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Colloqiua & Seminars
Current and Future Colloquia & Seminars
Past Colloquia & Seminars
Cosmological pathway to neutrino properties
Friday noon seminar
Martina Gerbino, Argonne National Laboratory
Brian Nord, The University of Chicago
The Mira Distance Ladder
Friday noon seminar
Caroline D Huang, Johns Hopkins University
Gravitational-wave Tests of General Relativity: Present and Future
Friday noon seminar
Kent Yagi, University of Virginia
Compact binary merger events recently discovered by the LIGO and Virgo Collaboration offer us excellent testbeds for probing fundamental physics. In this talk, I will first review the current status of testing extreme (strong and dynamical-field) gravity with gravitational waves from binary black hole and binary neutron star mergers that was previously inaccessible. In particular, I will explain how well one can probe various fundamental pillars in General Relativity. I will next review how these tests will improve in future and what kinds of new tests are possible once space-borne interferometers, such as LISA, are in operation. In particular, I will discuss the possibility of using multi-band gravitational-wave observations by combining space-borne and ground-based detections to further improve the power of probing extreme gravity with gravitational waves.
A Deep Learning Approach to Galaxy Cluster X-ray Masses
Friday noon seminar
Michelle Ntampaka, Harvard University
I will present a machine-learning approach for estimating galaxy cluster masses from Chandra x-ray mock observations. I will describe how a Convolutional Neural Network (CNN) -- a deep machine learning tool commonly used in image recognition tasks -- can be used to infer cluster masses from these images, reducing scatter in the mass estimates by up to 50%. I will also show an interpretation tool, inspired by Google DeepDream, that can be used to gain some physical insight into what the CNN sees.
Progress with the Giant Magellan Telescope
Patrick J McCarthy, GMTO
The Giant Magellan Telescope is in the construction phase in north-central Chile. I will provide an update on the status of the project and its potential in fields from exoplanets to first-light in the universe. I will also discuss community wide efforts to develop a case for federal participation in the project.
Charting the Molecular Gas of the Universe Across Cosmic Time
Friday noon seminar
Garrett K Keating, Harvard-Smithsonian Center for Astrophysics
In the early Universe, galaxies boasted large reservoirs of gas that fueled a cosmic bonfire of star formation, at rates an order of magnitude greater than that observed locally. But exploring the properties of molecular gas from this era is exceptionally difficult; even sensitive instruments like ALMA and VLA have only made dozens of detections, generally only probing the most massive and luminous of systems. Understanding the molecular gas of this early era calls for a survey that can probe thousands or millions of such galaxies: a difficult goal if targeting individual objects, but may achieved by measuring aggregate emission from many objects, through a power spectrum technique known as "CO intensity mapping." In this talk, I will discuss two intensity mapping experiments: the CO Power Spectrum Survey (COPSS); which resulted in a tentative detection of bulk molecular gas at z~3, and the Millimeter-wave Intensity Mapping Experiment (mmIME), an on-going experiment leveraging the combined power of SMA, ALMA, and VLA to survey molecular gas emission from about a billion years after the big bang to the present day (0.2 < z < 6). With these surveys, we hope to build a better understanding of how the natal star-forming molecular gas of the early Universe transforms from pristine material into the dusty, smoggy clouds found in modern galaxies like the Milky Way; a critical piece in understanding how galaxies have evolved into their present-day state.
First Event Horizon Telescope Results: M87
Friday noon seminar
Junhan Kim, University of Arizona
The Event Horizon Telescope (EHT) project is a very-long-baseline interferometry experiment to observe supermassive black holes at the highest resolution ever achieved, combining a network of radio telescopes around the world. Recently, the EHT announced its first results: imaging the nuclear black hole in the galaxy M87. The image reconstruction, and a geometric model fit directly to the interferometric data, reveal a bright emission ring around a central intensity depression, consistent with the expected shape for emission from around a Kerr black hole. In this talk, I will review the EHT results, including the instrument development and data processing that led to the first scientific outcome of the project.
Advancing CMB Cosmology: ACTPol and onward to CMB-S4
Jeff McMahon, University of Michigan
Measurements of the cosmic microwave background (CMB) are a powerful probe of the origin, contents, and evolution of our Universe. CMB measurements continue to improve according to a Moore's law under which the mapping speed of experiments improves by an order of magnitude roughly every five years. This rapid progression in our ability to measure the CMB has translated into a series of scientific advances including showing our universe to be spatially flat, constraining inflationary and alternative theories of the primordial universe, and providing a cornerstone for our precision knowledge of the Lambda-CDM model. Observations with the current generation of experiments, including Advanced ACTPol, will soon produce improved cosmological constraints. Building on this work, in the coming decade CMB-S4 will: pass critical thresholds in constraints on inflation and light relativistic species; provide improved measurements of dark energy, dark matter, neutrino masses, and a variety of astrophysical phenomena; and enable searches for new surprises.
