315 research outputs found
Existence Theorems for Hairy Black Holes in su(N) Einstein-Yang-Mills theories
We establish the existence of hairy black holes in su(N) Einstein-Yang-Mills
theories, described by N-1 parameters, corresponding to the nodes of the gauge
field functions.Comment: 64 pages, latex2e, minor changes on the nature of the parameters,
version to appear in J. Math. Phy
Instability of hairy black holes in spontaneously-broken Einstein-Yang-Mills-Higgs systems
The stability of a new class of hairy black hole solutions in the coupled
system of Einstein-Yang-Mills-Higgs is examined, generalising a method
suggested by Brodbeck and Straumann and collaborators, and Volkov and Gal'tsov.
The method maps the algebraic system of linearised radial perturbations of the
various field modes around the black hole solution into a coupled system of
radial equations of Schr\"odinger type. No detailed knowledge of the black hole
solution is required, except from the fact that the boundary conditions at the
physical space-time boundaries (horizons) must be such so as to guarantee the
{\it finiteness} of the various expressions involved. In this way, it is
demonstrated that the above Schr\"odinger equations have bound states, which
implies the instability of the associated black hole solution.Comment: 8 pages, LATE
Dark Energy as an off-shell Tachyon Background in four-dimensional Strings
We consider a time-dependent bosonic string in graviton, dilaton and tachyon
backgrounds, for which it was already shown that conformal invariance is
respected to all orders in alpha' and in any space-time dimension. Assuming
that the tachyon is off-shell, we show in this note that the specific
time-homogeneity of the corresponding space time effective action leads to a
power-law expanding Universe, that can be accelerating or decelerating,
depending on the range of parameter space considered. We interpret this result
as dark energy. We arrive at this result without requiring the knowledge of the
structure of the string effective action, which is not known to all orders in
alpha'. Moreover, in our approach, the background configurations are consistent
in a four-dimensional space time, without any need for extra dimensions.Comment: 10 pages, typos correcte
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