1,159 research outputs found
Evidence of the selection of tidal streams by northern rock sole (Lepidopsetta polyxystra) for transport in the eastern Bering Sea
Depth data from archival tags on northern rock sole (Lepidopsetta polyxystra) were examined to assess whether fish used tidal currents to aid horizontal migration. Two northern rock sole, out of 115 released with archival tags in the eastern Bering Sea, were recovered 314 and 667 days after release. Both fish made periodic excursions away from the bottom during mostly night-time hours, but also during particular phases of the tide cycle. One fish that was captured and released in an area of rotary currents made
vertical excursions that were correlated with tidal current direction. To test the hypothesis that the fish made vertical excursions to use tidal currents to aid migration,
a hypothetical migratory path was calculated using a tide model to predict the current direction and speed during periods when the fish was off the bottom. This migration
included limited movements from July through December, followed by a 200-km southern migration from January through February, then a return northward in March and
April. The successful application of tidal current information to predict a horizontal migratory path not
only provides evidence of selective tidal stream transport but indicates that vertical excursions were conducted
primarily to assist horizontal migration
Testing Emergent Gravity on Galaxy Cluster Scales
Verlinde's theory of Emergent Gravity (EG) describes gravity as an emergent
phenomenon rather than a fundamental force. Applying this reasoning in de
Sitter space leads to gravity behaving differently on galaxy and galaxy cluster
scales; this excess gravity might offer an alternative to dark matter. Here we
test these ideas using the data from the Coma cluster and from 58 stacked
galaxy clusters. The X-ray surface brightness measurements of the clusters at
along with the weak lensing data are used to test the theory.
We find that the simultaneous EG fits of the X-ray and weak lensing datasets
are significantly worse than those provided by General Relativity (with cold
dark matter). For the Coma cluster, the predictions from Emergent Gravity and
General Relativity agree in the range of 250 - 700 kpc, while at around 1 Mpc
scales, EG total mass predictions are larger by a factor of 2. For the cluster
stack the predictions are only in good agreement at around the 1 - 2 Mpc
scales, while for Mpc EG is in strong tension with the data.
According to the Bayesian information criterion analysis, GR is preferred in
all tested datasets; however, we also discuss possible modifications of EG that
greatly relax the tension with the data.Comment: 19 pages, 5 figures, 5 tables, accepted for publication on JCA
Acoustic Oscillations in the Early Universe and Today
During its first ~100,000 years, the universe was a fully ionized plasma with
a tight coupling by Thompson scattering between the photons and matter. The
trade--off between gravitational collapse and photon pressure causes acoustic
oscillations in this primordial fluid. These oscillations will leave
predictable imprints in the spectra of the cosmic microwave background and the
present day matter-density distribution. Recently, the BOOMERANG and MAXIMA
teams announced the detection of these acoustic oscillations in the cosmic
microwave background (observed at redshift ~1000). Here, we compare these CMB
detections with the corresponding acoustic oscillations in the matter-density
power spectrum (observed at redshift ~0.1). These consistent results, from two
different cosmological epochs, provide further support for our standard Hot Big
Bang model of the universe.Comment: To appear in the journal Science. 6 pages, 1 color figur
Testing chameleon gravity with the Coma cluster
We propose a novel method to test the gravitational interactions in the
outskirts of galaxy clusters. When gravity is modified, this is typically
accompanied by the introduction of an additional scalar degree of freedom,
which mediates an attractive fifth force. The presence of an extra
gravitational coupling, however, is tightly constrained by local measurements.
In chameleon modifications of gravity, local tests can be evaded by employing a
screening mechanism that suppresses the fifth force in dense environments.