In this talk I present the design and status of measurements with Advanced ACTPol and how we are building on this work to realize the next generations of experiments including Simons Observatory and ultimately CMB-S4. I will highlight the technological advances that underlie the rapid progress in measurements including: polarization sensitive detectors which simultaneously observe in multiple colors; metamaterial antireflection coated lenses and polarization modulators; and overall advances in experimental design. I will present preliminary new results from ACTPol and conclude with science forecasts for CMB-S4.
Dark Matter Strikes Back at the Galactic Center
Friday noon seminar
Rebecca K Leane, MIT
Statistical evidence has previously suggested that the Galactic Center GeV Excess (GCE) originates largely from point sources, and not from annihilating dark matter. In this talk, I will examine the impact of unmodeled source populations on identifying the true origin of the GCE, using non-Poissonian template fitting (NPTF) methods. In a proof-of-principle example with simulated data, I will show that unmodeled sources in the Fermi Bubbles lead to a dark matter signal being misattributed to point sources by the NPTF. I will reveal striking behavior consistent with such an effect in the real Fermi data, finding that an artificial injected dark matter signal is completely misattributed to point sources. I will conclude that dark matter may provide a dominant contribution to the GCE.
WIMP Dark Matter - dead or alive?
Lars Bergstrom, The Oskar Klein Center, Physics Dept., Stockholm University
The hunt for the identity of dark matter continues with undiminished strength, even though the most popular candidate during recent decades, the Weakly Interacting Massive Particle (WIMP), is facing pressure from non-observation in present-day experiments.
In this talk, a review of the situation, discussing some of the WIMP and non-WIMP possibilities, will be given and of some intriguing indications of signals be discussed that may in fact point to the survival of WIMP dark matter. Upcoming experiments may clarify in the next few years whether the dark matter problem thus has been solved, or if a change of paradigm is needed.
What can we learn about extreme matter from colliding neutron stars
Katerina Chatziioannou, Flatiron Institute
The first detection of the merger of two neutron stars offered, among a plethora of other things, the first constraints on the properties of extremely dense and cold matter with gravitational waves. In this talk I will outline how gravitational waves can be used to probe matter in conditions unattainable in human laboratories. I will describe how the data from neutron star merger GW170817 has been used to infer the radius of the merging bodies and the properties of the equation of state of supranuclear matter. I will further discuss future prospects for equation of state constrains from improved gravitational wave detectors, as well as the post-merger part of the gravitational wave signal. Finally, I will discuss how these systems can be used to gain novel insights about nuclear as well as fundamental physics.
Testing general relativity with LIGO and Virgo
Friday noon seminar
Maximiliano Isi, MIT
Gravity remains the least understood of the four fundamental forces. To address this, we would like to experimentally study the extreme phenomena in which the full nonlinear and dynamic richness of gravity comes into play. Gravitational waves are a unique tool to do just that: they carry uncorrupted information directly from strong-gravity objects (like black holes, or their mimickers), and reflect in their own basic properties (like polarization or speed) the fundamental structure of space and time. Thus, gravitational waves are an invaluable resource to experimentally test our best theory of gravity, Einstein's general relativity, in yet unexplored regimes. The ever-increasing number of signals detected by LIGO and Virgo has already allowed us to make progress in this direction. This includes precisely probing the strong-field orbital dynamics of compact-binary mergers, testing the nature of the resulting remnant object, limiting the mass of the graviton and constraining the speed and polarization of gravitational waves. In this talk, I will review the results obtained from the 11 existing confident detections (ten binary black holes and one binary neutron star merger) and explain their relevance in our effort to better understand gravity. I will then discuss the potential for these studies in the near future, as existing detectors reach their design sensitivities, and beyond.
Gravitational Wave Signatures of Dynamically Formed Black Hole Binaries
Zhong-Zhi Xianyu, Harvard University
Now that LIGO has revealed the existence of a large number of binary black holes, identifying their origin becomes an important challenge. For dynamically formed binaries which might reside in dense environments such as galactic centers or globular clusters, the binary orbits could possess observably large eccentricity at LIGO and future gravitational wave detectors. Measuring the eccentricity distribution accurately could help us probe the background and the formation of the mergers. I will describe an analytical approach to predict the eccentricity distribution in a dynamical channel with three-body interactions. Furthermore, I will show that the third-body-induced barycenter motion of the binaries and the eccentricity variations might be observable in future space gravitational wave detectors such as LISA which could provide direct information about the black hole binary environments and otherwise invisible ambient mass.
