7,777 research outputs found
Modulated Reheating and Large Non-Gaussianity in String Cosmology
A generic feature of the known string inflationary models is that the same
physics that makes the inflaton lighter than the Hubble scale during inflation
often also makes other scalars this light. These scalars can acquire
isocurvature fluctuations during inflation, and given that their VEVs determine
the mass spectrum and the coupling constants of the effective low-energy field
theory, these fluctuations give rise to couplings and masses that are modulated
from one Hubble patch to another. These seem just what is required to obtain
primordial adiabatic fluctuations through conversion into density perturbations
through the `modulation mechanism', wherein reheating takes place with
different efficiency in different regions of our Universe. Fluctuations
generated in this way can generically produce non-gaussianity larger than
obtained in single-field slow-roll inflation; potentially observable in the
near future. We provide here the first explicit example of the modulation
mechanism at work in string cosmology, within the framework of LARGE Volume
Type-IIB string flux compactifications. The inflationary dynamics involves two
light Kaehler moduli: a fibre divisor plays the role of the inflaton whose
decay rate to visible sector degrees of freedom is modulated by the primordial
fluctuations of a blow-up mode (which is made light by the use of
poly-instanton corrections). We find the challenges of embedding the mechanism
into a concrete UV completion constrains the properties of the non-gaussianity
that is found, since for generic values of the underlying parameters, the model
predicts a local bi-spectrum with fNL of order `a few'. However, a moderate
tuning of the parameters gives also rise to explicit examples with fNL O(20)
potentially observable by the Planck satellite.Comment: 42 pages, 2 figure
Consequences of Zeeman Degeneracy for van der Waals Blockade between Rydberg Atoms
We analyze the effects of Zeeman degeneracies on the long-range interactions
between like Rydberg atoms, with particular emphasis on applications to quantum
information processing using van der Waals blockade. We present a general
analysis of how degeneracies affect the primary error sources in blockade
experiments, emphasizing that blockade errors are sensitive primarily to the
weakest possible atom-atom interactions between the degenerate states, not the
mean interaction strength. We present explicit calculations of the van der
Waals potentials in the limit where the fine-structure interaction is large
compared to the atom-atom interactions. The results are presented for all
potential angular momentum channels invoving s, p, and d states. For most
channels there are one or more combinations of Zeeman levels that have
extremely small dipole-dipole interactions and are therefore poor candidates
for effective blockade experiments. Channels with promising properties are
identified and discussed. We also present numerical calculations of Rb and Cs
dipole matrix elements and relevant energy levels using quantum defect theory,
allowing for convenient quantitative estimates of the van der Waals
interactions to be made for principal quantum numbers up to 100. Finally, we
combine the blockade and van der Waals results to quantitatively analyze the
angular distribution of the blockade shift and its consequence for angular
momentum channels and geometries of particular interest for blockade
experiments with Rb.Comment: 16 figure
Coriolis force corrections to g-mode spectrum in 1D MHD model
The corrections to g-mode frequencies caused by the presence of a central
magnetic field and rotation of the Sun are calculated. The calculations are
carried out in the simple one dimensional magnetohydrodynamical model using the
approximations which allow one to find the purely analytical spectra of
magneto-gravity waves beyond the scope of the JWKB approximation and avoid in a
small background magnetic field the appearance of the cusp resonance which
locks a wave within the radiative zone. These analytic results are compared
with the satellite observations of the g-mode frequency shifts which are of the
order one per cent as given in the GOLF experiment at the SoHO board. The main
contribution turns out to be the magnetic frequency shift in the strong
magnetic field which obeys the used approximations. In particular, the fixed
magnetic field strength 700 KG results in the mentioned value of the frequency
shift for the g-mode of the radial order n=-10. The rotational shift due to the
Coriolis force appears to be small and does not exceed a fracton of per cent,
\alpha_\Omega < 0.003.Comment: RevTeX4, 9 pages, 4 eps figures; accepted for publication in
Astronomy Reports (Astronomicheskii Zhurnal
Brane-Antibrane Backreaction in Axion Monodromy Inflation
We calculate the interaction potential between D5 and anti-D5 branes wrapping
distant but homologous 2-cycles. The interaction potential is logarithmic in
the separation radius and does not decouple at infinity. We show that
logarithmic backreaction is generic for 5-branes wrapping distant but
homologous 2-cycles, and we argue that this destabilises models of axion
monodromy inflation involving NS5 brane-antibrane pairs in separate warped
throats towards an uncontrolled region.Comment: 12 page
Footballs, Conical Singularities and the Liouville Equation
We generalize the football shaped extra dimensions scenario to an arbitrary
number of branes. The problem is related to the solution of the Liouville
equation with singularities and explicit solutions are presented for the case
of three branes. The tensions of the branes do not need to be tuned with each
other but only satisfy mild global constraints.Comment: 15 pages, Refs. added, minor changes. Typo in eq. 4.3 corrected.