While the chameleon field may be screened in the interior of the cluster, its
outer region can still be affected by the extra force, introducing a deviation
between the hydrostatic and lensing mass of the cluster. Thus, the chameleon
modification can be tested by combining the gas and lensing measurements of the
cluster. We demonstrate the operability of our method with the Coma cluster,
for which both a lensing measurement and gas observations from the X-ray
surface brightness, the X-ray temperature, and the Sunyaev-Zel'dovich effect
are available. Using the joint observational data set, we perform a Markov
chain Monte Carlo analysis of the parameter space describing the different
profiles in both the Newtonian and chameleon scenarios. We report competitive
constraints on the chameleon field amplitude and its coupling strength to
matter. In the case of f(R) gravity, corresponding to a specific choice of the
coupling, we find an upper bound on the background field amplitude of
|f_{R0}|<6*10^{-5}, which is currently the tightest constraint on cosmological
scales.Comment: 27 pages, 8 figures, version accepted for publication in JCA
Every student counts: promoting numeracy and enhancing employability
This three-year project investigated factors that influence the development of undergraduates’ numeracy skills, with a view to identifying ways to improve them and thereby enhance student employability. Its aims and objectives were to ascertain: the generic numeracy skills in which employers expect their graduate recruits to be competent and the extent to which employers are using numeracy tests as part of graduate recruitment processes; the numeracy skills developed within a diversity of academic disciplines;
the prevalence of factors that influence undergraduates’ development of their numeracy skills; how the development of numeracy skills might be better supported within undergraduate curricula; and the extra-curricular support necessary to enhance undergraduates’ numeracy skills
Optimizing baryon acoustic oscillation surveys – I. Testing the concordance ΛCDM cosmology
We optimize the design of future spectroscopic redshift surveys for
constraining the dark energy via precision measurements of the baryon acoustic
oscillations (BAO), with particular emphasis on the design of the Wide-Field
Multi-Object Spectrograph (WFMOS). We develop a model that predicts the number
density of possible target galaxies as a function of exposure time and
redshift. We use this number counts model together with fitting formulae for
the accuracy of the BAO measurements to determine the effectiveness of
different surveys and instrument designs. We search through the available
survey parameter space to find the optimal survey with respect to the dark
energy equation-of-state parameters according to the Dark Energy Task Force
Figure-of-Merit, including predictions of future measurements from the Planck
satellite. We optimize the survey to test the LambdaCDM model, assuming that
galaxies are pre-selected using photometric redshifts to have a constant number
density with redshift, and using a non-linear cut-off for the matter power
spectrum that evolves with redshift. We find that line-emission galaxies are
strongly preferred as targets over continuum emission galaxies. The optimal
survey covers a redshift range 0.8 < z < 1.4, over the widest possible area
(6000 sq. degs from 1500 hours observing time). The most efficient number of
fibres for the spectrograph is 2,000, and the survey performance continues to
improve with the addition of extra fibres until a plateau is reached at 10,000
fibres. The optimal point in the survey parameter space is not highly peaked
and is not significantly affected by including constraints from upcoming
supernovae surveys and other BAO experiments.Comment: 15 pages, 9 figure
The design of a Space-borne multispectral canopy LiDAR to estimate global carbon stock and gross primary productivity
Understanding the dynamics of the global carbon cycle is one of the most challenging issues for the scientific community. The ability to measure the magnitude of terrestrial carbon sinks as well as monitoring the short and long term changes is vital for environmental decision making. Forests form a significant part of the terrestrial biosystem and understanding the global carbon cycle, Above Ground Biomass (AGB) and Gross Primary Productivity (GPP) are critical parameters. Current estimates of AGB and GPP are not adequate to support models of the global carbon cycle and more accurate estimates would improve predictions of the future and estimates of the likely behaviour of these sinks. Various vegetation indices have been proposed for the characterisation of forests including canopy height, canopy area, Normalised Difference Vegetation Index (NDVI) and Photochemical Reflectance Index (PRI). Both NDVI and PRI are obtained from a measure of reflectivity at specific wavelengths and have been estimated from passive measurements. The use of multi-spectral LiDAR to measure NDVI and PRI and their vertical distribution within the forest represents a significant improvement over current techniques. This paper describes an approach to the design of an advanced Multi-Spectral Canopy LiDAR, using four wavelengths for measuring the vertical profile of the canopy simultaneously. It is proposed that the instrument be placed on a satellite orbiting the Earth on a sun synchronous polar orbit to provide samples on a rectangular grid at an approximate separation of 1km with a suitable revisit frequency. The systems engineering concept design will be presented
The effects of velocities and lensing on moments of the Hubble diagram
We consider the dispersion on the supernova distance-redshift relation due to
peculiar velocities and gravitational lensing, and the sensitivity of these
effects to the amplitude of the matter power spectrum. We use the MeMo lensing
likelihood developed by Quartin, Marra & Amendola (2014), which accounts for
the characteristic non-Gaussian distribution caused by lensing magnification
with measurements of the first four central moments of the distribution of
magnitudes. We build on the MeMo likelihood by including the effects of
peculiar velocities directly into the model for the moments. In order to
measure the moments from sparse numbers of supernovae, we take a new approach
using Kernel Density Estimation to estimate the underlying probability density
function of the magnitude residuals. We also describe a bootstrap re-sampling
approach to estimate the data covariance matrix. We then apply the method to
the Joint Light-curve Analysis (JLA) supernova catalogue. When we impose only
that the intrinsic dispersion in magnitudes is independent of redshift, we find
at the one standard deviation level, although
we note that in tests on simulations, this model tends to overestimate the
magnitude of the intrinsic dispersion, and underestimate . We note
that the degeneracy between intrinsic dispersion and the effects of
is more pronounced when lensing and velocity effects are considered
simultaneously, due to a cancellation of redshift dependence when both effects
are included. Keeping the model of the intrinsic dispersion fixed as a Gaussian
distribution of width 0.14 mag, we find .Comment: 16 pages, updated to match version accepted in MNRA
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