80 research outputs found
Wakes in Dilatonic Current-Carrying Cosmic Strings
In this work, we present the gravitational field generated by a cosmic string
carrying a timelike current in the scalar-tensor gravities. The mechanism of
formation and evolution of wakes is fully investigated in this framework. We
show explicitly that the inclusion of electromagnetic properties for the string
induces logarithmic divergences in the accretion problem.Comment: Revised version to be published in the Phys. Rev.
Bridging the gap between energy and the environment
Meeting the world’s energy demand is a major challenge for society over the coming century. To identify the most sustainable energy pathways to meet this demand, analysis of energy systems on which policy is based must move beyond the current primary focus on carbon to include a broad range of ecosystem services on which human well-being depends. Incorporation of a broad set of ecosystem services into the design of energy policy will differentiates between energy technology options to identify policy options that reconcile national and international obligations to address climate change and the loss of biodiversity and ecosystem services. In this paper we consider our current understanding of the implications of energy systems for ecosystem services and identify key elements of an assessment. Analysis must consider the full life cycle of energy systems, the territorial and international footprint, use a consistent ecosystem service framework that incorporates the value of both market and non-market goods, and consider the spatial and temporal dynamics of both the energy and environmental system. While significant methodological challenges exist, the approach we detail can provide the holistic view of energy and ecosystem services interactions required to inform the future of global energy policy
A perspective on the landscape problem
I discuss the historical roots of the landscape problem and propose criteria
for its successful resolution. This provides a perspective to evaluate the
possibility to solve it in several of the speculative cosmological scenarios
under study including eternal inflation, cosmological natural selection and
cyclic cosmologies.Comment: Invited contribution for a special issue of Foundations of Physics
titled: Forty Years Of String Theory: Reflecting On the Foundations. 31
pages, no figure
The Structure of Cosmic String Wakes
The clustering of baryons and cold dark matter induced by a single moving
string is analyzed numerically making use of a new three-dimensional Eulerian
cosmological hydro code which is based on the PPM method to track the
baryons and the PIC method to evolve the dark matter particles.
A long straight string moving with a speed comparable to induces a planar
overdensity (a``wake"). Since the initial perturbation is a velocity kick
towards the plane behind the string and there is no initial Newtonian
gravitational line source, the baryons are trapped in the center of the wake,
leading to an enhanced baryon to dark matter ratio. The cold coherent flow
leads to very low post--shock temperatures of the baryonic fluid.
In contrast, long strings with a lot of small-scale structure (which can be
described by adding a Newtonian gravitational line source) move slowly and form
filamentary objects. The large central pressure due to the gravitational
potential causes the baryons to be expelled from the central regions and leads
to a relative deficit in the baryon to dark matter ratio. In this case, the
velocity of the baryons is larger, leading to high post-shock temperatures.Comment: 36 pages (including 19 figures), TeX (with phyzzx) type, mpeg
simulations available at http://www.damtp.cam.ac.uk/user/ats25
The fundamental constants and their variation: observational status and theoretical motivations
This article describes the various experimental bounds on the variation of
the fundamental constants of nature. After a discussion on the role of
fundamental constants, of their definition and link with metrology, the various
constraints on the variation of the fine structure constant, the gravitational,
weak and strong interactions couplings and the electron to proton mass ratio
are reviewed. This review aims (1) to provide the basics of each measurement,
(2) to show as clearly as possible why it constrains a given constant and (3)
to point out the underlying hypotheses. Such an investigation is of importance
to compare the different results, particularly in view of understanding the
recent claims of the detections of a variation of the fine structure constant
and of the electron to proton mass ratio in quasar absorption spectra. The
theoretical models leading to the prediction of such variation are also
reviewed, including Kaluza-Klein theories, string theories and other
alternative theories and cosmological implications of these results are
discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy
Grooming decisions under structural despotism: the impact of social rank & bystanders among wild male chimpanzees
Understanding the evolution of cooperation remains a central concern in studies of animal behaviour, with fundamental issues being how individuals avoid being cheated, or ‘short-changed’, and how partners are chosen. Economic decisions made during social interactions should depend upon the availability of potential partners nearby, as these bystanders generate temptations to defect from the current partner. The influence of bystanders is highlighted in two theoretical approaches, biological markets theory and parcelling, both economic models of behaviour. Here, we tested predictions of these models using the grooming behaviour of wild male chimpanzees, Pan troglodytes, living under strong structural despotism, where grooming is exchanged both for agonistic support and for itself, and so we provide the first investigation of both presence and value of bystanders on chimpanzees' grooming decisions. We found that male chimpanzees took into account the relative value (rank) of bystanders compared to that of their current partner, with this more important than bystander numbers. Highranking bystanders appeared to generate incentives to defect from a potentially cooperative interaction and we found that grooming effort was parcelled into discrete episodes, with smaller parcels used when a bystander outranked the current partner. The number of bystanders also generated a temptation to defect, as bidirectional (reciprocated) bouts were more likely to occur with fewer bystanders. Such bouts were more likely with smaller rank distances between groomer and recipient. We found no influence of grooming relationship on initial investment: groomers did not appear to trust that they would receive grooming in return, even from those with whom they had a history of strongly reciprocal grooming. Our findings are consistent with an economic-benefits, markets-based approach, but not a relationship model paradigm. Our work highlights the importance of considering the immediate social context (number and quality of bystanders) in studies of cooperation
The density parameter and the Anthropic Principle
In the context of open inflation, we calculate the probability distribution
for the density parameter . A large class of two field models of open
inflation do not lead to infinite open universes, but to an ensemble of
inflating islands of finite size, or ``quasi-open'' universes, where the
density parameter takes a range of values. Assuming we are typical observers,
the models make definite predictions for the value we are most likely
to observe. When compared with observations, these predictions can be used to
constrain the parameters of the models. We also argue that obsevers should not
be surprised to find themselves living at the time when curvature is about to
dominate.Comment: Revised version, to be published in Phys. Rev.
