Cosmology Lunchtime Seminar

semester, 2018


Monday, January 22nd 2018
1:25 pm:
To be announced.

Monday, January 29th 2018
12:15 pm:
There will be no seminar this week.

Monday, February 5th 2018
12:15 pm:
There will be no seminar this week.

Monday, February 12th 2018
12:15 pm:
Speaker: Lawrence Rudnick, UMN
Subject: The Stormy Life of Galaxy Clusters

This is a modified version of my plenary talk from the recent AAS Meeting. Regular cosmo seminar attendees will have seen some of this before, but this talk puts things into a larger, and hopefully useful, context.

Galaxy clusters, the largest gravitationally bound structures, hold the full history of their baryonic evolution, serve as important cosmological tools and allow us to probe unique physical regimes in their diffuse plasmas. With characteristic dynamical timescales of 107-109 years, these diffuse thermal and relativistic media continue to evolve, as dark matter drives major mergers and more gentle continuing accretion. The history of this assembly is encoded in the plasmas, and a wide range of observational and theoretical investigations are aimed at decoding their signatures. X-ray temperature and density variations, low Mach number shocks, and "cold front" discontinuities all illuminate clusters' continued evolution. Radio structures and spectra are passive indicators of merger shocks, while radio galaxy distortions reveal the complex motions in the intracluster medium. Deep in cluster cores, AGNs associated with brightest cluster galaxies provide ongoing energy, and perhaps even stabilize the intracluster medium. In this talk, we will recount this evolving picture of the stormy ICM, and suggest areas of likely advance in the coming years.


Monday, February 19th 2018
12:15 pm:
Speaker: Marcelo Alvarez, UC Berkeley, Berkeley Center for Cosmological Physics
Subject:  The Bright Future of Reionization with 21 cm and CMB Observations

In the coming decade we will witness the completion of CMB and 21 cm experiments that promise to lift the veil on reionization. Up until now, the details of reionization have remained shrouded in mystery across the chasm of space and time that separates us from the billion years after the big bang in which it occured, more than 12 billion years ago and 30 billion light years away due to cosmic expansion. CMB observations probe the distribution of what we think was a complicated network of growing and overlapping ionized bubbles created by UV and X-ray ancient dwarf galaxies and newborn supermassive black holes, while 21 cm observations probe the neutral patches left behind. As such, these two types of observations provide complementary information about the first billion years. I will discuss the exciting new prospects for understanding reionization by analyzing upcoming 21 cm and CMB observations jointly, emphasizing how simulations can help us avoid the pitfalls associated with teasing out the faint signals from nearby foregrounds, instrumental noise, and systematics.

Faculty Host: Shaul Hanany

Monday, February 26th 2018
12:15 pm:
There will be no seminar this week.

Monday, March 5th 2018
12:15 pm:
Speaker: Zewei Xiong, UMN
Subject:  Active-sterile neutrino oscillations in the neutrino-driven wind

Neutrino-driven winds from a proto-neutron star made in a core-collapse supernova have been studied extensively as a site for production of elements heavier than the Fe group. The absorption of electron neutrinos and antineutrinos on free nucleons not only provide the heating to drive the wind, but also determine the neutron-to-proton ratio, or equivalently the electron fraction of the wind, which is a critical parameter for nucleosynthesis. Flavor mixing between electron neutrino (antineutrino) and a sterile species that lacks normal weak interaction can potentially impact the dynamics and nucleosynthesis of the wind. We have implemented this active-sterile mixing in a steady-state model of the wind. We find that mixing with a sterile neutrino of ~1eV in mass can significantly affect the electron fraction and hence, nucleosynthesis in the wind.

Faculty Host: Yong-Zhong Qian

Monday, March 19th 2018
12:15 pm:
No Seminar This Week

Monday, March 26th 2018
12:15 pm:
Speaker: Liliya L.R. Williams, UMN
Subject: Relaxation of dark matter halos: distribution in energy and angular momentum

The problem of figuring out the properties of steady-state configuration of dark matter halos has a long history. I will recap some of that, including our contribution to it. I will summarize our results concerning relaxation in terms of energy, and discuss our ongoing work on incorporating angular momentum. I will also present a comparison of theoretical predictions with observations and simulations.


