4,557 research outputs found
Cosmological simulations with disformally coupled symmetron fields
We investigate statistical properties of the distribution of matter at
redshift zero in disformal gravity by using N-body simulations. The disformal
model studied here consists of a conformally coupled symmetron field with an
additional exponential disformal term. We conduct cosmological simulations to
discover the impact of the new disformal terms in the matter power spectrum,
halo mass function, and radial profile of the scalar field. We calculated the
disformal geodesic equation and the equation of motion for the scalar field. We
then implemented these equations into the N-body code ISIS, which is a modified
gravity version of the code RAMSES. The presence of a conformal symmetron field
increases both the power spectrum and mass function compared to standard
gravity on small scales. Our main finding is that the newly added disformal
terms tend to counteract these effects and can make the evolution slightly
closer to standard gravity. We finally show that the disformal terms give rise
to oscillations of the scalar field in the centre of the dark matter haloes.Comment: Updated version to reflect the journal accepted paper. Added one
figure. 7 pages, 7 figure
Very large scale structures in growing neutrino quintessence
A quintessence scalar field or cosmon interacting with neutrinos can have
important effects on cosmological structure formation. Within growing neutrino
models the coupling becomes effective only in recent times, when neutrinos
become non-relativistic, stopping the evolution of the cosmon. This can explain
why dark energy dominates the universe only in a rather recent epoch by
relating the present dark energy density to the small mass of neutrinos. Such
models predict the presence of stable neutrino lumps at supercluster scales
(~200 Mpc and bigger), caused by an attractive force between neutrinos which is
stronger than gravity and mediated by the cosmon. We present a method to follow
the initial non-linear formation of neutrino lumps in physical space, by
integrating numerically on a 3D grid non-linear evolution equations, until
virialization naturally occurs. As a first application, we show results for
cosmologies with final large neutrino average mass ~2 eV: in this case,
neutrino lumps indeed form and mimic very large cold dark matter structures,
with a typical gravitational potential 10^{-5} for a lump size ~10 Mpc, and
reaching larger values for lumps of about 200 Mpc. A rough estimate of the
cosmological gravitational potential at small k in the non-linear regime,
Phi_nu = 10^{-6} (k/k_0)^{-2}, 1.2x10^{-2} h/Mpc < k_0 < 7.8x10^{-2} h/Mpc,
turns out to be many orders of magnitude smaller than an extrapolation of the
linear evolution of density fluctuations. The size of the neutrino-induced
gravitational potential could modify the spectrum of CMB anisotropies for small
angular momenta.Comment: 17 pages, 16 figures, accepted for publication in Physical Review D,
minor changes and correction
Phase Transition and Monopoles Densities in a Nearest Neighbors Two-Dimensional Spin Ice Model
In this work, we show that, due to the alternating orientation of the spins
in the ground state of the artificial square spin ice, the influence of a set
of spins at a certain distance of a reference spin decreases faster than the
expected result for the long range dipolar interaction, justifying the use of
the nearest neighbor two dimensional square spin ice model as an effective
model. Using an extension of the model presented in ref. [Scientific Reports 5,
15875 (2015)], considering the influence of the eight nearest neighbors of each
spin on the lattice, we analyze the thermodynamics of the model and study the
monopoles and string densities dependence as a function of the temperature.Comment: 11 pages, 8 figure
On virialization with dark energy
We review the inclusion of dark energy into the formalism of spherical
collapse, and the virialization of a two-component system, made of matter and
dark energy. We compare two approaches in previous studies. The first assumes
that only the matter component virializes, e.g. as in the case of a classic
cosmological constant. The second approach allows the full system to virialize
as a whole. We show that the two approaches give fundamentally different
results for the final state of the system. This might be a signature
discriminating between the classic cosmological constant which cannot virialize
and a dynamical dark energy mimicking a cosmological constant. This signature
is independent of the measured value of the equation of state. An additional
issue which we address is energy non-conservation of the system, which
originates from the homogeneity assumption for the dark energy. We propose a
way to take this energy loss into account.Comment: 15 pages, 5 figures. Accepted for publication in JCA
The cosmological behavior of Bekenstein's modified theory of gravity
We study the background cosmology governed by the Tensor-Vector-Scalar theory
of gravity proposed by Bekenstein. We consider a broad family of potentials
that lead to modified gravity and calculate the evolution of the field
variables both numerically and analytically. We find a range of possible
behaviors, from scaling to the late time domination of either the additional
gravitational degrees of freedom or the background fluid.Comment: 10 pages, 8 figures, A few typos corrected in the text and figures.
Version published in PR
Diamagnetic response of cylindrical normal metal - superconductor proximity structures with low concentration of scattering centers
We have investigated the diamagnetic response of composite NS proximity
wires, consisting of a clean silver or copper coating, in good electrical
contact to a superconducting niobium or tantalum core. The samples show strong
induced diamagnetism in the normal layer, resulting in a nearly complete
Meissner screening at low temperatures. The temperature dependence of the
linear diamagnetic susceptibility data is successfully described by the
quasiclassical Eilenberger theory including elastic scattering characterised by
a mean free path l. Using the mean free path as the only fit parameter we found
values of l in the range 0.1-1 of the normal metal layer thickness d_N, which
are in rough agreement with the ones obtained from residual resistivity
measurements. The fits are satisfactory over the whole temperature range
between 5 mK and 7 K for values of d_N varying between 1.6 my m and 30 my m.
Although a finite mean free path is necessary to correctly describe the
temperature dependence of the linear response diamagnetic susceptibility, the
measured breakdown fields in the nonlinear regime follow the temperature and
thickness dependence given by the clean limit theory. However, there is a
discrepancy in the absolute values. We argue that in order to reach
quantitative agreement one needs to take into account the mean free path from
the fits of the linear response. [PACS numbers: 74.50.+r, 74.80.-g]Comment: 10 pages, 9 figure
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