Neutrino Non-Standard Interactions: Present bounds and future implications
Ivan Martinez-Soler, Fermilab and Northwestern U.
The observations of most of the neutrino experiments can be explained within the three-neutrino mixing scenario. Although the results of the different experiments are compatible, there are still some tensions in the determination of some parameters. For instance, in the solar mass difference, the latest global analysis indicates a $2sigma$ tension between the results of reactors and solar experiments. In the light of the future detectors, where neutrinos of high energy are going to be measured with high precision, the description of the neutrino evolution in matter is crucial. For that reason, the existence of non-standard interactions (NSI) between neutrinos and fermions would lead to a completely different description of the neutrino evolution. In this talk, I will discuss the latest bounds on NSI by using the available data from oscillation experiments and in combination with the coherent neutrino-nucleus scattering. In addition, this analysis will allow us to examine the status of the LMA-Dark solution. In the final part of the talk, I will address the implications of NSI in future observations.
Direct Detection of sub-GeV Dark Matter: A New Frontier
Rouven Essig, Stony Brook University
Dark matter makes up 85% of the matter in our Universe, but we have yet to learn its identity. While most experimental searches focus on Weakly Interacting Massive Particles (WIMPs) with masses above the proton (about 1 GeV/c^2), the theoretical landscape of possible dark-matter candidates has expanded significantly over the last decade, to consider masses from 10^-22 eV/c^2 up to the Planck mass, and even higher in the case of composite dark matter. A broad search program is therefore needed to maximize our chances of identifying dark matter. In this talk, I will discuss the search for dark matter with masses between about 500 keV/c^2 to 1 GeV/c^2, which has seen tremendous progress in the last few years. I will describe several direct-detection strategies that can probe this under-explored mass range, including searching for dark matter interactions with electrons in noble liquids, semiconductors, and scintillators, as well as searching for interactions with molecules to excite vibrational states. I will in particular highlight SENSEI, a funded experiment that will use new ultra-low-threshold silicon CCD detectors ("Skipper CCDs"). I will describe the first results from SENSEI and show how SENSEI, and the upcoming DAMIC-M, will probe vast new regions of parameter space in the next few years. I will also mention how placing a Skipper CCD on a satellite could probe strongly interacting sub-GeV dark matter.
The Population of Binary Black Holes from Gravitational-wave Observations
Chris Pankow, Northwestern University
I will present the current inventory of binary black holes (BBH) collected during the first and second observing runs of the LIGO/Virgo gravitational-wave interferometer network. The ten BBH observed to date provide the means to resolve questions about their formation and population properties. As such, I will also present new estimates of the mass, spin, and merger rate distributions of stellar mass BBH. All analyses consistently find merger rate distributions over the primary mass which predict almost no black holes above 45 solar masses. We also find that probes of the rate evolution with redshift prefer inclining or flat models. The inferred spin magnitude distribution strongly disfavors high spin magnitudes when the component spins are aligned to the orbital angular momentum. Finally, I will describe prospects for what the future might hold for BBH in future observing runs.
kSZ Cosmology without the optical depth degeneracy
Mathew S Madhavacheril, Princeton University
We show how kSZ tomography measures a bispectrum containing a cosmological power spectrum of the velocity field and an astrophysical power spectrum of the electron density. While these are degenerate up to an overall amplitude, scale-dependent effects on large scales are much better constrained by the inclusion of kSZon top of galaxy clustering while assuming nothing about the optical depth of galaxy clusters. This allows for factors of >2x improvement on the amplitude of local primordial non-gaussianity fNL with the absolute constraint from Simons Observatory + LSST crossing the theoretically interesting threshold of sigma(fNL)<1. We also discuss ways of measuring the (scale-independent) growth rate by breaking the optical depth degeneracy using either (1) redshift-space distortions or more ambitiously (2) the dispersion measures of fast radio bursts (FRBs).
Constraints on Quantum Gravity
Hirosi Ooguri, Caltech & Kavli IPMU
Superstring theory is our best candidate for the ultimate unification of general relativity and quantum mechanics. Although predictions of the theory are typically at extremely high energy and out of reach of current experiments and observations, several non-trivial constraints on its low energy effective theory have been found. Because of the unusual ultraviolet behavior of gravitational theory, the standard argument for separation of scales may not work for gravity, leading to robust low energy predictions of consistency requirements at high energy. In this colloquium talk, I will start by explaining why the unification of general relativity and quantum mechanics has been difficult. After introducing the holographic principle as our guide to the unification, I will discuss its use in finding constraints on symmetry in quantum gravity. I will also discuss other conjectures on low energy effective theories, collectively called swampland conditions, with various levels of rigors. They include the weak gravity conjecture, which gives a lower bound on Coulomb-type forces relative to the gravitational force, and the distance conjecture, which is about structure of the space of scalar fields. I will discuss consequences of the conjectures.
