26 research outputs found
Orthogonal non-Gaussianities from Dirac-Born-Infeld Galileon inflation
We study the cosmology of the multifield relativistic Galileon model in which
an induced gravity term is added to the Dirac-Born-Infeld action. We highlight
the physical insight that is gained by employing a bimetric perspective in
which the induced gravity and Einstein-Hilbert action are treated on equal
footing. We derive the conditions under which a phase of quasi exponential
inflation can be sustained and demonstrate the existence of a critical
background energy density above which cosmological fluctuations become ghosts.
At the non-linear level, this scenario provides the first concrete early
Universe model in which the shape of the bispectrum can be predominantly of
orthogonal type. More generally, we show that the shape and sign of the
primordial non-Gaussianities act as powerful discriminants of the precise
strength of the induced gravity.Comment: 5 pages, 1 figure. Matches published version + added references and
misprints correcte
Nonlinear perturbations of cosmological scalar fields with non-standard kinetic terms
We adopt a covariant formalism to derive exact evolution equations for
nonlinear perturbations, in a universe dominated by two scalar fields. These
scalar fields are characterized by non-canonical kinetic terms and an arbitrary
field space metric, a situation typically encountered in inflationary models
inspired by string theory. We decompose the nonlinear scalar perturbations into
adiabatic and entropy modes, generalizing the definition adopted in the linear
theory, and we derive the corresponding exact evolution equations. We also
obtain a nonlinear generalization of the curvature perturbation on uniform
density hypersurfaces, showing that on large scales it is sourced only by the
nonlinear version of the entropy perturbation. We then expand these equations
to second order in the perturbations, using a coordinate based formalism. Our
results are relatively compact and elegant and enable one to identify the new
effects coming from the non-canonical structure of the scalar fields
Lagrangian. We also explain how to analyze, in our formalism, the interesting
scenario of multifield Dirac-Born-Infeld inflation.Comment: 31 pages, no figures. Discussion on multi-field DBI inflation added.
Version to be published in JCA
Multi-field DBI inflation: introducing bulk forms and revisiting the gravitational wave constraints
We study multi-field Dirac-Born-Infeld (DBI) inflation models, taking into
account the NS-NS and R-R bulk fields present in generic flux
compactifications. We compute the second-order action, which governs the
behaviour of linear cosmological perturbations, as well as the third-order
action, which can be used to calculate non-Gaussianities in these models.
Remarkably, for scalar-type perturbations, we show that the contributions due
to the various form fields exactly cancel in both the second- and third-order
actions. Primordial perturbations and their non-Gaussianities are therefore
unaffected by the presence of form fields and our previous results are
unmodified. We also study vector-type perturbations associated with the U(1)
gauge field confined on the D3-brane, and discuss whether their quantum
fluctuations can be amplified. Finally, we revisit the gravitational wave
constraints on DBI inflation and show that an ultra-violet DBI multi-field
scenario is still compatible with data, in contrast with the single field case,
provided there is a transfer from entropy into adiabatic perturbations.Comment: 22 page
Primordial fluctuations and non-Gaussianities in multi-field DBI inflation
We study Dirac-Born-Infeld (DBI) inflation models with multiple scalar
fields. We show that the adiabatic and entropy modes propagate with a common
effective sound speed and are thus amplified at the sound horizon crossing. In
the small sound speed limit, we find that the amplitude of the entropy modes is
much higher than that of the adiabatic modes. We show that this could strongly
affect the observable curvature power spectrum as well as the amplitude of
non-Gaussianities, although their shape remains as in the single-field DBI
case.Comment: 4 page
Primordial black holes and their gravitational-wave signatures
In the recent years, primordial black holes (PBHs) have emerged as one of the
most interesting and hotly debated topics in cosmology. Among other
possibilities, PBHs could explain both some of the signals from binary black
hole mergers observed in gravitational wave detectors and an important
component of the dark matter in the Universe. Significant progress has been
achieved both on the theory side and from the point of view of observations,
including new models and more accurate calculations of PBH formation,
evolution, clustering, merger rates, as well as new astrophysical and
cosmological probes. In this work, we review, analyse and combine the latest
developments in order to perform end-to-end calculations of the various
gravitational wave signatures of PBHs. Different ways to distinguish PBHs from
stellar black holes are emphasized. Finally, we discuss their detectability
with LISA, the first planned gravitational-wave observatory in space.Comment: 161 pages, 47 figures, comments welcom
Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis
BACKGROUND: Rates of antimicrobial resistance (AMR) are rising globally and there is concern that increased migration is contributing to the burden of antibiotic resistance in Europe. However, the effect of migration on the burden of AMR in Europe has not yet been comprehensively examined. Therefore, we did a systematic review and meta-analysis to identify and synthesise data for AMR carriage or infection in migrants to Europe to examine differences in patterns of AMR across migrant groups and in different settings. METHODS: For this systematic review and meta-analysis, we searched MEDLINE, Embase, PubMed, and Scopus with no language restrictions from Jan 1, 2000, to Jan 18, 2017, for primary data from observational studies reporting antibacterial resistance in common bacterial pathogens among migrants to 21 European Union-15 and European Economic Area countries. To be eligible for inclusion, studies had to report data on carriage or infection with laboratory-confirmed antibiotic-resistant organisms in migrant populations. We extracted data from eligible studies and assessed quality using piloted, standardised forms. We did not examine drug resistance in tuberculosis and excluded articles solely reporting on this parameter. We also excluded articles in which migrant status was determined by ethnicity, country of birth of participants' parents, or was not defined, and articles in which data were not disaggregated by migrant status. Outcomes were carriage of or infection with antibiotic-resistant organisms. We used random-effects models to calculate the pooled prevalence of each outcome. The study protocol is registered with PROSPERO, number CRD42016043681. FINDINGS: We identified 2274 articles, of which 23 observational studies reporting on antibiotic resistance in 2319 migrants were included. The pooled prevalence of any AMR carriage or AMR infection in migrants was 25·4% (95% CI 19·1-31·8; I2 =98%), including meticillin-resistant Staphylococcus aureus (7·8%, 4·8-10·7; I2 =92%) and antibiotic-resistant Gram-negative bacteria (27·2%, 17·6-36·8; I2 =94%). The pooled prevalence of any AMR carriage or infection was higher in refugees and asylum seekers (33·0%, 18·3-47·6; I2 =98%) than in other migrant groups (6·6%, 1·8-11·3; I2 =92%). The pooled prevalence of antibiotic-resistant organisms was slightly higher in high-migrant community settings (33·1%, 11·1-55·1; I2 =96%) than in migrants in hospitals (24·3%, 16·1-32·6; I2 =98%). We did not find evidence of high rates of transmission of AMR from migrant to host populations. INTERPRETATION: Migrants are exposed to conditions favouring the emergence of drug resistance during transit and in host countries in Europe. Increased antibiotic resistance among refugees and asylum seekers and in high-migrant community settings (such as refugee camps and detention facilities) highlights the need for improved living conditions, access to health care, and initiatives to facilitate detection of and appropriate high-quality treatment for antibiotic-resistant infections during transit and in host countries. Protocols for the prevention and control of infection and for antibiotic surveillance need to be integrated in all aspects of health care, which should be accessible for all migrant groups, and should target determinants of AMR before, during, and after migration. FUNDING: UK National Institute for Health Research Imperial Biomedical Research Centre, Imperial College Healthcare Charity, the Wellcome Trust, and UK National Institute for Health Research Health Protection Research Unit in Healthcare-associated Infections and Antimictobial Resistance at Imperial College London