103 research outputs found
A Nearly Model-Independent Characterization of Dark Energy Properties as a Function of Redshift
Understanding the acceleration of the universe and its cause is one of the
key problems in physics and cosmology today, and is best studied using a
variety of mutually complementary approaches. Daly and Djorgovski (2003, 2004)
proposed a model independent approach to determine the expansion and
acceleration history of the universe and a number of important physical
parameters of the dark energy as functions of redshift directly from the data.
Here, we apply the method to explicitly determine the first and second
derivatives of the coordinate distance with respect to redshift and combine
them to solve for the acceleration of the universe and the kinetic and
potential energy density of the dark energy as functions of redshift. A data
set of 228 supernova and 20 radio galaxy measurements with redshifts from zero
to 1.79 is used for this study. The values we obtain are shown to be consistent
with the values expected in a standard Lambda Cold Dark Matter model.Comment: 5 pages, 8 figure
The XMM Cluster Survey: Evidence for energy injection at high redshift from evolution of the X-ray luminosity-temperature relation
We measure the evolution of the X-ray luminosity-temperature (L_X-T) relation
since z~1.5 using a sample of 211 serendipitously detected galaxy clusters with
spectroscopic redshifts drawn from the XMM Cluster Survey first data release
(XCS-DR1). This is the first study spanning this redshift range using a single,
large, homogeneous cluster sample. Using an orthogonal regression technique, we
find no evidence for evolution in the slope or intrinsic scatter of the
relation since z~1.5, finding both to be consistent with previous measurements
at z~0.1. However, the normalisation is seen to evolve negatively with respect
to the self-similar expectation: we find E(z)^{-1} L_X = 10^{44.67 +/- 0.09}
(T/5)^{3.04 +/- 0.16} (1+z)^{-1.5 +/- 0.5}, which is within 2 sigma of the zero
evolution case. We see milder, but still negative, evolution with respect to
self-similar when using a bisector regression technique. We compare our results
to numerical simulations, where we fit simulated cluster samples using the same
methods used on the XCS data. Our data favour models in which the majority of
the excess entropy required to explain the slope of the L_X-T relation is
injected at high redshift. Simulations in which AGN feedback is implemented
using prescriptions from current semi-analytic galaxy formation models predict
positive evolution of the normalisation, and differ from our data at more than
5 sigma. This suggests that more efficient feedback at high redshift may be
needed in these models.Comment: Accepted for publication in MNRAS; 12 pages, 6 figures; added
references to match published versio
Synergy between the Large Synoptic Survey Telescope and the Square Kilometre Array
We provide an overview of the science benefits of combining information from the Square Kilometre Array (SKA) and the Large Synoptic Survey Telescope (LSST). We first summarise the capabilities and timeline of the LSST and overview its science goals. We then discuss the science questions in common between the two projects, and how they can be best addressed by combining the data from both telescopes. We describe how weak gravitational lensing and galaxy clustering studies with LSST and SKA can provide improved constraints on the causes of the cosmological acceleration. We summarise the benefits to galaxy evolution studies of combining deep optical multi-band imaging with radio observations. Finally, we discuss the excellent match between one of the most unique features of the LSST, its temporal cadence in the optical waveband, and the time resolution of the SKA
Populations behind the source-subtracted cosmic infrared background anisotropies
While the upcoming telescopes will reveal correspondingly fainter, more
distant galaxies, a question will persist: what more is there that these
telescopes cannot see? One answer is the source-subtracted Cosmic Infrared
Background (CIB). The CIB is comprised of the collective light from all sources
remaining after known, resolved sources are accounted for. Ever-more-sensitive
surveys will identify the brightest of these, allowing them to be removed, and
- like peeling layers off an onion - reveal deeper layers of the CIB. In this
way it is possible to measure the contributions from populations not accessible
to direct telescopic observation. Measurement of fluctuations in the
source-subtracted CIB, i.e., the spatial power spectrum of the CIB after
subtracting resolved sources, provides a robust means of characterizing its
faint, and potentially new, populations. Studies over the past 15 years have
revealed source-subtracted CIB fluctuations on scales out to ~100' which cannot
be explained by extrapolating from known galaxy populations. Moreover, they
appear highly coherent with the unresolved Cosmic X-ray Background, hinting at
a significant population of accreting black holes among the CIB sources.