Early Dark Energy and the Hubble Tension
Friday noon seminar
Tanvi Karwal, Johns Hopkins University
Although the standard Lambda-CDM model of cosmology is in excellent agreement with the observed cosmic microwave background (CMB) power spectrum, its prediction for the current rate of expansion H0 of the Universe is in tension with observations of the local universe at > 3 sigma, with local measurements preferring a higher value. Systematic causes have been investigated and not found to be the culprit. Could this then indicate new physics?
My talk will present a new-physics solution to the Hubble tension that modifies the early expansion history of the Universe through the addition of an early dark energy (EDE) component. This behaves like a cosmological constant at early times and then dilutes quickly with redshift after some critical time. It therefore only influences the Universe over a small range in redshift.
This solution is successful because the Hubble tension can be translated into an equivalent tension in the size of the sound horizon.
If such an EDE becomes dynamical before recombination, it increases the pre-recombination expansion rate and decreases the sound horizon, shifting the expected peaks in the CMB power spectrum to smaller angular scales. These can be brought back in agreement with observations by an increase in the predicted value of H0, reducing the Hubble tension.
I will present two physical scalar-field models for such an EDE, and their success with resolving the Hubble tension while still finding a good fit to most cosmological datasets.
Constraining Self-Interacting Dark Matter with Galaxy Warps
Friday noon seminar
Kris Pardo, Princeton University
Self-interacting dark matter remains a viable and interesting model for dark matter. For some types of self-interactions, the passage of a galaxy through some background dark matter overdensity will cause a separation between the centroids of the collisionless stars and the dark matter halo of the galaxy, which experiences a drag force from the self-interactions. For stars arranged in a disk, this would cause a U-shaped warp. In this talk, I will discuss our efforts to place constraints on self-interacting dark matter by looking for these U-shaped warps in SDSS galaxies. Our preliminary results show that this method can place competitive constraints on the self-interaction cross section.
The Planck last release
Jean-Loup Puget, IAS Université Paris-sud
The Planck High frequency maps improvements will be described together with some of their associated cosmology results. Implications for future experiments will also be discussed.
Scalar fields and strong-field gravity: spontaneous scalarization of compact objects
Friday noon seminar
Hector Okada da Silva, Montana State University
General Relativity remains to this day our best description of gravitational phenomena. The theory has shown remarkable agreement with observations in situations ranging from the slow-velocities, weak-gravitational fields regime from the confines of our Solar System, to the highly nonlinear, dynamical regime of binary black holes mergers. Despite its tremendous successes, issues such as its quantization and the cosmological constant problem suggest that Einstein's theory might not be final theory of the gravitational interaction. Motivated by these questions, theorists have proposed a myriad of extensions to General Relativity over the decades. In this talk I will focus on theories with additional scalar fields. In particular, I will describe how some of these theories can evade Solar System constraints and yet yield interesting new phenomenology in the strong-gravity situations involving compact objects, i.e. neutron stars and black holes.
New Directions for Direct Detection of MeV-Scale Dark Matter
Noah Kurinsky, Fermi National Accelerator Laboratory
While the case for dark matter continues to strengthen from the astrophysical side, particle dark matter has so far eluded the current generation of experiments, designed to probe the SUSY-motivated mass range of GeV-TeV scale dark matter. Meanwhile, the LHC has ruled out the simpler SUSY models, and the simple picture of a weak-scale, 30 GeV supersymmetric dark matter particle has begun to fade. In this talk, I will discuss recent advances in the search for Sub-GeV dark matter down to MeV-scale masses, and the path forward to new technologies capable of probing down to keV-scale mass fermionic dark matter scattering and meV-scale mass bosonic dark matter absorption. These include, but are not limited to, the use of superconductors as well as novel semiconductors as the target medium and readout stages. The energy resolution required to search for low-mass dark matter makes these technologies interesting as general imaging techniques for infrared and UV astronomy, as well as for coherent neutrino scattering and other low-energy rare event search experiments, and I will briefly touch on applications of these new technologies to those fields.
