3,254 research outputs found
Relic density and CMB constraints on dark matter annihilation with Sommerfeld enhancement
We calculate how the relic density of dark matter particles is altered when
their annihilation is enhanced by the Sommerfeld mechanism due to a Yukawa
interaction between the annihilating particles. Maintaining a dark matter
abundance consistent with current observational bounds requires the
normalization of the s-wave annihilation cross section to be decreased compared
to a model without enhancement. The level of suppression depends on the
specific parameters of the particle model, with the kinetic decoupling
temperature having the most effect. We find that the cross section can be
reduced by as much as an order of magnitude for extreme cases. We also compute
the mu-type distortion of the CMB energy spectrum caused by energy injection
from such Sommerfeld-enhanced annihilation. Our results indicate that in the
vicinity of resonances, associated with bound states, distortions can be large
enough to be excluded by the upper limit |mu|<9.0x10^(-5) found by the
COBE/FIRAS experiment.Comment: 10 pages, 6 figures, accepted for publication in Physical Review D.
Corrections to eqs. 9,10,14 and 16. Figures updated accordingly. No major
changes to previous results. Website with online tools for Sommerfeld-related
calculations can be found at
http://www.mpa-garching.mpg.de/~vogelsma/sommerfeld
A Parallel Decomposition Scheme for Solving Long-Horizon Optimal Control Problems
We present a temporal decomposition scheme for solving long-horizon optimal
control problems. In the proposed scheme, the time domain is decomposed into a
set of subdomains with partially overlapping regions. Subproblems associated
with the subdomains are solved in parallel to obtain local primal-dual
trajectories that are assembled to obtain the global trajectories. We provide a
sufficient condition that guarantees convergence of the proposed scheme. This
condition states that the effect of perturbations on the boundary conditions
(i.e., initial state and terminal dual/adjoint variable) should decay
asymptotically as one moves away from the boundaries. This condition also
reveals that the scheme converges if the size of the overlap is sufficiently
large and that the convergence rate improves with the size of the overlap. We
prove that linear quadratic problems satisfy the asymptotic decay condition,
and we discuss numerical strategies to determine if the condition holds in more
general cases. We draw upon a non-convex optimal control problem to illustrate
the performance of the proposed scheme
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
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