Characterizing the source-subtracted CIB with high accuracy, and thereby
constraining the nature of the new populations, is feasible with upcoming
instruments and would produce critically important cosmological information in
the next decade. New coextensive deep and wide-area near-infrared, X-ray, and
microwave surveys will bring decisive opportunities to examine, with high
fidelity, the spatial spectrum and origin of the CIB fluctuations and their
cross-correlations with cosmic microwave and X-ray backgrounds, and determine
the formation epochs and the nature of the new sources (stellar nucleosynthetic
or accreting black holes).Comment: Science whitepaper submitted to the Astro2020 Decadal Surve
Scalar phantom energy as a cosmological dynamical system
Phantom energy can be visualized as a scalar field with a (non-canonical)
negative kinetic energy term. We use the dynamical system formalism to study
the attractor behavior of a cosmological model containing a phantom scalar
field endowed with an exponential potential of the form , and a perfect fluid with constant equation of
state ; the latter can be of the phantom type too. As in the canonical
case, three characteristic solutions can be identified. The scaling solution
exists but is either unstable or of no physical interest. Thus, there are only
two stable critical points which are of physical interest, corresponding to the
perfect fluid and scalar field dominated solutions, respectively. The most
interesting case arises for , which allows the
coexistence of the three solutions. The main features of each solution are
discussed in turn.Comment: 6 pages, 3 eps figures; uses RevTex4. New references added, and
changes made according to referee's suggestions. Matches published version in
JCA
Cosmology with Phase 1 of the Square Kilometre Array Red Book 2018: Technical specifications and performance forecasts
We present a detailed overview of the cosmological surveys that we aim to carry out with Phase 1 of the Square Kilometre Array (SKA1) and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5 000 deg2; a wide and deep continuum galaxy and HI intensity mapping (IM) survey over 20 000 deg2 from to 3; and a deep, high-redshift HI IM survey over 100 deg2 from to 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to . These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical and near-infrared (NIR) surveys like Euclid, LSST, and WFIRST leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1
Imprint of DES superstructures on the cosmic microwave background
Small temperature anisotropies in the cosmic microwave background (CMB) can be sourced by density perturbations via the late-time integrated Sachs-Wolfe (ISW) effect. Large voids and superclusters are excellent environments to make a localized measurement of this tiny imprint. In some cases excess signals have been reported. We probed these claims with an independent data set, using the first year data of the Dark Energy Survey (DES) in a different footprint, and using a different superstructure finding strategy. We identified 52 large voids and 102 superclusters at redshifts 0.2 < z < 0.65. We used the Jubilee simulation to a priori evaluate the optimal ISW measurement configuration for our compensated top-hat filtering technique, and then performed a stacking measurement of the CMB temperature field based on the DES data. For optimal configurations, we detected a cumulative cold imprint of voids with DeltaTf â -5.0 ± 3.7 muK and a hot imprint of superclusters DeltaTf â 5.1 ± 3.2 muK; this is Ë1.2sigma higher than the expected |DeltaTf| â 0.6 muK imprint of such superstructures in Lambda cold dark matter (LambdaCDM). If we instead use an a posteriori selected filter size (R/Rv = 0.6), we can find a temperature decrement as large as DeltaTf â -9.8 ± 4.7 muK for voids, which is Ë2sigma above LambdaCDM expectations and is comparable to previous measurements made using Sloan Digital Sky Survey superstructure data
Independent Predictors of Cardiac Mortality and Hospitalization for Heart Failure in a Multi-Ethnic Asian ST-segment Elevation Myocardial Infarction Population Treated by Primary Percutaneous Coronary Intervention
We aimed to identify independent predictors of cardiac mortality and hospitalization for heart failure (HHF) from a real-world, multi-ethnic Asian registry [the Singapore Myocardial Infarction Registry] of ST-segment elevation myocardial infarction (STEMI) patients treated by primary percutaneous coronary intervention. 11,546 eligible STEMI patients between 2008 and 2015 were identified. In-hospital, 30-day and 1-year cardiac mortality and 1-year HHF rates were 6.4%, 6.8%, 8.3% and 5.2%, respectively. From the derivation cohort (70% of patients), age, Killip class and cardiac arrest, creatinine, hemoglobin and troponin on admission and left ventricular ejection fraction (LVEF) during hospitalization were predictors of in-hospital, 30-day and 1-year cardiac mortality. Previous ischemic heart disease (IHD) was a predictor of in-hospital and 30-day cardiac mortality only, whereas diabetes was a predictor of 1-year cardiac mortality only. Age, previous IHD and diabetes, Killip class, creatinine, hemoglobin and troponin on admission, symptom-to-balloon-time and LVEF were predictors of 1-year HHF. The c-statistics were 0.921, 0.901, 0.881, 0.869, respectively. Applying these models to the validation cohort (30% of patients) showed good fit and discrimination (c-statistic 0.922, 0.913, 0.903 and 0.855 respectively; misclassification rate 14.0%, 14.7%, 16.2% and 24.0% respectively). These predictors could be incorporated into specific risk scores to stratify reperfused STEMI patients by their risk level for targeted intervention
The Accelerated Acceleration of the Universe
We present a simple mechanism which can mimic dark energy with an equation of
state w < -1 as deduced from the supernova data. We imagine that the universe
is accelerating under the control of a quintessence field, which is moving up a
very gently sloping potential. As a result, the potential energy and hence the
acceleration increases at lower redshifts. Fitting this behavior with a dark
energy model with constant w would require w<-1. In fact we find that the
choice of parameters which improves the fit to the SNe mimics w = -1.4 at low
redshifts. Running up the potential in fact provides the best fit to the SN
data for a generic quintessence model. However, unlike models with phantoms,
our model does not have negative energies or negative norm states. Future
searches for supernovae at low redshifts 0.1 < z < 0.5 and at high redshifts
z>1 may be a useful probe of our proposal.Comment: 14 pages, 5 figure
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