30,487 research outputs found
Bouncing solutions from generalized EoS
We present an exact analytical bouncing solution for a closed universe filled
with only one exotic fluid with negative pressure, obeying a Generalized
Equations of State (GEoS) of the form , where
, and are constants. In our solution and
and is kept as a free parameter. For particular values of
the initial conditions, we obtain that our solution obeys Null Energy Condition
(NEC), which allows us to reinterpret the matter source as that of a real
scalar field, , with a positive kinetic energy and a potential .
We compute numerically the scalar field as a function of time as well as its
potential , and find an analytical function for the potential that
fits very accurately with the numerical results obtained. The shape of this
potential can be well described by a Gaussian-type of function, and hence,
there is no spontaneous symmetry minimum of . We further show that the
bouncing scenario is structurally stable under small variations of the
parameter , such that a family of bouncing solutions can be find
numerically, in a small vicinity of the value .Comment: 12 pages, 12 figure
The golden ratio in Schwarzschild-Kottler black holes
In this paper we show that the golden ratio is present in the
Schwarzschild-Kottler metric. For null geodesics with maximal radial
acceleration, the turning points of the orbits are in the golden ratio . This is a general result which is independent of the value and
sign of the cosmological constant
Density excitations of a harmonically trapped ideal gas
The dynamic structure factor of a harmonically trapped Bose gas has been
calculated well above the Bose-Einstein condensation temperature by treating
the gas cloud as a canonical ensemble of noninteracting classical particles.
The static structure factor is found to vanish as wavenumber squared in the
long-wavelength limit. We also incorporate a relaxation mechanism
phenomenologically by including a stochastic friction force to study the
dynamic structure factor. A significant temperature dependence of the
density-fluctuation spectra is found. The Debye-Waller factor has been
calculated for the trapped thermal cloud as function of wavenumber and of
particle number. A substantial difference is found between clouds of small and
large particle number
The nuclear contacts and short range correlations in nuclei
Atomic nuclei are complex strongly interacting systems and their exact
theoretical description is a long-standing challenge. An approximate
description of nuclei can be achieved by separating its short and long range
structure. This separation of scales stands at the heart of the nuclear shell
model and effective field theories that describe the long-range structure of
the nucleus using a mean- field approximation. We present here an effective
description of the complementary short-range structure using contact terms and
stylized two-body asymptotic wave functions. The possibility to extract the
nuclear contacts from experimental data is presented. Regions in the two-body
momentum distribution dominated by high-momentum, close-proximity, nucleon
pairs are identified and compared to experimental data. The amount of
short-range correlated (SRC) nucleon pairs is determined and compared to
measurements. Non-combinatorial isospin symmetry for SRC pairs is identified.
The obtained one-body momentum distributions indicate dominance of SRC pairs
above the nuclear Fermi-momentum.Comment: Accepted for publication in Physics Letters. 6 pages, 2 figure
Testing a dissipative kinetic k-essence model
In this work, we present a study of a purely kinetic k-essence model,
characterized basically by a parameter in presence of a bulk
dissipative term, whose relationship between viscous pressure and energy
density of the background follows a polytropic type law , where , in principle, is a parameter without
restrictions. Analytical solutions for the energy density of the k-essence
field are found in two specific cases: and
, and then we show that these solutions posses the
same functional form than the non-viscous counterpart. Finally, both approach
are contrasted with observational data from type Ia supernova, and the most
recent Hubble parameter measurements, and therefore, the best values for the
parameters of the theory are founds.Comment: 9 pages, 5 figures, accepted in EPJ
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