A New Frontier in the Search for Dark Matter
Gordan Krnjaic, Fermilab
The gravitational evidence for the existence of dark matter is overwhelming; observations of galactic rotation curves, the CMB power spectrum, and light element abundances independently suggest that over 80% of all matter is "dark" and beyond the scope of the Standard Model. However, its particle nature is currently unknown, so discovering its potential non-gravitational interactions is a major priority in fundamental physics. In this talk, I will survey the landscape of light dark matter theories and and introduce an emerging field of fixed-target experiments that are poised to cover hitherto unexplored dark matter candidates with MeV-GeV masses. These new techniques involve direct dark matter production with proton, electron, and *muon* beams at various facilities including Fermilab, CERN, SLAC, and JLab. Exploring this mass range is essential for fully testing a broad, predictive class of theories in which dark matter abundance arises from dark-visible interactions in thermal equilibrium in the early universe.
Inflation with Spooky Correlations
Craig Hogan, The University of Chicago
Famous "information paradoxes" in black hole theory can be solved if quantum information on horizons is delocalized or "spooky", like states of entangled particles. Similar spooky correlations on the inflationary horizon are estimated to produce curvature perturbations with a dimensionless power spectral density given by the inflationary expansion rate H in Planck units, larger than standard inflaton fluctuations. Current measurements of the spectrum are used to derive constraints on parameters of the effective potential in a slow-roll background. A distinctive and robust new prediction, in the sense of being insensitive to the details of specific spooky models, is an exact directional antisymmetry, traceable directly to the nonlocality and directional correlation of initial conditions on the horizon, which is forbidden in standard models. Signatures of this primordial antisymmetry might already be measured in CMB anisotropy, and if they are indeed due to nearly-scale-invariant primordial spookiness, should also be observable in large scale 3D galaxy surveys, possibly even in existing data. DES may be the first dataset capable of detecting this direct signature of Planck scale quantum physics.
Searching for the aftermath of binary neutron star mergers
Michael W Coughlin, California Institute Of Technology
Binary neutron star mergers provide one of the richest laboratories for studying physics with ground-based interferometric gravitational-wave detectors such as advanced LIGO and Virgo. After such a merger, a compact remnant is left over whose nature depends primarily on the masses of the inspiralling objects and on the equation of state of nuclear matter. We will discuss the search for short and intermediate-duration post-merger signals from GW170817, as well as all-sky, all-time searches for the same. In addition, we will describe ongoing searches for the detection of transients like GW170817 in electromagnetic wavelengths. With the Zwicky Transient Facility recently achieving first light, it is now fruitful to use its unprecedented combination of depth, field of view, and survey cadence to perform Target of Opportunity observations. Using the 50 square degree field of view of the instrument, it is possible to follow-up events from systems like the Fermi Gamma-Ray Burst Monitor, where it can be necessary to cover thousands of square degrees. We will demonstrate on short gamma-ray bursts how it is possible to use this system to do follow-up on this scale.
How many numbers does it take to determine our Universe?
Michael Turner, KICP
Since 2013, the Planck Surveyor team has made a good case that it takes six numbers to describe the whole Universe (fewer than the ten digits in a phone number), based upon their all-sky map of the CMB. Others have different opinions: zero, one, two, six (a different), and nine to describe our Universe. As I will discuss, the choice of numbers reveals much about what we know and our aspirations, as well as how we think about the Universe. After exploring the landscape, I will advocate for zero numbers and discuss the path and strategy to get there.
Counting Stars: Developing Probabilistic Cataloging for Crowded Fields
Stephen Portillo, University of Washington
The depth of next generation surveys poses a great data analysis challenge: these surveys will suffer from crowding, making their images difficult to deblend and catalog. Sources in crowded fields are extremely covariant with their neighbors and blending makes even the number of sources ambiguous. Probabilistic cataloging returns an ensemble of catalogs inferred from the image and can address these difficulties. We present the first optical probabilistic catalog, cataloging a crowded Sloan Digital Sky Survey r band image cutout from Messier 2. By comparing to a DAOPHOT catalog of the same image and a Hubble Space Telescope catalog of the same region, we show that our catalog ensemble goes more than a magnitude deeper than DAOPHOT. We also present an algorithm for reducing this catalog ensemble to a condensed catalog that is similar to a traditional catalog, except it explicitly marginalizes over source-source covariances and nuisance parameters. We also detail efforts to make probabilistic cataloging more computationally efficient and extend it beyond point sources to extended objects. Probabilistic cataloging takes significant computational resources, but its performance compared to existing software in crowded fields make it a enticing method to pursue further.
Challenges for physical cosmology after Planck
Matias Zaldarriaga, Institute for Advanced Study
I will discuss the current status of physical cosmology after the latest Cosmic Microwave Background and other measurements. I will discuss the questions that still remain open in the field and how we might go about answering them. I will describe some recent theoretical developments that might contribute useful tools for overcoming some of the challenges that lie ahead.
