19,047 research outputs found
Unconventional cosmology on the (thick) brane
We consider the cosmology of a thick codimension 1 brane. We obtain the
matching conditions leading to the cosmological evolution equations and show
that when one includes matter with a pressure component along the extra
dimension in the brane energy-momentum tensor, the cosmology is of non-standard
type. In particular one can get acceleration when a dust of non-relativistic
matter particles is the only source for the (modified) Friedman equation. Our
equations would seem to violate the conservation of energy-momentum from a 4D
perspective, but in 5D the energy-momentum is conserved. One could write down
an effective conserved 4D energy-momentum tensor attaching a ``dark energy''
component to the energy-momentum tensor of matter that has pressure along the
extra dimension. This extra component could, on a cosmological scale, be
interpreted as matter-coupled quintessence. We comment on the effective 4D
description of this effect in terms of the time evolution of a scalar field
(the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change
Constraining f(R) Gravity as a Scalar Tensor Theory
We search for viable f(R) theories of gravity, making use of the equivalence
between such theories and scalar-tensor gravity. We find that models can be
made consistent with solar system constraints either by giving the scalar a
high mass or by exploiting the so-called chameleon effect. However, in both
cases, it appears likely that any late-time cosmic acceleration will be
observationally indistinguishable from acceleration caused by a cosmological
constant. We also explore further observational constraints from, e.g., big
bang nucleosynthesis and inflation.Comment: 15 pages, 5 figure
A Planck-like problem for quantum charged black holes
Motivated by the parallelism existing between the puzzles of classical
physics at the beginning of the XXth century and the current paradoxes in the
search of a quantum theory of gravity, we give, in analogy with Planck's black
body radiation problem, a solution for the exact Hawking flux of evaporating
Reissner-Nordstrom black holes. Our results show that when back-reaction
effects are fully taken into account the standard picture of black hole
evaporation is significantly altered, thus implying a possible resolution of
the information loss problem.Comment: 6 pages, LaTeX file, Awarded Fifth Prize in the Gravity Research
Foundation Essay Competition for 200
Spectroscopic characterisation of the stellar content of ultra diffuse galaxies
Understanding the peculiar properties of Ultra Diffuse Galaxies (UDGs) via
spectroscopic analysis is a challenging task requiring very deep observations
and exquisite data reduction. In this work we perform one of the most complete
characterisations of the stellar component of UDGs to date using deep optical
spectroscopic data from OSIRIS at GTC. We measure radial and rotation
velocities, star formation histories (SFH) and mean population parameters, such
as ages and metallicities, for a sample of five UDG candidates in the Coma
cluster. From the radial velocities, we confirm the Coma membership of these
galaxies. We find that their rotation properties, if detected at all, are
compatible with dwarf-like galaxies. The SFHs of the UDG are dominated by old
(~ 7 Gyr), metal-poor ([M/H] ~ -1.1) and alpha-enhanced ([Mg/Fe] ~ 0.4)
populations followed by a smooth or episodic decline which halted ~ 2 Gyr ago,
possibly a sign of cluster-induced quenching. We find no obvious correlation
between individual SFH shapes and any UDG morphological properties. The
recovered stellar properties for UDGs are similar to those found for DDO44, a
local UDG analogue resolved into stars. We conclude that the UDGs in our sample
are extended dwarfs whose properties are likely the outcome of both internal
processes, such as bursty SFHs and/or high-spin haloes, as well as
environmental effects within the Coma cluster.Comment: Accepted for publication in MNRA
NELIOTA: The wide-field, high-cadence lunar monitoring system at the prime focus of the Kryoneri telescope
We present the technical specifications and first results of the ESA-funded,
lunar monitoring project "NELIOTA" (NEO Lunar Impacts and Optical TrAnsients)
at the National Observatory of Athens, which aims to determine the
size-frequency distribution of small Near-Earth Objects (NEOs) via detection of
impact flashes on the surface of the Moon. For the purposes of this project a
twin camera instrument was specially designed and installed at the 1.2 m
Kryoneri telescope utilizing the fast-frame capabilities of scientific
Complementary Metal-Oxide Semiconductor detectors (sCMOS). The system provides
a wide field-of-view (17.0' 14.4') and simultaneous observations in
two photometric bands (R and I), reaching limiting magnitudes of 18.7 mag in 10
sec in both bands at a 2.5 signal-to-noise level. This makes it a unique
instrument that can be used for the detection of NEO impacts on the Moon, as
well as for any astronomy projects that demand high-cadence multicolor
observations. The wide field-of-view ensures that a large portion of the Moon
is observed, while the simultaneous, high-cadence, monitoring in two
photometric bands makes possible, for the first time, the determination of the
temperatures of the impacts on the Moon's surface and the validation of the
impact flashes from a single site. Considering the varying background level on
