25,677 research outputs found
Non-gaussianity from the second-order cosmological perturbation
Several conserved and/or gauge invariant quantities described as the
second-order curvature perturbation have been given in the literature. We
revisit various scenarios for the generation of second-order non-gaussianity in
the primordial curvature perturbation \zeta, employing for the first time a
unified notation and focusing on the normalisation f_{NL} of the bispectrum.
When the classical curvature perturbation first appears a few Hubble times
after horizon exit, |f_{NL}| is much less than 1 and is, therefore, negligible.
Thereafter \zeta (and hence f_{NL}) is conserved as long as the pressure is a
unique function of energy density (adiabatic pressure). Non-adiabatic pressure
comes presumably only from the effect of fields, other than the one pointing
along the inflationary trajectory, which are light during inflation (`light
non-inflaton fields'). During single-component inflation f_{NL} is constant,
but multi-component inflation might generate |f_{NL}| \sim 1 or bigger.
Preheating can affect f_{NL} only in atypical scenarios where it involves light
non-inflaton fields. The curvaton scenario typically gives f_{NL} \ll -1 or
f_{NL} = +5/4. The inhomogeneous reheating scenario can give a wide range of
values for f_{NL}. Unless there is a detection, observation can eventually
provide a limit |f_{NL}| \lsim 1, at which level it will be crucial to
calculate the precise observational limit using second order theory.Comment: Latex file in Revtex style. 13 pages, 1 figure. v2: minor changes.
Discussion in Subsection VI-A enlarged. References added. Conclusions
unchanged. v3: minor typographic changes. Correlated and uncorrelated \chi^2
non-gaussianity concepts and consequences introduced. Section VI-A enlarged.
Small change in Table I. References updated and added. Conclusions unchanged.
Version to appear in Physical Review
Momentum distributions of -particles from decaying low-lying C-resonances
The complex scaled hyperspherical adiabatic expansion method is used to
compute momentum and energy distributions of the three -particles
emerging from the decay of low-lying C-resonances. The large distance
continuum properties of the wave functions are crucial and must be accurately
calculated. We discuss separately decays of natural parity states: two ,
one , three , one , two , one , and one of each of
unnatural parity, , , , . The lowest natural parity state
of each decays predominantly sequentially via the Be ground
state whereas other states including unnatural parity states predominantly
decay directly to the continuum. We present Dalitz plots and systematic
detailed momentum correlations of the emerging -particles.Comment: 11 pages, 7 figures, accepted for publication in Physical Review
Non-gaussianity at tree and one-loop levels from vector field perturbations
We study the spectrum P_\zeta and bispectrum B_\zeta of the primordial
curvature perturbation \zeta when the latter is generated by scalar and vector
field perturbations. The tree-level and one-loop contributions from vector
field perturbations are worked out considering the possibility that the
one-loop contributions may be dominant over the tree level terms (both (either)
in P_\zeta and (or) in B_\zeta) and viceversa. The level of non-gaussianity in
the bispectrum, f_{NL}, is calculated and related to the level of statistical
anisotropy in the power spectrum, g_\zeta. For very small amounts of
statistical anisotropy in the power spectrum, the level of non-gaussianity may
be very high, in some cases exceeding the current observational limit.Comment: LaTeX file, 11 pages, Main body: 8 pages, References: 3 pages. v2:
Minor corrections. References added. Conclusions unchanged. v3: Minor
corrections. Some references added and others updated. Version accepted for
publication in Physical Review
Frequency and voltage partitioning in presence of renewable energy resources for power system (example: North Chile power network)
This paper investigates techniques for frequency and voltage partitioning of power network based on the
graph-theory. These methods divide the power system into distinguished regions to avoid the spread of disturbances
and to minimize the interaction between these regions for frequency and voltage control of power system. In case
of required active and reactive power for improving the performance of the power system, control can be performed
regionally instead of a centralized controller. In this paper, renewable energy sources are connected to the power
network to verify the effect of these sources on the power systems partitioning and performance. The number of
regions is found based on the frequency sensitivity for frequency partitioning and bus voltage for voltage partitioning to disturbances being applied to loads in each region. The methodology is applied to the north part of Chile power
network. The results show the performance and ability of graph frequency and voltage partitioning algorithm to divide
large scale power systems to smaller regions for applying decentralized controllers.Peer ReviewedPostprint (published version
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
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