7,653 research outputs found
Dynamic black holes through gravitational collapse: Analysis of multipole moment of the curvatures on the horizon
We have investigated several properties of rapidly rotating dynamic black
holes generated by gravitational collapse of rotating relativistic stars. At
present, numerical simulations of the binary black hole merger are able to
produce a Kerr black hole of J_final / M_final^2 up to = 0.91, of gravitational
collapse from uniformly rotating stars up to J_final / M_final^2 ~ 0.75, where
J_final is the total angular momentum and M_final the total gravitational mass
of the hole. We have succeeded in producing a dynamic black hole of spin
J_final / M_final^2 ~ 0.95 through the collapse of differentially rotating
relativistic stars. We have investigated those dynamic properties through
diagnosing multipole moment of the horizon, and found the following two
features. Firstly, two different definitions of the angular momentum of the
hole, the approximated Killing vector approach and dipole moment of the current
multipole approach, make no significant difference to our computational
results. Secondly, dynamic hole approaches a Kerr by gravitational radiation
within the order of a rotational period of an equilibrium star, although the
dynamic hole at the very forming stage deviates quite far from a Kerr. We have
also discussed a new phase of quasi-periodic waves in the gravitational
waveform after the ringdown in terms of multipole moment of the dynamic hole.Comment: 13 pages with 19 figures, revtex4-1.cls. Accepted for publication in
the Physical Review
A global picture of quantum de Sitter space
Perturbative gravity about a de Sitter background motivates a global picture
of quantum dynamics in `eternal de Sitter space,' the theory of states which
are asymptotically de Sitter to both future and past. Eternal de Sitter physics
is described by a finite dimensional Hilbert space in which each state is
precisely invariant under the full de Sitter group. This resolves a
previously-noted tension between de Sitter symmetry and finite entropy.
Observables, implications for Boltzmann brains, and Poincare recurrences are
briefly discussed.Comment: 17 pages, 1 figure. v2: minor changes, references added. v3: minor
changes to correspond to PRD versio
Anthropic reasoning in multiverse cosmology and string theory
Anthropic arguments in multiverse cosmology and string theory rely on the
weak anthropic principle (WAP). We show that the principle, though ultimately a
tautology, is nevertheless ambiguous. It can be reformulated in one of two
unambiguous ways, which we refer to as WAP_1 and WAP_2. We show that WAP_2, the
version most commonly used in anthropic reasoning, makes no physical
predictions unless supplemented by a further assumption of "typicality", and we
argue that this assumption is both misguided and unjustified. WAP_1, however,
requires no such supplementation; it directly implies that any theory that
assigns a non-zero probability to our universe predicts that we will observe
our universe with probability one. We argue, therefore, that WAP_1 is
preferable, and note that it has the benefit of avoiding the inductive
overreach characteristic of much anthropic reasoning.Comment: 7 pages. Expanded discussion of selection effects and some minor
clarifications, as publishe
X-ray Polarization Signatures of Compton Scattering in Magnetic Cataclysmic Variables
Compton scattering within the accretion column of magnetic cataclysmic
variables (mCVs) can induce a net polarization in the X-ray emission. We
investigate this process using Monte Carlo simulations and find that
significant polarization can arise as a result of the stratified flow structure
in the shock-ionized column. We find that the degree of linear polarization can
reach levels up to ~8% for systems with high accretion rates and low
white-dwarf masses, when viewed at large inclination angles with respect to the
accretion column axis. These levels are substantially higher than previously
predicted estimates using an accretion column model with uniform density and
temperature. We also find that for systems with a relatively low-mass white
