1,956 research outputs found

### More on Tachyon Cosmology in De Sitter Gravity

We aim to study rolling tachyon cosmological solutions in de Sitter gravity.
The solutions are taken to be flat FRW type and these are not time-reversal
symmetric. We find that cosmological constant of our universe has to be
fine-tuned at the level of the action itself, as in KKLT string
compactification. The rolling tachyon can give rise to required inflation with
suitable choice of the initial conditions which include nonvanishing Hubble
constant. We also determine an upper bound on the volume of the
compactification manifold.Comment: 15pp, 3 figures; references adde

### Gravitational Lensing Signature of Long Cosmic Strings

The gravitational lensing by long, wiggly cosmic strings is shown to produce
a large number of lensed images of a background source. In addition to pairs of
images on either side of the string, a number of small images outline the
string due to small-scale structure on the string. This image pattern could
provide a highly distinctive signature of cosmic strings. Since the optical
depth for multiple imaging of distant quasar sources by long strings may be
comparable to that by galaxies, these image patterns should be clearly
observable in the next generation of redshift surveys such as the Sloan Digital
Sky Survey.Comment: 4 pages, revtex with 3 postscript figures include

### Observing Long Cosmic Strings Through Gravitational Lensing

We consider the gravitational lensing produced by long cosmic strings formed
in a GUT scale phase transition. We derive a formula for the deflection of
photons which pass near the strings that reduces to an integral over the light
cone projection of the string configuration plus constant terms which are not
important for lensing. Our strings are produced by performing numerical
simulations of cosmic string networks in flat, Minkowski space ignoring the
effects of cosmological expansion. These strings have more small scale
structure than those from an expanding universe simulation - fractal dimension
1.3 for Minkowski versus 1.1 for expanding - but share the same qualitative
features. Lensing simulations show that for both point-like and extended
objects, strings produce patterns unlike more traditional lenses, and, in
particluar, the kinks in strings tend to generate demagnified images which
reside close to the string. Thus lensing acts as a probe of the small scale
structure of a string. Estimates of lensing probablity suggest that for string
energy densities consistant with string seeded structure formation, on the
order of tens of string lenses should be observed in the Sloan Digital Sky
Survey quasar catalog. We propose a search strategy in which string lenses
would be identified in the SDSS quasar survey, and the string nature of the
lens can be confirmed by the observation of nearby high redshift galaxies which
are also be lensed by the string.Comment: 24 pages revtex with 12 postscript firgure

### Scaling Property of the global string in the radiation dominated universe

We investigate the evolution of the global string network in the radiation
dominated universe by use of numerical simulations in 3+1 dimensions. We find
that the global string network settles down to the scaling regime where the
energy density of global strings, $\rho_{s}$, is given by $\rho_{s} = \xi \mu /
t^2$ with $\mu$ the string tension per unit length and the scaling parameter,
$\xi \sim (0.9-1.3)$, irrespective of the cosmic time. We also find that the
loop distribution function can be fitted with that predicted by the so-called
one scale model. Concretely, the number density, $n_{l}(t)$, of the loop with
the length, $l$, is given by $n_{l}(t) = \nu/[t^{3/2} (l + \kappa t)^{5/2}]$
where $\nu \sim 0.0865$ and $\kappa$ is related with the Nambu-Goldstone(NG)
boson radiation power from global strings, $P$, as $P = \kappa \mu$ with
$\kappa \sim 0.535$. Therefore, the loop production function also scales and
the typical scale of produced loops is nearly the horizon distance. Thus, the
evolution of the global string network in the radiation dominated universe can
be well described by the one scale model in contrast with that of the local
string network.Comment: 18 pages, 9 figures, to appear in Phys. Rev.

### Evolution of a global string network in a matter dominated universe

We evolve the network of global strings in the matter-dominated universe by
means of numerical simulations. The existence of the scaling solution is
confirmed as in the radiation-dominated universe but the scaling parameter
$\xi$ takes a slightly smaller value, $\xi \simeq 0.6 \pm 0.1$, which is
defined as $\xi = \rho_{s} t^{2} / \mu$ with $\rho_{s}$ the energy density of
global strings and $\mu$ the string tension per unit length. The change of
$\xi$ from the radiation to the matter-dominated universe is consistent with
that obtained by Albrecht and Turok by use of the one-scale model. We also
study the loop distribution function and find that it can be well fitted with
that predicted by the one-scale model, where the number density $n_{l}(t)$ of
the loop with the length $l$ is given by $n_{l}(t) = \nu/[t^2 (l + \kappa
t)^2]$ with $\nu \sim 0.040$ and $\kappa \sim 0.48$. Thus, the evolution of the
global string network in the matter-dominated universe can be well described by
the one-scale model as in the radiation-dominated universe.Comment: 10 pages, 5 figure

