2,240 research outputs found
Social learning and information sharing: an evolutionary simulation model of foraging in Norway rats
Social learning is distinguished from innate behaviour and individual learning as a behavioural strategy. We investigate simple mechanisms for social learning in an evolutionary simulation of food-preference copying in Norway rats. These animals learn preferences by interacting with conspecifics, but, unexpectedly, they fail to learn aversions after interacting with a poisoned demonstrator. They also follow each other for food sites. Simulation results show that failure to discriminate between sick and healthy demonstrators may be due to food toxicity in foraging environments. A seemingly complex instance of social information transmission is explained through the action of simple behaviours in an appropriately structured environment
Homogeneous Transitions during Inflation: a Description in Quantum Cosmology
The usual description of inflationary fluctuations uses the framework of quantum field theory (QFT) in curved spacetime, in which quantum fluctuations are superimposed on a classical background spacetime. Even for large fluctuations, such as those envisioned during a regime of eternal inflation, this framework is frequently used. In the present work we go one step beyond this description by quantising both the scalar field and the scale factor of the universe. Employing the Lorentzian path integral formulation of semi-classical gravity we restrict to a simplified minisuperspace setting by considering homogeneous transitions. This approach allows us to determine the dominant geometry and inflaton evolution contributing to such amplitudes. We find that for precisely specified initial scale factor and inflaton values (and uncertain momenta), two distinct saddle point geometries contribute to the amplitude, leading to interference effects. However, when the momenta of both scale factor and inflaton are specified with sufficient certainty, only a single saddle point is relevant and QFT in curved spacetime is applicable. In particular we find that for inflaton transitions up the potential, meaningful results are only obtained when the initial uncertainty in the inflaton value is large enough, allowing the dominant evolution to be a complexified slow-roll solution \emph{down} from a comparatively unlikely position higher up in the potential
Emergence and persistence of communities in coevolutionary networks
We investigate the emergence and persistence of communities through a
recently proposed mechanism of adaptive rewiring in coevolutionary networks. We
characterize the topological structures arising in a coevolutionary network
subject to an adaptive rewiring process and a node dynamics given by a simple
voterlike rule. We find that, for some values of the parameters describing the
adaptive rewiring process, a community structure emerges on a connected
network. We show that the emergence of communities is associated to a decrease
in the number of active links in the system, i.e. links that connect two nodes
in different states. The lifetime of the community structure state scales
exponentially with the size of the system. Additionally, we find that a small
noise in the node dynamics can sustain a diversity of states and a community
structure in time in a finite size system. Thus, large system size and/or local
noise can explain the persistence of communities and diversity in many real
systems.Comment: 6 pages, 5 figures, Accepted in EPL (2014
Unstable no-boundary fluctuations from sums over regular metrics
It was recently shown by Feldbrugge et al. that the no-boundary proposal, defined via a Lorentzian path integral and in minisuperspace, leads to unstable fluctuations, in disagreement with early universe observations. In these calculations many off-shell geometries summed over in the path integral in fact contain singularities, and the question arose whether the instability might ultimately be caused by these off-shell singularities. We address this question here by considering a sum over purely regular geometries, by extending a calculation pioneered by Halliwell and Louko. We confirm that the fluctuations are unstable, even in this restricted context which, arguably, is closer in spirit to the original proposal of Hartle and Hawking. Elucidating the reasons for the instability of the no-boundary proposal will hopefully show how to overcome these difficulties, or pave the way to new theories of initial conditions for the universe
1.4 GHz polarimetric observations of the two fields imaged by the DASI experiment
We present results of polarization observations at 1.4 GHz of the two fields
imaged by the DASI experiment (, and , ,
respectively). Data were taken with the Australia Telescope Compact Array with
3.4 arcmin resolution and mJy beam sensitivity. The emission
is dominated by point sources and we do not find evidence for diffuse
synchrotron radiation even after source subtraction. This allows to estimate an
upper limit of the diffuse polarized emission. The extrapolation to 30 GHz
suggests that the synchrotron radiation is lower than the polarized signal
measured by the DASI experiment by at least 2 orders of magnitude. This further
supports the conclusions drawn by the DASI team itself about the negligible
Galactic foreground contamination in their data set, improving by a factor
the upper limit estimated by Leitch et al. (2005).
The dominant point source emission allows us to estimate the contamination of
the CMB by extragalactic foregrounds. We computed the power spectrum of their
contribution and its extrapolation to 30 GHz provides a framework where the CMB
signal should dominate. However, our results do not match the conclusions of
the DASI team about the negligibility of point source contamination, suggesting
to take into account a source subtraction from the DASI data.Comment: 7 pages, six figures, submitted to MNRA
Shallow extra mixing in solar twins inferred from Be abundances
Lithium and beryllium are destroyed at different temperatures in stellar
interiors. As such, their relative abundances offer excellent probes of the
nature and extent of mixing processes within and below the convection zone. We
determine Be abundances for a sample of eight solar twins for which Li
abundances have previously been determined. The analyzed solar twins span a
very wide range of age, 0.5-8.2 Gyr, which enables us to study secular
evolution of Li and Be depletion. We gathered high-quality UVES/VLT spectra and
obtained Be abundances by spectral synthesis of the Be II 313 nm doublet. The
derived beryllium abundances exhibit no significant variation with age. The
more fragile Li, however, exhibits a monotonically decreasing abundance with
increasing age. Therefore, relatively shallow extra mixing below the convection
zone is necessary to simultaneously account for the observed Li and Be behavior
in the Sun and solar twins
The Solar Twin Planet Search II. A Jupiter twin around a solar twin
Through our HARPS radial velocity survey for planets around solar twin stars,
we have identified a promising Jupiter twin candidate around the star HIP11915.
We characterize this Keplerian signal and investigate its potential origins in
stellar activity. Our analysis indicates that HIP11915 hosts a Jupiter-mass
planet with a 3800-day orbital period and low eccentricity. Although we cannot
definitively rule out an activity cycle interpretation, we find that a planet
interpretation is more likely based on a joint analysis of RV and activity
index data. The challenges of long-period radial velocity signals addressed in
this paper are critical for the ongoing discovery of Jupiter-like exoplanets.
If planetary in nature, the signal investigated here represents a very close
analog to the solar system in terms of both Sun-like host star and Jupiter-like
planet.Comment: 8 pages, 5 figures; A&A accepted; typos corrected in this versio
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