15,202 research outputs found
Spectra of primordial fluctuations in two-perfect-fluid regular bounces
We introduce analytic solutions for a class of two components bouncing
models, where the bounce is triggered by a negative energy density perfect
fluid. The equation of state of the two components are constant in time, but
otherwise unrelated. By numerically integrating regular equations for scalar
cosmological perturbations, we find that the (would be) growing mode of the
Newtonian potential before the bounce never matches with the the growing mode
in the expanding stage. For the particular case of a negative energy density
component with a stiff equation of state we give a detailed analytic study,
which is in complete agreement with the numerical results. We also perform
analytic and numerical calculations for long wavelength tensor perturbations,
obtaining that, in most cases of interest, the tensor spectral index is
independent of the negative energy fluid and given by the spectral index of the
growing mode in the contracting stage. We compare our results with previous
investigations in the literature.Comment: 11 pages, 5 figure
Plane Gravitational Radiation from Neutrinos Source with Kalb-Ramond Coupling
In this work, we propose a model based on a non-minimal coupling of neutrinos
to a Kalb-Ramond field. The latter is taken as a possible source for
gravitational radiation. As an immediate illustration of this system, we have
studied the case where gravitational (plane) wave solutions behave as damped
harmonic oscillators.Comment: Presented at 7th Alexander Friedmann International Seminar on
Gravitation and Cosmology, Joao Pessoa, Brazil, 29-05 Jul 200
Conductivity of suspended and non-suspended graphene at finite gate voltage
We compute the DC and the optical conductivity of graphene for finite values
of the chemical potential by taking into account the effect of disorder, due to
mid-gap states (unitary scatterers) and charged impurities, and the effect of
both optical and acoustic phonons. The disorder due to mid-gap states is
treated in the coherent potential approximation (CPA, a self-consistent
approach based on the Dyson equation), whereas that due to charged impurities
is also treated via the Dyson equation, with the self-energy computed using
second order perturbation theory. The effect of the phonons is also included
via the Dyson equation, with the self energy computed using first order
perturbation theory. The self-energy due to phonons is computed both using the
bare electronic Green's function and the full electronic Green's function,
although we show that the effect of disorder on the phonon-propagator is
negligible. Our results are in qualitative agreement with recent experiments.
Quantitative agreement could be obtained if one assumes water molelcules under
the graphene substrate. We also comment on the electron-hole asymmetry observed
in the DC conductivity of suspended graphene.Comment: 13 pages, 11 figure
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