76 research outputs found
Cosmological evolution of finite temperature Bose-Einstein Condensate dark matter
Once the temperature of a bosonic gas is smaller than the critical, density
dependent, transition temperature, a Bose - Einstein Condensation process can
take place during the cosmological evolution of the Universe. Bose - Einstein
Condensates are very strong candidates for dark matter, since they can solve
some major issues in observational astrophysics, like, for example, the
galactic core/cusp problem. The presence of the dark matter condensates also
drastically affects the cosmic history of the Universe. In the present paper we
analyze the effects of the finite dark matter temperature on the cosmological
evolution of the Bose-Einstein Condensate dark matter systems. We formulate the
basic equations describing the finite temperature condensate, representing a
generalized Gross-Pitaevskii equation that takes into account the presence of
the thermal cloud in thermodynamic equilibrium with the condensate. The
temperature dependent equations of state of the thermal cloud and of the
condensate are explicitly obtained in an analytical form. By assuming a flat
Friedmann-Robertson-Walker (FRW) geometry, the cosmological evolution of the
finite temperature dark matter filled Universe is considered in detail in the
framework of a two interacting fluid dark matter model, describing the
transition from the initial thermal cloud to the low temperature condensate
state. The dynamics of the cosmological parameters during the finite
temperature dominated phase of the dark matter evolution are investigated in
detail, and it is shown that the presence of the thermal excitations leads to
an overall increase in the expansion rate of the Universe.Comment: 14 pages, 11 figures, accepted for publication in PR
Generalized Langevin equation with colored noise description of the stochastic oscillations of accretion disks
We consider a description of the stochastic oscillations of the general
relativistic accretion disks around compact astrophysical objects interacting
with their external medium based on a generalized Langevin equation with
colored noise, which accounts for the general memory and retarded effects of
the frictional force, and on the fluctuation-dissipation theorem. The presence
of the memory effects influences the response of the disk to external random
interactions, and modifies the dynamical behavior of the disk, as well as the
energy dissipation processes. The generalized Langevin equation of the motion
of the disk in the vertical direction is studied numerically, and the vertical
displacements, velocities and luminosities of the stochastically perturbed
disks are explicitly obtained for both the Schwarzschild and the Kerr cases.
The Power Spectral Distribution (PSD) of the disk luminosity is also obtained.
As a possible astrophysical application of the formalism we investigate the
possibility that the Intra Day Variability (IDV) of the Active Galactic Nuclei
(AGN) may be due to the stochastic disk instabilities. The perturbations due to
colored/nontrivially correlated noise induce a complicated disk dynamics, which
could explain some astrophysical observational features related to disk
variability.Comment: 17 pages, 10 figures, accepted for publication in EPJ
Generalized Langevin equation description of the stochastic oscillations of general relativistic disks
We consider a description of the stochastic oscillations of the general
relativistic accretion disks around compact astrophysical objects based on the
generalized Langevin equation, which accounts for the general retarded effects
of the frictional force, and on the fluctuation-dissipation theorems. The
vertical displacements, velocities and luminosities of the stochastically
perturbed disks are explicitly obtained for both the Schwarzschild and the Kerr
cases. The Power Spectral Distribution of the luminosity it is also obtained,
and it is shown that it has non-standard values. The theoretical predictions of
the model are compared with the observational data for the luminosity time
variation of the BL Lac S5 0716+714 object.Comment: 4 pages, two figures, accepted for publication in Journal of
Astrophysics and Astronom
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