159 research outputs found
Formation and evolution of cosmic D-strings
We study the formation of D and F-cosmic strings in D-brane annihilation
after brane inflation. We show that D-string formation by quantum de Sitter
fluctuations is severely suppressed, due to suppression of RR field
fluctuations in compact dimensions. We discuss the resonant mechanism of
production of D and F-strings, which are formed as magnetic and electric flux
tubes of the two orthogonal gauge fields living on the world-volume of the
unstable brane. We outline the subsequent cosmological evolution of the D-F
string network. We also compare the nature of these strings with the ordinary
cosmic strings and point out some differences and similarities.Comment: Added discussion and reference
Solitonic D-branes and brane annihilation
We point out some intriguing analogies between field theoretic solitons
(topological defects) and D-branes. Annihilating soliton-antisoliton pairs can
produce stable solitons of lower dimensionality. Solitons that localize
massless gauge fields in their world volume automatically imply the existence
of open flux tubes ending on them and closed flux tubes propagating in the
bulk. We discuss some aspects of this localization on explicit examples of
unstable wall-anti-wall systems. The annihilation of these walls can be
described in terms of tachyon condensation which renders the world-volume gauge
field non-dynamical. During this condensation the world volume gauge fields
(open string states) are resonantly excited. These can later decay into closed
strings, or get squeezed into a network flux tubes similar to a network of
cosmic strings formed at a cosmological phase transition. Although, as in the
-brane case, perturbatively one can find exact time-dependent solutions,
when the energy of the system stays localized in the plane of the original
soliton, such solutions are unstable with respect to decay into open and closed
string states. Thus, when a pair of such walls annihilates, the energy is
carried away (at least) by closed string excitations (``glueballs''), which are
the lowest energy excitations about the bulk vacuum. Suggested analogies can be
useful for the understanding of the complicated D-brane dynamics and of the
production of topological defects and reheating during brane collision in the
early universe.Comment: a typo correcte
Quantum effects in gravitational wave signals from cuspy superstrings
We study the gravitational emission, in Superstring Theory, from fundamental
strings exhibiting cusps. The classical computation of the gravitational
radiation signal from cuspy strings features strong bursts in the special null
directions associated to the cusps. We perform a quantum computation of the
gravitational radiation signal from a cuspy string, as measured in a
gravitational wave detector using matched filtering and located in the special
null direction associated to the cusp. We study the quantum statistics
(expectation value and variance) of the measured filtered signal and find that
it is very sharply peaked around the classical prediction. Ultimately, this
result follows from the fact that the detector is a low-pass filter which is
blind to the violent high-frequency quantum fluctuations of both the string
worldsheet, and the incoming gravitational field.Comment: 16 pages, no figur
Diluting Cosmological Constant In Infinite Volume Extra Dimensions
We argue that the cosmological constant problem can be solved in a braneworld
model with infinite-volume extra dimensions, avoiding no-go arguments
applicable to theories that are four-dimensional in the infrared. Gravity on
the brane becomes higher-dimensional at super-Hubble distances, which entails
that the relation between the acceleration rate and vacuum energy density flips
upside down compared to the conventional one. The acceleration rate decreases
with increasing the energy density. The experimentally acceptable rate is
obtained for the energy density larger than (1 TeV). The results are stable
under quantum corrections because supersymmetry is broken only on the brane and
stays exact in the bulk of infinite volume extra space. Consistency of 4D
gravity and cosmology on the brane requires the quantum gravity scale to be
around eV. Testable predictions emerging within this approach are:
(i) simultaneous modifications of gravity at sub-millimeter and the Hubble
scales; (ii) Hagedorn-type saturation in TeV energy collisions due to the Regge
spectrum with the spacing equal to eV.Comment: 36 pages, 1 eps fig; 4 refs and comment adde
Membranes in the two-Higgs standard model
We present some non-topological static wall solutions in two-Higgs extensions
