241 research outputs found
Cold Positrons from Decaying Dark Matter
Many models of dark matter contain more than one new particle beyond those in
the Standard Model. Often heavier particles decay into the lightest dark matter
particle as the Universe evolves. Here we explore the possibilities that arise
if one of the products in a (Heavy Particle) (Dark Matter) decay
is a positron, and the lifetime is shorter than the age of the Universe. The
positrons cool down by scattering off the cosmic microwave background and
eventually annihilate when they fall into Galactic potential wells. The
resulting 511 keV flux not only places constraints on this class of models but
might even be consistent with that observed by the INTEGRAL satellite.Comment: 20 pages, 7 figure
Surprising phenomena in a rich new class of inflationary models
We report on a new class of fast-roll inflationary models. In a huge part of
its parameter space, inflationary perturbations exhibit quite unusual phenomena
such as scalar and tensor modes freezing out at widely different times, as well
as scalar modes reentering the horizon during inflation. In another, narrower
range of parameters, this class of models agrees with observations. One
specific point in parameter space is characterized by extraordinary behavior of
the scalar perturbations. Freeze-out of scalar perturbations as well as
particle production at horizon crossing are absent. Also the behavior of the
perturbations around this quasi-de Sitter background is dual to a quantum field
theory in flat space-time. Finally, the form of the primordial power spectrum
is determined by the interaction between different modes of scalar
perturbations.Comment: 12 pages, 5 figures, 1 table, references + comments added, errors
corrected, conclusions unchanged, version published in JCA
On the Issue of the \zeta Series Convergence and Loop Corrections in the Generation of Observable Primordial Non-Gaussianity in Slow-Roll Inflation. Part II: the Trispectrum
We calculate the trispectrum T_\zeta of the primordial curvature perturbation
\zeta, generated during a {\it slow-roll} inflationary epoch by considering a
two-field quadratic model of inflation with {\it canonical} kinetic terms. We
consider loop contributions as well as tree level terms, and show that it is
possible to attain very high, {\it including observable}, values for the level
of non-gaussianity \tau_{NL} if T_\zeta is dominated by the one-loop
contribution. Special attention is paid to the claim in JCAP {\bf 0902}, 017
(2009) [arXiv:0812.0807 [astro-ph]] that, in the model studied in this paper
and for the specific inflationary trajectory we choose, the quantum
fluctuations of the fields overwhelm the classical evolution. We argue that
such a claim actually does not apply to our model, although more research is
needed in order to understand the role of quantum diffusion. We also consider
the probability that an observer in an ensemble of realizations of the density
field sees a non-gaussian distribution. In that respect, we show that the
probability associated to the chosen inflationary trajectory is non-negligible.
Finally, the levels of non-gaussianity f_{NL} and \tau_{NL} in the bispectrum
B_\zeta and trispectrum T_\zeta of \zeta, respectively, are also studied for
the case in which \zeta is not generated during inflation.Comment: LaTex File, 27 pages, 8 figures. v2: Previous Section 2 has been
removed. Two new sections (3 and 4) discussing the classicality condition
given by Byrnes, Choi, and Hall, in JCAP 0902, 017 (2009), and the
probability that an observer sees a non-gaussian distribution have been
added. v3: Version accepted for publication in Physical Review
TeV scale resonant leptogenesis from supersymmetry breaking
We propose a model of TeV-scale resonant leptogenesis based upon recent
models of the generation of light neutrino masses from supersymmetry-breaking
effects with TeV-scale right-handed (rhd) neutrinos, . The model leads to
naturally large cosmological lepton asymmetries via the resonant behaviour of
the one-loop self-energy contribution to decay. Our model addresses the
primary problems of previous phenomenological studies of low-energy
leptogenesis: a rational for TeV-scale rhd neutrinos with small Yukawa
couplings so that the out-of equilibrium condition for decay is
satisfied; the origin of the tiny, but non-zero mass splitting required between
at least two masses; and the necessary non-trivial breaking of flavour
symmetries in the rhd neutrino sector. The low mass-scale of the rhd neutrinos
and their superpartners, and the TeV-scale -terms automatically contained
within the model offer opportunities for partial direct experimental tests of
this leptogenesis mechanism at future colliders.Comment: 10 Pages latex, version for JHE
Modulating the Framework Negative Charge Density in the System [BDT−TTP•+]/[Re6S5Cl91-]/[Re6(S/Se)6Cl82-]/[Re6S7Cl73-]