Searching for Dark Matter Interactions in Cosmology
Kimberly Boddy, Johns Hopkins University
There is a substantial effort in the physics community to search for dark matter interactions with the Standard Model of particle physics. Collisions between dark matter particles and baryons exchange heat and momentum in the early Universe, enabling a search for dark matter interactions using cosmological observations in a parameter space that is highly complementary to that of direct detection. In this talk, I will describe the effects of scattering in the CMB power spectra and show constraints using Planck 2015 data, and I will discuss the implications of late-time scattering during the era of Cosmic Dawn.
New Results from BICEP/Keck
Colin Bischoff, University of Cincinnati
The BICEP/Keck series of telescopes make up a long-running program of small-aperture Cosmic Microwave Background polarimeters observing from the South Pole. I will describe new results that incorporate Keck Array observations from 2015, including our first 220 GHz data. These results improve the upper limit on the tensor-to-scalar ratio to r < 0.07 at 95% confidence and we explore the robustness of this constraint to complications in the dust foreground, instrumental systematics, and other variations in the analysis. The next steps forward in sensitivity will include 2016-2018 data from BICEP3 and Keck Array, followed by the four-telescope BICEP Array which will begin observing in 2020.
A Solution to the Cosmological Constant Problem
Surjeet Rajendran, UC Berkeley
The discovery of Dark Energy, a mysterious source that drives the accelerated expansion of the universe has created a major theoretical conundrum: the measured value of the dark energy is at least 60 orders of magnitude smaller than known theoretical contributions to it. What is the physics responsible for this extra-ordinary cancellation? It has been known for a long time that conventional symmetry based ideas that are often used to explain small numbers and precise cancellations are experimentally ruled out as solutions to this problem. In this talk, I will present a solution to the cosmological constant problem, where the problem is solved through cosmic evolution. This solution features novel cosmologies such as a Big Bounce that replaces the conventional Big Bang picture of the early universe. I will also discuss experimental techniques to search for these solutions in the laboratory.
Dark Matter in Disequilibrium and Implications for Direct Detection
Lina Necib, Caltech
Using two realizations of the Milky Way from the FIRE simulation, we find that the kinematics of dark matter follows closely the kinematics of accreted stars from the same mergers. We use this correspondence to build an empirical local velocity distribution of dark matter, by analyzing the Gaia second data release coupled with the ninth release from the Sloan Digital Sky Survey, and computing the velocity distribution of the accreted stars. We find that this velocity distribution is peaked at lower velocities than the generally assumed Maxwell Boltzmann distribution, due to the presence of a recent merger referred to as the Gaia Sausage, leading to a weakening of direct detection limits at dark matter masses less than 10 GeV.
Superfluids and the Cosmological Constant Problem
Jeremy Sakstein, University of Pennsylvania
The Lambda-CDM cosmological model is still the best-fit to current data, and numerous alternatives have recently been ruled out by the observation of gravitational waves and other small-scale probes. Theoretically, the cosmological constant (Lambda) suffers from a severe fine-tuning that needs to be understood in order for Lambda-CDM to be a satisfactory model. In this talk I will discuss a recent proposal for a model that may ameliorate the cosmological constant problem. In this model, a superfluid pervading the universe could counteract the large (unobserved) cosmological constant predicted by quantum mechanics. I will discuss the novel phenomenology predicted by the superfluid as well as future directions for testing this model.
Dark Energy: The Cosmological Constant in the Skies
Stephon Alexander, Brown University
After a pedagogical review of the cosmological constant problem and status of Dark Energy, I will present some new ideas, progress and challenges to account for the current acceleration of the universe and the smallness of the cosmological constant.
Astrophysical applications of coherent neutrino scattering
Louis Strigari, Texas A&M University
Neutrino-nucleus coherent scattering (CNS) is a long standing theoretical prediction of the Standard Model (SM), with experimental evidence for it just very recently being announced. CNS provides an important probe of physics beyond the SM, with a reach that can surpass the sensitivity of much larger scale detectors. In addition, it can open up a new window into neutrinoastrophysics, through studies of low energy neutrinos from the Sun, atmosphere, and supernovae. CNS is also vital for understanding and interpreting future particle dark matter searches. In this talk, I will discuss the prospects for learning about the nature of neutrinos and astrophysical sources from CNS detection, highlighting how astrophysical and terrestrial-based detections play important and complementary roles.