Version to be published in PR
Volume stabilization in a warped flux compactification model
We investigate the stability of the extra dimensions in a warped, codimension
two braneworld that is based upon an Einstein-Maxwell-dilaton theory with a
non-vanishing scalar field potential. The braneworld solution has two 3-branes,
which are located at the positions of the conical singularities. For this type
of brane solution the relative positions of the branes (the shape modulus) is
determined via the tension-deficit relations, if the brane tensions are fixed.
However, the volume of the extra dimensions (the volume modulus) is not fixed
in the context of the classical theory, implying we should take quantum
corrections into account. Hence, we discuss the one-loop effective potential of
the volume modulus for a massless, minimally coupled scalar field.Comment: 25 pages, 8 figures, typos correcte
Forbidden Transitions in a Magneto-Optical Trap
We report the first observation of a non-dipole transition in an ultra-cold
atomic vapor. We excite the 3P-4P electric quadrupole (E2) transition in
Na confined in a Magneto-Optical Trap(MOT), and demonstrate its
application to high-resolution spectroscopy by making the first measurement of
the hyperfine structure of the 4P level and extracting the magnetic
dipole constant A 30.6 0.1 MHz. We use cw OODR (Optical-Optical
Double Resonance) accompanied by photoinization to probe the transition
Scaling Solutions to 6D Gauged Chiral Supergravity
We construct explicitly time-dependent exact solutions to the field equations
of 6D gauged chiral supergravity, compactified to 4D in the presence of up to
two 3-branes situated within the extra dimensions. The solutions we find are
scaling solutions, and are plausibly attractors which represent the late-time
evolution of a broad class of initial conditions. By matching their near-brane
boundary conditions to physical brane properties we argue that these solutions
(together with the known maximally-symmetric solutions and a new class of
non-Lorentz-invariant static solutions, which we also present here) describe
the bulk geometry between a pair of 3-branes with non-trivial on-brane
equations of state.Comment: Contribution to the New Journal of Physics focus issue on Dark
Energy; 28 page
Neutrino Oscillations, Fluctuations and Solar Magneto-gravity Waves
This review has two parts. The first part summarizes the current
observational constraints on fluctuations in the solar medium deep within the
solar Radiative Zone, and shows how the KamLAND and SNO-salt data combine to
make the experimental determination of the neutrino oscillation parameters
largely insensitive to prior assumptions about the nature of these
oscillations. As part of a search for plausible sources of solar fluctuations
to which neutrinos could be sensitive, the second part of the talk summarizes a
preliminary analysis of the influence of magnetic fields on helioseismic waves.
Using simplifying assumptions which should apply to modes in the solar
radiative zone, we find a resonance between Alfven waves and helioseismic
g-modes which potentially modifies the solar density profile fairly
significantly over comparatively short distance scales, too narrow to be ruled
out by present-day analyses of p-wave helioseismic spectra.Comment: Plenary talk presented at AHEP 2003, Valencia, Spain, October 200
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