25,872 research outputs found
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
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
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
A redshift survey towards the CMB Cold Spot
We have carried out a redshift survey using the VIMOS spectrograph on the VLT
towards the Cosmic Microwave Background cold spot. A possible cause of the cold
spot is the Integrated Sachs-Wolfe effect imprinted by an extremely large void
(hundreds of Mpc in linear dimension) at intermediate or low redshifts. The
redshift distribution of over seven hundred z<1 emission-line galaxies drawn
from an I-band flux limited sample of galaxies in the direction of the cold
spot shows no evidence of a gap on scales of Delta-z> 0.05 as would be expected
if such a void existed at 0.35<z<1. There are troughs in the redshift
distribution on smaller scales (Delta-z ~0.01) indicating that smaller scale
voids may connect regions separated by several degrees towards the cold spot. A
comparison of this distribution with that generated from similarly-sized
subsamples drawn from widely-spaced pointings of the VVDS survey does not
indicate that the redshift distribution towards the cold spot is anomalous or
that these small gaps can be uniquely attributed to real voids.Comment: MNRAS in press, 6 page
What heavy quanta bounds could be inferred from a Higgs discovery?
The Higgs couplings can receive non-decoupling corrections due to heavy
quanta, and deviations from the SM can be used to test its presence. The
possible Higgs signal recently reported at LEP, with mh=115 GeV, severely
constrains the presence of heavy quanta, such as a heavy fourth family. At
Tevatron, the Higgs production by gluon fusion, followed by the decay h -> WW*,
can also be used to probe the existence of heavy colored particles, including
additional families, chiral sextet and octet quarks. Within the MSSM, we also
find that gluon fusion is a sensitive probe for the squark spectrum.Comment: 12 pages, 3 tables, 1 figure. Accepted in Mod. Phys. Lett. A (2001
Electrostatic attraction between cationic-anionic assemblies with surface compositional heterogeneities
Electrostatics plays a key role in biomolecular assembly. Oppositely charged
biomolecules, for instance, can co-assembled into functional units, such as DNA
and histone proteins into nucleosomes and actin-binding protein complexes into
cytoskeleton components, at appropriate ionic conditions. These
cationic-anionic co-assemblies often have surface charge heterogeneities that
result from the delicate balance between electrostatics and packing
constraints. Despite their importance, the precise role of surface charge
heterogeneities in the organization of cationic-anionic co-assemblies is not
well understood. We show here that co-assemblies with charge heterogeneities
strongly interact through polarization of the domains. We find that this leads
to symmetry breaking, which is important for functional capabilities, and
structural changes, which is crucial in the organization of co-assemblies. We
determine the range and strength of the attraction as a function of the
competition between the steric and hydrophobic constraints and electrostatic
interactions.Comment: JCP June/200
Anxiolytic effect of Mozart music over short and long photoperiods as part of environmental enrichment in captive Rattus norvegicus (Rodentia: Muridae)
Music is known to be able to elicit emotional changes, including anxiolytic effects on humans and animals. Photoperiod has also been reported to play an important role in the modulation of anxiety. In the present study, we examined whether the effect of music on anxiety is influenced by day length, comparing, short day (SD; 8:16 h light/dark) and long day (LD; 16:8 h light/dark) with controls (CD; 12:12 h light/dark). After 8 weeks of photoperiod treatment, rats were randomly assigned to 2 groups: silence and music. In the music group, rats were exposed to music 24 h before behavioral tests to quantify anxiety level. Exposure to Mozart music reduced anxiety in rats in the CD group. These effects of music were abolished by LD. Independently of music, rats exposed to SD exhibited higher levels of anxiety-like behavior than rats exposed to CD, in elevated plus-maze and open-field tests. The present findings suggest that the anxiolytic effects of Mozart music are photoperiod-dependent
Accelerating universes driven by bulk particles
We consider our universe as a 3d domain wall embedded in a 5d dimensional
Minkowski space-time. We address the problem of inflation and late time
acceleration driven by bulk particles colliding with the 3d domain wall. The
expansion of our universe is mainly related to these bulk particles. Since our
universe tends to be permeated by a large number of isolated structures, as
temperature diminishes with the expansion, we model our universe with a 3d
domain wall with increasing internal structures. These structures could be
unstable 2d domain walls evolving to fermi-balls which are candidates to cold
dark matter. The momentum transfer of bulk particles colliding with the 3d
domain wall is related to the reflection coefficient. We show a nontrivial
dependence of the reflection coefficient with the number of internal dark
matter structures inside the 3d domain wall. As the population of such
structures increases the velocity of the domain wall expansion also increases.
The expansion is exponential at early times and polynomial at late times. We
connect this picture with string/M-theory by considering BPS 3d domain walls
with structures which can appear through the bosonic sector of a
five-dimensional supergravity theory.Comment: To appear in Phys. Rev. D, 16 pages, 3 eps figures, minor changes and
references adde
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