842 research outputs found
WFMOS - Sounding the Dark Cosmos
Vast sound waves traveling through the relativistic plasma during the first
million years of the universe imprint a preferred scale in the density of
matter. We now have the ability to detect this characteristic fingerprint in
the clustering of galaxies at various redshifts and use it to measure the
acceleration of the expansion of the Universe. The Wide-Field Multi-Object
Spectrograph (WFMOS) would use this test to shed significant light on the true
nature of dark energy, the mysterious source of this cosmic acceleration. WFMOS
would also revolutionise studies of the kinematics of the Milky Way and provide
deep insights into the clustering of galaxies at redshifts up to z~4. In this
article we discuss the recent progress in large galaxy redshift surveys and
detail how WFMOS will help unravel the mystery of dark energy.Comment: 6 pages, pure pdf. An introduction to WFMOS and Baryon Acoustic
Oscillations for a general audienc
Atomistic Simulations of the Defect Chemistry and Self-Diffusion of Li-ion in LiAlO2
Lithium aluminate, LiAlO2, is a material that is presently being considered as a tritium breeder material in fusion reactors and coating material in Li-conducting electrodes. Here, we employ atomistic simulation techniques to show that the lowest energy intrinsic defect process is the cation anti-site defect (1.10 eV per defect). This was followed closely by the lithium Frenkel defect (1.44 eV per defect), which ensures a high lithium content in the material and inclination for lithium diffusion from formation of vacancies. Li self-diffusion is three dimensional and exhibits a curved pathway with a migration barrier of 0.53 eV. We considered a variety of dopants with charges +1 (Na, K and Rb), +2 (Mg, Ca, Sr and Ba), +3 (Ga, Fe, Co, Ni, Mn, Sc, Y and La) and +4 (Si, Ge, Ti, Zr and Ce) on the Al site. Dopants Mg2+ and Ge4+ can facilitate the formation of Li interstitials and Li vacancies, respectively. Trivalent dopants Fe3+, Ni3+ and Mn3+ prefer to occupy the Al site with exoergic solution energies meaning that they are candidate dopants for the synthesis of Li (Al, M) O2 (M = Fe, Ni and Mn) compounds
Constraining Scale-Dependent Non-Gaussianity with Future Large-Scale Structure and the CMB
We forecast combined future constraints from the cosmic microwave background
and large-scale structure on the models of primordial non-Gaussianity. We study
the generalized local model of non-Gaussianity, where the parameter f_NL is
promoted to a function of scale, and present the principal component analysis
applicable to an arbitrary form of f_NL(k). We emphasize the complementarity
between the CMB and LSS by using Planck, DES and BigBOSS surveys as examples,
forecast constraints on the power-law f_NL(k) model, and introduce the figure
of merit for measurements of scale-dependent non-Gaussianity.Comment: 28 pages, 8 figures, 2 tables; v2: references update
Large non-Gaussian Halo Bias from Single Field Inflation
We calculate Large Scale Structure observables for non-Gaussianity arising
from non-Bunch-Davies initial states in single field inflation. These scenarios
can have substantial primordial non-Gaussianity from squeezed (but observable)
momentum configurations. They generate a term in the halo bias that may be more
strongly scale-dependent than the contribution from the local ansatz. We also
discuss theoretical considerations required to generate an observable
signature.Comment: 30 pages, 14 figures, typos corrected and minor changes to match
published version JCAP09(2012)00
Measuring neutrino masses with a future galaxy survey
We perform a detailed forecast on how well a Euclid-like photometric galaxy
and cosmic shear survey will be able to constrain the absolute neutrino mass
scale. Adopting conservative assumptions about the survey specifications and
assuming complete ignorance of the galaxy bias, we estimate that the minimum
mass sum of sum m_nu ~ 0.06 eV in the normal hierarchy can be detected at 1.5
sigma to 2.5 sigma significance, depending on the model complexity, using a
combination of galaxy and cosmic shear power spectrum measurements in
conjunction with CMB temperature and polarisation observations from Planck.
