1,059 research outputs found
A no-go theorem for accelerating cosmologies from M-theory compactifications
It is known that four-dimensional cosmologies exhibiting transient phases of
acceleration can be obtained by compactifications of low-energy effective
string or M-theory on time-varying manifolds. In the four-dimensional theory,
the acceleration can be attributed to a quintessential scalar field with a
positive effective potential. Recently, Townsend has conjectured that the
potentials obtained by such compactifications cannot give rise to late-time
accelerating universes which possess future event horizons. Such a `no-go'
result would be desirable, since current string or M-theory seems unable to
provide an adequate description of space-times with future event horizons. In
this letter, we provide a proof of this conjecture for a class of warped
compactifications with a single scalar modulus parametrising the volume of the
compactification manifold.Comment: 11 pages, LaTeX; minor changes and clarifications mad
Monopoles, vortices and kinks in the framework of non-commutative geometry
Non-commutative differential geometry allows a scalar field to be regarded as
a gauge connection, albeit on a discrete space. We explain how the underlying
gauge principle corresponds to the independence of physics on the choice of
vacuum state, should it be non-unique. A consequence is that Yang-Mills-Higgs
theory can be reformulated as a generalised Yang-Mills gauge theory on
Euclidean space with a internal structure. By extending the Hodge star
operation to this non-commutative space, we are able to define the notion of
self-duality of the gauge curvature form in arbitrary dimensions. It turns out
that BPS monopoles, critically coupled vortices, and kinks are all self-dual
solutions in their respective dimensions. We then prove, within this unified
formalism, that static soliton solutions to the Yang-Mills-Higgs system exist
only in one, two and three spatial dimensions.Comment: 28 pages, Late
Bounds on negative energy densities in static space-times
Certain exotic phenomena in general relativity, such as backward time travel,
appear to require the presence of matter with negative energy. While quantum
fields are a possible source of negative energy densities, there are lower
bounds - known as quantum inequalities - that constrain their duration and
magnitude. In this paper, we derive new quantum inequalities for scalar fields
in static space-times, as measured by static observers with a choice of
sampling function. Unlike those previously derived by Pfenning and Ford, our
results do not assume any specific sampling function. We then calculate these
bounds in static three- and four-dimensional Robertson-Walker universes, the de
Sitter universe, and the Schwarzschild black hole. In each case, the new
inequality is stronger than that of Pfenning and Ford for their particular
choice of sampling function.Comment: 22 pages, 5 figures; LaTeX; minor changes mad
Algebraic analysis of Trivium-like ciphers
Trivium is a bit-based stream cipher in the final portfolio of the eSTREAM project. In this paper, we apply the approach of Berbain et al. to Trivium-like ciphers and perform new algebraic analyses on them, namely Trivium and its reduced versions: Trivium-N, Bivium-A and Bivium-B. In doing so, we answer an open question in the literature. We demonstrate a new algebraic attack on Bivium-A. This attack requires less time and memory than previous techniques which use the F4 algorithm to recover Bivium-A's initial state. Though our attacks on Bivium-B, Trivium and Trivium-N are worse than exhaustive keysearch, the systems of equations which are constructed are smaller and less complex compared to previous algebraic analysis. Factors which can affect the complexity of our attack on Trivium-like ciphers are discussed in detail
Mesh versus non-mesh for inguinal and femoral hernia repair (Protocol)
This is the protocol for a review and there is no abstract. The objectives are as follows: To evaluate the outcomes of inguinal and femoral hernia repair techniques in adults, specifically comparing closure with mesh versus without mesh
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