64 research outputs found
Tracing the Nature of Dark Energy with Galaxy Distribution
Dynamical Dark Energy (DE) is a viable alternative to the cosmological
constant. Yet, constructing tests to discriminate between Lambda and dynamical
DE models is difficult because the differences are not large. In this paper we
explore tests based on the galaxy mass function, the void probability function
(VPF), and the number of galaxy clusters. At high z the number density of
clusters shows large differences between DE models, but geometrical factors
reduce the differences substantially. We find that detecting a model dependence
in the cluster redshift distribution is a hard challenge. We show that the
galaxy redshift distribution is potentially a more sensitive characteristics.
We do so by populating dark matter halos in Nbody simulations with galaxies
using well-tested Halo Occupation Distribution (HOD). We also estimate the Void
Probability Function and find that, in samples with the same angular surface
density of galaxies in different models, the VPF is almost model independent
and cannot be used as a test for DE. Once again, geometry and cosmic evolution
compensate each other. By comparing VPF's for samples with fixed galaxy mass
limits, we find measurable differences.Comment: 12 pages, 11 figures, dependence on mass-luminosity relation
discussed, minor changes to match the accepted version by MNRA
Evaluation of resistive-plate-chamber-based TOF-PET applied to in-beam particle therapy monitoring
Particle therapy is a highly conformal radiotherapy technique which reduces the dose deposited to the surrounding normal tissues. In order to fully exploit its advantages, treatment monitoring is necessary to minimize uncertainties related to the dose delivery. Up to now, the only clinically feasible technique for the monitoring of therapeutic irradiation with particle beams is Positron Emission Tomography (PET). In this work we have compared a Resistive Plate Chamber (RPC)-based PET scanner with a scintillation-crystal-based PET scanner for this application. In general, the main advantages of the RPC-PET system are its excellent timing resolution, low cost, and the possibility of building large area systems. We simulated a partial-ring scannerbeam monitoring, which has an intrinsically low positron yield compared to diagnostic PET. In addition, for in-beam PET there is a further data loss due to the partial ring configuration. In order to improve the performance of the RPC-based scanner, an improved version of the RPC detector (modifying the thickness of the gas and glass layers), providing a larger sensitivity, has been simulated and compared with an axially extended version of the crystal-based device. The improved version of the RPC shows better performance than the prototype, but the extended version of the crystal-based PET outperforms all other options. based on an RPC prototype under construction within the Fondazione per Adroterapia Oncologica (TERA). For comparison with the crystal-based PET scanner we have chosen the geometry of a commercially available PET scanner, the Philips Gemini TF. The coincidence time resolution used in the simulations takes into account the current achievable values as well as expected improvements of both technologies. Several scenarios (including patient data) have been simulated to evaluate the performance of different scanners. Initial results have shown that the low sensitivity of the RPC hampers its application to hadro
Tracing the nature of dark energy with galaxy distribution
Dynamical dark energy (DE) is a viable alternative to the cosmological constant. Constructing tests to discriminate between Λ and dynamical DE models is difficult, however, because the differences are not large. In this paper we explore tests based on the galaxy mass function, the void probability function (VPF), and the number of galaxy clusters. At high z, the number density of clusters shows large differences between DE models, but geometrical factors reduce the differences substantially. We find that detecting a model dependence in the cluster redshift distribution is a significant challenge. We show that the galaxy redshift distribution is potentially a more sensitive characteristic. We do this by populating dark matter haloes in N-body simulations with galaxies using well-tested halo occupation distributions. We also estimate the VPF and find that samples with the same angular surface density of galaxies, in different models, exhibition almost model-independent VPF which therefore cannot be used as a test for DE. Once again, geometry and cosmic evolution compensate each other. By comparing VPFs for samples with fixed galaxy mass limits, we find measurable difference
Spectroscopic properties and radiation damage investigation of a diamond based Schottky diode for ion-beam therapy microdosimetry
In this work, a detailed analysis of the properties of a novel microdosimeter based on a synthetic single crystal diamond is reported. Focused ion microbeams were used to investigate the device spectropscopic properties as well as the induced radiation damageeffects. A diamond based Schottky diode was fabricated by chemical vapor deposition with a very thin detecting region, about 400 nm thick (approximately 1.4 μm water equivalent thickness), corresponding to the typical size in microdosimetric measurements. A 200 × 200 μm2 square metallic contact was patterned on the diamond surface by standard photolithography to define the sensitive area. Experimental measurements were carried out at the Ruder Boškovic′ Institute microbeam facility using 4 MeV carbon and 5 MeV silicon ions. Ion beam induced charge maps were employed to characterize the microdosimeter response in terms of its charge collection properties. A stable response with no evidence of polarization or memory effects was observed up to the maximum investigated ion beam flux of about 1.7 × 109 ions·cm−2·s−1. A homogeneity of the response about 6% was found over the sensitive region with a well-defined confinement of the response within the active area. Tests of the radiation damageeffect were performed by selectively irradiating small areas of the device with different ion fluences, up to about 1012 ions/cm2. An exponential decrease of the charge collection efficiency was observed with a characteristic decay constant of about 4.8 MGy and 1 MGy for C and Si ions, respectively. The experimental data were analyzed by means of GEANT4 Monte Carlo simulations. A direct correlation between the diamond damaging effect and the Non Ionizing Energy Loss (NIEL) fraction was found. In particular, an exponential decay of the charge collection efficiency with an exponential decay as a function of NIEL is observed, with a characteristic constant of about 9.3 kGy-NIEL for both carbon and silicon ions
Dynamical quintessence fields Press-Schechter mass function: detectability and effect on dark haloes
We present an investigations on the influence of dynamical quintessence field
models on the formation of non-linear structures. In particular, we focus on
the structure traced by the mass function. Our contribution builds on previous
studies by considering two potentials not treated previousy and investigating
the impact of the free parameters of two other models. Our approach emphasises
the physical insight into the key role of the evolution of the equation of
state. We use the variations in the spherical non-linear collapse caused by
quintessence as a function of the model's potential in a Press Schechter scheme
to obtain the differences in halo mass functions. The comparison is also done
with the more usual scenario CDM. We conclude the method should be
promissing for discrimination using cluster mass determination and that the key
role lies within the evolution of the equation of state.Comment: submitted to JCAP, 22pp, corrected typo in eq.(3.3
High accuracy power spectra including baryonic physics in dynamical Dark Energy models
The next generation mass probes will obtain information on non--linear power
spectra P(k,z) and their evolution, allowing us to investigate the nature of
Dark Energy. To exploit such data we need high precision simulations, extending
at least up to scales of k 10 h/Mpc, where the effects of baryons can no longer
be neglected.