With better knowledge of the galaxy bias, the significance of the detection
could potentially reach 5.4 sigma. Interestingly, neither Planck+shear nor
Planck+galaxy alone can achieve this level of sensitivity; it is the combined
effect of galaxy and cosmic shear power spectrum measurements that breaks the
persistent degeneracies between the neutrino mass, the physical matter density,
and the Hubble parameter. Notwithstanding this remarkable sensitivity to sum
m_nu, Euclid-like shear and galaxy data will not be sensitive to the exact mass
spectrum of the neutrino sector; no significant bias (< 1 sigma) in the
parameter estimation is induced by fitting inaccurate models of the neutrino
mass splittings to the mock data, nor does the goodness-of-fit of these models
suffer any significant degradation relative to the true one (Delta chi_eff ^2<
1).Comment: v1: 29 pages, 10 figures. v2: 33 pages, 12 figures; added sections on
shape evolution and constraints in more complex models, accepted for
publication in JCA
Automated cross-identifying radio to infrared surveys using the LRPY algorithm: A case study
Cross-identifying complex radio sources with optical or infra red (IR) counterparts in surveys such as the Australia Telescope Large Area Survey (ATLAS) has traditionally been performed manually. However, with new surveys from the Australian Square Kilometre Array Pathfinder detecting many tens of millions of radio sources, such an approach is no longer feasible. This paper presents new software (LRPY - Likelihood Ratio in PYTHON) to automate the process of cross-identifying radio sources with catalogues at other wavelengths. LRPY implements the likelihood ratio (LR) technique with a modification to account for two galaxies contributing to a sole measured radio component. We demonstrate LRPY by applying it to ATLAS DR3 and a Spitzer-based multiwavelength fusion catalogue, identifying 3848 matched sources via our LR-based selection criteria. A subset of 1987 sources have flux density values for all IRAC bands which allow us to use criteria to distinguish between active galactic nuclei (AGNs) and star-forming galaxies (SFG). We find that 936 radio sources (Ëś47 per cent) meet both of the Lacy and Stern AGN selection criteria. Of the matched sources, 295 have spectroscopic redshifts and we examine the radio to IR flux ratio versus redshift, proposing an AGN selection criterion below the Elvis radio-loud AGN limit for this dataset. Taking the union of all three AGNs selection criteria we identify 956 as AGNs (Ëś48 per cent). From this dataset, we find a decreasing fraction of AGNs with lower radio flux densities consistent with other results in the literature
Direct Dark Matter Search using CCDs
There is currently vast evidence for Dark Matter (DM) from astronomical
observations. However, in spite of tremendous efforts by large experimental
groups, there is no confirmed direct detection of the dark matter in our
galaxy. Recent experimental results and theoretical developments suggest the
possibility of a DM particle with mass below 10 GeV, such a particle would
escape most of the direct searches due to the large thresholds for the
detection of nuclear recoils typically used. In this work we study the
possibility of a new Dark Matter search with an unprecedented low threshold for
the detection of nuclear recoils using high-resistivity CCD detectors (hr-CCD).
Due to their extremely low readout noise and the relatively large active mass,
these detectors present a unique opportunity in this field.Comment: paper presented at the Taup2009 Conferenc
Sensitivity and Insensitivity of Galaxy Cluster Surveys to New Physics
We study the implications and limitations of galaxy cluster surveys for
constraining models of particle physics and gravity beyond the Standard Model.
Flux limited cluster counts probe the history of large scale structure
formation in the universe, and as such provide useful constraints on
cosmological parameters. As a result of uncertainties in some aspects of
cluster dynamics, cluster surveys are currently more useful for analyzing
physics that would affect the formation of structure than physics that would
modify the appearance of clusters. As an example we consider the Lambda-CDM
cosmology and dimming mechanisms, such as photon-axion mixing.Comment: 24 pages, 8 eps figures. References added, discussion of scatter in
relations between cluster observables lengthene
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