip never occurs.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11585/766612009-01-01T00:00:00ZQuantum cosmology and the evolution of inflationary spectrahttp://hdl.handle.net/11585/594918Titolo: Quantum cosmology and the evolution of inflationary spectra
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11585/5949182016-01-01T00:00:00ZMETHOD OF COMPARISON EQUATIONS AND GENERALIZED
ERMAKOV’S EQUATIONhttp://hdl.handle.net/11585/89825Titolo: METHOD OF COMPARISON EQUATIONS AND GENERALIZED
ERMAKOV’S EQUATION
Abstract: The Wentzel–Kramers–Brillouin (WKB) method is one of the main tools in quantum mechanics and quantum field theory. From the mathematical point of view, this method coincides with construction of asymptotic
expansions for a solution to the second-order differential equations with a small coefficient of the second derivative term . In the present work, we consider an interesting modification of the WKB method – the so-called method of comparison equations and analyze its relation with the Ermakov equation..
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11585/898252009-01-01T00:00:00ZChaotic spin precession in anisotropic universes and fermionic dark matterhttp://hdl.handle.net/11585/559904Titolo: Chaotic spin precession in anisotropic universes and fermionic dark matter
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11585/5599042016-01-01T00:00:00ZEffective action and heat kernel in a toy model of brane-induced gravityhttp://hdl.handle.net/11585/37031Titolo: Effective action and heat kernel in a toy model of brane-induced gravity
Abstract: We apply a recently suggested technique of the Neumann-Dirichlet reduction to a toy model of brane-induced gravity for the calculation of its quantum one-loop effective action. This model is represented by a massive scalar field in the (d+1)-dimensional flat bulk supplied with the d-dimensional kinetic term localized on a flat brane and mimicking the brane Einstein term of the Dvali-Gabadadze-Porrati (DGP) model. We obtain the inverse mass expansion of the effective action and its ultraviolet divergences which turn out to be nonvanishing for both even and odd spacetime dimensionality d. For the massless case, which corresponds to a limit of the toy DGP model, we obtain the Coleman-Weinberg type effective potential of the system. We also obtain the proper-time expansion of the heat kernel in this model associated with the generalized Neumann boundary conditions containing second-order tangential derivatives. We show that in addition to the usual integer and half-integer powers of the proper time this expansion exhibits, depending on the dimension d, either logarithmic terms or powers multiple of one quarter. This property is considered in the context of strong ellipticity of the boundary value problem, which can be violated when the Euclidean action of the theory is not positive definite.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11585/370312007-01-01T00:00:00ZQuantum cosmology with a big-brake singularityhttp://hdl.handle.net/11585/46903Titolo: Quantum cosmology with a big-brake singularity
Abstract: We investigate a cosmological model with a big-brake singularity in the future: while the first time derivative of the scale factor goes to zero, its second time derivative tends to minus infinity. Although we also discuss the classical version of the model in some detail, our main interest lies in its quantization. We formulate the Wheeler-DeWitt equation and derive solutions describing wave packets. We show that all such solutions vanish in the region of the classical singularity, a behavior which we interpret as singularity avoidance. We then discuss the same situation in loop quantum cosmology. While this leads to a different factor ordering, the singularity is avoided there, too.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11585/469032007-01-01T00:00:00Z