24 research outputs found
Strings as a Model for Parent and Baby Universes: Total Splitting Rates
Emission of hard microscopic string (graviton) by an excited macroscopic
string may be viewed as a model of branching of a -dimensional baby
universe off large parent one. We show that, apart from a trivial factor, the
total emission rate is not suppressed by the size of the macroscopic string.
This implies unsuppressed loss of quantum coherence in -dimensional
parent universe.Comment: 12 pages, LaTeX, 2 figures (bezier.sty
Gravity and antigravity in a brane world with metastable gravitons : Comment on hep-th/0002190 and hep-th/0003020
In the framework of a five-dimensional three-brane model with quasi-localized
gravitons we evaluate metric perturbations induced on the positive tension
brane by matter residing thereon. We find that at intermediate distances, the
effective four-dimensional theory coincides, up to small corrections, with
General Relativity. This is in accord with Csaki, Erlich and Hollowood and in
contrast to Dvali, Gabadadze and Porrati. We show, however, that at ultra-large
distances this effective four-dimensional theory becomes dramatically
different: conventional tensor gravity changes into scalar anti-gravity.Comment: 6 pages revtex, tex corrected, this paper should be read in tandem
with hep-th/000207
More about spontaneous Lorentz-violation and infrared modification of gravity
We consider a model with Lorentz-violating vector field condensates, in which
dispersion laws of all perturbations, including tensor modes, undergo
non-trivial modification in the infrared. The model is free of ghosts and
tachyons at high 3-momenta. At low 3-momenta there are ghosts, and at even
lower 3-momenta there exist tachyons. Still, with appropriate choice of
parameters, the model is phenomenologically acceptable. Beyond a certain large
distance scale and even larger time scale, the gravity of a static source
changes from that of General Relativity to that of van Dam--Veltman--Zakharov
limit of the Fierz--Pauli theory. Yet the late time cosmological evolution is
always determined by the standard Friedmann equation, modulo small correction
to the ``cosmological Planck mass'', so the modification of gravity cannot by
itself explain the accelerated expansion of the Universe. We argue that the
latter property is generic in a wide class of models with condensates.Comment: 15 pages, 1 figure, JHEP3.cls; Added reference
Wave function of the radion in a brane world
We calculate the linearized metric perturbation corresponding to a massless
four-dimensional scalar field, the radion, in a five-dimensional two-brane
model of Randall and Sundrum. In this way we obtain relative strengths of the
radion couplings to matter residing on each of the branes. The results are in
agreement with the analysis of Garriga and Tanaka of gravitational and
Brans--Dicke forces between matter on the branes. We also introduce a model
with infinite fifth dimension and ``almost'' confined graviton, and calculate
the radion properties in that model.Comment: 5 pages revtex, no figures, equation correcte
Quasi-localized states on noncommutative solitons
We consider noncommutative gauge theories which have zero mass states
propagating along both commutative and noncommutative dimensions. Solitons in
these theories generically carry U(m) gauge group on their world-volume. From
the point of view of string theory, these solitons correspond to
``branes within branes''. We show that once the world-volume U(m) gauge
theory is in the Higgs phase, light states become quasi-localized, rather than
strictly localized on the soliton, i.e. they mix with light bulk modes and have
finite widths to escape into the noncommutative dimensions. At small values of
U(m) symmetry breaking parameters, these widths are small compared to the
corresponding masses. Explicit examples considered are adjoint scalar field in
the background of a noncommutative vortex in U(1)-Higgs theory, and gauge
fields in instanton backgrounds in pure gauge noncommutative theories.Comment: 27 pages, references and comments added, final version to appear in
JHE
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
Geodesic (in) Completeness in General Metric Frames
The geometric concept of geodesic completeness depends on the choice of the metric field or “metric frame”. We develop a frame-invariant concept of “generalised geodesic completeness” or “time completeness”. It is based on the notion of physical time defined by counting oscillations for some physically allowed process. Oscillating solutions of wave functions for particles with varying mass permit the derivation of generalised geodesics and the associated notion of completeness. Time completeness involves aspects of particle physics and is no longer a purely geometric concept