Past Events

  • Dissertation Defense: "Exploring the Universe with the Atacama Cosmology Telescope: Polarization-Sensitive Measurements of the Cosmic Microwave Background"

    David Rittenhouse Laboratory, 3W2

    Marius Lungu (UPenn)

  • High Energy Theory Seminar: "BMS Invariant Fluids"

    David Rittenhouse Laboratory, 2N36

    Robert Penna (Columbia University)

    The Bondi-van der Burg-Metzner-Sachs (BMS) group is the asymptotic symmetry group of asymptotically flat spacetime.  It is related to flat gravity in the same way that the conformal group is related to anti de Sitter gravity.  However, while the conformal group is well known throughout field theory, the BMS group is rather mysterious.  We show that the BMS group is closely related to infinite dimensional symmetry groups governing fluid dynamics.  We use a relationship between gravitating systems and lower dimensional fluids on their boundaries to give a new derivation of

  • High Energy Theory Seminar: "Entanglement at a Scale and Recovery Maps"

    David Rittenhouse Laboratory, 2N36

    Nima Lashkari (MIT)

    I discuss the amount of information in quantum field theory states reduced to a region, that cannot be recovered from its subregion density matrices. One can reconstruct the density matrix from its subregions using recovery maps. The vacuum of a conformal field theory is a quantum Markov state and the fixed point of both transformations. I define the entanglement of scaling as a measure of entanglement at a scale, and mention connections with C-theorems proofs in 2, 3 and 4 dimensions.

  • Astro Seminar:"Probes of Primordial Black Hole Dark Matter"

    David Rittenhouse Laboratory, A4

    Ely Kovetz (Johns Hopkins)

    The LIGO observatory has reported several detections of gravitational waves from the coalescence of binary black holes. We consider the extraordinary possibility that the detected events involving heavier masses are mergers of primordial black holes making up the dark matter in the Universe.

  • Condensed Matter Seminar: "Tuning Quantum Materials with Uniaxial Strain"

    David Rittenhouse Laboratory, A4

    Abhay Narayan Pasupathy (Columbia University)

    What is the effect of stretching a crystal along a given direction by a small amount? In general, one might not expect much: a change in lattice constant, accompanied by corresponding changes in the electronic and vibrational properties of a crystal. I will describe a few cases of materials where the effect of stretching (ie, uniaxial strain) lead to large and unexpected effects.

  • HET & HEE Joint Seminar: "Composite Higgses"

    Center for Particle Cosmology

    Csaba Csaki (Cornell University)

  • High Energy Theory Seminar: "What Can Cosmology Tell Us About Gravity?"

    David Rittenhouse Laboratory, 2N36

    Levon Pogosian (Simon Fraser University)

    I will review the current status and future prospects of testing theories of gravity using data from large-scale structure surveys. I will then discuss certain aspects of the so-called model-independent tests of dark energy and modified gravity. One is the choice of priors when trying to constrain unknown functions of redshift. Another is the physical interpretation of such tests and their implications for particular types of modified gravity theories.

  • Physics Department Colloquium: "Orientational Transitions: From Liquid Crystals to Viral Capsids"

    David Rittenhouse Laboratory - A8

    Robijn Bruinsma (UCLA) hosted by Douglas Durian

    Lars Onsager showed in 1948 that there could be a new type of phase transition where a liquid loses rotational symmetry but retains its translational symmetry, unlike the freezing transition where a liquid loses both types of symmetry operations. He proposed that this is what takes place when a liquid becomes a nematic liquid crystal, a material now extensively used in displays.

  • High Energy Theory Seminar: "Spinning Cosmology"

    David Rittenhouse Laboratory, 2N36

    Andrei Khmelnitsky (ICTP)

    We study the signatures left by new light particles with a spin on the primordial cosmological fluctuations. In distinction from the pure de Sitter background, where fields with spin have to be massive enough to satisfy the Higuchi bound, the inflation allows to host spinning particles that are parametrically lighter than the expansion rate. I introduce the description of the particles with spin in the context of the Effective Field Theory of Inflation and discuss the main signatures left by such particles on the primordial fluctuations.

  • Condensed Matter Seminar: "A change in stripes for cholesteric shells via anchoring in moderation"

    David Rittenhouse Laboratory, A4

    Lisa Tran (University of Pennsylvania)

    Chirality, ubiquitous in complex biological systems, can be controlled and quantified in synthetic materials such as cholesteric liquid crystal (CLC) systems. In this work, we study spherical shells of CLC under weak anchoring conditions. We induce anchoring transitions at the inner and outer boundaries using two independent methods: by changing the surfactant concentration or by raising the temperature close to the clearing point.