Aspen Center for Physics

2020 Colloquia

30 minute talks followed by Q&A

  • June 16, 2020
    Deep Learning and Proteins
    Speaker: Lucy Colwell, Cambridge University
    A central challenge is to be able to predict functional properties of a protein from its sequence, and thus (i) discover new proteins with specific required functionality and (ii) better understand the functional effect of genomic mutations. Experimental breakthroughs in our ability to read and write DNA allows data on the relationship between sequence and function to be rapidly acquired. This data can be used to train and validate machine learning models that predict protein function from sequence. Because in many cases phenotypic changes are controlled by more than one amino acid, the mutations that separate different phenotypes may be epistatic, requiring us to build models that take the correlation structure into account. In this talk we show that such models rival the accuracy of existing hidden Markov models at sequence annotation, even when given relatively little training data. The representation of sequence space learned by the model can be used to build families that the model did not see during training. Finally, we report experimental confirmation that machine learning models can accurately identify variants of the AAV capsid protein that assemble integral capsids and package their genome, for applications in gene therapy.
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  • June 23, 2020
    The Formation and Growth of Supermassive Black Holes
    Speaker: Anna-Christina Eilers, MIT
    Quasars are the most luminous objects in the universe powered by accretion onto supermassive black holes (SMBHs). They can be observed at the earliest cosmic epochs, providing unique insights into the early phases of black hole, structure, and galaxy formation. Observations of these quasars demonstrate that they host SMBHs at their center, already less than ~1 Gyr after the Big Bang. It has been argued that in order to rapidly grow these SMBHs in such short amounts of cosmic time, they need to accrete matter over timescales comparable to the age of the universe, and thus the lifetime of quasars - the integrated time that galaxies shine as active quasars - is expected to be of order ~10^9 yr at a redshift of z~6, even if they accrete continuously at the theoretical maximum limit.

    I will present a new method to obtain constraints on the lifetime of high-redshift quasars with unprecedented precision, based on measurements of the sizes of ionized regions around quasars, known as proximity zones. The sizes of these proximity zones are sensitive to the lifetime of the quasars, because the intergalactic gas has a finite response time to the quasars� radiation. Applying this method to quasar spectra at z>6, we recently discover an unexpected population of very young quasars, indicating lifetimes of only ~10,000 years, which is several orders of magnitude shorter than expected. I will discuss the consequences of such short lifetimes on the quasars' ionizing power, their black hole mass accretion rates, and highlight tensions with current theoretical models for black hole formation. Furthermore, I will present several modifications to the current SMBH formation paradigm that might explain our results, e.g. super-critical mass accretion rates, massive initial black hole seeds in excess of stellar remnants, or highly obscured quasar growth phases. In the end I will show how we aim to disentangle the various scenarios by means of future observations with the upcoming James Webb Space Telescope, in order to shed new light onto the formation and growth of the first SMBHs in the universe.
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  • June 30, 2020
    Mixing, Climate and the Abyssal Ocean
    Speaker: Raffaele Ferrari, MIT
    Turbulent mixing plays a key role in shaping Earth's climate. This is particularly true in the abyssal ocean which stores massive amounts of heat and carbon. Oceanographers had long realized that the abyssal ocean is filled with cold and carbon-rich waters sinking to the ocean bottom near the poles. The puzzle was how these waters return back to the surface. In other words we have known for a long time how the ocean `breathes' heat and carbon in, but not how it `exhales'. Oceanographer Walter Munk proposed that these waters rise everywhere from the abyss as they are mixed with overlying warmer and lighter waters. Turbulent probes developed over the last forty years have finally provided evidence for turbulent mixing in the abyssal ocean, but in an unexpected twist have also shown that this mixing drives sinking rather than rising of waters. In this presentation we will discuss how this conundrum was resolved by realizing that abyssal waters rise toward the surface along thin boundary layers along the ocean margins.  We will then discuss the implications of this new paradigm for our understanding of Earth's climate.
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  • July 7, 2020
    Galactic Phylogenetics
    Speaker: Paula Jofre, Universidad Diego Portales
    Reconstructing the history of galaxies requires understanding the complexity of several astrophysical phenomena, from star formation to supernova yields and galaxy interaction. Combining this information is not trivial, leading us often to a fragmented story of how our own Galaxy assembled.

    Stars are the main carriers of the evolutionary information. They are the main producers of metals in the Universe, inheriting the modified chemical composition down for generations. With their chemical data from spectroscopic surveys, combined with their kinematics from Gaia and their ages from stellar models, we are starting to assemble huge stellar samples that are helping us to put the fragmented stories of Milky Way evolution into one.

