The IceCube neutrino observatory was designed to detect astrophysical neutrinos, which originate from outside of our solar system. IceCube has detected candidate astrophysical events, and measured a diffuse flux, but the source of these neutrinos so far remains unknown. Current approaches look for "hot spots" of neutrino events in the sky. It is also possible to describe a population of sources in terms of the number of observed events, forming a non-Poissonian statistical distribution. This distribution was used to show that the excess of gamma rays measured by Fermi-LAT around the galactic center was likely due to point sources rather than decaying dark matter. In this talk, I will present the application of this statistical method to the search for point sources in IceCube.
I will discuss a fundamental obstruction to any theory of the beginning of the universe, formulated as a semiclassical path integral. Hartle and Hawking’s no boundary proposal and Vilenkin’s tunneling proposal are examples of such theories. Each may be formulated as the quantum amplitude for obtaining a final 3-geometry by integrating over 4-geometries. The result is obtained using a new mathematical tool - Picard-Lefschetz theory - for defining the semiclassical path integral for gravity. The Lorentzian path integral for quantum cosmology with a positive cosmological constant is mathematically meaningful in this approach, but the Euclidean version is not. Framed in this way, the resulting framework and predictions are unique. Unfortunately, the outcome is that primordial gravitational wave fluctuations are unsuppressed. One can prove a general theorem to this effect, in a wide class of theories.
With quantum gas microscopy we are now able to take the control of ultracold quantum gases in an optical lattice to the next and ultimate level of high fidelity addressing, manipulation and readout of single particles. In my talk I will first give an introduction to this field of research and present an overview of recent experiments, including the first observation of an anti-ferromagnetic phase of Fermions in an optical lattice.
About the speaker
Prof. Markus Greiner from Harvard University in Cambridge, USA, is a pioneer in quantum simulation with ultracold atoms. He started his career with a PhD at the group of Theodor Hänsch in Munich, where he created the first strongly correlated atomic quantum gases, observing a quantum phase transition from a superfluid to a Mott insulator. In his postdoctoral research at JILA, Colorado, he worked with Prof. Deborah Jin and realized the first fermionic condensate, a new state of matter. In his research group at Harvard he invented quantum gas microscopy, and used the single particle control to carry out quantum simulations with ultracold atoms. Recently he realized the first cold atom Fermi-Hubbard antiferromagnet as well as doped magnets, entering a domain in which quantum models relevant to condensed matter physics can be studied in extremely difficult to calculate regimes.
The realization of multi-messenger astrophysics has opened up a new field of exploration of the most energetic phenomena in the universe. Messenger particles of all four of nature’s fundamental forces reach detectors on the ground and satellites in space. Finding coincident signals from these experiments in realtime will help us explore their sources. The Astrophysical Multimessenger Observatory Network (AMON) links multiple current and future high-energy neutrino, cosmic rays, and gamma rays observatories as well as gravitational wave facilities into a single virtual system, enabling near realtime coincidence searches for multi-messenger astrophysical transients and their electromagnetic counterparts and providing alerts to follow-up observatories. In this talk, I will present the science case, design elements, partner observatories, and status of the AMON project, followed by examples of AMON realtime searches for jointly emitting neutrino+gamma-ray transient sources.
I will discuss the theory behind, and constraints on, interactions between the dark sectors of cosmology. In the first part of the talk, I will discuss the relationship between fluid models and the field theoretical models that underlie such descriptions. This question is particularly important in light of suggestions that such interactions may help alleviate a number of current tensions between different cosmological datasets. I will describe how to construct consistent field theory models for an interacting dark sector that behave exactly like the coupled fluid ones, even at the level of linear perturbations, and can be trusted deep in the nonlinear regime.
In the second part of the talk, I will focus on robust constraints on such models that can be obtained using only mildly nonlinear scales. I will show that lensing and clustering of galaxies in combination with the Cosmic Microwave Background (CMB) is capable of probing the dark sector coupling to the few percent level for a given class of models, using only linear and quasi-linear Fourier modes. These scales can, in principle, be described by semi-analytical techniques such as the effective field theory of large-scale structure.
The much-anticipated joint detection of gravitational waves and electromagnetic radiation was achieved for the first time on August 17, 2017, for the binary neutron star merger GW170817. This event was detected by Advanced LIGO/Virgo, gamma-ray satellites, and dozens of telescopes on the ground and in space spanning from radio to X-rays. In this talk I will describe the exciting discovery of the optical counterpart, which in turn led to several detailed studies across the electromagnetic spectrum. The results of the observations carried out by our team include the first detailed study of a "kilonova", an optical/infrared counterpart powered by the radioactive decay of r-process nuclei synthesized in the merger, as well as the detection of an off-axis jet powering radio and X-ray emission. These results provide the first direct evidence that neutron star mergers are the dominant site for the r-process and are the progenitors of short GRBs. I will also describe how studies of the host galaxy shed light on the merger timescale, and describe initial constraints on the Hubble Constant from the combined GW and EM detection.
