23,659 research outputs found
The Swampland, Quintessence and the Vacuum Energy
It has recently been conjectured that string theory does not admit de Sitter
vacua, and that quintessence explains the current epoch of accelerated cosmic
expansion. A proposed, key prediction of this scenario is time-varying
couplings in the dark sector, induced by the evolving quintessence field. We
note that cosmological models with varying couplings suffer from severe
problems with quantum corrections, beyond those shared by all quintessence
models. The vacuum energy depends on all masses and couplings of the theory,
and even small variations of parameters can lead to overwhelmingly large
corrections to the effective potential. We find that quintessence models with
varying parameters can be realised in consistent quantum theories by either: 1)
enforcing exceptional levels of fine-tuning; 2) realising some unknown
mechanism that cancels all undesirable contributions to the effective potential
with unprecedented accuracy; or 3) ensuring that the quintessence field couples
exclusively to very light states, and does not backreact on heavy fields.Comment: 4
Fast spectroscopy and imaging with the FORS2 HIT mode
The HIgh-Time resolution (HIT) mode of FORS2 has 3 sub-modes that allow for
imaging and spectroscopy over a range of timescales from milliseconds up to
seconds. It is the only high time resolution spectroscopy mode available on an
8m class telescope. In imaging mode, it can be used to measure the pulse of
pulsars and spinning white dwarfs in a variety of high throughput broad- and
narrow-band filters. In spectroscopy mode it can take up to 10 spectra per
second using a novel ''shift-and-wait'' clocking pattern for the CCD. It takes
advantage of the user-designed masks which can be inserted into FORS2 to allow
any two targets within the 6.8' x 6.8' field of view of FORS2 to be selected. A
number of integration, or more precisely 'wait', times are available, which
together with the high throughput GRISMs can observe the entire optical
spectrum on a range of timescales.Comment: 19 pages, 7 figures, to appear in "High Time Resolution Astrophysics,
Galway 2006
Hyperinflation generalised: from its attractor mechanism to its tension with the `swampland conjectures'
In negatively curved field spaces, inflation can be realised even in steep
potentials. Hyperinflation invokes the `centrifugal force' of a field orbiting
the hyperbolic plane to sustain inflation. We generalise hyperinflation by
showing that it can be realised in models with any number of fields
(), and in broad classes of potentials that, in particular, don't
need to be rotationally symmetric. For example, hyperinflation can follow a
period of radial slow-roll inflation that undergoes geometric destabilisation,
yet this inflationary phase is not identical to the recently proposed scenario
of `side-tracked inflation'. We furthermore provide a detailed proof of the
attractor mechanism of (the original and generalised) hyperinflation, and
provide a novel set of characteristic, explicit models. We close by discussing
the compatibility of hyperinflation with observations and the recently much
discussed `swampland conjectures'. Observationally viable models can be
realised that satisfy either the `de Sitter conjecture' () or
the `distance conjecture' (), but satisfying both
simultaneously brings hyperinflation in some tension with successful reheating
after inflation. However, hyperinflation can get much closer to satisfying all
of these criteria than standard slow-roll inflation. Furthermore, while the
original model is in stark tension with the weak gravity conjecture,
generalisations can circumvent this issue.Comment: 26 pages, 3 figure
Dissipative particle dynamics: the equilibrium for finite time steps
Dissipative particle dynamics (DPD) is a relatively new technique which has
proved successful in the simulation of complex fluids. We caution that for the
equilibrium achieved by the DPD simulation of a simple fluid the temperature
depends strongly on the time step. An analytic expression for the dependence is
obtained and shown to agree well with simulation results.Comment: 5 pages, LaTeX, 1 Postscript figure, submitted to Europhys.Letts.,
Algebraic corrections made to final resul
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