2,138 research outputs found
An updated analysis of two classes of f(R) theories of gravity
The observed accelerated cosmic expansion can be a signature of
fourth\,-\,order gravity theories, where the acceleration of the Universe is a
consequence of departures from Einstein General Relativity, rather than the
sign of the existence of a fluid with negative pressure. In the
fourth\,-\,order gravity theories, the gravity Lagrangian is described by an
analytic function of the scalar curvature subject to the demanding
conditions that no detectable deviations from standard GR is observed on the
Solar System scale. Here we consider two classes of theories able to
pass Solar System tests and investigate their viability on cosmological scales.
To this end, we fit the theories to a large dataset including the combined
Hubble diagram of Type Ia Supernovae and Gamma Ray Bursts, the Hubble parameter
data from passively evolving red galaxies, Baryon Acoustic Oscillations
extracted from the seventh data release of the Sloan Digital Sky Survey (SDSS)
and the distance priors from the Wilkinson Microwave Anisotropy Probe seven
years (WMAP7) data. We find that both classes of fit very well this
large dataset with the present\,-\,day values of the matter density, Hubble
constant and deceleration parameter in agreement with previous estimates;
however, the strong degeneracy among the parameters prevents us from
strongly constraining their values. We also derive the growth factor , with the matter density
perturbation, and show that it can still be well approximated by . We finally constrain (on some representative
scales) and investigate its redshift dependence to see whether future data can
discriminate between these classes of theories and standard dark energy
models.Comment: 27 pages, 5 figures, 1 table, accepted for publication on JCAP. Note
that this paper updates and supersedes preprint arXiv:0907.468
Interior Optima and the Inada Conditions
We present a new proof of the interiority of the policy function based on the Inada conditions. It is based on supporting properties of concave functions.growth model ; Inada conditions ; policy function
Monetary Equilibrium and the Differentiability of the Value Function
In this study we offer a new approach to proving the differentiability of the value function, which complements and extends the literature on dynamic programming. This result is then applied to the analysis of equilibrium in the recent class of monetary economies developed in [13]. For this type of environments we demonstrate that the value function is differentiable and this guarantees that the marginal value of money balances is well defined.value function ; optimal plans ; money
Green synthesis of vanillin: Pervaporation and dialysis for process intensification in a membrane reactor
In the present work, two different membrane processes (pervaporation and dialysis) are compared in view of their utilization in a membrane reactor, where vanillin, which is probably the most important aroma of the food industry, is synthesized in a green and sustainable way. The utilized precursor (ferulic acid, which is possibly a natural product from agricultural wastes) is partially oxidized (photocatalytically or biologically) and the product is continuously recovered from the reacting solution by the membrane process to avoid its degradation. It is observed that pervaporation is much more selective towards vanillin than dialysis, but the permeate flux of dialysis is much higher. Furthermore, dialysis can work also at lower temperatures and can be used to continuously restore the consumed substrate into the reacting mixture. A mathematical model of the integrated process (reaction combined with membrane separation) reproduces quite satisfactorily the experimental results and can be used for the analysis and the design of the process
Mass - concentration relation and weak lensing peak counts
The statistics of peaks in weak lensing convergence maps is a promising tool
to investigate both the properties of dark matter haloes and constrain the
cosmological parameters. We study how the number of detectable peaks and its
scaling with redshift depend upon the cluster dark matter halo profiles and use
peak statistics to constrain the parameters of the mass - concentration (MC)
relation. We investigate which constraints the Euclid mission can set on the MC
coefficients also taking into account degeneracies with the cosmological
parameters. To this end, we first estimate the number of peaks and its redshift
distribution for different MC relations. We find that the steeper the mass
dependence and the larger the normalisation, the higher is the number of
detectable clusters, with the total number of peaks changing up to
depending on the MC relation. We then perform a Fisher matrix forecast of the
errors on the MC relation parameters as well as cosmological parameters. We
find that peak number counts detected by Euclid can determine the normalization
, the mass and redshift slopes and intrinsic scatter
of the MC relation to an unprecedented accuracy being
, , ,
if all cosmological parameters are assumed to
be known. Should we relax this severe assumption, constraints are degraded, but
remarkably good results can be restored setting only some of the parameters or
combining peak counts with Planck data. This precision can give insight on
competing scenarios of structure formation and evolution and on the role of
baryons in cluster assembling. Alternatively, for a fixed MC relation, future
peaks counts can perform as well as current BAO and SNeIa when combined with
Planck.Comment: 14 pages, 8 figures, accepted for publication on Astronomy &
Astrophysic
GazeDrone: Mobile Eye-Based Interaction in Public Space Without Augmenting the User
Gaze interaction holds a lot of promise for seamless human-computer interaction. At the same time, current wearable mobile eye trackers require user augmentation that negatively impacts natural user behavior while remote trackers require users to position themselves within a confined tracking range. We present GazeDrone, the first system that combines a camera-equipped aerial drone with a computational method to detect sidelong glances for spontaneous (calibration-free) gaze-based interaction with surrounding pervasive systems (e.g., public displays). GazeDrone does not require augmenting each user with on-body sensors and allows interaction from arbitrary positions, even while moving. We demonstrate that drone-supported gaze interaction is feasible and accurate for certain movement types. It is well-perceived by users, in particular while interacting from a fixed position as well as while moving orthogonally or diagonally to a display. We present design implications and discuss opportunities and challenges for drone-supported gaze interaction in public
Monetary Equilibrium and the Differentiability of the Value Function
In this study we offer a new approach to proving the differentiability of the value function, which complements and extends the literature on dynamic programming. This result is then applied to the analysis of equilibrium in the recent class of monetary economies developed in [Lagos, R., Wright, R., 2005. A unified framework for monetary theory and policy analysis. journal of Political Economy 113, 463-484]. For this type of environments we demonstrate that the value function is differentiable and this guarantees that the marginal value of money balances is well defined
Applicability of shape parameterizations for giant dipole resonance in warm and rapidly rotating nuclei
We investigate how well the shape parameterizations are applicable for
studying the giant dipole resonance (GDR) in nuclei, in the low temperature
and/or high spin regime. The shape fluctuations due to thermal effects in the
GDR observables are calculated using the actual free energies evaluated at
fixed spin and temperature. The results obtained are compared with Landau
theory calculations done by parameterizing the free energy. We exemplify that
the Landau theory could be inadequate where shell effects are dominating. This
discrepancy at low temperatures and high spins are well reflected in GDR
observables and hence insists on exact calculations in such cases.Comment: 10 pages, 2 figure
On thin-shell wormholes evolving in flat FRW spacetimes
We analize the stability of a class of thin-shell wormholes with spherical
symmetry evolving in flat FRW spacetimes. The wormholes considered here are
supported at the throat by a perfect fluid with equation of state
and have a physical radius equal to , where is a
time-dependent function describing the dynamics of the throat and is the
background scale factor. The study of wormhole stability is done by means of
the stability analysis of dynamic systems.Comment: 8 pages; to appear in MPL
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