6,824 research outputs found
Scale-factor duality in string Bianchi cosmologies
We apply the scale factor duality transformations introduced in the context
of the effective string theory to the anisotropic Bianchi-type models. We find
dual models for all the Bianchi-types [except for types and ] and
construct for each of them its explicit form starting from the exact original
solution of the field equations. It is emphasized that the dual Bianchi class
models require the loss of the initial homogeneity symmetry of the
dilatonic scalar field.Comment: 18 pages, no figure
Evolutionary Robotics: a new scientific tool for studying cognition
We survey developments in Artificial Neural Networks, in Behaviour-based Robotics and Evolutionary Algorithms that set the stage for Evolutionary Robotics in the 1990s. We examine the motivations for using ER as a scientific tool for studying minimal models of cognition, with the advantage of being capable of generating integrated sensorimotor systems with minimal (or controllable) prejudices. These systems must act as a whole in close coupling with their environments which is an essential aspect of real cognition that is often either bypassed or modelled poorly in other disciplines. We demonstrate with three example studies: homeostasis under visual inversion; the origins of learning; and the ontogenetic acquisition of entrainment
Peak intensity measurement of relativistic lasers via nonlinear Thomson scattering
The measurement of peak laser intensities exceeding 10^{20} \text{W/cm^2}
is in general a very challenging task. We suggest a simple method to accurately
measure such high intensities up to about 10^{23} \text{W/cm^2}, by
colliding a beam of ultrarelativistic electrons with the laser pulse. The
method exploits the high directionality of the radiation emitted by
ultrarelativistic electrons via nonlinear Thomson scattering. Initial electron
energies well within the reach of laser wake-field accelerators are required,
allowing in principle for an all-optical setup. Accuracies of the order of 10%
are theoretically envisaged.Comment: 4 pages, 2 figure
Leptogenesis for Pedestrians
During the process of thermal leptogenesis temperature decreases by about one
order of magnitude while the baryon asymmetry is generated. We present an
analytical description of this process so that the dependence on the neutrino
mass parameters becomes transparent. In the case of maximal CP asymmetry all
decay and scattering rates in the plasma are determined by the mass M_1 of the
decaying heavy Majorana neutrino, the effective light neutrino mass tilde{m}_1
and the absolute mass scale bar{m} of the light neutrinos. In the mass range
suggested by neutrino oscillations, m_{sol} \simeq 8*10^{-3} eV \lesssim
\tilde{m}_1 \lesssim m_{atm} \simeq 5*10^{-2} eV, leptogenesis is dominated
just by decays and inverse decays. The effect of all other scattering processes
lies within the theoretical uncertainty of present calculations. The final
baryon asymmetry is dominantly produced at a temperature T_B which can be about
one order of magnitude below the heavy neutrino mass M_1. We also derive an
analytical expression for the upper bound on the light neutrino masses implied
by successful leptogenesis.Comment: 55 pages, 14 figures include
A Bootstrap Stationarity Test for Predictive Regression Invalidity
In order for predictive regression tests to deliver asymptotically valid inference, account has to be taken of the degree of persistence of the predictors under test. There is also a maintained assumption that any predictability in the variable of interest is purely attributable to the predictors under test. Violation of this assumption by the omission of relevant persistent predictors renders the predictive regression invalid, and potentially also spurious, as both the finite sample and asymptotic size of the predictability tests can be significantly in ated. In response we propose a predictive regression invalidity test based on a stationarity testing approach. To allow for an unknown degree of persistence in the putative predictors, and for heteroskedasticity in the data, we implement our proposed test using a fixed regressor wild bootstrap procedure. We demonstrate the asymptotic validity of the proposed bootstrap test by proving that the limit distribution of the bootstrap statistic, conditional on the data, is the same as the limit null distribution of the statistic computed on the original data, conditional on the predictor. This corrects a long-standing error in the bootstrap literature whereby it is incorrectly argued that for strongly persistent regressors and test statistics akin to ours the validity of the fixed regressor bootstrap obtains through equivalence to an unconditional limit distribution. Our bootstrap results are therefore of interest in their own right and are likely to have applications beyond the present context. An illustration is given by re-examining the results relating to U.S. stock returns data in Campbell and Yogo (2006)
