8 research outputs found
Lorentz-violation and cosmological perturbations: a toy brane-world model
We study possible effects of Lorentz-violation on the generation of
cosmological perturbations at inflation by introducing a simple inflating
five-dimensional brane-world setup with violation of four-dimensional
Lorentz-invariance at an energy scale . We consider massless scalar field,
meant to mimic perturbations of inflaton and/or gravitational field, in this
background. At three-momenta below , there exists a zero mode localized on
the brane, whose behaviour coincides with that in four-dimensional theory. On
the contrary, at three-momenta above , the localized mode is absent and
physics is entirely five-dimensional. As three-momenta get redshifted, more
modes get localized on the brane, the phenomenon analogous to ``mode
generation''. We find that for , where is the inflationary Hubble
scale, the spectrum of perturbations coincides with that in four-dimensional
theory. For and time-dependent bulk parameters, the spectrum deviates,
possibly strongly, from the flat spectrum even for pure de Sitter inflation.Comment: 5 figures, iopart, minor changes, appendix adde
UV stable, Lorentz-violating dark energy with transient phantom era
Phantom fields with negative kinetic energy are often plagued by the vacuum
quantum instability in the ultraviolet region. We present a Lorentz-violating
dark energy model free from this problem and show that the crossing of the
cosmological constant boundary w=-1 to the phantom equation of state is
realized before reaching a de Sitter attractor. Another interesting feature is
a peculiar time-dependence of the effective Newton's constant; the magnitude of
this effect is naturally small but may be close to experimental limits. We also
derive momentum scales of instabilities at which tachyons or ghosts appear in
the infrared region around the present Hubble scale and clarify the conditions
under which tachyonic instabilities do not spoil homogeneity of the
present/future Universe.Comment: 22 pages, 7 figures; Presentation modified substantially, results and
conclusions unchanged. Journal versio
Ghost-free braneworld bigravity
We consider a generalisation of the DGP model, by adding a second brane with
localised curvature, and allowing for a bulk cosmological constant and brane
tensions. We study radion and graviton fluctuations in detail, enabling us to
check for ghosts and tachyons. By tuning our parameters accordingly, we find
bigravity models that are free from ghosts and tachyons. These models will lead
to large distance modifications of gravity that could be observable in the near
future.Comment: Dedicated to the memory of Ian Kogan. Version to appear in Classical
and Quantum Gravit
Gauge field localization on Abelian vortices in six dimensions
The vector and tensor fluctuations of vortices localizing gravity in the
context of the six-dimensional Abelian Higgs model are studied. These
string-like solutions break spontaneously six-dimensional Poincar\'e invariance
leading to a finite four-dimensional Planck mass and to a regular geometry both
in the bulk and on the core of the vortex. While the tensor modes of the metric
are decoupled and exhibit a normalizable zero mode, the vector fluctuations,
present in the gauge sector of the theory, are naturally coupled to the
graviphoton fields associated with the vector perturbations of the warped
geometry. Using the invariance under infinitesimal diffeomorphisms, it is found
that the zero modes of the graviphoton fields are never localized. On the
contrary,the fluctuations of the Abelian gauge field itself admit a
normalizable zero mode.Comment: 31 pages in Latex style with 3 figure