Showing papers by "Oliver F. Piattella published in 2011"

Bulk viscous cosmology with causal transport theory

Abstract: We consider cosmological scenarios originating from a single imperfect fluid with bulk viscosity and apply Eckart's and both the full and the truncated M?ller-Israel-Stewart's theories as descriptions of the non-equilibrium processes. Our principal objective is to investigate if the dynamical properties of Dark Matter and Dark Energy can be described by a single viscous fluid and how such description changes when a causal theory (M?ller-Israel-Stewart's, both in its full and truncated forms) is taken into account instead of Eckart's non-causal one. To this purpose, we find numerical solutions for the gravitational potential and compare its behaviour with the corresponding ?CDM case. Eckart's and the full causal theory seem to be disfavoured, whereas the truncated theory leads to results similar to those of the ?CDM model for a bulk viscous speed in the interval 10?11 cb2 10?8.

Bulk viscous cosmology with causal transport theory

Abstract: We consider cosmological scenarios originating from a single imperfect fluid with bulk viscosity and apply Eckart's and both the full and the truncated Muller-Israel-Stewart's theories as descriptions of the non-equilibrium processes. Our principal objective is to investigate if the dynamical properties of Dark Matter and Dark Energy can be described by a single viscous fluid and how such description changes when a causal theory (Muller-Israel-Stewart's, both in its full and truncated forms) is taken into account instead of Eckart's non-causal theory. To this purpose, we find numerical solutions for the gravitational potential and compare its behaviour with the corresponding LambdaCDM case. Eckart's and the full causal theory seem to be disfavoured, whereas the truncated theory leads to results similar to those of the LambdaCDM model for a bulk viscous speed in the interval 10^{-11} << c_b^2 < 10^{-8}. Tentatively relating such value to a square propagation velocity of the order of T/m of perturbations in a non-relativistic gas of particles with mass m at the epoch of matter-radiation equality, this may be compatible with a mass range 0.1 GeV < m << 100 GeV.

Unified Dark Matter scalar field models with fast transition

Abstract: We investigate the general properties of Unified Dark Matter (UDM) scalar field models with Lagrangians with a non-canonical kinetic term, looking specifically for models that can produce a fast transition between an early Einstein-de Sitter CDM-like era and a later Dark Energy like phase, similarly to the barotropic fluid UDM models in JCAP01(2010)014. However, while the background evolution can be very similar in the two cases, the perturbations are naturally adiabatic in fluid models, while in the scalar field case they are necessarily non-adiabatic. The new approach to building UDM Lagrangians proposed here allows to escape the common problem of the fine-tuning of the parameters which plague many UDM models. We analyse the properties of perturbations in our model, focusing on the the evolution of the effective speed of sound and that of the Jeans length. With this insight, we can set theoretical constraints on the parameters of the model, predicting sufficient conditions for the model to be viable. An interesting feature of our models is that what can be interpreted as wDE can be < −1 without violating the null energy conditions.

Scalar models for the generalized Chaplygin gas and the structure formation constraints

Abstract: The generalized Chaplygin gas model represents an attempt to unify dark matter and dark energy. It is characterized by a fluid with the equation of state p = −A/ρ α . It can be obtained from a generalization of the Dirac-Born-Infeld (DBI) action for a scalar, tachyonic field. At a background level, this model gives very good results, but it suffers from many drawbacks at the perturbative level. We show that, while for background analysis it is possible to consider any value of α, the perturbative analysis must be restricted to positive values of α. This restriction can be circumvented if the origin of the generalized Chaplygin gas is traced back to a self-interacting scalar field, instead of the DBI action. But, in doing so, the predictions coming from formation of large-scale structures reduce the generalized Chaplygin gas model to a kind of quintessence model, and the unification scenario is lost if the scalar field is the canonical one. However, if the unification condition is imposed from the beginning as a prior, the model may remain competitive. More interesting results concerning the unification program are obtained if a non-canonical self-interacting scalar field, inspired by Rastall’s theory of gravity, is invoked. In this case, an agreement with the background tests is possible.

