Optimized Constant Pressure Stochastic Dynamics

A recently proposed method for computer simulations in the isothermal-isobaric (NPT) ensemble, based on Langevin-type equations of motion for the particle coordinates and the ``piston'' degree of freedom, is re-derived by straightforward application of the standard Kramers-Moyal formalism. An integration scheme is developed which reduces to a time-reversible symplectic integrator in the limit of vanishing friction. This algorithm is hence expected to be quite stable for small friction, allowing for a large time step. We discuss the optimal choice of parameters, and present some numerical test results.Comment: 16 pages, 2 figures, submitted to J. Chem. Phy

Dilaton dominance in the early Universe dilutes Dark Matter relic abundances

The role of the dilaton field and its coupling to matter may result to a dilution of Dark Matter (DM) relic densities. This is to be contrasted with quintessence scenarios in which relic densities are augmented, due to modification of the expansion rate, since Universe is not radiation dominated at DM decoupling. Dilaton field, besides this, affects relic densities through its coupling to dust which tends to decrease relic abundances. Thus two separate mechanisms compete each other resulting, in general, to a decrease of the relic density. This feature may be welcome and can rescue the situation if Direct Dark Matter experiments point towards small neutralino-nucleon cross sections, implying small neutralino annihilation rates and hence large relic densities, at least in the popular supersymmetric scenarios. In the presence of a diluting mechanism both experimental constraints can be met. The role of the dilaton for this mechanism has been studied in the context of the non-critical string theory but in this work we follow a rather general approach assuming that the dilaton dominates only at early eras long before Big Bang Nucleosynthesis.Comment: 11 pages, Latex, 4 figures: Comments and references added, version to appear in Phys. Rev.

Non-critical String Cosmologies

Non-critical String Cosmologies are offered as an alternative to Standard Big Bang Cosmology. The new features encompassed within the dilaton dependent non-critical terms affect the dynamics of the Universe\'s evolution in an unconventional manner being in agreement with the cosmological data. Non-criticality is responsible for a late transition to acceleration at redshifts z=0.2. The role of the uncoupled rolling dilaton to relic abundance calculations is discussed. The uncoupled rolling dilaton dilutes the neutralino relic densities in supersymmetric theories by factors of ten, relaxing considerably the severe WMAP Dark Matter constraints, while at the same time leaves almost unaffected the baryon density in agreement with primordial Nucleosynthesis.Comment: 16 pages, 7 figures, conference tal

Nonthermal Supermassive Dark Matter

We discuss several cosmological production mechanisms for nonthermal supermassive dark matter and argue that dark matter may be elementary particles of mass much greater than the weak scale. Searches for dark matter should not be limited to weakly interacting particles with mass of the order of the weak scale, but should extend into the supermassive range as well.Comment: 11 page LaTeX file. No major changes. Version accepted by PR

Constraints on radiative decay of the 17-keV neutrino from COBE Measurements

It is shown that, for a nontrivial radiative decay channel of the 17-keV neutrino, the photons would distort the microwave background radiation through ionization of the universe. The constraint on the branching ratio of such decays from COBE measurements is found to be more stringent than that from other considerations. The limit on the branching ratio in terms of the Compton $y$ parameter is $B_\gamma < 1.5 \times 10^{-7} ({\tau_\nu \over 10^{11} sec})^{0.45} ({y \over 10^{-3}})^{1.11} h^{-1}$ for an $\Omega=1, \Omega_b=0.1$ universe.Comment: 7 pages. (figures will be sent on request) (To appear in Phys. Rev. D.

Validation Through Simulations of a Cn2 Profiler for the ESO/VLT Adaptive Optics Facility

The Adaptive Optics Facility (AOF) project envisages transforming one of the VLT units into an adaptive telescope and providing its ESO (European Southern Observatory) second generation instruments with turbulence corrected wavefronts. For MUSE and HAWK-I this correction will be achieved through the GALACSI and GRAAL AO modules working in conjunction with a 1170 actuators Deformable Secondary Mirror (DSM) and the new Laser Guide Star Facility (4LGSF). Multiple wavefront sensors will enable GLAO and LTAO capabilities, whose performance can greatly benefit from a knowledge about the stratification of the turbulence in the atmosphere. This work, totally based on end-to-end simulations, describes the validation tests conducted on a Cn2 profiler adapted for the AOF specifications. Because an absolute profile calibration is strongly dependent on a reliable knowledge of turbulence parameters r0 and L0, the tests presented here refer only to normalized output profiles. Uncertainties in the input parameters inherent to the code are tested as well as the profiler response to different turbulence distributions. It adopts a correction for the unseen turbulence, critical for the GRAAL mode, and highlights the effects of masking out parts of the corrected wavefront on the results. Simulations of data with typical turbulence profiles from Paranal were input to the profiler, showing that it is possible to identify reliably the input features for all the AOF modes.Comment: 15 pages, 12 figures, accepted for publication in the MNRAS Accepted 2015 January 22. Received 2015 January 21; in original form 2014 December

Scattering of Dirac and Majorana Fermions off Domain Walls

We investigate the interaction of fermions having both Dirac and left-handed and right-handed Majorana mass terms with vacuum domain walls. By solving the equations of motion in thin-wall approximation, we calculate the reflection and transmission coefficients for the scattering of fermions off walls.Comment: 6 pages, 1 figure, some typos corrected, one reference added, major revisions, title changed, version accepted for publication in Phys. Rev.

Effects of Unstable Dark Matter on Large-Scale Structure and Constraints from Future Surveys

In this paper we explore the effect of decaying dark matter (DDM) on large-scale structure and possible constraints from galaxy imaging surveys. DDM models have been studied, in part, as a way to address apparent discrepancies between the predictions of standard cold dark matter models and observations of galactic structure. Our study is aimed at developing independent constraints on these models. In such models, DDM decays into a less massive, stable dark matter (SDM) particle and a significantly lighter particle. The small mass splitting between the parent DDM and the daughter SDM provides the SDM with a recoil or "kick" velocity vk, inducing a free-streaming suppression of matter fluctuations. This suppression may be probed via weak lensing power spectra measured by a number of forthcoming imaging surveys that aim primarily to constrain dark energy. Using scales on which linear perturbation theory alone is valid (multipoles < 300), surveys like Euclid or LSST can be sensitive to vk > 90 km/s for lifetimes ~ 1-5 Gyr. To estimate more aggressive constraints, we model nonlinear corrections to lensing power using a simple halo evolution model that is in good agreement with numerical simulations. In our most ambitious forecasts, using multipoles < 3000, we find that imaging surveys can be sensitive to vk ~ 10 km/s for lifetimes < 10 Gyr. Lensing will provide a particularly interesting complement to existing constraints in that they will probe the long lifetime regime far better than contemporary techniques. A caveat to these ambitious forecasts is that the evolution of perturbations on nonlinear scales will need to be well calibrated by numerical simulations before they can be realized. This work motivates the pursuit of such a numerical simulation campaign to constrain dark matter with cosmological weak lensing.Comment: 15 pages, 7 figures. Submitted to PR