59 research outputs found
Generating solutions via sigma-models
We review recent development of solution-generating techniques for four and
five-dimensional Einstein equations coupled to vector and scalar fields. This
includes D=4 Einstein-Maxwell-dilaton-axion theory with multiple vector fields,
D=5 Einstein-Maxwell gravity with the Chern-Simons term (minimal
five-dimensional supergravity), and some other models which attracted attention
in connection with black rings. The method is based on reduction to
three-dimensional gravity coupled sigma-models with symmetric target spaces.
Our recent results open a way to construct the general charged black rings in
five-dimensional supergravity possibly coupled to vector multiplets.Comment: An updated version of the talk given at ICGA8 and published in the
Proceedings. 14 pages, ptpte
Quantized rotating Taub-bolt instantons
We argue that previously suggested metrics for rotating Taub-bolt instantons
do not satisfy all the necessary regularity conditions, and we present a family
of new regular rotating Taub-bolts labelled by an odd integer . There are
two types of rotating bolt solutions. The first infinite sequence starts with
non-rotating Taub-NUT with positive mass for and goes to
for (or for ), where is the rotation parameter.
For the second sequence of rotating Page bolts, the masses go through the
value for and asymptote to for .Comment: The paper has been withdrawn by the authors since the solutions
obtained were found not to be completely regular eithe
Solitons and black holes in Einstein-Born-Infeld-dilaton theory
Static spherically symmetric asymptotically flat solutions to the U(1)
Born-Infeld theory coupled to gravity and to a dilaton are investigated. The
dilaton enters in such a way that the theory is - symmetric for a unit
value of the dilaton coupling constant. We find globally regular solutions for
any value of the effective coupling which is the ratio of the gravitational and
dilaton couplings; they form a continuous sequence labeled by the sole free
parameter of the local solution near the origin. The allowed values of this
parameter are bounded from below, and, as the limiting value is approached, the
mass and the dilaton charge rise indefinitely and tend to a strict equality (in
suitable units). Together with the electric charge they saturate the BPS bound
of the corresponding linear U(1) theory. Beyond this boundary the solutions
become compact (singular), while the limiting solution at the exact boundary
value is non-compact and non-asymptotically flat. The black holes in this
theory exist for any value of the event horizon radius and also form a sequence
labelled by a continuously varying parameter restricted to a finite interval.
The singularity inside the black hole exhibits a power-law mass inflation.Comment: Latex, 16 pages, 6 .ps figure
Piercing of domain walls: new mechanism of gravitational radiation
Domain wall (DW) moving in media undergoes the friction force due to particle
scattering. However certain particles are not scattered, but perforate the
wall. As a result, the wall gets excited in the form of the branon wave, while
the particle experiences an acceleration jump. This gives rise to generation of
gravitational waves which we call "piercing gravitational radiation" (PGR).
Though this effect is of higher order in the gravitational constant than the
quadrupole radiation from the collapsing DWs, its amplitude is enhanced in the
case of relativistic particles or photons because of absence of the velocity
factor which is present in the quadrupole formula. We derive the
spectral-angular distribution of PGR within the simplified model of the weakly
gravitating particle-wall system in Minkowski space-time of arbitrary
dimensions. Within this model the radiation amplitude is obtained analytically.
The spectral-angular distribution of PGR in such an approach suffers from
infrared and ultraviolet divergences as well as from collinear divergence in
the case of a massless perforating particle. Different cut-off schemes
appropriate in various dimensions are discussed. Our results are applicable
both to cosmological DWs and to the braneworld models.Comment: 30 pages, 8 figure
Oxidation of D=3 cosets and Bonnor dualities in
Bonnor's map in General Relativity is duality between (dimensionally reduced)
vacuum gravity and static truncation of electro-vacuum theory. It was used as a
tool to generate an exact solution of electro-vacuum from some vacuum solution.
It can be expected that similar dualities will be useful for solution
generation in higher-dimensional theories too. Here we study such maps within a
class of theories in dimensions using oxidation of D=3 cosets and
consistent truncation of the corresponding theories. Our class includes those
theories whose D=3 symmetries are subgroups of . It contains
six-dimensional minimal supergravity, five-dimensional minimal and
supergravities and a number of four-dimensional theories which attracted
attention recently in the search of exact solutions. We give explicit
reduction/truncation formulas relating different theories in dimensions in terms of metrics and matter fields and discuss various alternative
duality chains between them.Comment: Two references added, Introduction slightly edited, mispints removed.
Journal versio
Conformal and kinetic couplings as two Jordan frames of the same theory
Non-minimal scalar-tensor (ST) theories may admit an Einstein frame
representation, where gravity is described by the Einstein-Hilbert action plus
the scalar sector. Some STs become just {\em minimal} Einstein-scalar (MES)
theories, notable examples are Brans-Dicke and . ST theories with {\em
derivative} coupling can also be reduced to an Einstein frame by disformal
transformations, but, as a rule, their scalar sector will contain higher
derivative terms. Here we draw attention to a new Palatini kinetically coupled
theory which can be reduced to pure MES by an invertible disformal
transformation. This theory can then be converted into the Jordan frame of
another ST, , which also admits an invertible transformation to MES.
Two theories, each of which is dual to MES, will be {\em sequentially dual} to
each other and can be considered as two different Jordan frames of the same
theory. Both chosen theories violate null energy condition. Transforming the
same singular MES solutions, we find the desingularization signs in both Jordan
frames, but these are more pronouned in the framework of kinetic theory,
leading, in the cosmological case, to Genesis-type behavior.Comment: 25 page
Rehabilitating space-times with NUTs
We revisit the Taub-NUT solution of the Einstein equations without time
periodicity condition, showing that the Misner string is still fully
transparent for geodesics. In this case, analytic continuation can be carried
out through both horizons leading to a Hausdorff spacetime without a central
singularity, and thus geodesically complete. Furthermore, we show that, in
spite of the presence of a region containing closed time-like curves, there are
no closed causal {\em geodesics}. Thus, some longstanding obstructions to
accept the Taub-NUT solution as physically relevant are removed.Comment: 9 pages revtex 4, Journal versio
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