About the speaker
Edo Berger is a Professor of Astronomy at Harvard University. His group's research covers a wide range of explosive and eruptive astrophysical phenomena, including gamma-ray bursts, tidal disruption events, super-luminous supernovae, and other optical transients (from the Pan-STARRS project and elsewhere), as well as magnetic activity in sub-stellar objects. They use observations across the electromagnetic spectrum - from radio to γ-rays - utilizing observatories around the world and in space.
Previously, Edo was a joint Hubble Postdoctoral Fellow (2004-2007) and Carnegie-Princeton Postdoctoral Fellow (2004-2008) at the Carnegie Observatories and Princeton University.
Edo received a PhD in Astrophysics from Caltech in 2004, with a thesis focused on multi-wavelength studies of gamma-ray bursts, their host galaxies, and type Ib/c core-collapse supernovae.
Since 2013 he have also been serving as the Director of Undergraduate Studies in the department of astronomy.
The modern S-matrix program offers an elegant approach to bootstrapping quantum field theories without the aid of an action. While most progress has centered on gravity and gauge theory, similar ideas apply to effective field theories (EFTs). Sans reference to symmetry or symmetry breaking, we show how certain EFTs can be derived directly from the properties of the tree-level S-matrix, carving out a theory space of consistent EFTs from first principles. Furthermore, we argue that the S-matrix encodes a hidden unification of gravity, gauge theory, and EFTs. In particular, starting from the tree-level S-matrix of the mother of all theories, gravity, we derive the S-matrices of gluons, pions, Galileons, and Born-Infeld photons. Many attributes of EFTs like soft theorems and double copy relations are revealed as inherited traits. At the level of the action, this procedure is equivalent to a peculiar version of dimensional reduction, and applied to Yang-Mills theory yields a new action for the nonlinear sigma model comprised of purely cubic interactions. Physically, this recasts pions as gluons in higher dimensions. This representation manifests an explicit duality between flavor and kinematics, and the square of this action yields a new action for the Galileon.
Compress almost anything to very high densities and electrons react with protons to make neutron rich matter. This material is at the heart of many fundamental questions in nuclear physics and astrophysics. What are the high-density phases of QCD? Where did the chemical elements come from? What is the structure of many compact and energetic objects in the heavens, and what determines their electromagnetic, neutrino, and gravitational-wave radiations? Recently, extensive gravitational wave and electromagnetic observations of the neutron star merger GW170817 have constrained the equation of state of neutron rich matter and strongly suggest that neutron star mergers are an important site of rapid neutron capture (r-process) nucleosynthesis of heavy elements such as gold and uranium. We discuss these historic developments and try and place them in a broad context. We describe how the thickness of the neutron skins of the 48Ca and 208Pb nuclei are being measured with parity violating electron scattering at Jefferson Laboratory. These skins depend on the pressure of neutron rich matter and have important implications for the structure of neutron stars. We expect many thousand neutrino events from the next galactic core collapse supernova (SN). Simple neutrino interactions suggest that the neutrino driven wind is not very neutron rich and thus, despite what is said in many textbooks, SN may not be the site of the r-process for heavy elements. Finally, GW170817, by suggesting the astrophysical conditions, has set the stage for the Facility for Rare Isotope Beams (FRIB) to perform a detailed study r-process nucleosynthesis. FRIB, a powerful radioactive beam accelerator under construction in Michigan, will produce, for the first time, many of the very neutron rich heavy nuclei that are involved in the r-process.
About the speaker
Chuck Horowitz earned a B.A. from Harvey Mudd College in 1978 and a Ph.D. from Stanford in 1981, then conducted post-doctoral research at Niels Bohr Institute at the University of Copenhagen. In 2007, while at IU, Horowitz's work received international attention with the announcement that his research had led to the first-ever modeling of the chemistry of a neutron star. Currently, Chuck's research is on neutron rich dense matter, neutron star crusts, neutrino interactions in supernova explosions, and laboratory measurements of nuclear properties important for astrophysics.
"Site-resolved microscopy of ultracold Fermi-Hubbard systems in new regimes"
The ability to probe and manipulate ultracold fermions in optical lattices at the atomic level using quantum gas microscopes has enabled quantitative studies of Fermi-Hubbard models in a temperature regime that is challenging for state-of-the-art numerical simulations. Experiments have focused on spin-balanced gases of repulsively interacting atoms with the hope of elucidating phenomena in high-temperature superconductors. In this talk, I will present experiments that explore the Hubbard model in two new regimes: repulsive gases with spin-imbalance and attractive spin-balanced gases. In the first regime, we observe canted antiferromagnetism at half-filling, with stronger correlations in the direction orthogonal to the magnetization. Away from half-filling, the polarization of the gas exhibits non-monotonic behavior with doping, resembling the behavior of the magnetic susceptibility of the cuprates. The attractive Hubbard model studied in the second set of experiments is the simplest theoretical model for studying pairing and superconductivity of fermions in a lattice. Our measurements on the normal state reveal checkerboard charge-density wave correlations close to half-filling. The charge-density-wave correlations are a sensitive thermometer in the low temperature regime relevant for future studies of inhomogeneous superfluid phases in spin-imbalanced attractive gases.