January 9, 2019 | 3:30 PM | ERC 161 | Wednesday colloquium How many numbers does it take to determine our Universe? Michael Turner, KICP
Video 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.
January 16, 2019 | 3:30 PM | ERC 161 | Astronomy Colloquium Cosmic Reionization Nick Gnedin, University of Chicago/Fermilab
Cosmic reionization - ionization of the bulk of cosmic gas by ultraviolet radiation from first galaxies and quasars - is the least explored epoch in cosmic history. While significant progress has been made recently with the HST Frontier Fields program, the major breakthrough is still in the future, but not a distant one. The launch of JWST will start a revolution in studies of cosmic reionization, and other advanced observational probes will follow soon. As observers are preparing for the flood of new data, theorists are currently busy revamping their tools to stay on par with future observations. This fortunate match between theory and observations will lead to a major breakthrough in this last cosmic frontier.
January 23, 2019 | 3:30 PM | ERC 161 | Wednesday colloquium A New Frontier in the Search for Dark Matter Gordan Krnjaic, Fermilab
Video 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.
January 30, 2019 | 3:30 PM | ERC 161 | Astronomy Colloquium CANCELLED Infrared Spectroscopy of Stars and Planets Ian Crossfield, MIT
Extrasolar planets and cool stars emit most of their light beyond the range of standard optical observations. These objects are often best studied using infrared spectroscopy. I will present recent results from my group on two topics: space-based IR spectroscopy of exoplanet atmospheres, and ground-based, high-resolution spectroscopy of both planets and stars. I will also conclude with a brief discussing of how future IR-optimized observatories will also enable exciting new science in these areas.
February 6, 2019 | 3:30 PM | ERC 161 | Wednesday colloquium The Planck last release Jean-Loup Puget, IAS Université Paris-sud
Video 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.
February 13, 2019 | 3:30 PM | ERC 161 | Astronomy Colloquium Extreme Astrophysics Roger Blandford, Stanford University
The electromagnetic spectrum has been opened up from meter radio waves to 100 TeV photons and augmented with 10 - 300 Hz gravitational wave, MeV - PeV neutrinos and MeV - ZeV (160 Joule) cosmic ray messages. Consequently, there is a high rate of discovery and understanding of phenomena whose explanation invokes accepted physics - classical (including general relativity), atomic, nuclear and particle (including QED) processes - in extreme environments. The richness of the discovery space can be epitomized by describing some new observations and ideas pertaining to relativistic jets formed by massive spinning black holes, Ultra High Energy Cosmic Rays accelerated by strong shock waves surrounding rich clusters of galaxies and Fast Radio Bursts, generated by neutron stars with 100 GT magnetic fields.
Video 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.
February 27, 2019 | 3:30 PM | ERC 161 | Astronomy Colloquium The Dynamic Infrared Sky Mansi Kasliwal, Caltech
The dynamic infrared sky is hitherto largely unexplored. The infrared is key to understand elusive stellar fates that are opaque, cold or dusty. The infrared unveiled the otherwise opaque heavy element nucleosynthesis in neutron star mergers. I will describe multiple projects to chart the time-domain in the infrared. I will begin with the SPitzer InfraRed Intensive Transients Survey (SPIRITS) −−− a systematic search of 194 nearby galaxies within 30 Mpc, on timescales ranging between a week to a year, to a depth of 20 mag with Spitzer's IRAC camera. SPIRITS has already uncovered over 131 explosive transients and over 2536 strong variables. Of these, 64 infrared transients are especially interesting as they have no optical counterparts whatsoever even with deep limits from Keck and HST. Interpretation of these new discoveries may include (i) deeply enshrouded supernovae, (ii) stellar mergers with dusty winds, (iii) 8--10 solar mass stars experiencing e-capture induced collapse in their cores, (iv) the birth of massive binaries that drive shocks in their molecular cloud, or (v) formation of stellar mass black holes. Motivated by the treasure trove of SPIRITS discoveries, we just commissioned Palomar Gattini-IR - a new 25 sq deg J-band camera to robotically chart the dynamic infrared sky. We have also begun building WINTER - a new 1 sq deg yJH-band camera on a new 1m telescope at Palomar Observatory.
March 6, 2019 | 3:30 PM | ERC 161 | Wednesday colloquium Direct Detection of sub-GeV Dark Matter: A New Frontier Rouven Essig, Stony Brook University
Video 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.
March 13, 2019 | 3:30 PM | ERC 161 | Astronomy Colloquium Simulating Galaxy Formation: Illustris, IllustrisTNG and Beyond Mark Vogelsberger, MIT
Cosmological simulations of galaxy formation have evolved significantly over the last years. In my talk, I will describe recent efforts to model the large-scale distribution of galaxies with cosmological hydrodynamics simulations. I will focus on the Illustris simulation, and our new simulation campaign, the IllustrisTNG project. After demonstrating the success of these simulations in terms of reproducing an enormous amount of observational data, I will also talk about their limitations and directions for further improvements over the next couple of years.