Precision Cosmology with the Cosmic Microwave Background from Chile
Sara M. Simon, University of Michigan
The cosmic microwave background (CMB) provides unparalleled views into the early universe and its later evolution. Recent and ongoing experiments have contributed to our understanding of neutrinos, dark energy, and dark matter through measurements of large scale structure imprinted on the CMB and constrained the conditions in the early universe, tightly restricting inflationary and other cosmological models through measurements of CMB polarization. Next-generation CMB experiments like Simons Observatory will further constrain the sum of the neutrino masses and number of relativistic species, expand our understanding of dark energy and dark matter, and set new constraints on cosmological models describing the first moments of the universe. The polarization in the CMB is faint, so future experiments must be orders of magnitude more sensitive. Additionally, both polarized foregrounds from synchrotron and dust emission and systematic effects from the instruments can create spurious polarization signals. Characterizing and removing foregrounds requires wide frequency coverage, while systematic effects must be modeled, mitigated and calibrated at unprecedented levels. I will discuss several advances in instrumentation and analysis that will be critical for this leap in performance.
Massive Neutrinos, Galaxy Clusters and the Lyman-alpha Forest
Friday noon seminar
Simeon Bird, UC Riverside
I'll present new efficient and accurate techniques for including massive neutrinos in N-body simulations, using a linear response (to the cold dark matter) approximation for the neutrinos. Then I'll talk about the potential for massive neutrinos to resolve some cosmological tensions within CMB observations galaxy clusters. Finally, I'll discuss how to detect features in quasar spectra using machine learning.
Fundamental Physics with the Simons Observatory
Brian Keating, UC San Diego
The Simons Observatory is a new cosmic microwave background experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. I will describe the scientific goals of the experiment, motivate its design, and forecast its performance. The Simons Observatory will measure the temperature and polarization anisotropy of the cosmic microwave background with arcminute resolution over approximately 40% of the sky in six frequency bands: 27, 39, 93, 145, 225 and 280 GHz. In its initial phase, three small-aperture (0.5-meter diameter) telescopes and one large-aperture (6-meter diameter) telescope will be fielded. These instruments will host a total of 60,000 cryogenic bolometer detectors. I will discuss some of the key science goals of the Simons Observatory, including the characterization of primordial fluctuations, determination of the number of relativistic species, and measuring the mass of neutrinos. I will also discuss other tests of fundamental physics -- some of which may be best measured using Cosmic Microwave Background observations such as the ones we are embarking upon.
Neutrino cosmology and large scale structure
Christiane Stefanie Lorenz, University of Oxford
In this talk, I will present studies of the model-dependence of cosmological neutrino mass constraints. In particular, I will focus on two phenomenological parameterizations of time-varying dark energy (early dark energy and barotropic dark energy) that can exhibit degeneracies with the cosmic neutrino background over extended periods of cosmic time. Moreover, I will show how the combination of multiple probes across cosmic time can help to distinguish between the two components. In addition, I will discuss how neutrino mass constraints can change in extended neutrino mass models, and how current tensions between low- and high-redshift cosmological data might be affected in these models. Finally, I will discuss whether lensing magnification and other relativistic effects that affect the galaxy distribution contain additional information about dark energy and neutrino parameters, and how much parameter constraints can be biased when these effects are neglected.
Cosmological results from the final data release of the Planck satellite
Silvia Galli, IAP
Planck is an ESA satellite aimed at the observation of the Cosmic Microwave Background.
This year, the Planck collaboration has released its final data and results. In this talk, I will describe the main results on cosmology from the mission, highlighting the changes with respect to previous releases, the agreement with other cosmological probes and the unsolved questions opened for the future.
Mapping the Milky Way in 6D with Gaia
Jo Bovy, The University of Toronto
One of the main goals of Gaia, a new astrometric satellite mission, is to provide an empirical measurement of the distribution of stars in the 6+N dimensional space of position, velocity, age, mass, elemental abundances, color, magnitude, etc.. Knowledge of this empirical distribution will allow the formation, evolution, and dynamics of the Milky Way to be strongly constrained. I will give an overview of the Gaia mission and discuss novel methods to map the Milky Way in position and velocity using the billion-star Gaia catalog. I will then discuss results on the stellar content and dynamics of the solar neighborhood from applying these techniques to Gaia's first and second data release. I will also discuss the implications of the structure in the velocity distribution in the extended solar neighborhood observed in Gaia's second data release.
Imaging supermassive black holes with the Event Horizon Telescope
Friday noon seminar
Lindy Blackburn, Harvard-Smithsonian CfA
The Event Horizon Telescope is an expanding global array of sub-mm radio telescopes designed to directly probe the spacetime geometry and radiative processes on event-horizon scales for the supermassive black holes at the center of our galaxy, Sgr A*, and at the center of M87. A major goal of the EHT is to measure the size and shape of the black hole "shadow," a characteristic signature of strong lensing at the event-horizon and a fundamental prediction of general relativity. In 2017, the EHT operated an 8-station array with both the South Pole Telescope and the ALMA array in Chile for the first time, and included a coordinated campaign of simultaneous ground and space-based multiwavelength observations. While analysis is ongoing, the data achieve an unprecedented 20 micro-arcsecond resolution and provide a direct view of the spatial structure of dynamical processes in the immediate vicinity of Sgr A*.