    In this talk I will first discuss the challenges our field is facing in assembling the chemical data from spectroscopic surveys into single consistent datasets. Then I will discuss how can we use phylogenetic trees in order to put this vast information together to study the different astrophysical phenomena shaping the evolution of our Galaxy.
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  • July 14, 2020
    Higher Order Topological Phases of Matter
    Speaker: Taylor Hughes, University of Illinois Urbana Champaign
  • Topological phases of matter have become a thriving sub-field of condensed matter physics. In this colloquium I will provide an elementary introduction to topological phases of matter, and the new discovery of higher order topology. After the basic discussion I will describe interesting connections between symmetry, geometry, and topology as well as how to create topological phases in artificial dimensions.
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  • July 21, 2020
    Pushing the Limits of Hydrodynamics
    Speaker: Pavel Kovtun, University of Victoria
  • Hydrodynamics is a well-established field with a venerable history. In this talk, I will focus on foundational aspect of hydrodynamics which came to light in recent years. Do the equations of hydrodynamics even make sense? To what degree can the crudeness of hydrodynamics be improved? What about the phenomena that hydrodynamics should describe but fails to? And what about the phenomena that hydrodynamics shouldn't describe, but does?
  • Watch the lecture.

  • July 28, 2020
    Classifying Quantum Field Theories
    Speaker: Ibrahima Bah, Johns Hopkins University
    Quantum Field Theories underlie our most basic understanding of nature and its diverse phenomena.  An interesting question about them is: can they all be classified?  With the help of string theory and geometric engineering of quantum field theories, this classification problem can be studied, at least, for supersymmetric quantum field theories.  In this talk, I will discuss what it means to classify quantum field theories, and review the various strategies that exploit classifying structures for geometries.
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  • August 4, 2020
    Topic: Nu Intersections: Neutrino Physics at a Crossroad
    Speaker: Andr� de Gouv�a, Northwestern University
  • Neutrinos are the least understood particles of our Standard Model for the fundamental building blocks of matter. Measuring neutrino properties and identifying how they inform our understanding of nature at the smallest distance scales is among the highest priorities of particle physics today. I discuss our current understanding of neutrinos, concentrating on the observation of neutrino oscillations and neutrino masses and all the open questions that came out of this remarkable discovery, more than 20 years ago.
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  • August 11, 2020
    Understanding Dark Matter throughout Cosmic History
    Speaker: Kimberly Boddy, Johns Hopkins University
  • There is overwhelming evidence for the existence of dark matter. It plays a crucial role in the formation of structure in the Universe, but little is known about its properties beyond gravitational effects. In this talk, I will give an overview of the traditional methods of investigating the particle nature of dark matter and discuss future prospects to explore beyond the standard paradigm. In particular, I will focus on how cosmological and astrophysical observations offer glimpses into different cosmic eras to help unravel the mystery of dark matter.
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  • August 18, 2020
    Electronic Topology Driven by Strong Correlations
    Speaker: Silke Buehler-Paschen, Vienna University of Technology
    Tuning the correlation strength of electronic materials has led to the identification and classification of novel quantum phases, thereby elucidating the underlying physics. To explore how this landscape transforms in the presence of nontrivial electronic topology represents a new frontier. After a general discussion of this background I will present our recent results on a Weyl semimetal driven by strong correlations, highlighting in particular the extreme topological response observed in both thermodynamic and transport properties.
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  • August 25, 2020
    Cosmology with the High-Resolution Microwave Sky
    Speaker: Mathew Madhavacheril, Perimeter Institute
  • Observations of the anisotropies of the cosmic microwave background (CMB) have provided a solid foundation for the standard cosmological model: a universe consisting primarily of dark matter, seeded by Gaussian primordial perturbations and experiencing a relatively recent acceleration in its expansion.  While the Planck satellite has measured large-scale temperature anisotropies to excellent precision, a wealth of information remains to be uncovered in CMB polarization and in small-scale secondary anisotropies due to gravitational lensing and the presence of ionized gas at late-times. I will review how ongoing and future high-resolution ground-based CMB experiments will provide valuable information on massive neutrinos, dark energy and the primordial fluctuations through these new windows, and additionally cross-examine curious disagreements between early-time and late-time measurements (e.g. the Hubble tension). I will also present new results from the Atacama Cosmology Telescope, a ground-based arcminute-resolution survey of the CMB.
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  • September 1, 2020
    Looking for Imprints of Microphysics on Large Scale Structure
    Speaker: Chanda Prescod-Weinstein, University of New Hampshire
  • In this talk, I will describe my efforts to understand the nature of the mysterious dark matter. I provide an overview of the general problem and then describe my current approach to it, which is to characterize the behavior of a proposed dark matter particle, the axion. I will give some insight into how I am using a range of tools � model building, computation, and high energy astrophysics � to get at the basic question of �what is the statistical mechanics of axion dark matter?�
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