of the standard model. They are classically stable in a large region of
parameter space, compatible with perturbative unitarity and with present
phenomenological bounds.Comment: 7 pages, latex, 3 figures available upon reques
Inflating magnetically charged braneworlds
Numerical solutions of Einstein, scalar, and gauge field equations are found
for static and inflating defects in a higher-dimensional spacetime. The defects
have -dimensional core and magnetic monopole configuration in
extra dimensions. For symmetry-breaking scale below the critical value
, the defects are characterized by a flat worldsheet geometry and
asymptotically flat extra dimensions. The critical scale is comparable
to the higher-dimensional Planck scale and has some dependence on the gauge and
scalar couplings. For , the extra dimensions degenerate into a
`cigar', and for all static solutions are singular. The
singularity can be removed if the requirement of staticity is relaxed and
defect cores are allowed to inflate. The inflating solutions have de Sitter
worldsheets and cigar geometry in the extra dimensions. Exact analytic
solutions describing the asymptotic behavior of these inflating monopoles are
found and the parameter space of these solutions is analyzed.Comment: 35 pages, revtex, 18 eps figure
Topologically Stable Electroweak Flux Tube
We show that for a large range of parameters in a
electroweak theory with two Higgs doublets there may exist classically stable
flux tubes of Z boson magnetic field. In a limit of an extra global symmetry, these flux-tubes become topologically stable. These results are
automatically valid even if is gauged.Comment: 10 pages, LATE
Bogomol'nyi Bounds for Gravitational Cosmic Strings
We present a new method for finding lower bounds on the energy of topological
cosmic string solutions in gravitational field theories. This new method
produces bounds that are valid over the entire space of solutions, unlike the
traditional approach, where the bounds obtained are only valid for
cylindrically symmetric solutions. This method is shown to be a generalisation
of the well-known Bogomol'nyi procedure for non-gravitational theories and as
such, it can be used to find gravitational Bogomol'nyi bounds for models
wherever the traditional Bogomol'nyi procedure can be applied in the
non-gravitational limit. Furthermore, this method yields Bogomol'nyi equations
that do not rule out the existence of asymmetric bound-saturating solutions.Comment: 17 pages - final version (accepted for publication in JHEP
Stealth Branes
We discuss the brane world model of Dvali, Gabadadze and Porrati in which
branes evolve in an infinite bulk and the brane curvature term is added to the
action. If Z_2 symmetry between the two sides of the brane is not imposed, we
show that the model admits the existence of "stealth branes" which follow the
standard 4D internal evolution and have no gravitational effect on the bulk
space. Stealth branes can nucleate spontaneosly in a Minkowski bulk. This
process is described by the standard 4D quantum cosmology formalism with
tunneling boundary conditions for the brane world wave function. The notorious
ambiguity in the choice of boundary conditions is fixed in this case due to the
presence of the embedding spacetime. We also point to some problematic aspects
of models admitting stealth brane solutions.Comment: 24 pages; Final version, to appear in Phys. Rev. D. The discussion of
"embeddability obstruction" is removed (thanks to Takahiro Tanaka who
convinced us that there is no such obstruction
The Power of Brane-Induced Gravity
We study the role of the brane-induced graviton kinetic term in theories with
large extra dimensions. In five dimensions we construct a model with a
TeV-scale fundamental Planck mass and a {\it flat} extra dimension the size of
which can be astronomically large. 4D gravity on the brane is mediated by a
massless zero-mode, whereas the couplings of the heavy Kaluza-Klein modes to
ordinary matter are suppressed. The model can manifest itself through the
predicted deviations from Einstein theory in long distance precision
measurements of the planetary orbits. The bulk states can be a rather exotic
form of dark matter, which at sub-solar distances interact via strong 5D
gravitational force. We show that the induced term changes dramatically the
phenomenology of sub-millimeter extra dimensions. For instance, high-energy
constraints from star cooling or cosmology can be substantially relaxed.Comment: 24 pages, 4 eps figures; v2 typos corrected; v3 1 ref. added; PRD
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