Measuring Space-Time Geometry over the Ages

Theorists are often told to express things in the "observational plane". One can do this for space-time geometry, considering "visual" observations of matter in our universe by a single observer over time, with no assumptions about isometries, initial conditions, nor any particular relation between matter and geometry, such as Einstein's equations. Using observables as coordinates naturally leads to a parametrization of space-time geometry in terms of other observables, which in turn prescribes an observational program to measure the geometry. Under the assumption of vorticity-free matter flow we describe this observational program, which includes measurements of gravitational lensing, proper motion, and redshift drift. Only 15% of the curvature information can be extracted without long time baseline observations, and this increases to 35% with observations that will take decades. The rest would likely require centuries of observations. The formalism developed is exact, non-perturbative, and more general than the usual cosmological analysis.Comment: Originally written for the Gravity Research Foundation 2012 Awards for Essays on Gravitation and received Honorable Mentio

Bose-Einstein condensation in complex networks

The evolution of many complex systems, including the world wide web, business and citation networks is encoded in the dynamic web describing the interactions between the system's constituents. Despite their irreversible and non-equilibrium nature these networks follow Bose statistics and can undergo Bose-Einstein condensation. Addressing the dynamical properties of these non-equilibrium systems within the framework of equilibrium quantum gases predicts that the 'first-mover-advantage', 'fit-get-rich' and 'winner-takes-all' phenomena observed in competitive systems are thermodynamically distinct phases of the underlying evolving networks

Search for variable gamma-ray emission from the Galactic plane in the Fermi data

High-energy gamma-ray emission from the Galactic plane above ~100 MeV is composed of three main contributions: diffuse emission from cosmic ray interactions in the interstellar medium, emission from extended sources, such as supernova remnants and pulsar wind nebulae, and emission from isolated compact source populations. The diffuse emission and emission from the extended sources provide the dominant contribution to the flux almost everywhere in the inner Galaxy, preventing the detection of isolated compact sources. In spite of this difficulty, compact sources in the Galactic plane can be singled out based on the variability properties of their gamma-ray emission. Our aim is to find sources in the Fermi data that show long-term variability. We performed a systematic study of the emission variability from the Galactic plane, by constructing the variability maps. We find that emission from several directions along the Galactic plane is significantly variable on a time scale of months. These directions include, in addition to known variable Galactic sources and background blazars, the Galactic ridge region at positive Galactic longitudes and several regions containing young pulsars. We argue that variability on the time scale of months may be common to pulsars, originating from the inner parts of pulsar wind nebulae, similarly to what is observed in the Crab pulsar.Comment: 4 pages, 4 figures, accepted to Astronomy & Astrophysic

Tomonaga-Luttinger physics in electronic quantum circuits

In one-dimensional conductors, interactions result in correlated electronic systems. At low energy, a hallmark signature of the so-called Tomonaga-Luttinger liquids (TLL) is the universal conductance curve predicted in presence of an impurity. A seemingly different topic is the quantum laws of electricity, when distinct quantum conductors are assembled in a circuit. In particular, the conductances are suppressed at low energy, a phenomenon called dynamical Coulomb blockade (DCB). Here we investigate the conductance of mesoscopic circuits constituted by a short single-channel quantum conductor in series with a resistance, and demonstrate a proposed link to TLL physics. We reformulate and establish experimentally a recently derived phenomenological expression for the conductance using a wide range of circuits, including carbon nanotube data obtained elsewhere. By confronting both conductance data and phenomenological expression with the universal TLL curve, we demonstrate experimentally the predicted mapping between DCB and the transport across a TLL with an impurity.Comment: 9p,6fig+SI; to be published in nature comm; v2: mapping extended to finite range interactions, added discussion and SI material, added reference

Status of sonic boom methodology and understanding

In January 1988, approximately 60 representatives of industry, academia, government, and the military gathered at NASA-Langley for a 2 day workshop on the state-of-the-art of sonic boom physics, methodology, and understanding. The purpose of the workshop was to assess the sonic boom area, to determine areas where additional sonic boom research is needed, and to establish some strategies and priorities in this sonic boom research. Attendees included many internationally recognized sonic boom experts who had been very active in the Supersonic Transport (SST) and Supersonic Cruise Aircraft Research Programs of the 60's and 70's. Summaries of the assessed state-of-the-art and the research needs in theory, minimization, atmospheric effects during propagation, and human response are given

