411 research outputs found
Testing Modified Dark Matter with Galaxy Clusters: Does Dark Matter know about the Cosmological Constant?
We discuss the possibility that the cold dark matter mass profiles contain
information on the cosmological constant, and that such information constrains
the nature of cold dark matter (CDM). We call this approach Modified Dark
Matter (MDM). In particular, we examine the ability of MDM to explain the
observed mass profiles of 13 galaxy clusters. Using general arguments from
gravitational thermodynamics, we provide a theoretical justification for our
MDM mass profile and successfully compare it to the NFW mass profiles both on
cluster and galactic scales. Our results suggest that indeed the CDM mass
profiles contain information about the cosmological constant in a non-trivial
way
Testing MONDian Dark Matter with Galactic Rotation Curves
MONDian dark matter (MDM) is a new form of dark matter quantum that naturally
accounts for Milgrom's scaling, usually associated with modified Newtonian
dynamics (MOND), and theoretically behaves like cold dark matter (CDM) at
cluster and cosmic scales. In this paper, we provide the first observational
test of MDM by fitting rotation curves to a sample of 30 local spiral galaxies
(z approximately 0.003). For comparison, we also fit the galactic rotation
curves using MOND, and CDM. We find that all three models fit the data well.
The rotation curves predicted by MDM and MOND are virtually indistinguishable
over the range of observed radii (~1 to 30 kpc). The best-fit MDM and CDM
density profiles are compared. We also compare with MDM the dark matter density
profiles arising from MOND if Milgrom's formula is interpreted as Newtonian
gravity with an extra source term instead of as a modification of inertia. We
find that discrepancies between MDM and MOND will occur near the center of a
typical spiral galaxy. In these regions, instead of continuing to rise sharply,
the MDM mass density turns over and drops as we approach the center of the
galaxy. Our results show that MDM, which restricts the nature of the dark
matter quantum by accounting for Milgrom's scaling, accurately reproduces
observed rotation curves.Comment: Preprint number IPMU13-0147. Accepted for publication in Ap
CLUMP-3D. Testing CDM with galaxy cluster shapes
The CDM model of structure formation makes strong predictions on
concentration and shape of DM (dark matter) halos, which are determined by mass
accretion processes. Comparison between predicted shapes and observations
provides a geometric test of the CDM model. Accurate and precise
measurements needs a full three-dimensional analysis of the cluster mass
distribution. We accomplish this with a multi-probe 3D analysis of the X-ray
regular CLASH (Cluster Lensing And Supernova survey with Hubble) clusters
combining strong and weak lensing, X-ray photometry and spectroscopy, and the
Sunyaev-Zel'dovich effect. The cluster shapes and concentrations are consistent
with CDM predictions. The CLASH clusters are randomly oriented, as
expected given the sample selection criteria. Shapes agree with numerical
results for DM-only halos, which hints at baryonic physics being not so
effective in making halos rounder.Comment: v2: 8 pages, in press on ApJL. Extended discussion on regularity. One
of three new companion papers of the CLUMP-3D project (Keiichi Umetsu et al.,
arxiv:1804.00664; I-Non Chiu et al., arXiv:1804.00676
Diagnostic quality assessment of compressed sensing accelerated magnetic resonance neuroimaging.
PURPOSE: To determine the efficacy of compressed sensing (CS) reconstructions for specific clinical magnetic resonance neuroimaging applications beyond more conventional acceleration techniques such as parallel imaging (PI) and low-resolution acquisitions.
MATERIALS AND METHODS: Raw k-space data were acquired from five healthy volunteers on a 3T scanner using a 32-channel head coil using T2 -FLAIR, FIESTA-C, time of flight (TOF), and spoiled gradient echo (SPGR) sequences. In a series of blinded studies, three radiologists independently evaluated CS, PI (GRAPPA), and low-resolution images at up to 5Ă— accelerations. Synthetic T2 -FLAIR images with artificial lesions were used to assess diagnostic accuracy for CS reconstructions.
RESULTS: CS reconstructions were of diagnostically acceptable quality at up to 4Ă— acceleration for T2 -FLAIR and FIESTA-C (average qualitative scores 3.7 and 4.3, respectively, on a 5-point scale at 4Ă— acceleration), and at up to 3Ă— acceleration for TOF and SPGR (average scores 4.0 and 3.7, respectively, at 3Ă— acceleration). The qualitative scores for CS reconstructions were significantly better than low-resolution images for T2 -FLAIR, FIESTA-C, and TOF and significantly better than GRAPPA for TOF and SPGR (Wilcoxon signed rank test, P \u3c 0.05) with no significant difference found otherwise. Diagnostic accuracy was acceptable for both CS and low-resolution images at up to 3Ă— acceleration (area under the ROC curve 0.97 and 0.96, respectively.)
