Rebels Leading London: the mayoralties of Ken Livingstone and Boris Johnson compared

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Generalized HydroDynamics on an Atom Chip

The emergence of a special type of fluid-like behavior at large scales in one-dimensional (1d) quantum integrable systems, theoretically predicted in 2016, is established experimentally, by monitoring the time evolution of the in situ density profile of a single 1d cloud of $^{87}{\rm Rb}$ atoms trapped on an atom chip after a quench of the longitudinal trapping potential. The theory can be viewed as a dynamical extension of the thermodynamics of Yang and Yang, and applies to the whole range of repulsion strength and temperature of the gas. The measurements, performed on weakly interacting atomic clouds that lie at the crossover between the quasicondensate and the ideal Bose gas regimes, are in very good agreement with the 2016 theory. This contrasts with the previously existing 'conventional' hydrodynamic approach---that relies on the assumption of local thermal equilibrium---, which is unable to reproduce the experimental data.Comment: v1: 6+11 pages, 4+4 figures. v2: published version, 6+11 pages, 4+6 figure

Is the cosmic microwave background really non-Gaussian?

Two recent papers have claimed detection of non-Gaussian features in the COBE DMR sky maps of the cosmic microwave background. We confirm these results, but argue that Gaussianity is still not convincingly ruled out. Since a score of non-Gaussianity tests have now been published, one might expect some mildly significant results even by chance. Moreover, in the case of one measure which yields a detection, a bispectrum statistic, we find that if the non-Gaussian feature is real, it may well be due to detector noise rather than a non-Gaussian sky signal, since a signal-to-noise analysis localizes it to angular scales smaller than the beam. We study its spatial origin in case it is nonetheless due to a sky signal (eg, a cosmic string wake or flat-spectrum foreground contaminant). It appears highly localized in the direction b=39.5, l=257, since removing a mere 5 pixels inside a single COBE beam area centered there makes the effect statistically insignificant. We also test Guassianity with an eigenmode analysis which allows a sky map to be treated as a random number generator. A battery of tests of this generator all yield results consistent with Gaussianity.Comment: Revised to match accepted ApJL version. 4 pages with 2 figs included. Links and color fig at http://www.sns.ias.edu/~max/gaussianity_frames.html or from [email protected]