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.Fri, 04 Dec 2020 02:14:53 GMT2020-12-04T02:14:53Z101141A Non-Singular One-Loop Wave Function of the Universe From a New Eigenvalue Asymptotics in Quantum Gravityhttp://hdl.handle.net/11585/23711.1Titolo: A Non-Singular One-Loop Wave Function of the Universe From a New Eigenvalue Asymptotics in Quantum Gravity
Abstract: Recent work on Euclidean quantum gravity on the four-ball has proved regularity at the origin of the generalized zeta-function built from eigenvalues for metric and ghost modes, when diffeomorphism-invariant boundary conditions are imposed in the de Donder gauge. The hardest part of the analysis involves one of the four sectors for scalar-type perturbations, the eigenvalues of which are obtained by squaring up roots of a linear combination of Bessel functions of integer adjacent orders, with a coefficient of linear combination depending on the unknown roots. This paper obtains, first, approximate analytic formulae for such roots for all values of the order of Bessel functions. For this purpose, both the descending series for Bessel functions and their uniform asymptotic expansion at large order are used. The resulting generalized zeta-function is also built, and another check of regularity at the origin is obtained. For the first time in the literature on quantum gravity on manifolds with boundary, a vanishing one-loop wave function of the Universe is found in the limit of small three-geometry, which suggests a quantum avoidance of the cosmological singularity driven by full diffeomorphism invariance of the boundary-value problem for one-loop quantum theory.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/11585/23711.12005-01-01T00:00:00ZStochastic cosmology, theories of perturbations and Lifshitz gravityhttp://hdl.handle.net/11585/526725Titolo: Stochastic cosmology, theories of perturbations and Lifshitz gravity
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11585/5267252015-01-01T00:00:00ZSpherical "top-hat" collapse in general-Chaplygin-gas-dominated universeshttp://hdl.handle.net/11585/115177Titolo: Spherical "top-hat" collapse in general-Chaplygin-gas-dominated universes
Abstract: We expand previous works on the spherical `top-hat' collapse (SC-TH) framework in generalized Chaplygin gas (gCg) dominated universes. Here we allow the collapse in all energetic components within the system. We analyze the non-linear stages of collapse for various choices of parameter alpha of the gCg model introducing an exact formulation for the so-called effective sound speed. We show that, within the SC-TH framework, the growth of the structure becomes faster with increasing values of alpha.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/11585/1151772012-01-01T00:00:00ZDynamical dark energy and spontaneously generated gravityhttp://hdl.handle.net/11585/119114.1Titolo: Dynamical dark energy and spontaneously generated gravity
Abstract: We study the cosmological evolution of an induced gravity model with a scale symmetry breaking
potential for the scalar field and the presence of barotropic fluids. The radiation to matter transition,
following inflation and reheating, influences the dynamics of such a field through its non-minimal
coupling. Indeed one finds, as a consequence of such a transition, that the scalar field is shifted from
the potential minimum (which is associated with a zero cosmological constant). We illustrate how, under
certain conditions on the potential, such a dynamics can lead to a suitable amount of dark energy
explaining the present accelerated expansion. In such an approach, however, for long enough times, the
dark energy will disappear.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/11585/119114.12012-01-01T00:00:00ZParadox of soft singularity crossing and its resolution by distributional cosmological quantitities.http://hdl.handle.net/11585/127030Titolo: Paradox of soft singularity crossing and its resolution by distributional cosmological quantitities.
Abstract: A cosmological model of a flat Friedmann universe filled with a mixture of anti-Chaplygin gas and dust-like matter exhibits a future soft singularity, where the pressure of the anti-Chaplygin gas diverges (while its energy density is finite). Despite infinite tidal forces the geodesics pass through the singularity. Due to the dust component, the Hubble parameter has a non-zero value at the encounter with the singularity, therefore the dust implies further expansion. With continued expansion however, the energy density and the pressure of the anti-Chaplygin gas would become ill-defined, hence from the point of view of the anti-Chaplygin gas only a contraction is allowed. Paradoxically, the universe in this cosmological model would have to expand and contract simultaneously. This obviosly could not happen. We solve the paradox by redefining the anti-Chaplygin gas in a distributional sense. Then a contraction could follow the expansion phase at the singularity at the price of a jump in the Hubble parameter. Although such an abrupt change is not common in any cosmological evolution, we explicitly show that the set of Friedmann, Raychaudhuri and continuity equations are all obeyed both at the singularity and in its vicinity. We also prove that the Israel junction conditions are obeyed through the singular spatial hypersurface. In particular we enounce and prove a more general form of the Lanczos equation.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/11585/1270302012-01-01T00:00:00ZScalar field potentials for closed and open cosmological models.http://hdl.handle.net/11585/126181Titolo: Scalar field potentials for closed and open cosmological models.
Abstract: We develop a technique for the reconstruction of the potential for a scalar field or a tachyon field, reproducing a given cosmological evolution in a closed and open isotropic cosmological models. Such potentials are explicitly written down for the cases of the evolutions driven by a generic barotropic fluid and by radiation plus a cosmological constant, for the case of a scalar field. For tachyon and pseudo-tachyon fields the potentials are reconstructed for some special cases, corresponding to particular values of the barotropic index.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/11585/1261812012-01-01T00:00:00ZComment about the vanishing of the vacuum energy in the Wess-Zumino modelhttp://hdl.handle.net/11585/645189Titolo: Comment about the vanishing of the vacuum energy in the Wess-Zumino model
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11585/6451892018-01-01T00:00:00ZQuantum gravity, time, bounces, and matterhttp://hdl.handle.net/11585/645195.1Titolo: Quantum gravity, time, bounces, and matter
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/11585/645195.12018-01-01T00:00:00ZInflation scenario via the Standard Model Higgs boson and LHC.http://hdl.handle.net/11585/65185Titolo: Inflation scenario via the Standard Model Higgs boson and LHC.
Abstract: We consider a quantum corrected inflation scenario driven by a generic GUT or Standard Model type particle model whose scalar field playing the role of an inflaton has a strong non-minimal coupling to gravity. We show that currently widely accepted bounds on the Higgs mass falsify the suggestion of the paper arXiv:0710.3755 (where the role of radiative corrections was underestimated) that the Standard Model Higgs boson can serve as the inflaton. However, if the Higgs mass could be raised to $sim 230$ GeV, then the Standard Model could generate an inflationary scenario with the spectral index of the primordial perturbation spectrum $n_ssimeq 0.935$ (barely matching present observational data) and the very low tensor-to-scalar perturbation ratio $rsimeq 0.0006$.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/651852008-01-01T00:00:00ZThe Higgs field as an inflatonhttp://hdl.handle.net/11585/127031Titolo: The Higgs field as an inflaton
Abstract: It is shown that the Higgs boson can also serve as an inflaton in the early Universe if one assumes its strong non-minimal coupling to gravity and takes quantum corrections from Standard Model particles to the tree action into account. This builds a bridge between modern cosmology and particle physics without introducing any new particles. We predict the range for allowed values of the Higgs mass in this scenario which can be tested at LHC. This contribution is based on the recent work.^1
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/11585/1270312012-01-01T00:00:00Z