January 11, 2019 | 12:00 PM | ERC 401 | Friday noon seminar 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.
January 18, 2019 | 12:00 PM | ERC 401 | Friday noon seminar 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.
January 25, 2019 | 12:00 PM | ERC 401 | Friday noon seminar 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.
February 1, 2019 | 12:00 PM | ERC 401 | Friday noon seminar Scalar fields and strong-field gravity: spontaneous scalarization of compact objects 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.
February 8, 2019 | 12:00 PM | ERC 401 | Friday noon seminar Constraining Self-Interacting Dark Matter with Galaxy Warps 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.
February 15, 2019 | 12:00 PM | ERC 401 | Friday noon seminar Early Dark Energy and the Hubble Tension 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.
February 22, 2019 | 12:00 PM | ERC 401 | Friday noon seminar 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).
March 1, 2019 | 12:00 PM | ERC 401 | Friday noon seminar The Population of Binary Black Holes from Gravitational-wave Observations Chris Pankow, Northwestern University
Artist's conception shows two merging black holes similar to those detected by LIGO.
Image Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)
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.
March 15, 2019 | 12:00 PM | ERC 401 | Friday noon seminar 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.
March 22, 2019 | 12:00 PM | ERC 401 | Friday noon seminar 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.
March 6, 2019 | 12:00 PM | ERC 401 | Open Group seminar Probing the non-Gaussian density field with clusters of galaxies Steffen Hagstotz, Oskar Klein Centre and Stockholm University
Considerable effort in cosmology today is focused on understanding the statistical nature and evolution of the (dark matter) density field that underlies the observed large-scale structure. Information about this field is mostly phrased in terms of two-point statistics, such as the power spectrum of galaxies or weak lensing, essentially approximating the large-scale structure as a Gaussian random field. However, the Universe is far more complex than that: Gravitational collapse turns the simple initial conditions into the cosmic web consisting of halos, filaments and large voids we see today. In my talk, I will show how we can use the abundance of galaxy clusters residing in the the 'knots' of the cosmic web to probe the non-Gaussian shape of the density field. This gives us insights into the physics of structure formation, and provides at the same time a new method to search for deviations from the cosmological standard model.
February 5, 2019 | 12:00 PM | ERC 576 | Special Seminar The Progenitors and Descendants of Peculiar Thermonuclear Supernovae Josiah Schwab, University of California, Santa Cruz
As transient surveys push deeper, wider and faster, new classes of events are revealed. I will discuss recent progress made in understanding the Type Iax supernovae, a class of thermonuclear events similar to Type Ia supernovae, but with distinct and intriguing differences. Multiple lines of evidence, both theoretical and observational, are now converging to give us a coherent picture of these systems. While many questions remain, these events are exemplars of how the extragalactic discovery and follow up of transient events can be combined with galactic studies of stellar populations to understand their progenitor systems and the remnants they leave behind.
February 12, 2019 | 12:00 PM | ERC 576 | Special Seminar Moving Mesh Astrophysics Paul Duffell, Harvard University
Novel methods in recent years have been developed for numerically solving the hydrodynamical and MHD equations relevant to all kinds of astrophysical flows. I will first (briefly) present one such computational technique, where the numerical grid follows the MHD flow using a "moving mesh". I will then present some astrophysical scenarios for which I have applied this method, including planet formation and high-energy transients such as supernovae and gamma ray bursts.
February 14, 2019 | 12:00 PM | ERC 576 | Special Seminar Fundamental processes in plasma astrophysics: How laser-driven laboratory experiments can shed light on the workings of the Cosmos Petros Tzeferacos, University of Chicago
I present the exciting fundamental science in magnetized astrophysical plasmas that the Chicago-Oxford team is accomplishing through concerted application of the FLASH code and laboratory plasma astrophysics experiments. In particular, I discuss recent breakthrough experiments in the study of turbulent dynamo, a ubiquitous astrophysical process thought to be responsible for the present-day magnetization of numerous celestial objects but that had eluded laboratory plasma physicists for decades. These experiments have enabled us to explore dynamo in various regimes that are important in astrophysics, providing us with a new tool to validate or falsify our theoretical understanding. I will also describe how these experiments are enabling the first laboratory investigations of cosmic ray (CR) acceleration mechanisms and the diffusive transport of extragalactic and ultra-high energy CRs - a key step towards identifying their sources and explaining the dipole anisotropy recently detected by the Pierre Auger Observatory.