A First Look at the Auriga-California Giant Molecular Cloud With Herschel and the CSO: Census of the Young Stellar Objects and the Dense Gas
We have mapped the Auriga/California molecular cloud with the Herschel PACS
and SPIRE cameras and the Bolocam 1.1 mm camera on the Caltech Submillimeter
Observatory (CSO) with the eventual goal of quantifying the star formation and
cloud structure in this Giant Molecular Cloud (GMC) that is comparable in size
and mass to the Orion GMC, but which appears to be forming far fewer stars. We
have tabulated 60 compact 70/160um sources that are likely pre-main-sequence
objects and correlated those with Spitzer and WISE mid-IR sources. At 1.1 mm we
find 18 cold, compact sources and discuss their properties. The most important
result from this part of our study is that we find a modest number of
additional compact young objects beyond those identified at shorter wavelengths
with Spitzer. We also describe the dust column density and temperature
structure derived from our photometric maps. The column density peaks at a few
x 10^22 cm^-2 (N_H2) and is distributed in a clear filamentary structure along
which nearly all the pre-main-sequence objects are found. We compare the YSO
surface density to the gas column density and find a strong non-linear
correlation between them. The dust temperature in the densest parts of the
filaments drops to ~10K from values ~ 14--15K in the low density parts of the
cloud. We also derive the cumulative mass fraction and probability density
function of material in the cloud which we compare with similar data on other
star-forming clouds.Comment: in press Astrophysical Journal, 201
Properties of the Youngest Protostars in Perseus, Serpens, and Ophiuchus
We present an unbiased census of deeply embedded protostars in Perseus,
Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam
continuum and Spitzer Legacy surveys. We identify protostellar candidates based
on their mid-infrared properties, correlate their positions with 1.1 mm core
positions, and construct well-sampled SEDs using our extensive wavelength
coverage (lam=1.25-1100 micron). Source classification based on the bolometric
temperature yields a total of 39 Class 0 and 89 Class I sources in the three
cloud sample. We compare to protostellar evolutionary models using the
bolometric temperature-luminosity diagram, finding a population of low
luminosity Class I sources that are inconsistent with constant or monotonically
decreasing mass accretion rates. This result argues strongly for episodic
accretion during the Class I phase, with more than 50% of sources in a
``sub-Shu'' (dM/dt < 1e-6 Msun/yr) accretion state. Average spectra are
compared to protostellar radiative transfer models, which match the observed
spectra fairly well in Stage 0, but predict too much near-IR and too little
mid-IR flux in Stage I. Finally, the relative number of Class 0 and Class I
sources are used to estimate the lifetime of the Class 0 phase; the three cloud
average yields a Class 0 lifetime of 1.7e5 yr, ruling out an extremely rapid
early accretion phase. Correcting photometry for extinction results in a
somewhat shorter lifetime (1.1e5 yr). In Ophiuchus, however, we find very few
Class 0 sources (N(Class0)/N(ClassI)=0.1-0.2), similar to previous studies of
that cloud. The observations suggest a consistent picture of nearly constant
average accretion rate through the entire embedded phase, with accretion
becoming episodic by at least the Class I stage, and possibly earlier.Comment: 31 pages, 19 figures, 8 tables; accepted for publication in Ap
Lepton Asymmetry of the Universe and Charged Quark-Gluon Plasma
The lepton asymmetry of order one is not excluded experimentally and, if
present, can lead to interesting phenomena in the early Universe. It is shown
that, when the temperature is above the quark-hadron transition, the lepton
asymmetry induces chemical potentials for electric charge and for baryon
number, and makes the quark-gluon plasma electrically charged.Comment: 7 pages, 2 figures; v2: misprints in eqs. 10 and 13 correcte
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