2,456 research outputs found
From k-essence to generalised Galileons
We determine the most general scalar field theories which have an action that
depends on derivatives of order two or less, and have equations of motion that
stay second order and lower on flat space-time. We show that those theories can
all be obtained from linear combinations of Lagrangians made by multiplying a
particular form of the Galileon Lagrangian by an arbitrary scalar function of
the scalar field and its first derivatives. We also obtain curved space-time
extensions of those theories which have second order field equations for both
the metric and the scalar field. This provide the most general extension, under
the condition that field equations stay second order, of k-essence, Galileons,
k-Mouflage as well as of the kinetically braided scalars. It also gives the
most general action for a scalar classicalizer, which has second order field
equations. We discuss the relation between our construction and the Euler
hierachies of Fairlie et al, showing in particular that Euler hierachies allow
one to obtain the most general theory when the latter is shift symmetric. As a
simple application of our formalism, we give the covariantized version of the
conformal Galileon.Comment: 25 page
Light from Cosmic Strings
The time-dependent metric of a cosmic string leads to an effective
interaction between the string and photons - the "gravitational Aharonov-Bohm"
effect -- and causes cosmic strings to emit light. We evaluate the radiation of
pairs of photons from cosmic strings and find that the emission from cusps,
kinks and kink-kink collisions occurs with a flat spectrum at all frequencies
up to the string scale. Further, cusps emit a beam of photons, kinks emit along
a curve, and the emission at a kink-kink collision is in all directions. The
emission of light from cosmic strings could provide an important new
observational signature of cosmic strings that is within reach of current
experiments for a range of string tensions.Comment: 8 pages, 1 figur
Prospective surveillance of invasive group a streptococcal disease, Fiji, 2005-2007.
We undertook a prospective active surveillance study of invasive group A streptococcal (GAS) disease in Fiji over a 23-month period, 2005-2007. We identified 64 cases of invasive GAS disease, which represents an average annualized all-ages incidence of 9.9 cases/100,000 population per year (95% confidence interval [CI] 7.6-12.6). Rates were highest in those >65 years of age and in those <5 years, particularly in infants, for whom the incidence was 44.9/100,000 (95% CI 18.1-92.5). The case-fatality rate was 32% and was associated with increasing age and underlying coexisting disease, including diabetes and renal disease. Fifty-five of the GAS isolates underwent emm sequence typing; the types were highly diverse, with 38 different emm subtypes and no particular dominant type. Our data support the view that invasive GAS disease is common in developing countries and deserves increased public health attention
Collisions of strings with Y junctions
We study the dynamics of Nambu--Goto strings with junctions at which three
strings meet. In particular, we exhibit one simple exact solution and examine
the process of intercommuting of two straight strings, in which they exchange
partners but become joined by a third string. We show that there are important
kinematical constraints on this process. The exchange cannot occur if the
strings meet with very large relative velocity. This may have important
implications for the evolution of cosmic superstring networks and non-abelian
string networks.Comment: 4 pages, 1 figure, uses revtex 4. Clarifying comments added to
correct a conceptual error, reference updated. Version accepted by Phys Rev
Letters, with additional references and minor change
Brane Gas Inflation
We consider the brane gas picture of the early universe. At later stages,
when there are no winding modes and the background is free to expand, we show
that a moving 3-brane, which we identify with our universe, can inflate even
though it is radiation-dominated. The crucial ingredients for successful
inflation are the coupling to the dilaton and the equation of state of the
bulk. If we suppose the brane initially forms in a collision of
higher-dimensional branes, then the spectrum of primordial density fluctuations
naturally has a thermal origin.Comment: 4 pages, 1 figur
PickCells: A Physically Reconfigurable Cell-composed Touchscreen
Touchscreens are the predominant medium for interactions with digital services; however, their current fixed form factor narrows the scope for rich physical interactions by limiting interaction possibilities to a single, planar surface. In this paper we introduce the concept of PickCells, a fully reconfigurable device concept composed of cells, that breaks the mould of rigid screens and explores a modular system that affords rich sets of tangible interactions and novel acrossdevice relationships. Through a series of co-design activities – involving HCI experts and potential end-users of such systems – we synthesised a design space aimed at inspiring future research, giving researchers and designers a framework in which to explore modular screen interactions. The design space we propose unifies existing works on modular touch surfaces under a general framework and broadens horizons by opening up unexplored spaces providing new interaction possibilities. In this paper, we present the PickCells concept, a design space of modular touch surfaces, and propose a toolkit for quick scenario prototyping
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