297 research outputs found
Supersymmetric k-defects
In supersymmetric theories, topological defects can have nontrivial behaviors
determined purely by whether or not supersymmetry is restored in the defect
core. A well-known example of this is that some supersymmetric cosmic strings
are automatically superconducting, leading to important cosmological effects
and constraints. We investigate the impact of nontrivial kinetic interactions,
present in a number of particle physics models of interest in cosmology, on the
relationship between supersymmetry and supercurrents on strings. We find that
in some cases it is possible for superconductivity to be disrupted by the extra
interactions.Comment: 12 page
The Ghost Condensate in N=1 Supergravity
We present the theory of a supersymmetric ghost condensate coupled to N=1
supergravity. This is accomplished using a general formalism for constructing
locally supersymmetric higher-derivative chiral superfield actions. The theory
admits a ghost condensate vacuum in de Sitter spacetime. Expanded around this
vacuum, the scalar sector of the theory is shown to be ghost-free with no
spatial gradient instabilities. By direct calculation, the fermion sector is
found to consist of a massless chiral fermion and a massless gravitino. By
analyzing the supersymmetry transformations, we find that the chiral fermion
transforms inhomogeneously, indicating that the ghost condensate vacuum
spontaneously breaks local supersymmetry with this field as the Goldstone
fermion. Although potentially able to get a mass through the super-Higgs
effect, the vanishing superpotential in the ghost condensate theory renders the
gravitino massless. Thus local supersymmetry is broken without the super-Higgs
effect taking place. This is in agreement with, and gives an explanation for,
the direct calculation.Comment: 41 pages, v2: minor corrections to section IV.D, conclusions
unchange
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Non-Singular Bouncing Cosmology: Consistency of the Effective Description
We explicitly confirm that spatially flat non-singular bouncing cosmologies
make sense as effective theories. The presence of a non-singular bounce in a
spatially flat universe implies a temporary violation of the null energy
condition, which can be achieved through a phase of ghost condensation. We
calculate the scale of strong coupling and demonstrate that the
ghost-condensate bounce remains trustworthy throughout, and that all
perturbation modes within the regime of validity of the effective description
remain under control. For this purpose we require the perturbed action up to
third order in perturbations, which we calculate in both flat and co-moving
gauge -- since these two gauges allow us to highlight different physical
aspects. Our conclusion is that there exist healthy descriptions of
non-singular bouncing cosmologies providing a viable resolution of the big-bang
singularities in cosmological models. Our results also suggest a variant of
ekpyrotic cosmology, in which entropy perturbations are generated during the
contracting phase, but are only converted into curvature perturbations after
the bounce.Comment: 43 pages, 18 figure
Towards a Solution of the Negative Mode Problem in Quantum Tunnelling with Gravity
In the absence of gravity, one can prove that tunnelling instantons exhibit
exactly one negative mode in their spectrum of fluctuations. It is precisely
the existence of this tunnelling negative mode that warrants an interpretation
of these solutions as mediating the decay of a metastable vacuum. In the
presence of gravity the situation is much more subtle, not least because of
diffeomorphism invariance. New complications arise here: in particular, the
kinetic term of the fluctuations can change sign somewhere along the instanton.
We show that in this case the mode functions remain non-singular, and the
tunnelling negative mode continues to exist. Moreover, the eigenvalues vary
continuously when the potential is varied such that the kinetic term of the
fluctuations switches sign. However, the negative kinetic term implies the
additional existence of an infinite tower of negative modes, whose significance
and interpretation remain elusive.Comment: 32 pages, 13 figures, v3: more references added, published versio
Supersymmetric Cubic Galileons Have Ghosts
Galileons are higher-derivative theories of a real scalar which nevertheless
admit second order equations of motion. They have interesting applications as
dark energy models and in early universe cosmology, and have been conjectured
to arise as descriptions of brane dynamics in string theory. In the present
paper, we study the bosonic sector of globally N=1 supersymmetric extensions of
the cubic Galileon Lagrangian in detail. Supersymmetry requires that the
Galileon scalar now becomes paired with a second real scalar field. We prove
that the presence of this second scalar causes the equations of motion to
become higher than second order, thus leading to the appearance of ghosts. We
also analyze the energy scales up to which, in an effective field theory
description, the ghosts can be tamed.Comment: 18 pages, v4: discussion updated in light of arXiv:1306.2961 by
Farakos et a
A Cosmological Super-Bounce
We study a model for a non-singular cosmic bounce in N=1 supergravity, based
on supergravity versions of the ghost condensate and cubic Galileon scalar
field theories. The bounce is preceded by an ekpyrotic contracting phase which
prevents the growth of anisotropies in the approach to the bounce, and allows
for the generation of scale-invariant density perturbations that carry over
into the expanding phase of the universe. We present the conditions required
for the bounce to be free of ghost excitations, as well as the tunings that are
necessary in order for the model to be in agreement with cosmological
observations. All of these conditions can be met. Our model thus provides a
proof-of-principle that non-singular bounces are viable in supergravity,
despite the fact that during the bounce the null energy condition is violated.Comment: 38 pages, 13 figures, v3: typos correcte
Cosmological Perturbations Through a Non-Singular Ghost-Condensate/Galileon Bounce
We study the propagation of super-horizon cosmological perturbations in a
non-singular bounce spacetime. The model we consider combines a ghost
condensate with a Galileon term in order to induce a ghost-free bounce. Our
calculation is performed in harmonic gauge, which ensures that the linearized
equations of motion remain well-defined and non-singular throughout. We find
that, despite the fact that near the bounce the speed of sound becomes
imaginary, super-horizon curvature perturbations remain essentially constant
across the bounce. In fact, we show that there is a time close to the bounce
where curvature perturbations of all wavelengths are required to be momentarily
exactly constant. We relate our calculations to those performed in other
gauges, and comment on the relation to previous results in the literature.Comment: 28 pages, 16 figures, v2: references adde
User cost in oil production
The assumption of an initial fixed mineral stock is superfluous and wrong. User cost (resource rent) in mineral production is the present value of expected increases in development cost. It can be measured as the difference between in-ground market value and development cost, or estimated approximately from current development cost. For private or national-income accounting, mineral reserves should be treated as a renewable inventory. Adjustment for change in inventory may increase or decrease the income of a mineral producer, but an increase is more likely.Supported by the National Science Foundation. Supported by the Center for Energy Policy Research of the M.I.T. Energy Laboratory
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