Beyond the Boost
Siavash Yasini, University of Southern California
Our peculiar motion with respect to the cosmic microwave background (CMB) changes the observed frequency and incoming angle of the CMB photons due to the Doppler and aberration effects. The most prominent signature of these motion-induced effects on the CMB is a kinematic dipole, which is observationally indistinguishable from any intrinsic dipole that the CMB might possess. Due to this degeneracy -- and the fact that we theoretically expect the intrinsic dipole of the CMB to be subdominant with respect to the kinematic component -- the 3mK dipole of the CMB is commonly interpreted as an entirely kinematic effect. Consequently, the frame in which the entire dipole of the CMB vanishes is customarily defined as the CMB rest frame. However, if the intrinsic dipole of the CMB is non-zero, this definition would not be appropriate anymore, unless we can properly separate the intrinsic and kinematic components of the dipole. In this talk, I will demonstrate how we can achieve this goal using spectral measurements of the monopole and quadrupole moments of the CMB. I will also describe the impact of the Doppler and aberration effects on the CMB power spectrum (especially on the small angular scales) and their relevance as an observational bias for the current and future surveys. Our recently developed "Generalized Doppler and Aberration Kernel" formalism can be used to measure and remove the motion-induced effects from any arbitrary frequency-dependent cosmological observable.
The Progenitor of the Milky Way's Halo
Friday noon seminar
Vasily Belokurov, University of Cambridge/CCA, NYC
We map the composition of the Galactic stellar halo in 7 dimensions spanned by phase-space coordinates and chemical abundances. The local halo appears to be dominated by stars on highly eccentric orbits. These stars are more metal-rich than typically assumed for the Galactic halo and were likely deposited into the Milky Way during an ancient massive accretion event. Using numerical simulations of the stellar halo formation we deduce that this merger must have happened between 8 and 11 Gyrs ago, during the epoch of the Galactic disk formation. This formation scenario for the MW halo has a number of implications for the studies of the evolution of the Galaxy in general and the measurements of the local Dark Matter matter distribution in particular.
The State of Small-Scale "Crises" In Dark Matter
Philip F Hopkins, California Institute of Technology
The most fundamental unsolved problems in star and galaxy formation revolve around "feedback" from massive stars (and black holes). I'll review how new generations of theoretical models combine new numerical methods and physics, to try to realistically model the diverse physics of the ISM, star formation, and feedback, on a wide range of scales from those of individual proto-stars to the inter-galactic medium. Feedback produces galactic outflows and perturbs galactic structure in ways which fundamentally perturb the nature of dark matter cores and 'cusps', re-shaping rotation curves and suppressing the central densities of low-mass galaxies. I'll discuss a variety of small-scale "crises" in cold dark matter models: "cusp-core," "missing satellites," "too big to fail," and more, and show that these "crises" tend to simply vanish as higher resolution and more treatments of known physics are included in simulations. However, I will show that there are robust, testable predictions of CDM as compared to other models such as self-interacting or ultra-light scalar field or "warm" dark matter, but these may require fundamentally new observations.
The early Universe: preparing theory for observations
Friday noon seminar
Emanuela Dimastrogiovanni, Case Western Reserve University
I will describe some interesting scenarios for the generation of gravitational waves from inflation and their characteristic imprints, which can be tested with upcoming B-mode observations as well as with interferometers. In the second part of my talk I provide an overview of the physics of CMB spectral distortions and discuss what we can learn from those about the early universe.
Microwave Multiplexing of Superconducting Sensors
John A B Mates, University of Colorado, Boulder
Superconducting detectors provide by far the most sensitive measurement of long-wavelength radiation for astronomy and cosmology, with detector noise falling below that of the astronomical signals in the mid-to-late 1990s, depending on the wavelength of interest. To measure better and faster, we have therefore assembled cameras with increasingly large arrays of detectors.
Since the 90s, the size of superconducting detector arrays has followed a Moore's Law trend, which is set to continue into the 100,000 pixel range with instruments like the Simons Observatory and CMB-S4. Perhaps the greatest challenge to continuing this trend is the need to bring the signals from the detector arrays out of a 100 mK cryostat on a much smaller number of wires.
I will present the emerging technique of multiplexing these superconducting sensors using superconducting microresonators. We can use this new scheme with both superconducting Transition-Edge Sensors (TESs) and Microwave Kinetic Inductance Detectors (MKIDs) to read out thousands of highly-sensitive detectors per coaxial cable. This capability will enable new instruments for astronomy and precision cosmology.