Monday, April 2nd 2018
12:15 pm:
Speaker: Matt Gomer, UMN
Subject:  Lensing degeneracies and their effect on H_0 measurements

At present, there is a 3σ- discrepancy between the local measurements of H0- and those derived from CMB observations. To explore whether or not this is significant, the current goal is to constrain H0- to within 1%, optimally using methods independent from these two. One potential method is to use the time delays between multiple images of gravitationally lensed system as an absolute measure of distance independent of either the distance ladder or CMB modelling. In order to accomplish this at the 1% level, it is critical that the lens system is modelled to similar precision. Unfortunately, there exist lensing degeneracies such as the mass sheet degeneracy which can result in the same set of observables using different mass distributions. Unless these degeneracies can be broken, measurements of H0- will be biased by an unknown amount. In this talk, I detail current efforts to quantify these biases and explore what the future holds for this method.

Faculty Host: Liliya L.R. Williams

Monday, April 9th 2018
12:15 pm:
Speaker: Jose Diego, Consejo Superior de Investigaciones Cientificas, CSIC
Subject: The Universe at Extreme Magnification

Galaxies and galaxy clusters can magnify objects at cosmic distances by large factors. To first order the maximum possible magnification depends on i) the mass of the lens and ii) the size of the background object. Small objects, like stars, can be magnified by factors of several thousand when the lens is a galaxy cluster. Kelly et al. 2018 discovered the first star at cosmological distance magnified by such extreme factors. I will show how the properties of the magnification can be altered by intervening microlenses (from the macrolens) and how this can be used to constrain the amount of dark matter that is in compact form. Gravitational Waves (GW) are another example, where the signal originates from an incredibly small source. I will show how the most massive events detected by LIGO can be re-interpreted as strongly lensed events at z~1-2 by groups of galaxies or small clusters at z~0.3.

Faculty Host: Patrick Kelly

Monday, April 16th 2018
12:15 pm:
No Seminar this Week

Monday, April 23rd 2018
12:15 pm:
Speaker: Qi Wen, UMN
Subject: Broadband anti-reflection coatings using “moth-eye” structures in millimeter and submillimeter astronomy

Wasting is evil, let alone wasting the light signal from billions years ago that can potentially reveal the secrets of the Universe. Unfortunately, optical elements in a telescope, such as lenses and filters, reflect part of the light back to sky. In millimeter and submillimeter astronomy, broadband anti-reflection coatings (ARC) are more desired than ever for foreground modeling. In this talk, I will introduce the subwavelength structures (SWS) or so called “moth-eye” structures as an emerging type of broadband ARC in millimeter and submillimeter astronomy.

Faculty Host: Shaul Hanany

Monday, April 30th 2018
12:15 pm:
Speaker:  Andre Luiz De Gouvea, Northwestern
Subject: Chiral Dark Sectors, Neutrino Masses, and Dark Matter

The Physics behind nonzero neutrino masses and the nature of the so-called dark matter remain elusive. Both, however, point to new degrees of freedom that couple very weakly to the known Standard Model degrees of freedom. I explore the possibility that these new degrees of freedom are the matter particles of a new chiral gauge symmetry under which the Standard Model degrees of freedom are gauge singlets. In more detail, I discuss on mechanism for constructing chiral gauge theories and present two concrete models: one based on a U(1) chiral gauge theory, the other on an SU(3)xSU(2) chiral gauge theory.

Faculty Host: M. Claudia Scarlata
4:40 pm:
Speaker: David-Michael Poehlmann
Subject: Gadolinium-loaded Plastic Scintillator for Neutron Detection
Faculty Host: Paul Crowell

Monday, May 7th 2018
12:15 pm:
There will be no seminar this week.