the Moon's surface we demonstrate that the NELIOTA system can detect NEO impact
flashes at a 2.5 signal-to-noise level of ~12.4 mag in the I-band and R-band
for observations made at low lunar phases ~0.1. We report 31 NEO impact flashes
detected during the first year of the NELIOTA campaign. The faintest flash was
at 11.24 mag in the R-band (about two magnitudes fainter than ever observed
before) at lunar phase 0.32. Our observations suggest a detection rate of events .Comment: Accepted for publication in A&
Element specific characterization of heterogeneous magnetism in (Ga,Fe)N films
We employ x-ray spectroscopy to characterize the distribution and magnetism
of particular alloy constituents in (Ga,Fe)N films grown by metal organic vapor
phase epitaxy. Furthermore, photoelectron microscopy gives direct evidence for
the aggregation of Fe ions, leading to the formation of Fe-rich nanoregions
adjacent to the samples surface. A sizable x-ray magnetic circular dichroism
(XMCD) signal at the Fe L-edges in remanence and at moderate magnetic fields at
300 K links the high temperature ferromagnetism with the Fe(3d) states. The
XMCD response at the N K-edge highlights that the N(2p) states carry
considerable spin polarization. We conclude that FeN{\delta} nanocrystals, with
\delta > 0.25, stabilize the ferromagnetic response of the films.Comment: 4 pages, 3 figures, 1 tabl
Bohmian transmission and reflection dwell times without trajectory sampling
Within the framework of Bohmian mechanics dwell times find a straightforward
formulation. The computation of associated probabilities and distributions
however needs the explicit knowledge of a relevant sample of trajectories and
therefore implies formidable numerical effort. Here a trajectory free
formulation for the average transmission and reflection dwell times within
static spatial intervals [a,b] is given for one-dimensional scattering
problems. This formulation reduces the computation time to less than 5% of the
computation time by means of trajectory sampling.Comment: 14 pages, 7 figures; v2: published version, significantly revised and
shortened (former sections 2 and 3 omitted, appendix A added, simplified
mathematics
Ellipticals at z=0 from Self-Consistent Hydrodynamical Simulations: Clues on Age Effects in their Stellar Populations
We present results of a study of the stellar age distributions in the sample
of elliptical-like objects (ELOs) identified at z=0 in four simulations
operating in the context of a concordance cosmological model. The simulations
show that the formation of most stars in each ELO of the sample is a
consequence of violent dynamical events, either fast multiclump collapse at
high z, or mergers at lower z. This second way can explain the age spread as
well as the dynamical peculiarities observed in some ellipticals, but its
relative weight is never dominant and decreases as the ELO mass at the halo
scale, , increases, to such an extent that some recent mergers
contributing an important fraction to the total ELO mass can possibly
contribute only a small fraction of new born stars. More massive objects have
older means and narrower spreads in their stellar age distributions than less
massive ones. The ELO sample shows also a tight correlation between
and the central stellar l.o.s. velocity dispersion, . This gives
a trend of the means and spreads of ELO stellar populations with
that is consistent, even quantitatively, with the age effects observationally
detected in the stellar populations of elliptical galaxies. Therefore, these
effects can be explained as the observational manifestation of the intrinsic
correlations found in the ELO sample between and the properties of
the stellar age distribution, on the one hand, and and
, on the other hand. These correlations hint, for the first time,
at a possible way to reconcile age effects in ellipticals, and, particularly,
the increase of ratios with , with the
hierarchical clustering paradigm.Comment: 13 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
Instability of brane cosmological solutions with flux compactifications
We discuss the stability of the higher-dimensional de Sitter (dS) brane
solutions with two-dimensional internal space in the Einstein-Maxwel theory. We
show that an instability appears in the scalar-type perturbations with respect
to the dS spacetime. We derive a differential relation which has the very
similar structure to the ordinary laws of thermodynamics as an extension of the
work for the six-dimensional model [20]. In this relation, the area of dS
horizon (integrated over the two internal dimensions) exactly behaves as the
thermodynamical entropy. The dynamically unstable solutions are in the
thermodynamically unstable branch. An unstable dS compactification either
evolves toward a stable configuration or two-dimensional internal space is
decompactified. These dS brane solutions are equivalent to the accelerating
cosmological solutions in the six-dimensional Einstein-Maxwell-dilaton theory
via dimensional reduction. Thus, if the seed higher-dimensional solution is
unstable, the corresponding six-dimensional solution is also unstable. From the
effective four-dimensional point of view, a cosmological evolution from an
unstable cosmological solution in higher dimensions may be seen as a process of
the transition from the initial cosmological inflation to the current dark
energy dominated Universe.Comment: 11 pages, 3 figures, references added, to appear in CQ
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