### The University of Florida Theoretical Astrophysics Seminar is held on Wednesdays at 1:45pm in Room 2165 of the New Physics Building, unless otherwise noted.

### *** Please note the unusual times and locations of many seminars and special colloquia for Spring 2018. Most seminars are being held at 12:30pm this semester.***

## Spring 2018 Schedule

#### January 16: Special Colloquium -- Katerina Chatziioannou (Perimeter)

#### [4:00pm, NPB 2205]

### Title: Astrophysics and nuclear physics with gravitational waves from binary neutron star coalescences

### Abstract:

Gravitational waves from the coalescence of two neutron stars were detected for the first time in August. The detection of both gravitational and electromagnetic radiation from a single astrophysical source showcased the uniqueness and importance of these systems as probes of astrophysics, nuclear physics, and fundamental theory. In this talk I will describe the properties of the gravitational wave signal emitted when two neutron stars merge and what we can learn from each stage of the coalescence. I will argue that in order to efficiently characterize these complicated systems we need to combine knowledge of astrophysics, nuclear physics, general relativity, and data analysis; I will discuss three relevant examples. Finally, I will discuss my future plans for improvements in the analysis and understanding of neutron star coalescences.

#### January 17: Katerina Chatziioannou (Perimeter)

#### [12:30pm, NPB 2165]

### Title: Spin-precessing compact binaries and their Gravitational Waves

### Abstract:

The recent direct detection of gravitational waves has brought the promise of a novel source of information about the Universe. Gravitational waves from compact binaries primarily depend on the masses of the binary components, however the binary components' spins play an important role too. Interactions between the spins and the orbital angular momentum of the binary give rise to modulations in the amplitude and phase of the wave. These spin-induced precessional modulations are hard to model and measure, yet they carry invaluable information about the binary and the environment that created it. In this talk I will describe efforts to analytically model these spin-precessional modulations accurately and efficiently. I will also describe how this can be used to compute gravitational waveforms that describe the entire evolution of a compact binary as seen by LIGO, including the violent merger and eventual ringdown.

#### January 18: Special Colloquium -- Leo Stein (Caltech)

#### [4:00pm, NPB 1002]

### Title: Probing strong-field gravity: Black holes and mergers in general relativity and beyond

### Abstract:

General relativity-Einstein's theory of gravitation-has been studied for more than 100 years. Over the past century, we have learned that the theory agrees with all available experimental and observational tests. At the same time we know that the theory is incomplete, as it leads to inconsistencies when coupled with quantum mechanics.

The strong-field regime is our best hope to study GR, both observationally and theoretically, and thus understand how to correct its shortcoming. In this talk, I will discuss investigations in the strong field, including black holes and neutron stars, in GR and theories beyond GR. The main focus will be predicting gravitational waves from merging black holes beyond GR. These predictions will allow for the most rigorous testing of general relativity, using LIGO, in the dynamical strong-field regime.

#### January 19: Leo Stein (Caltech)

#### [12:30pm, NPB 2165]

### Title: Separating metric perturbations in near-horizon extremal Kerr, and deformations from stringy interactions

### Abstract:

Linear perturbation theory is a powerful tool for studying black holes (BHs). Spherically symmetric (Schwarzschild) BHs are straightforward to study because the high degree of symmetry leads to separation of the field equations. However, generic rotating (Kerr) BHs do not have enough symmetry for the metric equations to separate. Researchers have relied on the Teukolsky formalism to solve for curvature perturbations in Kerr, but this leads to a very complicated metric reconstruction procedure. In this talk, I will discuss separating the metric perturbation equations in a special limit of Kerr named near-horizon extremal Kerr (NHEK), which has more symmetry than Kerr. I will (i) give an overview of the geometry of NHEK, (ii) demonstrate the method of highest/lowest weight to build symmetry-adapted basis functions, (iii) explain why the equations separate, and (iv) present an application: solving for deformations to NHEK due to "stringy" interactions. After showing the metric deformation solutions arising from these stringy interaction [dynamical Chern-Simons (dCS) and Einstein-dilaton-Gauss-Bonnet (EdGB) theories], I will present some physical implications: corrections to orbits, horizon areas, and horizon entropies. The increased horizon entropy is a signature of the additional microscopic degrees of freedom that become available as the black hole shrinks towards a new length scale.

