141 research outputs found
A common solution to the cosmic ray anisotropy and gradient problems
Multichannel Cosmic Ray (CR) spectra and the large scale CR anisotropy can
hardly be made compatible in the framework of conventional isotropic and
homogeneous propagation models. These models also have problems explaining the
longitude distribution and the radial emissivity gradient of the -ray
galactic interstellar emission. We argue here that accounting for a well
physically motivated correlation between the CR escape time and the spatially
dependent magnetic turbulence power can naturally solve both problems. Indeed,
by exploiting this correlation we find propagation models that fit a wide set
of CR primary and secondary spectra, and consistently reproduce the CR
anisotropy in the energy range 10^2 - 10^4 \GeV and the -ray
longitude distribution recently measured by Fermi-LAT.Comment: 4 pages, 3 figures. v2: Accepted in Phys. Rev. Let
Gamma-ray polarization constraints on Planck scale violations of special relativity
Using recent polarimetric observations of the Crab Nebula in the hard X-ray
band by INTEGRAL, we show that the absence of vacuum birefringence effects
constrains O(E/M) Lorentz violation in QED to the level |\xi| < 9x10^{-10} at
three sigma CL, tightening by more than three orders of magnitude previous
constraints. We show that planned X-ray polarimeters have the potential the
potential to probe |\xi|~ 10^{-16} by detecting polarization in active galaxies
at red-shift ~1.Comment: 4 pages, 3 figure
On Modified Dispersion Relations and the Chandrasekhar Mass Limit
Modified dispersion relations from effective field theory are shown to alter
the Chandrasekhar mass limit. At exceptionally high densities, the
modifications affect the pressure of a degenerate electron gas and can increase
or decrease the mass limit, depending on the sign of the modifications. These
changes to the mass limit are unlikely to be relevant for the astrophysics of
white dwarf or neutron stars due to well-known dynamical instabilities that
occur at lower densities. Generalizations to frameworks other than effective
field theory are discussed.Comment: 14 pages, 2 figures; v2: version accepted for publication, minor
changes; v3: note added correcting comments on the applicability of the
calculation to the DSR context, references added, results unchange
High-resolution bioelectrical imaging of Aβ-induced network dysfunction on CMOS-MEAs for neurotoxicity and rescue studies
Neurotoxicity and the accumulation of extracellular amyloid-beta1-42 (A\u3b2) peptides are associated with the development of Alzheimer's disease (AD) and correlate with neuronal activity and network dysfunctions, ultimately leading to cellular death. However, research on neurodegenerative diseases is hampered by the paucity of reliable readouts and experimental models to study such functional decline from an early onset and to test rescue strategies within networks at cellular resolution. To overcome this important obstacle, we demonstrate a simple yet powerful in vitro AD model based on a rat hippocampal cell culture system that exploits large-scale neuronal recordings from 4096-electrodes on CMOS-chips for electrophysiological quantifications. This model allows us to monitor network activity changes at the cellular level and to uniquely uncover the early activity-dependent deterioration induced by A\u3b2-neurotoxicity. We also demonstrate the potential of this in vitro model to test a plausible hypothesis underlying the A\u3b2-neurotoxicity and to assay potential therapeutic approaches. Specifically, by quantifying N-methyl D-aspartate (NMDA) concentration-dependent effects in comparison with low-concentration allogenic-A\u3b2, we confirm the role of extrasynaptic-NMDA receptors activation that may contribute to A\u3b2-neurotoxicity. Finally, we assess the potential rescue of neural stem cells (NSCs) and of two pharmacotherapies, memantine and saffron, for reversing A\u3b2-neurotoxicity and rescuing network-wide firing
D-Foam Phenomenology: Dark Energy, the Velocity of Light and a Possible D-Void
In a D-brane model of space-time foam, there are contributions to the dark
energy that depend on the D-brane velocities and on the density of D-particle
defects. The latter may also reduce the speeds of photons linearly with their
energies, establishing a phenomenological connection with astrophysical probes
of the universality of the velocity of light. Specifically, the cosmological
dark energy density measured at the present epoch may be linked to the apparent
retardation of energetic photons propagating from nearby AGNs. However, this
nascent field of `D-foam phenomenology' may be complicated by a dependence of
the D-particle density on the cosmological epoch. A reduced density of
D-particles at redshifts z ~ 1 - a `D-void' - would increase the dark energy
while suppressing the vacuum refractive index, and thereby might reconcile the
AGN measurements with the relatively small retardation seen for the energetic
photons propagating from GRB 090510, as measured by the Fermi satellite.Comment: 10 pages, 3 figure
Large-scale, high-resolution electrophysiological imaging of field potentials in brain slices with microelectronic multielectrode arrays
Multielectrode arrays (MEAs) are extensively used for electrophysiological studies on brain slices, but the spatial resolution and field of recording of conventional arrays are limited by the low number of electrodes available. Here, we present a large-scale array recording simultaneously from 4096 electrodes used to study propagating spontaneous and evoked network activity in acute murine cortico-hippocampal brain slices at unprecedented spatial and temporal resolution. We demonstrate that multiple chemically induced epileptiform episodes in the mouse cortex and hippocampus can be classified according to their spatio-temporal dynamics. Additionally, the large-scale and high-density features of our recording system enable the topological localization and quantification of the effects of antiepileptic drugs in local neuronal microcircuits, based on the distinct field potential propagation patterns. This novel high-resolution approach paves the way to detailed electrophysiological studies in brain circuits spanning spatial scales from single neurons up to the entire slice network
Lorentz Violation of Quantum Gravity
A quantum gravity theory which becomes renormalizable at short distances due
to a spontaneous symmetry breaking of Lorentz invariance and diffeomorphism
invariance is studied. A breaking of Lorentz invariance with the breaking
patterns and , describing 3+1 and 2+1
quantum gravity, respectively, is proposed. A complex time dependent
Schr\"odinger equation (generalized Wheeler-DeWitt equation) for the wave
function of the universe exists in the spontaneously broken symmetry phase at
Planck energy and in the early universe, uniting quantum mechanics and general
relativity. An explanation of the second law of thermodynamics and the
spontaneous creation of matter in the early universe can be obtained in the
symmetry broken phase of gravity.Comment: 10 pages, minor change and reference added. Typos corrected. To be
published in Class. Quant. Grav
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