21 research outputs found
Through the Looking Glass: Why the "Cosmic Horizon" is not a horizon
The present standard model of cosmology, CDM, contains some
intriguing coincidences. Not only are the dominant contributions to the energy
density approximately of the same order at the present epoch, but we note that
contrary to the emergence of cosmic acceleration as a recent phenomenon, the
time averaged value of the deceleration parameter over the age of the universe
is nearly zero. Curious features like these in CDM give rise to a
number of alternate cosmologies being proposed to remove them, including models
with an equation of state w = -1/3. In this paper, we examine the validity of
some of these alternate models and we also address some persistent
misconceptions about the Hubble sphere and the event horizon that lead to
erroneous conclusions about cosmology.Comment: Accepted for publication by MNRAS, 6 pages, 3 figure
How does the Hubble Sphere limit our view of the Universe?
It has recently been claimed that the Hubble Sphere represents a previously
unknown limit to our view of the universe, with light we detect today coming
from a proper distance less than this "Cosmic Horizon" at the present time. By
considering the paths of light rays in several cosmologies, we show that this
claim is not generally true. In particular, in cosmologies dominated by phantom
energy (with an equation of state of \omega < -1) the proper distance to the
Hubble Sphere decreases, and light rays can cross it more than once in both
directions; such behaviour further diminishes the claim that the Hubble Sphere
is a fundamental, but unrecognised, horizon in the universe.Comment: 4 pages, 4 figures: Accepted for publication in Monthly Notices of
the Royal Astronomical Society Letter
Another coincidence problem for CDM?
Over the last nine years of cosmic microwave background observations, the
Wilkinson Microwave Anisotropy Probe () results were consistent with a
CDM cosmological model in which the age of the Universe is one Hubble
time, and the time-averaged value of the deceleration parameter is consistent
with zero. This curious observation has been put forward as a new coincidence
problem for the CDM concordance cosmology, which is in fact a
`greater' coincidence than the near equality of the density parameters of
matter and the cosmological constant. At the moment of writing these conference
proceedings, the Planck Collaboration has released its first cosmological data,
which revealed a small shift in the CDM cosmological parameters when
compared to . We show that under the assumption of a spatially flat
CDM cosmology, Planck's results remove this coincidence problem for
CDM at greater than 99\% confidence level.Comment: 3 pages, 1 figure, proceedings of the 13th Marcel Grossmann meetin
Building blocks of the Milky Way's accreted spheroid
In the CDM model of structure formation, a stellar spheroid grows by
the assembly of smaller galaxies, the so-called building blocks. Combining the
Munich-Groningen semi-analytical model of galaxy formation with the high
resolution Aquarius simulations of dark matter haloes, we study the assembly
history of the stellar spheroids of six Milky Way-mass galaxies, focussing on
building block properties such as mass, age and metallicity. These properties
are compared to those of the surviving satellites in the same models. We find
that the building blocks have higher star formation rates on average, and this
is especially the case for the more massive objects. At high redshift these
dominate in star formation over the satellites, whose star formation timescales
are longer on average. These differences ought to result in a larger
-element enhancement from Type II supernovae in the building blocks
(compared to the satellites) by the time Type Ia supernovae would start to
enrich them in iron, explaining the observational trends. Interestingly, there
are some variations in the star formation timescales of the building blocks
amongst the simulated haloes, indicating that [/Fe] abundances in
spheroids of other galaxies might differ from those in our own Milky Way.Comment: 14 pages, 11 figures, accepted for publication in MNRA
Semi-analytic modelling of the europium production by neutron star mergers in the halo of the Milky Way
Neutron star mergers (NSM) are likely to be the main production sites for the
rapid (r-) neutron capture process elements. We study the r-process enrichment
of the stellar halo of the Milky Way through NSM, by tracing the typical
r-process element Eu in the Munich-Groningen semi-analytic galaxy formation
model, applied to three high resolution Aquarius dark matter simulations. In
particular, we investigate the effect of the kick velocities that neutron star
binaries receive upon their formation, in the building block galaxies (BBs)
that partly formed the stellar halo by merging with our Galaxy. When this kick
is large enough to overcome the escape velocity of the BB, the NSM takes place
outside the BB with the consequence that there is no r-process enrichment. We
find that a standard distribution of NS kick velocities decreases [Eu/Mg]
abundances of halo stars by ~dex compared to models where NS do not
receive a kick. With low NS kick velocities, our simulations match observed
[Eu/Mg] abundances of halo stars reasonably well, for stars with metallicities
[Mg/H]. Only in Aquarius halo B-2 also the lower metallicity stars
have [Eu/Mg] values similar to observations. We conclude that our assumption of
instantaneous mixing is most likely inaccurate for modelling the r-process
enrichment of the Galactic halo, or an additional production site for r-process
elements is necessary to explain the presence of low-metallicity halo stars
with high Eu abundances.Comment: 15 pages, 9 figures, accepted for publication in MNRA
Symbol Digit Modalities Test Variant in a Smartphone App for Persons With Multiple Sclerosis:Validation Study
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A two-minute walking test with a smartphone app for persons with multiple sclerosis:Validation study
BACKGROUND: Walking disturbances are a common dysfunction in persons with multiple sclerosis (MS). The 2-Minute Walking Test (2MWT) is widely used to quantify walking speed. We implemented a smartphone-based 2MWT (s2MWT) in MS sherpa, an app for persons with MS. When performing the s2MWT, users of the app are instructed to walk as fast as safely possible for 2 minutes in the open air, while the app records their movement and calculates the distance walked. OBJECTIVE: The aim of this study is to investigate the concurrent validity and test-retest reliability of the MS sherpa s2MWT. METHODS: We performed a validation study on 25 persons with relapsing-remitting MS and 79 healthy control (HC) participants. In the HC group, 21 participants were matched to the persons with MS based on age, gender, and education and these followed the same assessment schedule as the persons with MS (the HC-matched group), whereas 58 participants had a less intense assessment schedule to determine reference values (the HC-normative group). Intraclass correlation coefficients (ICCs) were determined between the distance measured by the s2MWT and the distance measured using distance markers on the pavement during these s2MWT assessments. ICCs were also determined for test-retest reliability and derived from 10 smartphone tests per study participant, with 3 days in between each test. We interviewed 7 study participants with MS regarding their experiences with the s2MWT. RESULTS: In total, 755 s2MWTs were completed. The adherence rate for the persons with MS and the participants in the HC-matched group was 92.4% (425/460). The calculated distance walked on the s2MWT was, on average, 8.43 m or 5% (SD 18.9 m or 11%) higher than the distance measured using distance markers (n=43). An ICC of 0.817 was found for the concurrent validity of the s2MWT in the combined analysis of persons with MS and HC participants. Average ICCs of 9 test-retest reliability analyses of the s2MWT for persons with MS and the participants in the HC-matched group were 0.648 (SD 0.150) and 0.600 (SD 0.090), respectively, whereas the average ICC of 2 test-retest reliability analyses of the s2MWT for the participants in the HC-normative group was 0.700 (SD 0.029). The interviewed study participants found the s2MWT easy to perform, but they also expressed that the test results can be confronting and that a pressure to reach a certain distance can be experienced. CONCLUSIONS: The high correlation between s2MWT distance and the conventional 2MWT distance indicates a good concurrent validity. Similarly, high correlations underpin a good test-retest reliability of the s2MWT. We conclude that the s2MWT can be used to measure the distance that the persons with MS walk in 2 minutes outdoors near their home, from which both clinical studies and clinical practice can benefit