265 research outputs found
Removing the Big Bang Singularity: The role of the generalized uncertainty principle in quantum gravity
The possibility of avoiding the big bang singularity by means of a
generalized uncertainty principle is investigated. In relation with this
matter, the statistical mechanics of a free-particle system obeying the
generalized uncertainty principle is studied and it is shown that the entropy
of the system has a finite value in the infinite temperature limit. It is then
argued that negative temperatures and negative pressures are possible in this
system. Finally, it is shown that this model can remove the big bang
singularity.Comment: 8 pages, Accepted for publication in Astrophysics & Space Scienc
Penrose limit and string quantization in AdS_3 \times S^3
We consider corrections to the Penrose limit of AdS_3 \times S^3 with NS-NS
flux which are due to the terms next to leading order in inverse radius
expansion. The worldsheet theory of a lightcone string is interacting due to
the presence of quartic terms in the action. Perturbative corrections to the
spectrum are shown to agree with the results from the exact quantization in
AdS_3 \times S^3.Comment: 18 pages v2: typos fixed, reference added, to appear in JHE
Kaluza-Klein supergravity on AdS_3 x S^3
We construct a Chern-Simons type gauged N=8 supergravity in three spacetime
dimensions with gauge group SO(4) x T_\infty over the infinite dimensional
coset space SO(8,\infty)/(SO(8) x SO(\infty)), where T_\infty is an infinite
dimensional translation subgroup of SO(8,\infty). This theory describes the
effective interactions of the (infinitely many) supermultiplets contained in
the two spin-1 Kaluza-Klein towers arising in the compactification of N=(2,0)
supergravity in six dimensions on AdS_3 x S^3 with the massless supergravity
multiplet. After the elimination of the gauge fields associated with T_\infty,
one is left with a Yang Mills type gauged supergravity with gauge group SO(4),
and in the vacuum the symmetry is broken to the (super-)isometry group of AdS_3
x S^3, with infinitely many fields acquiring masses by a variant of the
Brout-Englert-Higgs effect.Comment: LaTeX2e, 24 pages; v2: references update
Synthesizing greenhouse gas fluxes across nine European peatlands and shrublands: responses to climatic and environmental changes
In this study, we compare annual fluxes of methane
(CH4), nitrous oxide (N2O) and soil respiratory carbon dioxide
(CO2) measured at nine European peatlands (n = 4) and
shrublands (n = 5). The sites range from northern Sweden
to Spain, covering a span in mean annual air temperature
from 0 to 16 �C, and in annual precipitation from 300 to
1300mmyr−1. The effects of climate change, including temperature
increase and prolonged drought, were tested at five
shrubland sites. At one peatland site, the long-term (>30 yr)
effect of drainage was assessed, while increased nitrogen deposition
was investigated at three peatland sites.
The shrublands were generally sinks for atmospheric CH4,
whereas the peatlands were CH4 sources, with fluxes ranging
from −519 to +6890 mgCH4-Cm−2 yr−1 across the studied
ecosystems. At the peatland sites, annual CH4 emission
increased with mean annual air temperature, while
a negative relationship was found between net CH4 uptake
and the soil carbon stock at the shrubland sites. Annual
N2O fluxes were generally small ranging from −14
to 42 mgN2O-Nm−2 yr−1. Highest N2O emission occurred
at the sites that had highest nitrate (NO−
3 ) concentration
in the soil water. Furthermore, experimentally increased
NO−
3 deposition led to increased N2O efflux, whereas prolonged
drought and long-term drainage reduced the N2O efflux.
Soil CO2 emissions in control plots ranged from 310
to 732 gCO2-Cm−2 yr−1. Drought and long-term drainage generally reduced the soil CO2 efflux, except at a hydric
shrubland where drought tended to increase soil respiration.
In terms of fractional importance of each greenhouse gas
to the total numerical global warming response, the change in
CO2 efflux dominated the response in all treatments (ranging
71–96 %), except for NO−
3 addition where 89% was due to
change in CH4 emissions. Thus, in European peatlands and
shrublands the effect on global warming induced by the investigated
anthropogenic disturbances will be dominated by
variations in soil CO2 fluxes
Ambiguity in the evaluation of the effective action on the cone
An ambiguity in the computation of the one-loop effective action for fields
living on a cone is illustrated. It is shown that the ambiguity arises due to
the non-commutativity of the regularization of ultraviolet and (conical)
boundary divergencies.Comment: REVTeX file, 10 pages. Comments on recent papers have been adde
Unitarity bounds on low scale quantum gravity
We study the unitarity of models with low scale quantum gravity both in four
dimensions and in models with a large extra-dimensional volume. We find that
models with low scale quantum gravity have problems with unitarity below the
scale at which gravity becomes strong. An important consequence of our work is
that their first signal at the Large Hadron Collider would not be of a
gravitational nature such as graviton emission or small black holes, but rather
linked to the mechanism which fixes the unitarity problem. We also study models
with scalar fields with non minimal couplings to the Ricci scalar. We consider
the strength of gravity in these models and study the consequences for
inflation models with non-minimally coupled scalar fields. We show that a
single scalar field with a large non-minimal coupling can lower the Planck mass
in the TeV region. In that model, it is possible to lower the scale at which
gravity becomes strong down to 14 TeV without violating unitarity below that
scale.Comment: 15 page
Diurnal Variation in Martian Dust Devil Activity
We show that the dust devil parameterisation in use in most Mars Global Circulation Models (MGCMs) results in an unexpectedly high level of dust devil activity during morning hours.
Prior expectations of the diurnal variation of Martian dust devils are based mainly upon the observed behaviour of terrestrial dust devils: i.e. that the majority occur during the afternoon. We instead find that large areas of the Martian surface experience dust devil activity during the morning in our MGCM, and that many locations experience a peak in dust devil activity before mid-sol.
We find that the diurnal variation in dust devil activity is governed by near-surface wind speeds. Within the range of daylight hours, higher wind speeds tend to produce higher levels of dust devil activity, rather than the activity simply being governed by the availability of heat at the planet's surface, which peaks in early afternoon.
Evidence for whether the phenomenon we observe is real or an artefact of the parameterisation is inconclusive. We compare our results with surface-based observations of Martian dust devil timings and obtain a good match with the majority of surveys. We do not find a good match with orbital observations, which identify a diurnal distribution more closely matching that of terrestrial dust devils, but orbital observations have limited temporal coverage, biased towards the early afternoon.
We propose that the generally accepted description of dust devil behaviour on Mars is incomplete, and that theories of dust devil formation may need to be modified specifically for the Martian environment. Further surveys of dust devil observations are required to support any such modifications. These surveys should include both surface and orbital observations, and the range of observations must encompass the full diurnal period and consider the wider meteorological context surrounding the observations
String Theory on AdS Orbifolds
We consider worldsheet string theory on orbifolds of associated
with conical singularities. If the orbifold action includes a similar twist of
, supersymmetry is preserved, and there is a moduli space of vacua arising
from blowup modes of the orbifold singularity. We exhibit the spectrum,
including the properties of twisted sectors and states obtained by fractional
spectral flow. A subalgebra of the spacetime superconformal symmetry remains
intact after the quotient, and serves as the spacetime symmetry algebra
of the orbifold.Comment: 37 pages, 3 eps figures. v2: Substantial revision to section 7, on
spacetime CFT interpretatio
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