28,943 research outputs found
Dark energy: a quantum fossil from the inflationary Universe?
The discovery of dark energy (DE) as the physical cause for the accelerated
expansion of the Universe is the most remarkable experimental finding of modern
cosmology. However, it leads to insurmountable theoretical difficulties from
the point of view of fundamental physics. Inflation, on the other hand,
constitutes another crucial ingredient, which seems necessary to solve other
cosmological conundrums and provides the primeval quantum seeds for structure
formation. One may wonder if there is any deep relationship between these two
paradigms. In this work, we suggest that the existence of the DE in the present
Universe could be linked to the quantum field theoretical mechanism that may
have triggered primordial inflation in the early Universe. This mechanism,
based on quantum conformal symmetry, induces a logarithmic,
asymptotically-free, running of the gravitational coupling. If this evolution
persists in the present Universe, and if matter is conserved, the general
covariance of Einstein's equations demands the existence of dynamical DE in the
form of a running cosmological term whose variation follows a power law of the
redshift.Comment: LaTeX, 14 pages, extended discussion. References added. Accepted in
J. Phys. A: Mathematical and Theoretica
Design study of a fission-electric cell reactor
Fission electric cell reactor to generation of power in spac
Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem
Dynamical dark energy (DE) has been proposed to explain various aspects of
the cosmological constant (CC) problem(s). For example, it is very difficult to
accept that a strictly constant Lambda-term constitutes the ultimate
explanation for the DE in our Universe. It is also hard to acquiesce in the
idea that we accidentally happen to live in an epoch where the CC contributes
an energy density value right in the ballpark of the rapidly diluting matter
density. It should perhaps be more plausible to conceive that the vacuum
energy, is actually a dynamical quantity as the Universe itself. More
generally, we could even entertain the possibility that the total DE is in fact
a mixture of vacuum energy and other dynamical components (e.g. fields, higher
order terms in the effective action etc) which can be represented collectively
by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore,
acts as a dynamical DE component different from the vacuum energy. While it can
actually behave phantom-like by itself, the overall DE fluid may effectively
appear as standard quintessence, or even mimic at present an almost exact CC
behavior. Thanks to the versatility of such cosmic fluid we can show that a
composite DE system of this sort (``LXCDM'') may have a key to resolving the
mysterious coincidence problem.Comment: LaTeX, 13 pages, 5 figure
An associative memory for the on-line recognition and prediction of temporal sequences
This paper presents the design of an associative memory with feedback that is
capable of on-line temporal sequence learning. A framework for on-line sequence
learning has been proposed, and different sequence learning models have been
analysed according to this framework. The network model is an associative
memory with a separate store for the sequence context of a symbol. A sparse
distributed memory is used to gain scalability. The context store combines the
functionality of a neural layer with a shift register. The sensitivity of the
machine to the sequence context is controllable, resulting in different
characteristic behaviours. The model can store and predict on-line sequences of
various types and length. Numerical simulations on the model have been carried
out to determine its properties.Comment: Published in IJCNN 2005, Montreal, Canad
Vacuum effective action and inflation
We consider vacuum quantum effects in the Early Universe, which may lead to
inflation. The inflation is a direct consequence of the supposition that, at
high energies, all the particles can be described by the weakly interacting,
massless, conformally invariant fields. We discuss, from the effective field
theory point of view, the stability of inflation, transition to the FRW
solution, and also possibility to study metric and density perturbations.Comment: 6 pages, LaTeX, no figures. Contribution to the Proceedings of the X
Jorge Andre Swieca school in Particles and Fields. To be published in World
Scientifi
Color superconducting matter in a magnetic field
We investigate the effect of a magnetic field on cold dense three-flavor
quark matter using an effective model with four-Fermi interactions with
electric and color neutrality taken into account. The gap parameters Delta_1,
Delta_2, and Delta_3 representing respectively the predominant pairing between
down and strange (d-s) quarks, strange and up (s-u) quarks, and up and down
(u-d) quarks, show the de Haas-van Alphen effect, i.e. oscillatory behavior as
a function of the modified magnetic field B that can penetrate the color
superconducting medium. Without applying electric and color neutrality we find
Delta_2 \approx Delta_3 >> Delta_1 for 2 e B / mu_q^2, where e is the modified
electromagnetic coupling constant and mu_q is one third of the baryon chemical
potential. Because the average Fermi surface for each pairing is affected by
taking into account neutrality, the gap structure changes drastically in this
case; we find Delta_1 >> Delta_2 \approx Delta_3 for 2 e B > mu_q^2. We point
out that the magnetic fields as strong as presumably existing inside magnetars
might induce significant deviations from the gap structure Delta_1 \approx
Delta_2 \approx Delta_3 at zero magnetic field.Comment: 5 pages, 3 figure
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