648 research outputs found
Radiation from a D-dimensional collision of shock waves: first order perturbation theory
We study the spacetime obtained by superimposing two equal Aichelburg-Sexl
shock waves in D dimensions traveling, head-on, in opposite directions.
Considering the collision in a boosted frame, one shock becomes stronger than
the other, and a perturbative framework to compute the metric in the future of
the collision is setup. The geometry is given, in first order perturbation
theory, as an integral solution, in terms of initial data on the null surface
where the strong shock has support. We then extract the radiation emitted in
the collision by using a D-dimensional generalisation of the Landau-Lifschitz
pseudo-tensor and compute the percentage of the initial centre of mass energy
epsilon emitted as gravitational waves. In D=4 we find epsilon=25.0%, in
agreement with the result of D'Eath and Payne. As D increases, this percentage
increases monotonically, reaching 40.0% in D=10. Our result is always within
the bound obtained from apparent horizons by Penrose, in D=4, yielding 29.3%,
and Eardley and Giddings, in D> 4, which also increases monotonically with
dimension, reaching 41.2% in D=10. We also present the wave forms and provide a
physical interpretation for the observed peaks, in terms of the null generators
of the shocks.Comment: 27 pages, 11 figures; v2 some corrections, including D dependent
factor in epsilon; matches version accepted in JHE
'Mindless markers of the nation': The routine flagging of nationhood across the visual environment
The visual environment has increasingly been used as a lens with which to understand wider processes of social and economic change with studies employing in-depth qualitative approaches to focus on, for example, gentrification or trans-national networks. This exploratory paper offers an alternative perspective by using a novel method, quantitative photo mapping, to examine the extent to which a particular socio-cultural marker, the nation, is âflaggedâ across three contrasting sites in Britain. As a multi-national state with an increasingly diverse population, Britain offers a particularly fruitful case study, drawing in debates around devolution, European integration and Commonwealth migration. In contributing to wider debates around banal nationalism, the paper notes the extent to which nations are increasingly articulated through commerce, consumption and market exchange and the overall significance of everyday markers (signs, objects, infrastructure) in naturalising a national view of the world
Mass Dependent Evolution and the Light Gluino Existence
There is an intriguing discrepancy between \alpha_s(M_Z) values measured
directly at the CERN -factory and low-energy (at few GeV) measurements
transformed to by a massless QCD \alpha_s(Q) evolution relation.
There exists an attempt to reconcile this discrepancy by introducing a light
gluino \gl in the MSSM.
We study in detail the influence of heavy thresholds on \alpha_s(Q)
evolution. First, we consruct the "exact" explicit solution to the
mass-dependent two-loop RG equation for the running \alpha_s(Q). This solution
describes heavy thresholds smoothly. Second, we use this solution to
recalculate anew \alpha_s(M_Z) values corresponding to "low-energy" input data.
Our analysis demonstrates that using {\it mass-dependent RG procedure}
generally produces corrections of two types: Asymptotic correction due to
effective shift of threshold position; Local threshold correction only for the
case when input experiment lies in the close vicinity of heavy particle
threshold: .
Both effects result in the effective shift of the \asmz values of the order
of . However, the second one could be enhanced when the gluino mass is
close to a heavy quark mass. For such a case the sum effect could be important
for the discussion of the light gluino existence as it further changes the
\gl mass.Comment: 13, Late
Colliding AdS gravitational shock waves in various dimensions and holography
The formation of marginally trapped surfaces in the off-center collision of
two shock waves on AdS_D (with D=4,5,6,7 and 8) is studied numerically. We
focus on the case when the two waves collide with nonvanishing impact parameter
while the sources are located at the same value of the holographic coordinate.
In all cases a critical value of the impact parameter is found above which no
trapped surface is formed. The numerical results show the existence of a simple
scaling relation between the critical impact parameter and the energy of the
colliding waves. Using the isometries of AdS_D we relate the solutions obtained
to the ones describing the collision of two waves with a purely holographic
impact parameter. This provides a gravitational dual for the head-on collision
of two lumps of energy of unequal size.Comment: 25 pages, 11 figures. v2: minor changes, typos corrected. To appear
in JHE
Fragility of the Free-Energy Landscape of a Directed Polymer in Random Media
We examine the sensitiveness of the free-energy landscape of a directed
polymer in random media with respect to various kinds of infinitesimally weak
perturbation including the intriguing case of temperature-chaos. To this end,
we combine the replica Bethe ansatz approach outlined in cond-mat/0112384, the
mapping to a modified Sinai model and numerically exact calculations by the
transfer-matrix method. Our results imply that for all the perturbations under
study there is a slow crossover from a weakly perturbed regime where rare
events take place to a strongly perturbed regime at larger length scales beyond
the so called overlap length where typical events take place leading to chaos,
i.e. a complete reshuffling of the free-energy landscape. Within the replica
space, the evidence for chaos is found in the factorization of the replicated
partition function induced by infinitesimal perturbations. This is the reflex
of explicit replica symmetry breaking.Comment: 29 pages, Revtex4, ps figure
A hierarchical model for aging
We present a one dimensional model for diffusion on a hierarchical tree
structure. It is shown that this model exhibits aging phenomena although no
disorder is present. The origin of aging in this model is therefore the
hierarchical structure of phase space.Comment: 10 pages LaTeX, 4 postscript-figures include
Critical Trapped Surfaces Formation in the Collision of Ultrarelativistic Charges in (A)dS
We study the formation of marginally trapped surfaces in the head-on
collision of two ultrarelativistic charges in space-time. The metric of
ultrarelativistic charged particles in is obtained by boosting
Reissner-Nordstr\"om space-time to the speed of light. We show that
formation of trapped surfaces on the past light cone is only possible when
charge is below certain critical - situation similar to the collision of two
ultrarelativistic charges in Minkowski space-time. This critical value depends
on the energy of colliding particles and the value of a cosmological constant.
There is richer structure of critical domains in case. In this case
already for chargeless particles there is a critical value of the cosmological
constant only below which trapped surfaces formation is possible. Appearance of
arbitrary small nonzero charge significantly changes the physical picture.
Critical effect which has been observed in the neutral case does not take place
more. If the value of the charge is not very large solution to the equation on
trapped surface exists for any values of cosmological radius and energy density
of shock waves. Increasing of the charge leads to decrease of the trapped
surface area, and at some critical point the formation of trapped surfaces of
the type mentioned above becomes impossible.Comment: 30 pages, Latex, 7 figures, Refs. added and typos correcte
Scaling properties in off equilibrium dynamical processes
In the present paper, we analyze the consequences of scaling hypotheses on
dynamic functions, as two times correlations . We show, under general
conditions, that must obey the following scaling behavior , where the scaling variable is
and , two
undetermined functions. The presence of a non constant exponent
signals the appearance of multiscaling properties in the dynamics.Comment: 6 pages, no figure
The information paradox and the locality bound
Hawking's argument for information loss in black hole evaporation rests on
the assumption of independent Hilbert spaces for the interior and exterior of a
black hole. We argue that such independence cannot be established without
incorporating strong gravitational effects that undermine locality and
invalidate the use of quantum field theory in a semiclassical background
geometry. These considerations should also play a role in a deeper
understanding of horizon complementarity.Comment: 21 pages, harvmac; v2-3. minor corrections, references adde
An attempt to delineate very low frequency electromagnetic signals associated with earthquakes
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