### Entanglement of electrons in interacting molecules

Quantum entanglement is a concept commonly used with reference to the
existence of certain correlations in quantum systems that have no classical
interpretation. It is a useful resource to enhance the mutual information of
memory channels or to accelerate some quantum processes as, for example, the
factorization in Shor's Algorithm. Moreover, entanglement is a physical
observable directly measured by the von Neumann entropy of the system. We have
used this concept in order to give a physical meaning to the electron
correlation energy in systems of interacting electrons. The electronic
correlation is not directly observable, since it is defined as the difference
between the exact ground state energy of the many--electrons Schroedinger
equation and the Hartree--Fock energy. We have calculated the correlation
energy and compared with the entanglement, as functions of the nucleus--nucleus
separation using, for the hydrogen molecule, the Configuration Interaction
method. Then, in the same spirit, we have analyzed a dimer of ethylene, which
represents the simplest organic conjugate system, changing the relative
orientation and distance of the molecules, in order to obtain the configuration
corresponding to maximum entanglement.Comment: 15 pages, 7 figures, standard late

### Variation of cosmic ray injection across supernova shocks

The injection rate of suprathermal protons into the diffusive shock
acceleration process should vary strongly over the surface of supernova remnant
shocks. These variations and the absolute value of the injection rate are
investigated. In the simplest case, like for SN 1006, the shock can be
approximated as being spherical in a uniform large-scale magnetic field. The
injection rate depends strongly on the shock obliquity and diminishes as the
angle between the ambient field and the shock normal increases. Therefore
efficient particle injection, which leads to conversion of a significant
fraction of the kinetic energy at a shock surface element, arises only in
relatively small regions near the "poles", reducing the overall CR production.
The sizes of these regions depend strongly on the random background field and
the Alfven wave turbulence generated due to CR streaming instability. For the
cases of SN 1006 and Tycho's SNR they correspond to about 20, and for Cas A to
between 10 and 20 percent of the entire shock surface. In first approximation,
the CR production rate, calculated under the assumption of spherical symmetry,
has therefore to be renormalized by this factor, while the shock as such
remains roughly spherical.Comment: 10 pages, 4 figures. To appear in Astronomy and Astrophyics;
Corrected typos, references change

### Linear and non-linear perturbations in dark energy models

I review the linear and second-order perturbation theory in dark energy
models with explicit interaction to matter in view of applications to N-body
simulations and non-linear phenomena. Several new or generalized results are
obtained: the general equations for the linear perturbation growth; an
analytical expression for the bias induced by a species-dependent interaction;
the Yukawa correction to the gravitational potential due to dark energy
interaction; the second-order perturbation equations in coupled dark energy and
their Newtonian limit. I also show that a density-dependent effective dark
energy mass arises if the dark energy coupling is varying.Comment: 12 pages, submitted to Phys. Rev; v2: added a ref. and corrected a
typ

### Error threshold in optimal coding, numerical criteria and classes of universalities for complexity

The free energy of the Random Energy Model at the transition point between
ferromagnetic and spin glass phases is calculated. At this point, equivalent to
the decoding error threshold in optimal codes, free energy has finite size
corrections proportional to the square root of the number of degrees. The
response of the magnetization to the ferromagnetic couplings is maximal at the
values of magnetization equal to half. We give several criteria of complexity
and define different universality classes. According to our classification, at
the lowest class of complexity are random graph, Markov Models and Hidden
Markov Models. At the next level is Sherrington-Kirkpatrick spin glass,
connected with neuron-network models. On a higher level are critical theories,
spin glass phase of Random Energy Model, percolation, self organized
criticality (SOC). The top level class involves HOT design, error threshold in
optimal coding, language, and, maybe, financial market. Alive systems are also
related with the last class. A concept of anti-resonance is suggested for the
complex systems.Comment: 17 page

### Affine Constellations Without Mutually Unbiased Counterparts

It has been conjectured that a complete set of mutually unbiased bases in a
space of dimension d exists if and only if there is an affine plane of order d.
We introduce affine constellations and compare their existence properties with
those of mutually unbiased constellations, mostly in dimension six. The
observed discrepancies make a deeper relation between the two existence
problems unlikely.Comment: 8 page

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