dwarf accreting at a high accretion rate, the polarization properties may be
insensitive to the magnetic field, since most of the scattering occurs at the
base of the accretion column where the density structure is determined mainly
by bremsstrahlung cooling instead of cyclotron cooling.Comment: 7 pages, 8 figures, accepted by MNRA
Oxygen Limitation and Tissue Metabolic Potential of the African Fish Barbus neumayeri: Roles of Native Habitat and Acclimatization
Background: Oxygen availability in aquatic habitats is a major environmental factor influencing the ecology, behaviour, and physiology of fishes. This study evaluates the contribution of source population and hypoxic acclimatization of the African fish, Barbus neumayeri, in determining growth and tissue metabolic enzyme activities. Individuals were collected from two sites differing dramatically in concentration of dissolved oxygen (DO), Rwembaita Swamp (annual average DO 1.35 mgO2 L-1) and Inlet Stream West (annual average DO 5.58 mgO2 L-1) in Kibale National Park, Uganda, and reciprocally transplanted using a cage experiment in the field, allowing us to maintain individuals under natural conditions of oxygen, food availability, and flow. Fish were maintained under these conditions for four weeks and sampled for growth rate and the activities of phosphofructokinase (PFK), lactate dehydrogenase (LDH), citrate synthase (CS), and cytochrome c oxidase (CCO) in four tissues, liver, heart, brain, and skeletal muscle. Results: Acclimatization to the low DO site resulted in lower growth rates, lower activities of the aerobic enzyme CCO in heart, and higher activities of the glycolytic enzyme PFK in heart and skeletal muscle. The activity of LDH in liver tissue was correlated with site of origin, being higher in fish collected from a hypoxic habitat, regardless of acclimatization treatment. Conclusions: Our results suggest that the influence of site of origin and hypoxic acclimatization in determining enzyme activity differs among enzymes and tissues, but both factors contribute to higher glycolytic capacity and lower aerobic capacity in B. neumayeri under naturally-occurring conditions of oxygen limitation
Critical review of the impacts of grazing intensity on soil organic carbon storage and other soil quality indicators in extensively managed grasslands
Acknowledgements This work contributes to the N-Circle project (grant number BB/N013484/1), and CINAg (BB/N013468/1) Virtual Joint Centres on Agricultural Nitrogen (funded by the Newton Fund via UK BBSRC/NERC), U-GRASS (grant number NE/M016900/1), the Belmont Forum/FACCE-JPI DEVIL project (grant number NE/M021327/1), Soils-R-GGREAT (grant number NE/P019455/1), ADVENT (grant number NE/M019713/1), Sêr Cymru LCEE-NRN project, Climate-Smart Grass and the Scottish Government’s Strategic Research Programme.Peer reviewedPublisher PD
The Association Between Smartphone Addiction and Sleep: A UK Cross-Sectional Study of Young Adults
Background: In a large UK study we investigated the relationship between smartphone addiction and sleep quality in a young adult population.
Methods: We undertook a large UK cross-sectional observational study of 1,043 participants aged 18 to 30 between January 21st and February 30th 2019. Participants completed the Smartphone Addiction Scale Short Version, an adapted Pittsburgh Sleep Quality Score Index and reported smartphone use reduction strategies using both in-person (n = 968) and online (n = 75) questionnaires. A crude and adjusted logistic regression was fitted to assess risk factors for smartphone addiction, and the association between smartphone addiction and poor sleep.
Results: One thousand seventy one questionnaires were returned, of which 1,043 participants were included, with median age 21.1 [interquartile range (IQR) 19–22]. Seven hundred and sixty three (73.2%) were female, and 406 reported smartphone addiction (38.9%). A large proportion of participants disclosed poor sleep (61.6%), and in those with smartphone addiction, 68.7% had poor sleep quality, compared to 57.1% of those without. Smartphone addiction was associated with poor sleep (aOR = 1.41, 95%CI: 1.06–1.87, p = 0.018).