Monday, September 17th 2018
12:15 pm:
Speaker: Matt Fritts, University of Minnesota
Subject: Detector R&D for SuperCDMS

SuperCDMS is a direct-detection dark matter search employing cryogenic semiconductor detectors. One focus for SuperCDMS is low-mass WIMPs that would deposit very low energies in a detector; thus the detectors are designed for low energy thresholds and high energy resolution. Recent R&D efforts have taken resolution to the limit of single electron-hole pairs produced by ionizing events. WIMP searches with low-threshold detectors depend on the ionization yield for nuclear-recoil events, which is not well known at such low energies. One attempt to measure this is based on neutron capture. If WIMPs turn out to have high masses, then high exposures become important, and larger detectors will be needed. Recently 150mm-diameter CDMS-style detectors have been made and tested. I will report on our R&D efforts in each of these areas.

Faculty Host: Vuk Mandic

Monday, September 24th 2018
12:15 pm:
Speaker: Joshua A. Frieman, Fermilab
Subject: Probing Cosmology with the Dark Energy Survey

I will overview the Dark Energy Survey (DES) project and highlight its early science results, focusing on cosmology results from the first year of the survey. The DES collaboration built and is using the 570-megapixel Dark Energy Camera on the Blanco 4-meter telescope at NOAO's Cerro Tololo Inter-American Observatory in Chile to carry out a deep, wide-area, multi-band optical survey of several hundred million galaxies and a time-domain survey that has discovered several thousand supernovae. The survey started in Aug. 2013 and will finish early next year. DES was designed to address the questions: why is the expansion of the Universe speeding up? Is cosmic acceleration due to dark energy or does it require a modification of General Relativity? DES is addressing these questions by measuring the history of cosmic expansion and the growth of structure through multiple complementary techniques: galaxy clusters, the large-scale galaxy distribution, weak and strong gravitational lensing, and supernova distances, as well as through cross-correlation with other data sets, particularly the cosmic microwave background. I will also briefly discuss how the DES data are being used in unexpected ways, from probing dark matter with newly discovered ultra-faint dwarf galaxies to a new probe of the Hubble constant using the kilonova counterpart of a binary neutron star gravitational-wave source.

Faculty Host: Vuk Mandic

Monday, October 1st 2018
12:15 pm:
Speaker: Iary Davidzon, IPAC/Caltech
Subject: Probing the early universe *without* JWST

The James Webb Space Telescope (JWST) will not be the only breakthrough for high redshift (z>~3) studies. Exquisite data are coming from Subaru and Spitzer telescopes, eventually covering a large area of the sky (~25 sq deg) at an unprecedented depth. Probing such a large cosmic volume will allow to collect a statistically significant sample of rare, massive galaxies still undetected in pencil-beam HST surveys. In this talk I present "pathfinder results" from the COSMOS field, showing the importance of rare massive objects to constrain star formation efficiency and stellar-to-halo mass relationship at z>3. In the same context, I also discuss present limitations from an observational point of view and how they affect our understanding of galaxy evolution (e.g. the so-called "Eddington bias").

Faculty Host: M. Claudia Scarlata

Monday, October 8th 2018
12:15 pm:
Speaker: Ali Kheirandish, UW - Madison
Subject: The first evidence for the origin of high-energy cosmic neutrinos: multimessenger observation of a flaring blazar

For the first time since the discovery of high-energy cosmic neutrinos in IceCube, a distant gamma ray blazar was identified as a high-energy neutrino source. This represents the first evidence for the origin of very high energy cosmic rays. In this talk, I will review the status of high-energy cosmic neutrino flux measurements and how a multimessenger campaign led to the identification of blazar TXS 0506+056 as the origin of a high energy neutrino. Furthermore, I will discuss how archival data searches showed that the high-energy neutrino flux from the source is dominated by a neutrino burst, which implies flaring sources strongly contribute to the cosmic ray flux. I investigate the contribution of a subclass of flaring blazars to the high-energy neutrino flux and speculate on its connection to the very high-energy cosmic ray observations.