Non-invasive vibrational mode spectroscopy of ion Coulomb crystals through resonant collective coupling to an optical cavity field

We report on a novel non-invasive method to determine the normal mode frequencies of ion Coulomb crystals in traps based on the resonance enhanced collective coupling between the electronic states of the ions and an optical cavity field at the single photon level. Excitations of the normal modes are observed through a Doppler broadening of the resonance. An excellent agreement with the predictions of a zero-temperature uniformly charged liquid plasma model is found. The technique opens up for investigations of the heating and damping of cold plasma modes, as well as the coupling between them.Comment: 4 pages, 4 figure

Study of the decay B^0→D^(*+)ωπ^-

We report on a study of the decay B^0→D^(*+)ωπ^- with the BABAR detector at the PEP-II B-factory at the Stanford Linear Accelerator Center. Based on a sample of 232×10^6 BB decays, we measure the branching fraction B(B^0→D^(*+)ωπ^-)=(2.88±0.21(stat.)±0.31(syst.))×10^(-3). We study the invariant mass spectrum of the ωπ^- system in this decay. This spectrum is in good agreement with expectations based on factorization and the measured spectrum in τ-→ωπ-ν_τ. We also measure the polarization of the D^(*+) as a function of the ωπ^- mass. In the mass region 1.1 to 1.9 GeV we measure the fraction of longitudinal polarization of the D^(*+) to be ΓL/Γ=0.654±0.042(stat.)±0.016(syst.). This is in agreement with the expectations from heavy-quark effective theory and factorization assuming that the decay proceeds as B^(-0)→D^(*+)ρ(1450)-, ρ(1450)^-→ωπ^-

X-ray Localization of the Globular Cluster G1 with XMM-Newton

We present an accurate X-ray position of the massive globular cluster G1 by using XMM-Newton and the Hubble Space Telescope (HST). The X-ray emission of G1 has been detected recently with XMM-Newton. There are two possibilities for the origin of the X-ray emission. It can be either due to accretion of the central intermediate-mass black hole, or by ordinary low-mass X-ray binaries. The precise location of the X-ray emission might distinguish between these two scenarios. By refining the astrometry of the XMM-Newton and HST data, we reduced the XMM-Newton error circle to 1.5". Despite the smaller error circle, the precision is not sufficient to distinguish an intermediate-mass black hole and luminous low-mass X-ray binaries. This result, however, suggests that future Chandra observations may reveal the origin of the X-ray emission.Comment: 4 pages, 2 figures; accepted for publication in Ap

Compressive Inverse Scattering II. SISO Measurements with Born scatterers

Inverse scattering methods capable of compressive imaging are proposed and analyzed. The methods employ randomly and repeatedly (multiple-shot) the single-input-single-output (SISO) measurements in which the probe frequencies, the incident and the sampling directions are related in a precise way and are capable of recovering exactly scatterers of sufficiently low sparsity. For point targets, various sampling techniques are proposed to transform the scattering matrix into the random Fourier matrix. The results for point targets are then extended to the case of localized extended targets by interpolating from grid points. In particular, an explicit error bound is derived for the piece-wise constant interpolation which is shown to be a practical way of discretizing localized extended targets and enabling the compressed sensing techniques. For distributed extended targets, the Littlewood-Paley basis is used in analysis. A specially designed sampling scheme then transforms the scattering matrix into a block-diagonal matrix with each block being the random Fourier matrix corresponding to one of the multiple dyadic scales of the extended target. In other words by the Littlewood-Paley basis and the proposed sampling scheme the different dyadic scales of the target are decoupled and therefore can be reconstructed scale-by-scale by the proposed method. Moreover, with probes of any single frequency \om the coefficients in the Littlewood-Paley expansion for scales up to \om/(2\pi) can be exactly recovered.Comment: Add a new section (Section 3) on localized extended target