February 18, 2019 | 3:00 PM | ERC 401 | Special Seminar Technosignatures: What Are They, And How Might We Find Them? Jill Tarter, SETI Institute
In 1973 Arthur C. Clarke (British engineer turned science fiction author) formulated his three laws: 1. When a distinguished but elderly scientist states that something is possible, they are almost certainly right. When they state that something is impossible, they are very probably wrong. 2. The only way of discovering the limits of the possible is to venture a little way past them into the impossible. 3. Any sufficiently advanced technology is indistinguishable from magic. Since 1960, and more recently 1999, SETI (Search for ExtraTerrestrial Intelligence) researchers have been searching for that "magic" in the form of radio, and now optical, electromagnetic signals. These searches need to continue and grow utilizing the exponentially increasing capabilities of computing, but within the SETI field, we've always reserved the right to get smarter. In 2014, Karl Schroeder (Canadian futurist and science fiction author) suggested a variant of the Third Law; Any sufficiently advanced technology is indistinguishable from Nature. What opportunities does this increased scope of the Third Law offer to our own 21st century search for life beyond Earth? As we design and implement the next generations of ground and space based observatories, some of whose primary goals are the imaging of exoplanets and the spectroscopic analysis of their atmospheres, we should consider how we might distinguish between the byproducts of microbes and mathematicians. We are vigorously discussing/debating the right instruments to develop and fly to find biosignatures - how can we find the technosignatures of the mathematicians?
February 19, 2019 | 12:00 PM | ERC 576 | Special Seminar The Growth and Merger of Supermassive Black Holes Vivian U, University of California, Riverside
The upcoming decades will present exciting opportunities to explore the physics of merging supermassive black holes from the multi-messenger perspective. As the gravitational wave spotlight includes mergers of more massive black holes than those discovered thus far, the electromagnetic identification of these candidates and subsequent detailed follow-ups will become routine. Thus, a study of the small-scale environment hosting these events provides the necessary groundwork for future investigations. This detailed study is best carried out in nearby galaxies where we have the ability to resolve the nuclear environment to within tens of parsecs spatial resolution. In this talk, I will highlight results from our Keck OSIRIS AO LIRG Analysis survey, the first key survey to probe the nuclear gas kinematics in nearby interacting galaxies on scales comparable to the sphere-of-influence regions of the central black holes. I will also outline a time-domain reverberation mapping study that examines the broad line regions of accreting black holes, and how it may be applied to identify binary candidates given upcoming large surveys. The power of high-resolution studies in dissecting how systems dynamically evolve will become indispensable for understanding the astrophysics of merging supermassive black holes as we enter an exciting era of astronomy with the imminence of the James Webb Space Telescope, 30-meter class telescopes, and beyond.
February 26, 2019 | 12:00 PM | ERC 576 | Special Seminar Exploring the Formation and Evolution of Planetary Systems Marta Bryan, University of California, Berkeley
Over the past two decades thousands of planets with an extraordinary diversity of properties have been discovered orbiting nearby stars. Many of these exoplanetary systems challenge our narrative for how planets form and evolve, motivating the search for observational clues to the underlying mechanisms that led to this diversity. In this talk I will describe my work using a wide range of techniques to uncover these underlying mechanisms. First, I will constrain the physics of gas giant formation and evolution by discerning population statistics, system architectures, rotation rates, and atmospheric compositions of gas giant planets. I will then discuss the effect that outer gas giants have on the inner architectures of planetary systems by exploring differences in inner planet masses, separations, multiplicities, and orbital properties. Finally, I will highlight the key role that next generation instruments and telescopes such as the GMT will play by extending these novel observations to entirely new classes of planets.
March 5, 2019 | 12:00 PM | ERC 576 | Special Seminar Modeling Variability and non-Kerr Spacetime Effects in Black Hole Images Lia Medeiros, University of California- Santa Barbara
The Event Horizon Telescope (EHT), a mm-wavelength very long baseline interferometer (VLBI), aims to take the first ever resolved image of a black hole at event horizon scales. I will discuss how I use numerical simulations to characterize the effect of intrinsic source variability and deviations from the Kerr geometry on interferometric observables. I show that intrinsic source variability will significantly affect conventional image reconstruction techniques and that variability must be taken in to account for both image synthesis and model fitting. Furthermore, I explore the utility of Principal Component Analysis (PCA) to characterize the structural variability in GRMHD simulations of Sgr A* and find that simulations can be compactly represented with a PCA-derived basis of eigenimages. This allows for detailed comparisons between variable observations and time-dependent models. Finally, I use parametrized metrics that deviate from the Kerr metric and that can be used to approximate several modified gravity theories, to simulate a large number of black hole shadows. I apply PCA to the set of shadows and show that only a handful of "eigen-shadows" are necessary to reconstruct the full set of non-Kerr and Kerr shadows.