7,057 research outputs found
Uncertainty relations for cosmological particle creation and existence of large fluctuations in reheating
We derive an uncertainty relation for the energy density and pressure of a
quantum scalar field in a time-dependent, homogeneous and isotropic, classical
background, which implies the existence of large fluctuations comparable to
their vacuum expectation values. A similar uncertainty relation is known to
hold for the field square since the field can be viewed as a Gaussian random
variable. We discuss possible implications of these results for the reheating
process in scalar field driven inflationary models, where reheating is achieved
by the decay of the coherently oscillating inflaton field. Specifically we
argue that the evolution after backreaction can seriously be altered by the
existence of these fluctuations. For example, in one model the coherence of the
inflaton oscillations is found to be completely lost in a very short time after
backreaction starts. Therefore we argue that entering a smooth phase in thermal
equilibrium is questionable in such models and reheating might destroy the
smoothness attained by inflation.Comment: 6 pages, essay written for the Gravity Research Foundation 2011
Awards for Essays on Gravitation, Received Honorable Mentio
On the Geometric Properties of AdS Instantons
According to the positive energy conjecture of Horowitz and Myers, there is a
specific supergravity solution, AdS soliton, which has minimum energy among all
asymptotically locally AdS solutions with the same boundary conditions. Related
to the issue of semiclassical stability of AdS soliton in the context of pure
gravity with a negative cosmological constant, physical boundary conditions are
determined for an instanton solution which would be responsible for vacuum
decay by barrier penetration. Certain geometric properties of instantons are
studied, using Hermitian differential operators. On a -dimensional
instanton, it is shown that there are harmonic functions. A class of
instanton solutions, obeying more restrictive boundary conditions, is proved to
have Killing vectors which also commute. All but one of the Killing
vectors are duals of harmonic one-forms, which are gradients of harmonic
functions, and do not have any fixed points.Comment: 22 pages, Latex, short comments and a reference adde
Scalar Absorption by Noncommutative D3-branes
The classical cross section for low energy absorption of the RR-scalar by a
stack of noncommutative D3-branes in the large NS B-field limit is calculated.
In the spirit of AdS/CFT correspondence, this cross section is related to two
point function of a certain operator in noncommutative Yang-Mills theory.
Compared at the same gauge coupling, the result agrees with that of obtained
from ordinary D3-branes. This is consistent with the expectation that ordinary
and noncommutative Yang-Mills theories are equivalent below the
noncommutativity scale, but it is a nontrivial prediction above this scale.Comment: 13 pages, Latex, corrected some typos (in the text
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