308 research outputs found
Higher Derivative Field Theories: Degeneracy Conditions and Classes
We provide a full analysis of ghost free higher derivative field theories
with coupled degrees of freedom. Assuming the absence of gauge symmetries, we
derive the degeneracy conditions in order to evade the Ostrogradsky ghosts, and
analyze which (non)trivial classes of solutions this allows for. It is shown
explicitly how Lorentz invariance avoids the propagation of "half" degrees of
freedom. Moreover, for a large class of theories, we construct the field
redefinitions and/or (extended) contact transformations that put the theory in
a manifestly first order form. Finally, we identify which class of theories
cannot be brought to first order form by such transformations.Comment: 26 pages, 1 figure. v2: minor changes, references added, matches
version published in JHE
FRW Cosmological Perturbations in Massive Bigravity
Cosmological perturbations of FRW solutions in ghost free massive bigravity,
including also a second matter sector, are studied in detail. At early time, we
find that sub horizon exponential instabilities are unavoidable and they lead
to a premature departure from the perturbative regime of cosmological
perturbations.Comment: latex, 14 page
Smart Procurement Of Naturally Generated Energy (SPONGE) for PHEV's
In this paper we propose a new engine management system for hybrid vehicles
to enable energy providers and car manufacturers to provide new services.
Energy forecasts are used to collaboratively orchestrate the behaviour of
engine management systems of a fleet of PHEV's to absorb oncoming energy in an
smart manner. Cooperative algorithms are suggested to manage the energy
absorption in an optimal manner for a fleet of vehicles, and the mobility
simulator SUMO is used to show simple simulations to support the efficacy of
the proposed idea.Comment: Updated typos with respect to previous versio
Finite Energy of Black Holes in Massive Gravity
In GR the static gravitational potential of a self-gravitating body goes as
1/r at large distances and any slower decrease leads to infinity energy. We
show that in a class of four-dimensional massive gravity theories there exists
spherically symmetric solutions with finite total energy, featuring an
asymptotic behavior slower than 1/r and generically of the form .
This suggests that configurations with nonstandard asymptotics may well turn
out to be physical. The effect is due to an extra field coupled only
gravitationally, which allows for modifications of the static potential
generated by matter, while counterbalancing the apparently infinite energy
budget.Comment: 4 page
Spherically Symmetric Solutions in Ghost-Free Massive Gravity
Recently, a class of theories of massive gravity has been shown to be
ghost-free. We study the spherically symmetric solutions in the bigravity
formulation of such theories. In general, the solutions admit both a Lorentz
invariant and a Lorentz breaking asymptotically flat behaviour and also fall in
two branches. In the first branch, all solutions can be found analitycally and
are Schwarzschild-like, with no modification as is found for other classes of
theories. In the second branch, exact solutions are hard to find, and relying
on perturbation theory, Yukawa-like modifications of the static potential are
found. The general structure of the solutions suggests that the bigravity
formulation of massive gravity is crucial and more than a tool.Comment: 15 pages. Some change in the reference
An Assessment on the Use of Stationary Vehicles as a Support to Cooperative Positioning
In this paper, we consider the use of stationary vehicles as tools to enhance
the localisation capabilities of moving vehicles in a VANET. We examine the
idea in terms of its potential benefits, technical requirements, algorithmic
design and experimental evaluation. Simulation results are given to illustrate
the efficacy of the technique.Comment: This version of the paper is an updated version of the initial
submission, where some initial comments of reviewers have been taken into
accoun
Smart Procurement of Naturally Generated Energy (SPONGE) for Plug-in Hybrid Electric Buses
We discuss a recently introduced ECO-driving concept known as SPONGE in the
context of Plug-in Hybrid Electric Buses (PHEB)'s.Examples are given to
illustrate the benefits of this approach to ECO-driving. Finally, distributed
algorithms to realise SPONGE are discussed, paying attention to the privacy
implications of the underlying optimisation problems.Comment: This paper is recently submitted to the IEEE Transactions on
Automation Science and Engineerin
Vainshtein mechanism after GW170817
The almost simultaneous detection of gravitational waves and a short
gamma-ray burst from a neutron star merger has put a tight constraint on the
difference between the speed of gravity and light. In the four-dimensional
scalar-tensor theory with second order equations of motion, the Horndeski
theory, this translates into a significant reduction of the viable parameter
space of the theory. Recently, extensions of Horndeski theory, which are free
from Ostrogradsky ghosts despite the presence of higher order derivatives in
the equations of motion, have been identified and classified exploiting the
degeneracy criterium. In these new theories, the fifth force mediated by the
scalar field must be suppressed in order to evade the stringent Solar System
constraints. We study the Vainshtein mechanism in the most general degenerate
higher order scalar-tensor theory in which light and gravity propagate at the
same speed. We find that the Vainshtein mechanism generally works outside a
matter source but it is broken inside matter, similarly to beyond Horndeski
theories. This leaves interesting possibilities to test these theories that are
compatible with gravitational wave observations using astrophysical objects.Comment: 5 pages, no figure, added references, corrected typos, accepted for
publication in Physical Review
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