Conclusions: Using a validated instrument, 39% young adults reported smartphone addiction. Smartphone addiction was associated with poor sleep, independent of duration of usage, indicating that length of time should not be used as a proxy for harmful usage
Temporal properties of the short gamma-ray bursts
A temporal analysis has been performed on a sample of 100 bright gamma-ray
bursts (GRBs) with T90<2s from the BATSE current catalog. The GRBs were
denoised using a median filter and subjected to an automated pulse selection
algorithm as an objective way of idenitifing the effects of neighbouring
pulses. The rise times, fall times, FWHM, pulse amplitudes and areas were
measured and the frequency distributions are presented here. All are consistent
with lognormal distributions. The distribution of the time intervals between
pulses is not random but consistent with a lognormal distribution. The time
intervals between pulses and pulse amplitudes are highly correlated with each
other. These results are in excellent agreement with a similar analysis that
revealed lognormal distributions for pulse properties and correlated time
intervals between pulses in bright GRBs with T90>2s. The two sub-classes of
GRBs appear to have the same emission mechanism which is probably caused by
internal shocks. They may not have the same progenitors because of the generic
nature of the fireball model.Comment: 4 pages, 7 figure
Radiative Shock-Induced Collapse of Intergalactic Clouds
Accumulating observational evidence for a number of radio galaxies suggests
an association between their jets and regions of active star formation. The
standard picture is that shocks generated by the jet propagate through an
inhomogeneous medium and trigger the collapse of overdense clouds, which then
become active star-forming regions. In this contribution, we report on recent
hydrodynamic simulations of radiative shock-cloud interactions using two
different cooling models: an equilibrium cooling-curve model assuming solar
metallicities and a non-equilibrium chemistry model appropriate for primordial
gas clouds. We consider a range of initial cloud densities and shock speeds in
order to quantify the role of cooling in the evolution. Our results indicate
that for moderate cloud densities (>1 cm^{-3}) and shock Mach numbers (<20),
cooling processes can be highly efficient and result in more than 50% of the
initial cloud mass cooling to below 100 K. We also use our results to estimate
the final H_2 mass fraction for the simulations that use the non-equilibrium
chemistry package. This is an important measurement, since H_2 is the dominant
coolant for a primordial gas cloud. We find peak H_2 mass fractions of >0.01
and total H_2 mass fractions of >10^{-5} for the cloud gas. Finally, we compare
our results with the observations of jet-induced star formation in
``Minkowski's Object.'' We conclude that its morphology, star formation rate (~
0.3M_solar/yr) and stellar mass (~ 1.2 x 10^7 M_solar) can be explained by the
interaction of a 90,000 km/s jet with an ensemble of moderately dense (~ 10
cm^{-3}), warm (10^4 K) intergalactic clouds in the vicinity of its associated
radio galaxy at the center of the galaxy cluster.Comment: 30 pages, 7 figures, submitted to Astrophysical Journa
Gedanken Worlds without Higgs: QCD-Induced Electroweak Symmetry Breaking
To illuminate how electroweak symmetry breaking shapes the physical world, we
investigate toy models in which no Higgs fields or other constructs are
introduced to induce spontaneous symmetry breaking. Two models incorporate the
standard SU(3)_c x SU(2)_L x U(1)_Y gauge symmetry and fermion content similar
to that of the standard model. The first class--like the standard electroweak
theory--contains no bare mass terms, so the spontaneous breaking of chiral
symmetry within quantum chromodynamics is the only source of electroweak
symmetry breaking. The second class adds bare fermion masses sufficiently small
that QCD remains the dominant source of electroweak symmetry breaking and the
model can serve as a well-behaved low-energy effective field theory to energies
somewhat above the hadronic scale. A third class of models is based on the
left-right--symmetric SU(3)_c x SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group. In
a fourth class of models, built on SU(4)_{PS} x SU(2)_L x SU(2)_R gauge
symmetry, lepton number is treated as a fourth color. Many interesting
characteristics of the models stem from the fact that the effective strength of
the weak interactions is much closer to that of the residual strong
interactions than in the real world. The Higgs-free models not only provide
informative contrasts to the real world, but also lead us to consider
intriguing issues in the application of field theory to the real world.Comment: 20 pages, no figures, uses RevTeX; typos correcte
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