2,590 research outputs found
S-matrix for s-wave gravitational scattering
In the s-wave approximation the 4D Einstein gravity with scalar fields can be
reduced to an effective 2D dilaton gravity coupled nonminimally to the matter
fields. We study the leading order (tree level) vertices. The 4-particle matrix
element is calculated explicitly. It is interpreted as scattering with
formation of a virtual black hole state. As one novel feature we predict the
gravitational decay of s-waves.Comment: 9 pages, 1 figure, added clarifying comments in the introduction, the
conclusion, and the virtual black hole sectio
Two-Dilaton Theories in Two Dimensions from Dimensional Reduction
Dimensional reduction of generalized gravity theories or string theories
generically yields dilaton fields in the lower-dimensional effective theory.
Thus at the level of D=4 theories, and cosmology many models contain more than
just one scalar field (e.g. inflaton, Higgs, quintessence). Our present work is
restricted to two-dimensional gravity theories with only two dilatons which
nevertheless allow a large class of physical applications.
The notions of factorizability, simplicity and conformal simplicity, Einstein
form and Jordan form are the basis of an adequate classification. We show that
practically all physically motivated models belong either to the class of
factorizable simple theories (e.g. dimensionally reduced gravity, bosonic
string) or to factorizable conformally simple theories (e.g. spherically
reduced Scalar-Tensor theories). For these theories a first order formulation
is constructed straightforwardly. As a consequence an absolute conservation law
can be established.Comment: 23 pages, 1 tabl
Effective Action and Hawking Flux from Covariant Perturbation Theory
The computation of the radiation flux related to the Hawking temperature of a
Schwarzschild Black Hole or another geometric background is still well-known to
be fraught with a number of delicate problems. In spherical reduction, as shown
by one of the present authors (W. K.) with D.V. Vassilevich, the correct black
body radiation follows when two ``basic components'' (conformal anomaly and a
``dilaton'' anomaly) are used as input in the integrated energy-momentum
conservation equation. The main new element in the present work is the use of a
quite different method, the covariant perturbation theory of Barvinsky and
Vilkovisky, to establish directly the full effective action which determines
these basic components. In the derivation of W. K. and D.V. Vassilevich the
computation of the dilaton anomaly implied one potentially doubtful
intermediate step which can be avoided here. Moreover, the present approach
also is sensitive to IR (renormalisation) effects. We realize that the
effective action naturally leads to expectation values in the Boulware vacuum
which, making use of the conservation equation, suffice for the computation of
the Hawking flux in other quantum states, in particular for the relevant Unruh
state. Thus, a rather comprehensive discussion of the effects of (UV and IR)
renormalisation upon radiation flux and energy density is possible.Comment: 26 page
Hawking radiation from dilaton gravity in 1 + 1 dimensions: a pedagogical review
Hawking radiation in d=4 is regarded as a well understood quantum theoretical
feature of Black Holes or of other geometric backgrounds with an event horizon.
On the other hand, the dilaton theory emerging after spherical reduction and
generalized dilaton theories only during the last years became the subject of
numerous studies which unveiled a surprisingly difficult situation. Recently we
have found some solution to the problem of Hawking flux in spherically reduced
gravity which has the merit of using a minimal input. It leads to exact
cancellation of negative contributions to this radiative flux, encountered in
other approaches at infinity, so that our result asymptotically coincides with
the one of minimally coupled scalars. The use of an integrated action is
avoided - although we have been able to present also that quantity in a closed
expression. This short review also summarizes and critically discusses recent
activities in this field, including the problem of ``conformal frames'' for the
background and questions which seem to be open in our own approach as well as
in others.Comment: latex2e, to appear in Annalen der Physi
Renormalizability of the open string sigma model and emergence of D-branes
Rederiving the one-loop divergences for the most general coupling of the open
string sigma model by the heat kernel technique, we distinguish the classical
background field from the mean field of the effective action. The latter is
arbitrary, i.e. does not fulfil the boundary conditions. As a consequence a new
divergent counter term strongly suggests the introduction of another external
one-form field (beside the usual gauge field), coupled to the normal derivative
at the boundary. Actually such a field has been proposed in the literature for
different reasons, but its full impact never seems to have thoroughly
investigated before. The beta function for the resulting renormalizable model
is calculated and the consequences are discussed, including the ones for the
Born-Infeld action. The most exciting property of the new coupling is that it
enters the coefficient in front of the normal derivative in Neumann boundary
conditions. For certain values of the background fields this coefficient
vanishes, leading to Dirichlet boundary conditions. This provides a natural
mechanism for the emergence of D-branes.Comment: 24 pages, a reference and discussion (about 1 page, sec. 3.3 and 4.1)
added, typos correcte
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