29,088 research outputs found
Gravitino Production Suppressed by Dynamics of Sgoldstino
In supersymmetric theories, the gravitino is abundantly produced in the early
Universe from thermal scattering, resulting in a strong upper bound on the
reheat temperature after inflation. We point out that the gravitino problem may
be absent or very mild due to the early dynamics of a supersymmetry breaking
field, i.e. a sgoldstino. In models of low scale mediation, the field value of
the sgoldstino determines the mediation scale and is in general different in
the early Universe from the present one. A large initial field value since the
era of the inflationary reheating suppresses the gravitino production
significantly. We investigate in detail the cosmological evolution of the
sgoldstino and show that the reheat temperature may be much higher than the
conventional upper bound, restoring the compatibility with thermal
leptogenesis.Comment: 23 pages, 3 figures; v2: discussions added and one figure updated,
matches version published in JHE
The Decay of Passive Scalars Under the Action of Single Scale Smooth Velocity Fields in Bounded 2D Domains : From non self similar pdf's to self similar eigenmodes
We examine the decay of passive scalars with small, but non zero, diffusivity
in bounded 2D domains. The velocity fields responsible for advection are smooth
(i.e., they have bounded gradients) and of a single large scale. Moreover, the
scale of the velocity field is taken to be similar to the size of the entire
domain. The importance of the initial scale of variation of the scalar field
with respect to that of the velocity field is strongly emphasized. If these
scales are comparable and the velocity field is time periodic, we see the
formation of a periodic scalar eigenmode. The eigenmode is numerically realized
by means of a deterministic 2D map on a lattice. Analytical justification for
the eigenmode is available from theorems in the dynamo literature. Weakening
the notion of an eigenmode to mean statistical stationarity, we provide
numerical evidence that the eigenmode solution also holds for aperiodic flows
(represented by random maps). Turning to the evolution of an initially small
scale scalar field, we demonstrate the transition from an evolving (i.e., {\it
non} self similar) pdf to a stationary (self similar) pdf as the scale of
variation of the scalar field progresses from being small to being comparable
to that of the velocity field (and of the domain). Furthermore, the {\it non}
self similar regime itself consists of two stages. Both the stages are examined
and the coupling between diffusion and the distribution of the Finite Time
Lyapunov Exponents is shown to be responsible for the pdf evolution.Comment: 21 pages (2 col. format), 11 figures. Accepted, to appear in PR
Surface Quasigeostrophic Turbulence : The Study of an Active Scalar
We study the statistical and geometrical properties of the potential
temperature (PT) field in the Surface Quasigeostrophic (SQG) system of
equations. In addition to extracting information in a global sense via tools
such as the power spectrum, the g-beta spectrum and the structure functions we
explore the local nature of the PT field by means of the wavelet transform
method. The primary indication is that an initially smooth PT field becomes
rough (within specified scales), though in a qualitatively sparse fashion.
Similarly, initially 1D iso-PT contours (i.e., PT level sets) are seen to
acquire a fractal nature. Moreover, the dimensions of the iso-PT contours
satisfy existing analytical bounds. The expectation that the roughness will
manifest itself in the singular nature of the gradient fields is confirmed via
the multifractal nature of the dissipation field. Following earlier work on the
subject, the singular and oscillatory nature of the gradient field is
investigated by examining the scaling of a probability measure and a sign
singular measure respectively. A physically motivated derivation of the
relations between the variety of scaling exponents is presented, the aim being
to bring out some of the underlying assumptions which seem to have gone
unnoticed in previous presentations. Apart from concentrating on specific
properties of the SQG system, a broader theme of the paper is a comparison of
the diagnostic inertial range properties of the SQG system with both the 2D and
3D Euler equations.Comment: 26 pages, 11 figures. To appear in Chao
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