CONCLUSION: Mild to moderate accelerations are possible for those sequences by a combined CS and PI reconstruction. Nevertheless, for certain sequences/applications one might mildly reduce the acquisition time by appropriately reducing the imaging resolution rather than the more complicated CS reconstruction. J. Magn. Reson. Imaging 2016;44:433-444
Quantum Gravity and Dark Matter
We propose a connection between global physics and local galactic dynamics
via quantum gravity. The salient features of cold dark matter (CDM) and
modified Newtonian dynamics (MOND) are combined into a unified scheme by
introducing the concept of MONDian dark matter which behaves like CDM at
cluster and cosmological scales but emulates MOND at the galactic scale.Comment: This paper has been awarded the Fifth Prize in the 2011 Essay
Competition of the Gravity Research Foundatio
Echocardiographic Manifestations in Patients with Cardiac Sarcoidosis
BackgroundCardiac sarcoidosis is a life-threatening disease with protean clinical manifestations, including congestive heart failure (CHF), conduction disturbance, ventricular arrhythmia and sudden death. Nonetheless, it is difficult to diagnose cardiac sarcoidosis in the clinical setting. Some echocardiographic findings of cardiac sarcoidosis associated with other diagnostic tools (201thallium scintigraphy, 67gallium citrate scan, serum markers and others) may be helpful upon early suspicion and diagnosis of cardiac sarcoidosis.Materials and MethodsFifty-two patients (36 female) with cardiac sarcoidosis, with a mean age of 48 ± 14 years (range, 21–70 yr), underwent a series of echocardiographic follow-up (mean, 88 ± 48 mo) examinations, and important echocardiographic parameters and findings were recorded.ResultsThere were left ventricular (LV) regional wall motion abnormalities (RWMAs) noted in 40 (localized in 16, multiple in 24), dilatation of the LV with impaired LV contractility in 28, thinning of the basal interventricular septum (IVS) in 27, thinning of LV free wall in 18, apical aneurysm in 12, apical thrombus in two, mimicking hypertrophic cardiomyopathy (HCM) in two, pericardial effusion (PE) in two (with cardiac tamponade in one), and LV wall thinning and aneurysm formation after steroid therapy for cardiac sarcoidosis in two of 43 patients.ConclusionThinning of the basal IVS or LV free wall is a specific echocardiographic finding of cardiac sarcoidosis. Other echocardiographic findings of cardiac sarcoidosis may mimic coronary artery disease (LV RWMA or apical aneurysm), CHF, or HCM. PE and thinning of the LV wall after steroid therapy were also noted in rare situations
Solving the Interpretational-Confounding and Interpretational-Ambiguity Problems of Formative Construct Modeling in Behavioral Research: Proposing a Two-Stage Fixed-Weight Redundancy Approach
Recently, information systems research has devoted increasing attention to formative measurements. However, current approaches to modeling formative constructs have potential validity problems and thus limited applicability. Here, we highlight two major problems in formative measurement—interpretational confounding and interpretational ambiguity—and propose a novel resolution. Interpretational confounding occurs when using the traditional free-estimation approach, because the weights of different formative indicators vary as the dependent variable changes, resulting in the distortion of the measurement weights of the focal formative construct and thus jeopardizing the generalizability of empirical tests. Another way to alleviate the interpretational-confounding issue is to include the multiple indicators multiple causes (MIMIC) construct in the path model (i.e., MIMIC-path). Unfortunately, this method has led to the second major problem of interpretational ambiguity, the existence of more than one potential explanation of the formative model. More specifically, reflective indicators in the MIMIC model can be viewed as (1) indicators of the MIMIC construct, (2) dependent variables of the formative construct, or (3) indicators of a reflective construct affected by independent variables (formative indicators). To resolve these issues, we propose a two-stage fixed-weight redundancy model (FWRM) approach. We demonstrate the applicability of the FWRM approach with a set of survey data. We conducted a simulation study evaluating the FWRM approach by comparing it with the commonly used free-estimation and MIMIC-path methods. The results indicate that our FWRM approach can indeed improve the validity of formative construct modeling by mitigating confounding and ambiguity issues
Exploring Individuals’ Loyalty To Online Support Groups From the Perspective Of Social Support
Prior research indicates that social support allows individuals to have additional skills to handle stresses, enhance self-esteem, and increase psychological well-being. Online social networks have become an emerging and important source of social support. However, little research has been done to explore how IT usage may improve the well-being and life quality of individuals, especially patients. Little research has been done to empirically examine individuals’ loyalty toward online social groups from the perspective of social support. This study aims to develop an integrated research model to the impact of online social support on psychological well-being and group identification, and individuals’ subsequent loyalty toward online social groups. Online social support group was operationalized as a second-order construct with four components: informational, emotional, esteem, and network supports. Data collected from 537 users of online support groups for mothers in Taiwan provide strong support for the research model. The results indicate that online social support is positively associated with psychological well-being and group identification, which in turn have a positive effect on moms’ loyalty toward online social groups. Implications for theory and practice and suggestions for future research are provided
Spacetime Emergence and General Covariance Transmutation
Spacetime emergence refers to the notion that classical spacetime "emerges"
as an approximate macroscopic entity from a non-spatio-temporal structure
present in a more complete theory of interacting fundamental constituents. In
this article, we propose a novel mechanism involving the "soldering" of
internal and external spaces for the emergence of spacetime and the twin
transmutation of general covariance. In the context of string theory, this
mechanism points to a critical four dimensional spacetime background.Comment: 11 pages, v2: version to appear in MPL
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