2,315 research outputs found
Cosmological attractors in massive gravity
We study Lorentz-violating models of massive gravity which preserve rotations
and are invariant under time-dependent shifts of the spatial coordinates. In
the linear approximation the Newtonian potential in these models has an extra
``confining'' term proportional to the distance from the source. We argue that
during cosmological expansion the Universe may be driven to an attractor point
with larger symmetry which includes particular simultaneous dilatations of time
and space coordinates. The confining term in the potential vanishes as one
approaches the attractor. In the vicinity of the attractor the extra
contribution is present in the Friedmann equation which, in a certain range of
parameters, gives rise to the cosmic acceleration.Comment: 26 pages, 1 figur
Is the electric charge conserved in brane world?
We discuss whether electric charge conservation may not hold in
four-dimensional world in models with infinite extra dimensions, i.e., whether
escape of charged particles from our brane is consistent with effectively
four-dimensional electrodynamics on the brane. We introduce a setup with photon
localized on the brane and show that charge leakage into extra dimension is
allowed within this setup. The electric field induced on the brane by escaping
charge does not obey four-dimensional Maxwell's equations; this field gradually
disappears in a causal way. We also speculate on the possibility of the escape
of colored particles and formation of colorless free quark states on the brane.Comment: 14 pages, 4 figures, misprint correcte
Quasilocalized gravity without asymptotic flatness
We present a toy model of a generic five-dimensional warped geometry in which
the 4D graviton is not fully localized on the brane. Studying the tensor sector
of metric perturbation around this background, we find that its contribution to
the effective gravitational potential is of 4D type (1/r) at the intermediate
scales and that at the large scales it becomes 1/r^{1+alpha}, 0<alpha=< 1 being
a function of the parameters of the model (alpha=1 corresponds to the
asymptotically flat geometry). Large-distance behavior of the potential is
therefore not necessarily five-dimensional. Our analysis applies also to the
case of quasilocalized massless particles other than graviton.Comment: 9 pages, 1 figure; to be published in Phys. Rev.
Comment on 4D Lorentz invariance violations in the brane-world
The brane-world scenario offers the possibility for signals to travel outside
our visible universe and reenter it. We find the condition for a signal emitted
from the brane to return to the brane. We study the propagation of such signals
and show that, as seen by a 4D observer, these signals arrive earlier than
light traveling along the brane. We also study the horizon problem and find
that, while the bulk signals can travel far enough to homogenize the visible
universe, it is unlikely that they have a significant effect since they are
redshifted in the gravitational field of the bulk black hole.Comment: 21 pages, 6 figures, REVTEX, New section adde
Creating semiclassical black holes in collider experiments and keeping them on a string
We argue that a simple modification of the TeV scale quantum gravity scenario
allows production of semiclassical black holes in particle collisions at the
LHC. The key idea is that in models with large extra dimensions the strength of
gravity in the bulk can be higher than on the brane where we live. A well-known
example of this situation is the case of warped extra dimensions. Even if the
energy of the collision is not sufficient to create a black hole on the brane,
it may be enough to produce a particle which accelerates into the bulk up to
trans-Planckian energy and creates a large black hole there. In a concrete
model we consider, the black hole is formed in a collision of the particle with
its own image at an orbifold plane. When the particle in question carries some
Standard Model gauge charges the created black hole gets attached to our brane
by a string of the gauge flux. For a 4-dimensional observer such system looks
as a long-lived charged state with the mass continuously decreasing due to
Hawking evaporation of the black hole. This provides a distinctive signature of
black hole formation in our scenario.Comment: Journal version, a misprint correcte
The quantum mechanics of perfect fluids
We consider the canonical quantization of an ordinary fluid. The resulting
long-distance effective field theory is derivatively coupled, and therefore
strongly coupled in the UV. The system however exhibits a number of
peculiarities, associated with the vortex degrees of freedom. On the one hand,
these have formally a vanishing strong-coupling energy scale, thus suggesting
that the effective theory's regime of validity is vanishingly narrow. On the
other hand, we prove an analog of Coleman's theorem, whereby the semiclassical
vacuum has no quantum counterpart, thus suggesting that the vortex premature
strong-coupling phenomenon stems from a bad identification of the ground state
and of the perturbative degrees of freedom. Finally, vortices break the usual
connection between short distances and high energies, thus potentially
impairing the unitarity of the effective theory.Comment: 35 page
Galactic anisotropy of Ultra-High Energy Cosmic Rays produced by CDM-related mechanisms
We briefly review current theoretical and experimental status of Ultra-High
Energy Cosmic Rays. We show that ``top-down'' mechanisms of UHE CR which
involve heavy relic particle-like objects predict Galactic anisotropy of
highest energy cosmic rays at the level of minimum . This anisotropy
is large enough to be either observed or ruled out in the next generation of
experiments.Comment: 10 pages; talk presented at the 10th International Seminar
``Quarks-98'', Suzdal, Russia, May 18-24, 1998; to appear in the Proceeding
Energy's and amplitudes' positivity
In QFT, the null energy condition (NEC) for a classical field configuration
is usually associated with that configuration's stability against small
perturbations, and with the sub-luminality of these. Here, we exhibit an
effective field theory that allows for stable NEC-violating solutions with
exactly luminal excitations only. The model is the recently introduced
`galileon', or more precisely its conformally invariant version. We show that
the theory's low-energy S-matrix obeys standard positivity as implied by
dispersion relations. However we also show that if the relevant NEC-violating
solution is inside the effective theory, then other (generic) solutions allow
for superluminal signal propagation. While the usual association between
sub-luminality and positivity is not obeyed by our example, that between NEC
and sub-luminality is, albeit in a less direct way than usual.Comment: 21 pages. v2: Typos in eq. (2.41) and (2.41) corrected; discussion of
section 2.3 modified accordingly. Other sections and conclusions unchanged.
Matches the Erratum published in JHE
Generation of 10^15 - 10^17 eV photons by UHE CR in the Galactic magnetic filed
We show that the deep expected in the diffuse photon spectrum above the
threshold of e+e- pair production, i.e., at energies 10^15 - 10^17 eV, may be
absent due to the synchrotron radiation by the electron component of the
extragalactic Ultra-High Energy Cosmic Rays (UHE CR) in the Galactic magnetic
filed. The mechanism we propose requires small (less than 2x10^-12 G)
extragalactic magnetic fields and large fraction of photons in the UHE CR. For
a typical photon flux expected in top-down scenarios of UHE CR, the predicted
flux in the region of the deep is close to the existing experimental limit. The
sensitivity of our mechanism to the extragalactic magnetic field may be used to
improve existing bounds on the latter by two orders of magnitude.Comment: 11 pages, LaTeX, 1 .ps figure. Numerical error corrected; references
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