198 research outputs found
Black hole gas in the early universe
We consider the early universe at temperatures close to the fundamental scale
of gravity (M_D << M_Planck) in models with extra dimensions. At such
temperatures a small fraction of particles will experience transplanckian
collisions that may result in microscopic black holes (BHs). BHs colder than
the environment will gain mass, and as they grow their temperature drops
further. We study the dynamics of a system (a black hole gas) defined by
radiation at a given temperature coupled to a distribution of BHs of different
mass. Our analysis includes the production of BHs in photon-photon collisions,
BH evaporation, the absorption of radiation, collisions of two BHs to give a
larger one, and the effects of the expansion. We show that the system may
follow two different generic paths depending on the initial temperature of the
plasma.Comment: 17 pages, version to appear in JCA
Constraining the primordial spectrum of metric perturbations from gravitino and moduli production
We consider the production of gravitinos and moduli fields from quantum
vacuum fluctuations induced by the presence of scalar metric perturbations at
the end of inflation. We obtain the corresponding occupation numbers, up to
first order in perturbation theory, in terms of the power spectrum of the
metric perturbations. We compute the limits imposed by nucleosynthesis on the
spectral index for different models with constant . The results show
that, in certain cases, such limits can be as strong as , which is
more stringent than those coming from primordial black hole production.Comment: 16 pages, LaTeX, 5 figures. Corrected figures, new references
included. Final version to appear in Phys. Rev.
ADHM/Nahm Construction of Localized Solitons in Noncommutative Gauge Theories
We study the relationship between ADHM/Nahm construction and ``solution
generating technique'' of BPS solitons in noncommutative gauge theories.
ADHM/Nahm construction and ``solution generating technique'' are the most
strong ways to construct exact BPS solitons. Localized solitons are the
solitons which are generated by the ``solution generating technique.'' The
shift operators which play crucial roles in ``solution generating technique''
naturally appear in ADHM/Nahm construction and we can construct various exact
localized solitons including new solitons: localized periodic instantons
(=localized calorons) and localized doubly-periodic instantons. Nahm
construction also gives rise to BPS fluxons straightforwardly from the
appropriate input Nahm data which is expected from the D-brane picture of BPS
fluxons. We also show that the Fourier-transformed soliton of the localized
caloron in the zero-period limit exactly coincides with the BPS fluxon.Comment: 30 pages, LaTeX, 3 figures; v3: minor changes, references added; v4:
references added, version to appear in PR
Health behaviours associated with healthy body composition among Aboriginal adolescents in Australia in the ‘Next Generation: Youth Well-being study’
OnlinePublThis study described the distribution of healthy body composition among Aboriginal adolescents in Australia aged 10–24 years and examined associations with health behaviours and self-rated health. Data were crosssectional from the ‘Next Generation: Youth Well-being study’ baseline (N = 1294). We used robust Poisson regression to quantify associations of self-reported health behaviours (physical activity, screen time, sleep, consumption of vegetables, fruit, soft drinks and fast food, and tobacco smoking and alcohol) and self-rated health to healthy body mass index (BMI) and waist/height ratio (WHtR). Overall, 48% of participants had healthy BMI and 64% healthy WHtR, with healthy body composition more common among younger adolescents. Higher physical activity was associated with healthy body composition (5–7 days last week vs none; adjusted prevalence ratio (aPR) healthy BMI 1.31 [95% CI 1.05–1.64], and healthy WHtR 1.30 [1.10–1.54]), as was recommended sleep duration (vs not; aPR healthy BMI 1.56 [1.19–2.05], and healthy WHtR 1.37 [1.13–1.67]). There was a trend for higher proportion of healthy body composition with more frequent fast food consumption. Healthy body composition was also associated with higher self-rated health (‘very good/excellent’ vs ‘poor/fair’; aPR healthy BMI 1.87 [1.45–2.42], and healthy WHtR 1.71 [1.40–2.10]). Culturally appropriate community health interventions with a focus on physical activity and sleep may hold promise for improving body composition among Aboriginal adolescents.Christopher D. McKay, Lina Gubhaju, Alison J. Gibberd, Bridgette J. McNamara, Rona Macniven, Grace Joshy, Robert Roseby, Robyn Williams, Aryati Yashadhana, Ted Fields, Bobby Porykali, Peter Azzopardi, Emily Banks, Sandra J. Eade
Categorizing Different Approaches to the Cosmological Constant Problem
We have found that proposals addressing the old cosmological constant problem
come in various categories. The aim of this paper is to identify as many
different, credible mechanisms as possible and to provide them with a code for
future reference. We find that they all can be classified into five different
schemes of which we indicate the advantages and drawbacks.
Besides, we add a new approach based on a symmetry principle mapping real to
imaginary spacetime.Comment: updated version, accepted for publicatio
The Earth: Plasma Sources, Losses, and Transport Processes
This paper reviews the state of knowledge concerning the source of magnetospheric plasma at Earth. Source of plasma, its acceleration and transport throughout the system, its consequences on system dynamics, and its loss are all discussed. Both observational and modeling advances since the last time this subject was covered in detail (Hultqvist et al., Magnetospheric Plasma Sources and Losses, 1999) are addressed
Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET
The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR
Relationship of edge localized mode burst times with divertor flux loop signal phase in JET
A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM
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