Faculty Host: Vuk Mandic

Monday, October 15th 2018
12:15 pm:
Speaker: Chris Pankow, Northwestern
Subject: Gravitational-wave Astrophysics with Compact Binary Mergers

The past three years have encompassed a meteoric rise of gravitational-wave astronomy with the activation of second generation gravitational-wave interferometers and the subsequent direct detection of GW150914 --- a gravitational-wave transient from a merging binary black hole. To date, two observing runs have been conducted, the second of which inaugurated a three instrument, worldwide network. I will report the key results driving the birth and growth of gravitational-wave astronomy: stellar mass black hole binaries and their measured masses and spins, the implications for compact binary astrophysics, and the foundation for future population studies. August of 2017 also added a highly anticipated component to multi-messenger astronomy. GW170817, the first binary neutron star detected with gravitational waves, kicked off a monumental joint electromagnetic and gravitational-wave observational campaign --- I will highlight ongoing studies of this watershed event. Finally, I will review the progression towards a more sensitive gravitational-wave network of up to five interferometers and prospects for compact binary detection in the next few years.

Faculty Host: Vuk Mandic

Monday, October 22nd 2018
12:15 pm:
Speaker: Wenlei Chen, UMN
Subject: Novel search for TeV-initiated pair cascades in the intergalactic medium

The observed magnetic fields in galaxies and galaxy clusters are believed to result from dynamo amplification of weak magnetic field seeds whose origin remains a long-standing open question. Beams of TeV gamma-rays from blazar jets can be used to infer the intensity, coherence length, and helicity of the intergalactic magnetic field. Intergalactic magnetic fields deflect the electron-positron pairs produced by TeV gamma-rays from blazars, resulting in broadened beams of secondary GeV gamma-rays known as pair halos. Such pair-cascades develop along the projected direction of the blazar jet, which is known from imaging radio observations. We searched for GeV pair halos in Fermi-LAT data around 12 high-synchrotron-peaked BL Lacs with well-determined jet orientation from VLBA radio observations. Our study exploits the expected asymmetry of blazar pair halos and uses advanced simulations of the pair cascades to improve the sensitivity of previous studies and increase the signal to noise. Although we find no significant detection, a 2-sigma hint for an extended pair halo along the direction of the jet appears in the stacked LAT data in the 30-300 GeV energy range, corresponding to an intergalactic magnetic field with strength of about 1E-15 Gauss. This magnetic field value is consistent with similar hints from independent studies using LAT data. We will present the results of our analysis and discuss the limitations of pair-halo searches due to astrophysical uncertainties. Finally, assuming that the apparent convergence on B ~ 1E-15 G is not coincidental, we will outline a clear path towards a positive detection of blazar pair halos with future space-borne and ground-based gamma-ray observatories.

Faculty Host: Patrick Kelly

Monday, October 29th 2018
12:15 pm:
Speaker: Clem Pryke, UMN
Subject: New BICEP/Keck Results
Faculty Host: Vuk Mandic

Monday, November 5th 2018
12:15 pm:
Speaker: Guillermo Fernandez Moroni, Fermilab
Subject: TBD (CCD Detectors)
Faculty Host: Vuk Mandic

Monday, November 12th 2018
12:15 pm:
Speaker: TBD
Subject: TBD
Faculty Host: Vuk Mandic

Monday, November 19th 2018
12:15 pm:
Speaker: Soumyajit Mandal, Case Western Reserve University
Subject: Ultra-steep-spectrum Sources in Galaxy Clusters
Faculty Host: Thomas W. Jones

Monday, November 26th 2018
12:15 pm:
Speaker: Sharan Banagiri, UMN
Subject: TBD (long duration transient searches with LIGO/Virgo)
Faculty Host: Vuk Mandic

Monday, December 3rd 2018
12:15 pm:
Speaker: Karl Young, UMN
Subject: TBD
Faculty Host: Shaul Hanany

Monday, December 10th 2018
12:15 pm:
Speaker: Rich Ormiston, UMN
Subject: TBD (Machine Learning and LIGO)
Faculty Host: Vuk Mandic

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