6,820 research outputs found
Relative entropy as a measure of inhomogeneity in general relativity
We introduce the notion of relative volume entropy for two spacetimes with
preferred compact spacelike foliations. This is accomplished by applying the
notion of Kullback-Leibler divergence to the volume elements induced on
spacelike slices. The resulting quantity gives a lower bound on the number of
bits which are necessary to describe one metric given the other. For
illustration, we study some examples, in particular gravitational waves, and
conclude that the relative volume entropy is a suitable device for quantitative
comparison of the inhomogeneity of two spacetimes.Comment: 15 pages, 7 figure
Towards the QCD phase diagram from analytical continuation
We calculate the QCD cross-over temperature, the equation of state and
fluctuations of conserved charges at finite density by analytical continuation
from imaginary to real chemical potentials. Our calculations are based on new
continuum extrapolated lattice simulations using the 4stout staggered actions
with a lattice resolution up to . The simulation parameters are tuned
such that the strangeness neutrality is maintained, as it is in heavy ion
collisions.Comment: 4 pages, 2 figures, Proceedings of the Quark Matter 2015 conference,
Kobe, Japa
Flux Compactifications: Stability and Implications for Cosmology
We study the dynamics of the size of an extra-dimensional manifold stabilised
by fluxes. Inspecting the potential for the 4D field associated with this size
(the radion), we obtain the conditions under which it can be stabilised and
show that stable compactifications on hyperbolic manifolds necessarily have a
negative four-dimensional cosmological constant, in contradiction with
experimental observations. Assuming compactification on a positively curved
(spherical) manifold we find that the radion has a mass of the order of the
compactification scale, M_c, and Planck suppressed couplings. We also show that
the model becomes unstable and the extra dimensions decompactify when the
four-dimensional curvature is higher than a maximum value. This in particular
sets an upper bound on the scale of inflation in these models: V_max \sim M_c^2
M_P^2, independently of whether the radion or other field is responsible for
inflation. We comment on other possible contributions to the radion potential
as well as finite temperature effects and their impact on the bounds obtained.Comment: 16 pages, 1 figure, LaTeX; v2: typos fixed and references adde
Conservation equation on braneworlds in six dimensions
We study braneworlds in six-dimensional Einstein-Gauss-Bonnet gravity. The
Gauss-Bonnet term is crucial for the equations to be well-posed in six
dimensions when non-trivial matter on the brane is included (the also involved
induced gravity term is not significant for their structure), and the matching
conditions of the braneworld are known. We show that the energy-momentum of the
brane is always conserved, independently of any regular bulk energy-momentum
tensor, contrary to the situation of the five-dimensional case.Comment: References added, minor changes, 3 pages, RevTeX, to app. in Class.
Quant. Gra
The Intrinsic Two-Dimensional Size of Sagittarius A*
We report the detection of the two-dimensional structure of the radio source
associated with the Galactic Center black hole, Sagittarius A*, obtained from
Very Long Baseline Array (VLBA) observations at a wavelength of 7mm. The
intrinsic source is modeled as an elliptical Gaussian with major axis size 35.4
x 12.6 R_S in position angle 95 deg East of North. This morphology can be
interpreted in the context of both jet and accretion disk models for the radio
emission. There is supporting evidence in large angular-scale multi-wavelength
observations for both source models for a preferred axis near 95 deg. We also
place a maximum peak-to-peak change of 15% in the intrinsic major axis size
over five different epochs. Three observations were triggered by detection of
near infrared (NIR) flares and one was simultaneous with a large X-ray flare
detected by NuSTAR. The absence of simultaneous and quasi-simultaneous flares
indicates that not all high energy events produce variability at radio
wavelengths. This supports the conclusion that NIR and X-ray flares are
primarily due to electron excitation and not to an enhanced accretion rate onto
the black hole.Comment: accepted for publication in Ap
Quantum Noise Randomized Ciphers
We review the notion of a classical random cipher and its advantages. We
sharpen the usual description of random ciphers to a particular mathematical
characterization suggested by the salient feature responsible for their
increased security. We describe a concrete system known as AlphaEta and show
that it is equivalent to a random cipher in which the required randomization is
effected by coherent-state quantum noise. We describe the currently known
security features of AlphaEta and similar systems, including lower bounds on
the unicity distances against ciphertext-only and known-plaintext attacks. We
show how AlphaEta used in conjunction with any standard stream cipher such as
AES (Advanced Encryption Standard) provides an additional, qualitatively
different layer of security from physical encryption against known-plaintext
attacks on the key. We refute some claims in the literature that AlphaEta is
equivalent to a non-random stream cipher.Comment: Accepted for publication in Phys. Rev. A; Discussion augmented and
re-organized; Section 5 contains a detailed response to 'T. Nishioka, T.
Hasegawa, H. Ishizuka, K. Imafuku, H. Imai: Phys. Lett. A 327 (2004) 28-32
/quant-ph/0310168' & 'T. Nishioka, T. Hasegawa, H. Ishizuka, K. Imafuku, H.
Imai: Phys. Lett. A 346 (2005) 7
Moduli Stabilization in Brane Gas Cosmology with Superpotentials
In the context of brane gas cosmology in superstring theory, we show why it
is impossible to simultaneously stabilize the dilaton and the radion with a
general gas of strings (including massless modes) and D-branes. Although this
requires invoking a different mechanism to stabilize these moduli fields, we
find that the brane gas can still play a crucial role in the early universe in
assisting moduli stabilization. We show that a modest energy density of
specific types of brane gas can solve the overshoot problem that typically
afflicts potentials arising from gaugino condensation.Comment: minor changes to match the journal versio
Classical Stabilization of Homogeneous Extra Dimensions
If spacetime possesses extra dimensions of size and curvature radii much
larger than the Planck or string scales, the dynamics of these extra dimensions
should be governed by classical general relativity. We argue that in general
relativity, it is highly nontrivial to obtain solutions where the extra
dimensions are static and are dynamically stable to small perturbations. We
also illustrate that intuition on equilibrium and stability built up from
non-gravitational physics can be highly misleading. For all static, homogeneous
solutions satisfying the null energy condition, we show that the Ricci
curvature of space must be nonnegative in all directions. Much of our analysis
focuses on a class of spacetime models where space consists of a product of
homogeneous and isotropic geometries. A dimensional reduction of these models
is performed, and their stability to perturbations that preserve the spatial
symmetries is analyzed. We conclude that the only physically realistic examples
of classically stabilized large extra dimensions are those in which the
extra-dimensional manifold is positively curved.Comment: 25 pages; minor changes, improved reference
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