9,724 research outputs found
Astrophysical Configurations with Background Cosmology: Probing Dark Energy at Astrophysical Scales
We explore the effects of a positive cosmological constant on astrophysical
and cosmological configurations described by a polytropic equation of state. We
derive the conditions for equilibrium and stability of such configurations and
consider some astrophysical examples where our analysis may be relevant. We
show that in the presence of the cosmological constant the isothermal sphere is
not a viable astrophysical model since the density in this model does not go
asymptotically to zero. The cosmological constant implies that, for polytropic
index smaller than five, the central density has to exceed a certain minimal
value in terms of the vacuum density in order to guarantee the existence of a
finite size object. We examine such configurations together with effects of
in other exotic possibilities, such as neutrino and boson stars, and
we compare our results to N-body simulations. The astrophysical properties and
configurations found in this article are specific features resulting from the
existence of a dark energy component. Hence, if found in nature would be an
independent probe of a cosmological constant, complementary to other
observations.Comment: 23 pages, 11 figures, 2 tables. Reference added. Mon. Not. Roy.
Astro. Soc in prin
Socialism without liberation: Land Reclamation Projects in Guinea-Bissau
One of the outstanding aims of most liberation movements has been to increase the economic well-being of their people, Guinea-Bissau being no exception in this respect. How far has the new Nation State succeeded in fulfilling this aim? A comparative analysis of the implementation of land reclamation projects during colonial and post-colonial times reveals astonishing similarities: especially the centralization of development efforts in the hands of administrators disconnected from the grassroots, lack of target group analysis and misconceptions about the aims and needs, as well as the resources, of the population involved in the development efforts, on the part of the administration. The effects of this negative conditioning process of 'development' over many years on the chances of cooperation between peasants and the administration are still largely unknown. Any development planner who wants to encourage the local population to take their future into their own hands, would have to take account of this negative conditioning process
Open source environment to define constraints in route planning for GIS-T
Route planning for transportation systems is strongly related to shortest path algorithms, an optimization problem extensively studied in the literature. To find the shortest path in a network one usually assigns weights to each branch to represent the difficulty of taking such branch. The weights construct a linear preference function ordering the variety of alternatives from the most to the least attractive.Postprint (published version
Transition into a low temperature superconducting phase of unconventional pinning in Sr_2RuO_4
We have found a sharp transition in the vortex creep rates at a temperature
T^\ast=0.05 T_c in a single crystal of Sr_2RuO_4 (T_{c}=1.03 K) by means of
magnetic relaxation measurements. For T<T^\ast, the initial creep rates drop to
undetectable low levels. One explanation for this transition into a phase with
such extremely low vortex creep is that the low-temperature phase of Sr_2RuO_4
breaks time reversal symmetry. In that case, degenerate domain walls separating
discreetly degenerate states of a superconductor can act as very strong pinning
centers.Comment: 3 pages, 2 figure
Big-bang nucleosynthesis and gamma-ray constraints on cosmic strings with a large Higgs condensate
We consider constraints on cosmic strings from their emission of Higgs particles, in the case that the strings have a Higgs condensate with amplitude of order the string mass scale, assuming that a fraction of the energy of the condensate can be turned into radiation near cusps. The injection of energy by the decaying Higgs particles affects the light element abundances predicted by standard big-bang nucleosynthesis (BBN) and also contributes to the diffuse gamma-ray background (DGRB) in the Universe today. We examine the two main string scenarios (Nambu-Goto and field theory) and find that the primordial helium and deuterium abundances strongly constrain the string tension and the efficiency of the emission process in the NG scenario, while the strongest BBN constraint in the FT scenario comes from the deuterium abundance. The Fermi-LAT measurement of the DGRB constrains the field theory scenario even more strongly than previously estimated from EGRET data, requiring that the product of the string tension μ and Newton’s constant G is bounded by Gμ≲2.7×10−11β−2ft, where β2ft is the fraction of the strings’ energy going into Higgs particles
ISIS: a new N-body cosmological code with scalar fields based on RAMSES. Code presentation and application to the shapes of clusters
Several extensions of the standard cosmological model include scalar fields
as new degrees of freedom in the underlying gravitational theory. A particular
class of these scalar field theories include screening mechanisms intended to
hide the scalar field below observational limits in the solar system, but not
on galactic scales, where data still gives freedom to find possible signatures
of their presence. In order to make predictions to compare with observations
coming from galactic and clusters scales (i.e. in the non-linear regime of
cosmological evolution), cosmological N-body simulations are needed, for which
codes that can solve for the scalar field must be developed. We present a new
implementation of scalar-tensor theories of gravity which include screening
mechanisms. The code is based in the already existing code RAMSES, to which we
have added a non-linear multigrid solver that can treat a large class of scalar
tensor theories of modified gravity. We present details of the implementation
and the tests that we made to it. As application of the new code, we have
studied the influence that two particular modified gravity theories, the
symmetron and gravity, have on the shape of cluster sized dark matter
halos and found consistent results with previous estimations made with a static
analysis.Comment: 13 pages, 6 figures, matches version accepted for publication in A&
Cosmology of Chameleons with Power-Law Couplings
In chameleon field theories a scalar field can couple to matter with
gravitational strength and still evade local gravity constraints due to a
combination of self-interactions and the couplings to matter. Originally, these
theories were proposed with a constant coupling to matter, however, the
chameleon mechanism also extends to the case where the coupling becomes
field-dependent. We study the cosmology of chameleon models with power-law
couplings and power-law potentials. It is found that these generalized
chameleons, when viable, have a background expansion very close to LCDM, but
can in some special cases enhance the growth of the linear perturbations at low
redshifts. For the models we consider it is found that this region of the
parameter space is ruled out by local gravity constraints. Imposing a coupling
to dark matter only, the local constraints are avoided, and it is possible to
have observable signatures on the linear matter perturbations.Comment: 12 pages, 5 figures. ApJ in prin
Degeneracies between Modified Gravity and Baryonic Physics
In order to determine the observable signatures of modified gravity theories,
it is important to consider the effect of baryonic physics. We use a modified
version of the ISIS code to run cosmological hydrodynamic simulations to study
degeneracies between modified gravity and radiative hydrodynamical processes.
Of these, one was the standard CDM model and four were variations of
the Symmetron model. For each model we ran three variations of baryonic
processes: non-radiative hydrodynamics; cooling and star formation; and
cooling, star formation, and supernova feedback. We construct stacked gas
density, temperature, and dark matter density profiles of the halos in the
simulations, and study the differences between them. We find that both
radiative variations of the models show degeneracies between their processes
and at least two of the three parameters defining the Symmetron model.Comment: 9 pages, 4 figures, matches version accepted to A&
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