Reconstructing $f(R,T)$ gravity from holographic dark energy

Abstract: We consider cosmological scenarios based on $f(R,T)$ theories of gravity ($R$ is the Ricci scalar and $T$ is the trace of the energy-momentum tensor) and numerically reconstruct the function $f(R,T)$ which is able to reproduce the same expansion history generated, in the standard General Relativity theory, by dark matter and holographic dark energy. We consider two special $f(R,T)$ models: in the first instance, we investigate the modification $R + 2f(T)$, i.e. the usual Einstein-Hilbert term plus a $f(T)$ correction. In the second instance, we consider a $f(R)+\lambda T$ theory, i.e. a $T$ correction to the renown $f(R)$ theory of gravity.

CMB-Galaxy correlation in Unified Dark Matter Scalar Field Cosmologies

Abstract: We present an analysis of the cross-correlation between the CMB and the large-scale structure (LSS) of the Universe in Unified Dark Matter (UDM) scalar field cosmologies. We work out the predicted cross-correlation function in UDM models, which depends on the speed of sound of the unified component, and compare it with observations from six galaxy catalogues (NVSS, HEAO, 2MASS, and SDSS main galaxies, luminous red galaxies, and quasars). We sample the value of the speed of sound and perform a likelihood analysis, finding that the UDM model is as likely as the ΛCDM, and is compatible with observations for a range of values of c∞ (the value of the sound speed at late times) on which structure formation depends. In particular, we obtain an upper bound of c∞2 ≤ 0.009 at 95% confidence level, meaning that the ΛCDM model, for which c∞2 = 0, is a good fit to the data, while the posterior probability distribution peaks at the value c∞2 = 10−4 . Finally, we study the time dependence of the deviation from ΛCDM via a tomographic analysis using a mock redshift distribution and we find that the largest deviation is for low-redshift sources, suggesting that future low-z surveys will be best suited to constrain UDM models.

CMB-Galaxy correlation in Unified Dark Matter Scalar Field Cosmologies

Abstract: We present an analysis of the cross-correlation between the CMB and the large-scale structure (LSS) of the Universe in Unified Dark Matter (UDM) scalar field cosmologies. We work out the predicted cross-correlation function in UDM models, which depends on the speed of sound of the unified component, and compare it with observations from six galaxy catalogues (NVSS, HEAO, 2MASS, and SDSS main galaxies, luminous red galaxies, and quasars). We sample the value of the speed of sound and perform a likelihood analysis, finding that the UDM model is as likely as the LambdaCDM, and is compatible with observations for a range of values of c_\infinity (the value of the sound speed at late times) on which structure formation depends. In particular, we obtain an upper bound of c_\infinity^2 \leq 0.009 at 95% confidence level, meaning that the LambdaCDM model, for which c_\infinity^2 = 0, is a good fit to the data, while the posterior probability distribution peaks at the value c_\infinity^2=10^(-4) . Finally, we study the time dependence of the deviation from LambdaCDM via a tomographic analysis using a mock redshift distribution and we find that the largest deviation is for low-redshift sources, suggesting that future low-z surveys will be best suited to constrain UDM models.

High Energy Processes in the Vicinity of the Kerr's Black Hole Horizon

Abstract: Two particle collisions close to the horizon of the rotating nonextremal black hole are analyzed. It is shown that high energy of the order of the Grand Unification scale in the centre of mass of colliding particles can be obtained when there is a multiple collision – the particle from the accretion disc gets the critical momentum in first collision with the other particle close to the horizon and then there is a second collision of the critical particle with the ordinary one. High energy occurs due to a great relative velocity of two particles and a large Lorentz factor. The dependence of the relative velocity on the distance to horizon is analyzed, the time of movement from the point in the accretion disc to the point of scattering with large energy as well as the time of back movement to the Earth are calculated. It is shown that they have reasonable order.

High energy processes in the vicinity of the Kerr's black hole horizon

Abstract: Two particle collisions close to the horizon of the rotating nonextremal black hole are analyzed. It is shown that high energy of the order of the Grand Unification scale in the centre of mass of colliding particles can be obtained when there is a multiple collision - the particle from the accretion disc gets the critical momentum in first collision with the other particle close to the horizon and then there is a second collision of the critical particle with the ordinary one. High energy occurs due to a great relative velocity of two particles and a large Lorentz factor. The dependence of the relative velocity on the distance to horizon is analyzed, the time of movement from the point in the accretion disc to the point of scattering with large energy as well as the time of back movement to the Earth are calculated. It is shown that they have reasonable order.

