1,457 research outputs found
New interactions in the dark sector mediated by dark energy
Cosmological observations have revealed the existence of a dark matter sector, which is commonly assumed to be made up of one particle species only. However, this sector might be more complicated than we currently believe: there might be more than one dark matter species (for example, two components of cold dark matter or a mixture of hot and cold dark matter) and there may be new interactions between these particles. In this paper we study the possibility of multiple dark matter species and interactions mediated by a dark energy field. We study both the background and the perturbation evolution in these scenarios. We find that the background evolution of a system of multiple dark matter particles (with constant couplings) mimics a single fluid with a time-varying coupling parameter. However, this is no longer true on the perturbative level. We study the case of attractive and repulsive forces as well as a mixture of cold and hot dark matter particles
Slow roll inflation in the presence of a dark energy coupling
In models of coupled dark energy, in which a dark energy scalar field couples to other matter components, it is natural to expect a coupling to the inflaton as well. We explore the consequences of such a coupling in the context of single-field slow-roll inflation. Assuming an exponential potential for the quintessence field we show that the coupling to the inflaton causes the quintessence field to be attracted toward the minimum of the effective potential. If the coupling is large enough, the field is heavy and is located at the minimum. We show how this affects the expansion rate and the slow-roll of the inflaton field, and therefore the primordial perturbations generated during inflation. We further show that the coupling has an important impact on the processes of reheating and preheating
Magnetocaloric effect in nano- and polycrystalline manganite
samples were prepared in nano- and polycrystalline
forms by sol-gel and solid state reaction methods, respectively, and
structurally characterized by synchrotron X-ray diffraction. The magnetic
properties determined by ac susceptibility and dc magnetization measurements
are discussed. The magnetocaloric effect in this nanocrystalline manganite is
spread over a broader temperature interval than in the polycrystalline case.
The relative cooling power of the poly- and nanocrystalline manganites is used
to evaluate a possible application for magnetic cooling below room temperature.Comment: 6 pages, 5 (double) figures, 1 table, 16 references; submitted to
Appl. Phys.
Cosmic String Network Evolution in arbitrary Friedmann-Lemaitre models
We use the velocity-dependent one-scale model by Martins & Shellard to
investigate the evolution of a GUT long cosmic string network in arbitrary
Friedmann-Lemaitre models. Four representative models are used to show that in
general there is no scaling solution. The implications for structure formation
are briefly discussed.Comment: 8 pages, 4 postscript figures included, submitted to Phys. Rev.
Affine Constellations Without Mutually Unbiased Counterparts
It has been conjectured that a complete set of mutually unbiased bases in a
space of dimension d exists if and only if there is an affine plane of order d.
We introduce affine constellations and compare their existence properties with
those of mutually unbiased constellations, mostly in dimension six. The
observed discrepancies make a deeper relation between the two existence
problems unlikely.Comment: 8 page
The Initial Mass Function of the Stellar Association NGC 602 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations
We present our photometric study of the stellar association NGC 602 in the
wing of the Small Magellanic Cloud (SMC). The data were taken in the filters
F555W and F814W using the Advanced Camera for Surveys (ACS) on-board the Hubble
Space Telescope (HST). Photometry was performed using the ACS module of the
stellar photometry package DOLPHOT. We detected more than 5,500 stars with a
magnitude range of 14 \lsim m_{555} \lsim 28 mag. Three prominent stellar
concentrations are identified with star counts in the observed field, the
association NGC 602 itself, and two clusters, one of them not being currently
in any known catalog. The Color-Magnitude Diagrams (CMDs) of both clusters show
features typical for young open clusters, while that of the association reveals
bright main sequence (MS) and faint pre-main sequence (PMS) stars as the
members of the system. We construct the initial mass spectrum (IMS) of the
association by applying an age-independent method of counting the PMS stars
within evolutionary tracks, while for the bright MS stars we transform their
magnitudes to masses with the use of mass-luminosity relations. The IMS of NGC
602 is found to be well represented by a single-power law, corresponding to an
Initial Mass Function (IMF) of slope \Gamma\approx -1.2 for 1 \lsim M/M{\solar}
\lsim 45. This indicates that the shape of the IMF of a star forming system in
the SMC for stars with masses higher than 1 M{\solar} seems to be quite similar
to the field IMF in the solar neighborhood.Comment: Accepted for publication in ApJ, 13 pages, 14 figures, emulateapj.cls
LaTeX style, full resolution version available on
http://www.astro.uni-bonn.de/~dgoulier/Science/NGC602/ms.pd
Large-scale cosmological perturbations on the brane
In brane-world cosmologies of Randall-Sundrum type, we show that evolution of
large-scale curvature perturbations may be determined on the brane, without
solving the bulk perturbation equations. The influence of the bulk
gravitational field on the brane is felt through a projected Weyl tensor which
behaves effectively like an imperfect radiation fluid with anisotropic stress.
We define curvature perturbations on uniform density surfaces for both the
matter and Weyl fluids, and show that their evolution on large scales follows
directly from the energy conservation equations for each fluid. The total
curvature perturbation is not necessarily constant for adiabatic matter
perturbations, but can change due to the Weyl entropy perturbation. To relate
this curvature perturbation to the longitudinal gauge metric potentials
requires knowledge of the Weyl anisotropic stress which is not determined by
the equations on the brane. We discuss the implications for large-angle
anisotropies on the cosmic microwave background sky.Comment: 13 pages, latex with revtex, no figure
Scalar perturbations from brane-world inflation
We investigate the scalar metric perturbations about a de Sitter brane
universe in a 5-dimensional anti de Sitter bulk. We compare the master-variable
formalism, describing metric perturbations in a 5-dimensional longitudinal
gauge, with results in a Gaussian normal gauge. For a vacuum brane (with
constant brane tension) there is a continuum of normalizable Kaluza-Klein
modes, with m>3H/2, which remain in the vacuum state. A light radion mode, with
m=\sqrt{2}H, satisfies the boundary conditions for two branes but is not
normalizable in the single-brane case. When matter is introduced (as a test
field) on the brane, this mode, together with the zero-mode and an infinite
ladder of discrete tachyonic modes, become normalizable. However, the boundary
condition requires the self-consistent 4-dimensional evolution of scalar field
perturbations on the brane and the dangerous growing modes are not excited.
These normalizable discrete modes introduce corrections at first-order to the
scalar field perturbations computed in a slow-roll expansion. On super-Hubble
scales, the correction is smaller than slow-roll corrections to the de Sitter
background. However on small scales the corrections can become significant.Comment: 15 page
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