In this paper, we present a series of large scale hydrodynamical simulations
for LCDM and dynamical Dark Energy (dDE) models, in which the equation of state
parameter is z-dependent. The simulations include gas cooling, star formation
and Supernovae feedback. They closely approximate the observed star formation
rate and the observationally derived star/Dark Matter mass ratio in collapsed
systems. Baryon dynamics cause spectral shifts exceeding 1% at k > 2-3 h/Mpc
compared to pure n-body simulations in the LCDM simulations. This agrees with
previous studies, although we find a smaller effect (~50%) on the power
spectrum amplitude at higher k's. dDE exhibits similar behavior, even though
the dDE simulations produce ~20% less stars than the analogous LCDM
cosmologies. Finally, we show that the technique introduced in Casarini et al.
to obtain spectra for any cosmology from constant-w models at any
redshift still holds when gas physics is taken into account. While this
relieves the need to explore the entire functional space of dark energy state
equations, we illustrate a severe risk that future data analysis could lead to
misinterpretation of the DE state equation.Comment: 12 pages, 13 figures, minor changes to match the accepted version,
MNRAS in pres
Imprints of dynamical dark energy on weak-lensing measurements
We show that simple models of scalar-field dark energy leave a generic
enhancement in the weak-lensing power spectrum when compared to the LCDM
prediction. In particular, we calculate the linear-scale enhancement in the
convergence (or cosmic-shear) power spectrum for two best-fit models of
scalar-field dark energy, namely, the Ratra-Peebles and SUGRA-type
quintessence. Our calculations are based on linear perturbation theory, using
gauge-invariant variables with carefully defined adiabatic initial conditions.
We find that geometric effects enhance the lensing power spectrum on a broad
range of scales, whilst the clustering of dark energy gives rise to additional
power on large scales. The dark-energy power spectrum for these models are also
explicitly obtained. On degree scales, the total enhancement may be as large as
30-40% for sources at redshift ~1. We argue that there are realistic prospects
for detecting such an enhancement using the next generation of large
telescopes.Comment: 10 pages, 8 figures, replacement matches version published in MNRA
Decoupling Dark Energy from Matter
We examine the embedding of dark energy in high energy models based upon supergravity and extend the usual phenomenological setting comprising an observable sector and a hidden supersymmetry breaking sector by including a third sector leading to the acceleration of the expansion of the universe. We find that gravitational constraints on the non-existence of a fifth force naturally imply that the dark energy sector must possess an approximate shift symmetry. When exact, the shift symmetry provides an example of a dark energy sector with a runaway potential and a nearly massless dark energy field whose coupling to matter is very weak, contrary to the usual lore that dark energy fields must couple strongly to matter and lead to gravitational inconsistencies. Moreover, the shape of the potential is stable under one-loop radiative corrections. When the shift symmetry is slightly broken by higher order terms in the Kähler potential, the coupling to matter remains small. However, the cosmological dynamics are largely affected by the shift symmetry breaking operators leading to the appearance of a minimum of the scalar potential such that dark energy behaves like an effective cosmological constant from very early on in the history of the universe
Cluster number counts dependence on dark energy inhomogeneities and coupling to dark matter
Cluster number counts can be used to test dark energy models. We investigate
dark energy candidates which are coupled to dark matter. We analyze the cluster
number counts dependence on the amount of dark matter coupled to dark energy.
Further more, we study how dark energy inhomogeneities affect cluster
abundances. It is shown that increasing the coupling reduces significantly the
cluster number counts, and that dark energy inhomogeneities increases cluster
abundances. Wiggles in cluster number counts are shown to be a specific
signature of coupled dark energy models. Future observations will possibly
detect such oscillations and discriminate among the different dark energy
models.Comment: 9 pages, 8 figures. Further extensions on section on discriminating
models with future surveys. Accepted for publication in Mon. Not. Roy. Astro.
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