Gravitational potential evolution in unified dark matter scalar field cosmologies: an analytical approach

Abstract: We investigate the time evolution of the gravitational potential Φ for a special class of non-adiabatic Unified Dark matter Models described by scalar field Lagrangians. These models predict the same background evolution as in the ΛCDM and possess a non-vanishing speed of sound. We provide a very accurate approximation of Φ, valid after the recombination epoch, in the form of a Bessel function of the first kind. This approximation may be useful for a future deeper analysis of Unified Dark Matter scalar field models.

Quantum properties of the Dirac field on BTZ black hole backgrounds

Abstract: We consider a Dirac field on a (1 + 2)-dimensional uncharged BTZ black hole background. We first find out the Dirac Hamiltonian, and study its self-adjointness properties. We find that, in analogy to the Kerr–Newman–AdS Dirac Hamiltonian in (1 + 3) dimensions, essential self-adjointness on C∞0(r+, ∞)2 of the reduced (radial) Hamiltonian is implemented only if a suitable relation between the mass μ of the Dirac field and the cosmological radius l holds true. The very presence of a boundary-like behaviour of r = ∞ is at the root of this problem. Also, we determine in a complete way the qualitative spectral properties for the non-extremal case, for which we can infer the absence of quantum bound states for the Dirac field. Next, we investigate the possibility of a quantum loss of angular momentum for the (1 + 2)-dimensional uncharged BTZ black hole. Unlike the corresponding stationary four-dimensional solutions, the formal treatment of the level crossing mechanism is much simpler. We find that, even in the extremal case, no level crossing takes place. Therefore, no quantum loss of angular momentum via particle pair production is allowed.

Gravitational potential evolution in Unified Dark Matter Scalar Field Cosmologies: an analytical approach

Abstract: We investigate the time evolution of the gravitational potential Phi for a special class of non-adiabatic Unified Dark matter Models described by scalar field lagrangians. These models predict the same background evolution as in the LambdaCDM and possess a non-vanishing speed of sound. We provide a very accurate approximation of Phi, valid after the recombination epoch, in the form of a Bessel function of the first kind. This approximation may be useful for a future deeper analysis of Unified Dark Matter scalar field models.

High energy processes in the vicinity of the kerr's black hole horizon

Abstract: Two particle collisions close to the horizon of the rotating nonextremal black hole are analyzed. It is shown that high energy of the order of the Grand Unification scale in the centre of mass of colliding particles can be obtained when there is a multiple collision – the particle from the accretion disc gets the critical momentum in first collision with the other particle close to the horizon and then there is a second collision of the critical particle with the ordinary one. High energy occurs due to a great relative velocity of two particles and a large Lorentz factor. The dependence of the relative velocity on the distance to horizon is analyzed, the time of movement from the point in the accretion disc to the point of scattering with large energy as well as the time of back movement to the Earth are calculated. It is shown that they have reasonable order.

Two variants of the step-harmonic potential

Abstract: Following the general scheme presented in [Am. J. Phys. 78 (8), 842–850 (2010)] we analyze the quantum-mechanical behavior of a system described by a one-dimensional asymmetric potential constituted by a step plus i) a linear barrier or ii) an exponential barrier. We explicitly solve the energy eigenvalue equation by means of the integral representation method, which allows us to classify the independent solutions as equivalence classes of homotopic paths in the complex plane. We discuss the structure of the bound states as function of the height U0 of the step and we study the propagation of a sharp-peaked wave packet reflected by the barrier. For both the linear and the exponential barrier we provide an explicit formula for the delay time �(E) as a function of the peak energy E. We display the resonant behavior of �(E) at energies close to U0. By analyzing the asymptotic behavior for large energies of the eigenfunctions of the continuous spectrum we also show that, as expected, �(E) approaches the classical value for E ! 1 , thus diverging for the step-linear case and vanishing for the step-exponential one.