32 research outputs found
On boosted space-times with cosmological constant and their ultrarelativistic limit
The problem of deriving a shock-wave geometry with cosmological constant by
boosting a Schwarzschild-de Sitter (or anti-de Sitter) black hole is
re-examined. Unlike previous work in the literature, we deal with the exact
Schwarzschild-de Sitter (or anti-de Sitter) metric. By virtue of peculiar
cancellations in this exact calculation, where the metric does not depend
linearly on the mass parameter, we find a singularity of distributional nature
on a null hypersurface, which corresponds to a shock-wave geometry derived in a
fully non-perturbative way. The result agrees with previous calculations, where
the metric had been linearized in the mass parameter.Comment: 13 pages, Latex file. In the final version, the presentation has been
improved, and two references have been adde
On off-shell bosonic string amplitudes
We give a simple prescription for computing, in the framework of the bosonic
string theory, off-shell one-loop amplitudes with any number of external
massless particles, both for the open and for the closed string. We discuss
their properties and, in particular, for the two-string one-loop amplitudes we
show their being transverse.Comment: 12 pages, Latex. One reference added. Introduction and conclusions
expanded. Some other minor changes in the tex
N=1 Matter from Fractional Branes
We study a bound state of fractional D3-branes localized inside the
world-volume of fractional D7-branes on the orbifold C^3/Z_2 x Z_2. We
determine the open string spectrum that leads to N=1 U(N1)xU(N2)xU(N3)xU(N4)
gauge theory with matter having the number of D7-branes as a flavor index. We
derive the linearized boundary action of the D7-brane on this orbifold using
the boundary state formalism and we discuss the tadpole cancellation. After
computing the asymptotic expression of the supergravity solution the anomalies
of the gauge theory are reproduced.Comment: LaTeX 20 pages, 1 figure, small changes and references adde
Stretched Horizon and Entropy of Superstars
Amongst the class of supergravity solutions found by Lin, Lunin and
Maldacena, we consider pure and mixed state configurations generated by phase
space densities in the dual fermionic picture. A one-to-one map is constructed
between the phase space densities and piecewise monotonic curves, which
generalize the Young diagrams corresponding to pure states. Within the
fermionic phase space picture, a microscopic formula for the entropy of mixed
states is proposed. Considering thermal ensembles, agreement is found between
the thermodynamic and the proposed microscopic entropies. Furthermore, we study
fluctuations in thermodynamic ensembles for the superstar and compare the
entropy of these ensembles with the area of stretched horizons predicted by the
mean fluctuation size.Comment: 21 pages, 3 figures, 2 references adde