An expanded M_bh-sigma diagram, and a new calibration of active galactic nuclei masses
[Abridged] We present an updated and improved M_bh-sigma diagram containing
64 galaxies for which M_bh measurements (not just upper limits) are available.
Due to new and increased black hole masses at the high-mass end, and a better
representation of barred galaxies at the low-mass end, the "classical" (all
morphological type) M_bh-sigma relation for predicting black hole masses is
log(M_bh/M_Sun) = (5.13+/-0.34)log[sigma/200] + (8.13+/-0.05), with an rms
scatter of 0.43 dex. Modifying the regression analysis to correct for a
hitherto over-looked sample bias in which black holes with masses <10^6 M_Sun
are not (yet) detectable, the relation steepens further to give log(M_bh/M_Sun)
= (5.95+/-0.44)log[sigma/200] + (8.15+/-0.06). We have also updated the
"barless" and "elliptical-only" M_bh-sigma relations introduced by Graham and
Hu in 2008 due to the offset nature of barred/disc galaxies. These relations
have a total scatter as low as 0.34 dex and currently define the upper envelope
of points in the M_bh-sigma diagram. These relations also have a slope
consistent with the value 5, in agreement with the prediction by Silk & Rees
based on feedback from massive black holes in bulges built by
monolithic-collapse.
Using updated virial products and velocity dispersions from 28 active
galactic nuclei, we determine that the optimal scaling factor f - which brings
their virial products in line with the 64 directly measured black hole masses -
is 2.8^{+0.7}_{-0.5}. This is roughly half the value reported by Onken et al.
and Woo et al., and consequently halves the mass estimates of most
high-redshift quasars. We have explored the results after separating the
samples into barred and non-barred galaxies, and we have also developed a
preliminary corrective term to the velocity dispersion based on bar dynamics.Comment: 17 pages. MNRAS, in pres
Structure and Formation of Elliptical and Spheroidal Galaxies
New surface photometry of all known elliptical galaxies in the Virgo cluster
is added to published data to derive composite profiles over large dynamic
ranges. Sersic functions fit them remarkably well. Effective brightnesses and
radii are derived via Sersic fits and by integrating the profiles
nonparametrically. We strongly confirm two dichotomies: (1) Elliptical galaxies
from cDs to M32 form a tight sequence in Fundamental Plane parameter space that
is almost perpendicular to the sequence of spheroidal galaxies from NGC 205 to
Draco. This is consistent with our understanding of their different formation
processes: mergers for Es and conversion of late-type galaxies into spheroidals
by environmental effects and by energy feedback from supernovae. (2)
Ellipticals come in two varieties: e.g., our 10 brightest Es have cuspy cores;
our 17 fainter Es do not have cores. We find a new distinct component in
coreless Es. All have extra light at the center above the inward extrapolation
of the outer Sersic profile. We suggest that extra light is made by starbursts
in dissipational (wet) mergers, as in numerical simulations. Three other new
aspects also point to an explanation of how the E-E dichotomy formed: extra
light Es were made in wet mergers while core Es were made in dry mergers. We
confirm that core Es do and extra light Es generally do not contain X-ray gas.
This suggests why the E-E dichotomy arose. Only core Es and their progenitors
are massive enough to retain hot gas that can make dry mergers dry and protect
old star populations from late star formation.Comment: 94 pages, 77 figures from 170 Postscript files; requires
emulateapj.sty, apjfonts.sty, and psfig.sty; accepted for publication in
ApJS; for a version with full resolution figures, see
http://chandra.as.utexas.edu/~kormendy/kfcb.htm
Demonstration of the Blood-Stage Plasmodium falciparum Controlled Human Malaria Infection Model to Assess Efficacy of the P. falciparum Apical Membrane Antigen 1 Vaccine, FMP2.1/AS01
We study whether the relationship between the state unemployment rate at the time of conception
and infant health, infant mortality and maternal characteristics in the United States
has changed over the years 1980-2004. We use microdata on births and deaths for years
1980-2004 and find that the relationship between the state unemployment rate at the time of
conception and infant mortality and birthweight changes over time and is stronger for blacks
than whites. For years 1980-1989 increases in the state unemployment rate are associated
with a decline in infant mortality among blacks, an effect driven by mortality from gestational
development and birth weight, and complications of placenta while in utero. In contrast,
state economic conditions are unrelated to black infant mortality in years 1990-2004 and
white infant mortality in any period, although effects vary by cause of death. We explore potential
mechanisms for our findings and, including mothers younger than 18 in the analysis,
uncover evidence of age-related maternal selection in response to the business cycle. In
particular, in years 1980-1989 an increase in the unemployment rate at the time of conception
is associated with fewer babies born to young mothers. The magnitude and direction of
the relationship between business cycles and infant mortality differs by race and period.
Age-related selection into motherhood in response to the business cycle is a possible explanation
for this changing relationship
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