402 research outputs found
Phenomenological and statistical analysis of fracture in polycrystalline aluminum oxide
Phenomenological and statistical analysis of fracture in polycrystalline aluminum oxid
Chasing Nomadic Worlds: A New Class of Deep Space Missions
Nomadic worlds, i.e., objects not gravitationally bound to any star(s), are
of great interest to planetary science and astrobiology. They have garnered
attention recently due to constraints derived from microlensing surveys and the
recent discovery of interstellar planetesimals. In this paper, we roughly
estimate the prevalence of nomadic worlds with radii of . The cumulative number density
appears to follow a heuristic power law given by . Therefore, smaller objects are probably much more numerous
than larger rocky nomadic planets, and statistically more likely to have
members relatively close to the inner Solar system. Our results suggest that
tens to hundreds of planet-sized nomadic worlds might populate the spherical
volume centered on Earth and circumscribed by Proxima Centauri, and may thus
comprise closer interstellar targets than any planets bound to stars. For the
first time, we systematically analyze the feasibility of exploring these
unbounded objects via deep space missions. We investigate what near-future
propulsion systems could allow us to reach nomadic worlds of radius in a
-year flight timescale. Objects with km are within the purview
of multiple propulsion methods such as electric sails, laser electric
propulsion, and solar sails. In contrast, nomadic worlds with
km are accessible by laser sails (and perhaps nuclear fusion), thereby
underscoring their vast potential for deep space exploration.Comment: 22 pages including "Highlights" page; accepted by Acta Astronautic
Moduli-Space Approximation for BPS Brane-Worlds
We develop the moduli-space approximation for the low energy regime of
BPS-branes with a bulk scalar field to obtain an effective four-dimensional
action describing the system. An arbitrary BPS potential is used and account is
taken of the presence of matter in the branes and small supersymmetry breaking
terms. The resulting effective theory is a bi-scalar tensor theory of gravity.
In this theory, the scalar degrees of freedom can be stabilized naturally
without the introduction of additional mechanisms other than the appropriate
BPS potential. We place observational constraints on the shape of the potential
and the global configuration of branes.Comment: 10 pages, 1 figur
Cosmic String Wakes in Scalar-Tensor Gravities
The formation and evolution of cosmic string wakes in the framework of a
scalar-tensor gravity are investigated in this work. We consider a simple model
in which cold dark matter flows past an ordinary string and we treat this
motion in the Zel'dovich approximation. We make a comaprison between our
results and previous results obtained in the context of General Relativity. We
propose a mechanism in which the contribution of the scalar field to the
evolution of the wakes may lead to a cosmological observation.Comment: Replaced version to be published in the Classical and Quantum Gravit
Low Energy Branes, Effective Theory and Cosmology
The low energy regime of cosmological BPS-brane configurations with a bulk
scalar field is studied. We construct a systematic method to obtain
five-dimensional solutions to the full system of equations governing the
geometry and dynamics of the bulk. This is done for an arbitrary bulk scalar
field potential and taking into account the presence of matter on the branes.
The method, valid in the low energy regime, is a linear expansion of the system
about the static vacuum solution. Additionally, we develop a four-dimensional
effective theory describing the evolution of the system. At the lowest order in
the expansion, the effective theory is a bi-scalar tensor theory of gravity.
One of the main features of this theory is that the scalar fields can be
stabilized naturally without the introduction of additional mechanisms,
allowing satisfactory agreement between the model and current observational
constraints. The special case of the Randall-Sundrum model is discussed.Comment: 19 pages, 4 figure
Vacuum Polarization in the Spacetime of a Scalar-Tensor Cosmic String
We study the vacuum polarization effect in the spacetime generated by a
magnetic flux cosmic string in the framework of a scalar-tensor gravity. The
vacuum expectation values of the energy-momentum tensor of a conformally
coupled scalar field are calculated. The dilaton's contribution to the vacuum
polarization effect is shown explicitly.Comment: 11 pages, LATEX file, 2 eps figure
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