A series of 2D metals, β-(BDT-TTP)6[Re6Se6Cl8]·(CHCl2−CHCl2)2, 2; β-(ST-TTP)6[Re6S6Cl8]·(CH2Cl−CHCl2)2, 3; β-(BDT-TTP)7[Re6S6Cl8]0.5[Re6S7Cl7]0.5·(CH2Cl2), 4; β-(BDT-TTP)7[Re6Se6Cl8]0.5[Re6S7Cl7]0.5·(CH2Cl2), 5; β-(BDT-TTP)8[Re6S7Cl7]·(CH2Cl2)4, 6 (BDT-TTP and ST-TTP are 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene and 2-(1,3-diselenol-2-ylidene)-5(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene, respectively) is reported to have one single β-slab layered topology despite successive increases of the cluster anion negative charge. The charge density within the templating composite inorganic−neutral molecule slab is shown to remain above a threshold of ca. one negative charge per square nanometer, that is, for cluster anions with two negative charges and higher. Conversely, discrete stacks are shown to be stabilized instead in the semiconducting salts (BDT-TTP)2[Re6S5Cl9], 1 where the cluster anion bears one negative charge only. The electronic structure of salts 2−6 is shown to be very stable and kept almost intact across the series. The templating strategy is shown to fulfill its anticipated potential for deliberate installment of incommensurate band fillings in molecular metals. The deliberate admixture of the 6:1 and 8:1 structures yields novel phases with a 7:1 stoichiometry with the anticipated crystal and electronic structures. The action at the organic−inorganic interface triggered by changing the anion charge yet keeping its shape and volume identical, which ultimately governs the shape of the unit cell, is of paramount importance in defining the Fermi surface of these metallic salts. The present BDT-TTP salts thus provide a series of materials with strongly related but subtly different Fermi surfaces worthy of many physical studies. Shubnikov−de Haas measurements are expected to be particularly interesting since they are especially sensitive to the details of the Fermi surface
A semi-analytical approach to perturbations in mutated hilltop inflation
We study cosmological perturbations and observational aspects for mutated
hilltop model of inflation. Employing mostly analytical treatment, we evaluate
observable parameters during inflation as well as post-inflationary
perturbations. This further leads to exploring observational aspects related to
Cosmic Microwave Background (CMB) radiation. This semi-analytical treatment
reduces complications related to numerical computation to some extent for
studying the different phenomena related to CMB angular power spectrum for
mutated hilltop inflation.Comment: 7 pages, 2 figures. Improved version to appear in IJMP
Dynamical compactification from de Sitter space
We show that D-dimensional de Sitter space is unstable to the nucleation of
non-singular geometries containing spacetime regions with different numbers of
macroscopic dimensions, leading to a dynamical mechanism of compactification.
These and other solutions to Einstein gravity with flux and a cosmological
constant are constructed by performing a dimensional reduction under the
assumption of q-dimensional spherical symmetry in the full D-dimensional
geometry. In addition to the familiar black holes, black branes, and
compactification solutions we identify a number of new geometries, some of
which are completely non-singular. The dynamical compactification mechanism
populates lower-dimensional vacua very differently from false vacuum eternal
inflation, which occurs entirely within the context of four-dimensions. We
outline the phenomenology of the nucleation rates, finding that the
dimensionality of the vacuum plays a key role and that among vacua of the same
dimensionality, the rate is highest for smaller values of the cosmological
constant. We consider the cosmological constant problem and propose a novel
model of slow-roll inflation that is triggered by the compactification process.Comment: Revtex. 41 pages with 24 embedded figures. Minor corrections and
added reference
CMB Anisotropies at Second Order I
We present the computation of the full system of Boltzmann equations at
second-order describing the evolution of the photon, baryon and cold dark
matter fluids. These equations allow to follow the time evolution of the Cosmic
Microwave Background (CMB) anisotropies at second-order at all angular scales
from the early epoch, when the cosmological perturbations were generated, to
the present through the recombination era. This paper sets the stage for the
computation of the full second-order radiation transfer function at all scales
and for a a generic set of initial conditions specifying the level of
primordial non-Gaussianity. In a companion paper, we will present the
computation of the three-point correlation function at recombination which is
so relevant for the issue of non-Gaussianity in the CMB anisotropies.Comment: 26 pages, LaTeX file, typos correcte
A Hydrodynamical Approach to CMB mu-distortions
Spectral distortion of the cosmic microwave background provides a unique
opportunity to probe primordial perturbations on very small scales by
performing large-scale measurements. We discuss in a systematic and pedagogic
way all the relevant physical phenomena involved in the production and
evolution of the mu-type spectral distortion. Our main results agree with
previous estimates (in particular we show that a recently found factor of 3/4
arises from relativistic corrections to the wave energy). We also discuss
several subleading corrections such as adiabatic cooling and the effects of
bulk viscosity, baryon loading and photon heat conduction. Finally we calculate
the transfer function for mu-distortions between the end of the mu-era and now.Comment: 45 page
Action approach to cosmological perturbations: the 2nd order metric in matter dominance
We study nonlinear cosmological perturbations during the post-inflationary
evolution, using the equivalence between a perfect barotropic fluid and a
derivatively coupled scalar field with Lagrangian [-(\partial
\phi)^2]^[(1+w)/2w]. Since this Lagrangian is just a special case of
k-inflation, this approach is analogous to the one employed in the study of
non-Gaussianities from inflation. We use this method to derive the second order
metric during matter dominance in the comoving gauge directly as function of
the primordial inflationary perturbation \zeta. Going to Poisson gauge, we
recover the metric previously derived in the literature.Comment: 16 pages. New version, typo fixed in eq. (91
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