3,109 research outputs found
Can a galaxy redshift survey measure dark energy clustering?
(abridged) A wide-field galaxy redshift survey allows one to probe galaxy
clustering at largest spatial scales, which carries an invaluable information
on horizon-scale physics complementarily to the cosmic microwave background
(CMB). Assuming the planned survey consisting of z~1 and z~3 surveys with areas
of 2000 and 300 square degrees, respectively, we study the prospects for
probing dark energy clustering from the measured galaxy power spectrum,
assuming the dynamical properties of dark energy are specified in terms of the
equation of state and the effective sound speed c_e in the context of an
adiabatic cold dark matter dominated model. The dark energy clustering adds a
power to the galaxy power spectrum amplitude at spatial scales greater than the
sound horizon, and the enhancement is sensitive to redshift evolution of the
net dark energy density, i.e. the equation of state. We find that the galaxy
survey, when combined with Planck, can distinguish dark energy clustering from
a smooth dark energy model such as the quintessence model (c_e=1), when
c_e<0.04 (0.02) in the case of the constant equation of state w_0=-0.9 (-0.95).
An ultimate full-sky survey of z~1 galaxies allows the detection when c_e<0.08
(0.04) for w_0=0.9 (-0.95). We also investigate a degeneracy between the dark
energy clustering and the non-relativistic neutrinos implied from the neutrino
oscillation experiments, because the two effects both induce a scale-dependent
modification in the galaxy power spectrum shape at largest spatial scales
accessible from the galaxy survey. It is shown that a wider redshift coverage
can efficiently separate the two effects by utilizing the different redshift
dependences, where dark energy clustering is apparent only at low redshifts
z<1.Comment: 14 pages, 7 figures; minor changes to match the published versio
The Important Role of Cosmic-Ray Re-Acceleration
In the last decades, the improvement of high energy instruments has enabled a
deeper understanding of the Cosmic Ray origin issue. In particular, the
gamma-ray satellites AGILE (Astrorivelatore Gamma ad Immagini LEggero) and
Fermi-LAT (Fermi-Large Area Telescope) have strongly contributed to the
confirmation of direct involvement of Supernova Remnants in Cosmic Ray
energization. Despite several attempts to fit experimental data assuming the
presence of freshly accelerated particles, the scientific community is now
aware that the role of pre-existing Cosmic Ray re-acceleration cannot be
neglected. In this work, we highlight the importance of pre-existing Cosmic Ray
re-acceleration in the Galaxy showing its fundamental contribution in middle
aged Supernova Remnant shocks and in the forward shock of stellar winds.Comment: 16 pages, 4 figure
Understanding hadronic gamma-ray emission from supernova remnants
We aim to test the plausibility of a theoretical framework in which the
gamma-ray emission detected from supernova remnants may be of hadronic origin,
i.e., due to the decay of neutral pions produced in nuclear collisions
involving relativistic nuclei. In particular, we investigate the effects
induced by magnetic field amplification on the expected particle spectra,
outlining a phenomenological scenario consistent with both the underlying
Physics and the larger and larger amount of observational data provided by the
present generation of gamma experiments, which seem to indicate rather steep
spectra for the accelerated particles. In addition, in order to study to study
how pre-supernova winds might affect the expected emission in this class of
sources, the time-dependent gamma-ray luminosity of a remnant with a massive
progenitor is worked out. Solid points and limitations of the proposed scenario
are finally discussed in a critical way.Comment: 30 pages, 5 figures; Several comments, references and a figure added.
Some typos correcte
SpatioTemporal Feature Integration and Model Fusion for Full Reference Video Quality Assessment
Perceptual video quality assessment models are either frame-based or
video-based, i.e., they apply spatiotemporal filtering or motion estimation to
capture temporal video distortions. Despite their good performance on video
quality databases, video-based approaches are time-consuming and harder to
efficiently deploy. To balance between high performance and computational
efficiency, Netflix developed the Video Multi-method Assessment Fusion (VMAF)
framework, which integrates multiple quality-aware features to predict video
quality. Nevertheless, this fusion framework does not fully exploit temporal
video quality measurements which are relevant to temporal video distortions. To
this end, we propose two improvements to the VMAF framework: SpatioTemporal
VMAF and Ensemble VMAF. Both algorithms exploit efficient temporal video
features which are fed into a single or multiple regression models. To train
our models, we designed a large subjective database and evaluated the proposed
models against state-of-the-art approaches. The compared algorithms will be
made available as part of the open source package in
https://github.com/Netflix/vmaf
Gamma ray emission from SNR RX J1713.7-3946 and the origin of galactic cosmic rays
We calculate the flux of non-thermal radiations from the supernova remnant RX
J1713.7-3946 in the context of the non-linear theory of particle acceleration
at shocks, which allows us to take into account self-consistently the dynamical
reaction of the accelerated particles, the generation of magnetic fields in the
shock proximity and the dynamical reaction of the magnetic field on the plasma.
When the fraction of particles which get accelerated is of order , we find that the strength of the magnetic field obtained as a result
of streaming instability induced by cosmic rays is compatible with the
interpretation of the X-ray emitting filaments being produced by strong
synchrotron losses in magnetic fields. If the X-ray filaments
are explained in alternative ways, the constraint on the magnetic field
downstream of the shock disappears and the HESS data can be marginally fit with
ICS of relativistic electrons off a complex population of photons, tailored to
comprise CMB and ambient IR/Optical photons. The fit, typically poor at the
highest energies, requires a large density of target photons within the
remnant; only a fraction of order of the background particles
gets accelerated; the local magnetic field is of order and the
maximum energy of protons is much lower than the knee energy. Current HESS
gamma ray observations combined with recent X-ray observations by Suzaku do not
allow as yet to draw a definitive conclusion on whether RX J1713.7-3946 is an
efficient cosmic ray accelerator, although at the present time a hadronic
interpretation of HESS data seems more likely. We discuss the implications of
our results for the GLAST gamma ray telescope, which should be able to
discriminate the two scenarios discussed above.Comment: Accepted for Publication in MNRA
Revealing modified gravity signal in matter and halo hierarchical clustering
We use a set of N-body simulations employing a modified gravity (MG) model
with Vainshtein screening to study matter and halo hierarchical clustering. As
test-case scenarios we consider two normal branch Dvali-Gabadadze-Porrati
(nDGP) gravity models with mild and strong growth rate enhancement. We study
higher-order correlation functions up to and associated
hierarchical amplitudes . We find that
the matter PDFs are strongly affected by the fifth-force on scales up to
Mpc, and the deviations from GR are maximised at . For reduced
cumulants , we find that at small scales Mpc the MG is
characterised by lower values, with the deviation growing from in the
reduced skewness up to even in . To study the halo clustering we
use a simple abundance matching and divide haloes into thee fixed number
density samples. The halo two-point functions are weakly affected, with a
relative boost of the order of a few percent appearing only at the smallest
pair separations (Mpc). In contrast, we find a strong MG signal
in 's, which are enhanced compared to GR. The strong model exhibits a
level signal at various scales for all halo samples and in all
cumulants. In this context, we find that the reduced kurtosis to be an
especially promising cosmological probe of MG. Even the mild nDGP model leaves
a imprint at small scales Mpc, while the stronger model
deviates from a GR-signature at nearly all scales with a significance of
. Since the signal is persistent in all halo samples and over a range
of scales, we advocate that the reduced kurtosis estimated from galaxy
catalogues can potentially constitute a strong MG-model discriminatory as well
as GR self-consistency test.Comment: 19 pages, 11 figures, comments are welcom
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