#### January 23: Special Colloquium -- Anastasia Fialkov (ITC/Harvard)

#### [4:00pm, NPB 2205]

### Title: Shining Light into the Cosmic Dark Ages

### Abstract:

The first billion years of cosmic history hold the key to unlocking many of the remaining mysteries in astronomy, cosmology and physics. However, due to the lack of direct observations, we know almost nothing about this era. Fortunately, observational effort is on the way to map the Epochs of Reionization and Cosmic Dawn including the Hydrogen Epoch of Reionization Array (radio), the James Webb Space Telescope (infrared), and the next generation X-ray observatories such as Athena and the projected Lynx mission. The wealth of data that these projects will generate will revolutionize our knowledge of the first billion years. In my talk I will describe what we can learn about the first billion years using the redshifted 21-cm line of neutral hydrogen. I will also mention other potentially interesting tracers of the Cosmic Dawn such as the cosmic X-ray background.

#### January 24: Anastasia Fialkov (ITC/Harvard)

#### [12:30pm, NPB 2165]

### Title: Probing Cosmic Dawn

### Abstract:

21-cm signal of neutral hydrogen is predicted to be a powerful probe of the epochs of Reionization and Cosmic Dawn. In my talk I will discuss prospects for constraining astrophysics and cosmology using this line. Another potential source of information on Reionization are fast radio bursts. I will discuss cosmological modeling of the population and prospects on constraining Reionization using these sources as tracers. Finally, I will talk about my ongoing research including cosmological implications of axion-like dark matter of mass ~ 1e-22 eV. I will show preliminary results from hydrodynamical simulations of galaxy formation in an axion universe.

#### January 29: Special Colloquium -- Nick Stone (Columbia)

#### [12:30pm, NPB 2205]

### Title: General Relativity and Dynamics in Galactic Centers

### Abstract:

Black holes were for decades one of the most controversial predictions of Einstein's theory of general relativity (GR). We now know that black holes are not only ubiquitous in typical galaxies, but that the accretion of baryonic matter onto black holes powers the most luminous sources of electromagnetic radiation in the Universe. However, the electromagnetic output from a single black hole pales in comparison to the gravitational wave radiation from the merger of two black holes, which can reach a nontrivial fraction of the Planck luminosity! In this talk, I will review our knowledge of astrophysical black holes and their potential utility as laboratories for probing GR. I will then describe my own, primarily theoretical, research into the luminous sources of electromagnetic and gravitational radiation that may originate from black holes. I will focus especially on black holes in the centers of galaxies, as these extreme environments (i) are the primary astrophysical homes of supermassive black holes, with masses millions to billions times that of our own Sun, and (ii) are a rare example of collisional stellar systems, where frequent close encounters between combinations of stars and compact objects occur. With the tools of stellar kinetic theory (i.e. treating stellar distributions as weakly collisional plasmas that evolve via a nonlinear Fokker-Planck equation), I will show that galactic nuclei are dynamical factories that manufacture some of the most extreme sources of radiation in the universe, such as X-ray binaries, tidal disruption events, and black hole mergers.

#### January 30: Nick Stone (Columbia)

#### [12:30pm, NPB 2165]

### Title: Black-Hole-Powered Transients in the Era of Time Domain Astrophysics

### Abstract:

Recent wide field astronomical surveys (e.g. Pan-STARRS, PTF, and ASASSN) have inaugurated the era of time domain astronomy, in which previously rare outbursts of optical light can now be found with some regularity. The transient flares found by these surveys have many physical origins (novae, supernovae, etc.) but many arise from accretion onto black holes (BHs). The discoveries of time domain surveys complement the recent breakthroughs of the LIGO-Virgo collaboration, which have likewise given us the ability to detect new types of transient signals from BHs in the Universe. I will discuss in detail my ongoing and future efforts to understand the astrophysical origins of BH-powered transients (in both electromagnetic and gravitational wave radiation), focusing primarily on the dense stellar systems that dynamically produce unstable combinations of stars and BHs.

#### February 7: Luc Blanchet (IAP)

#### [12:30pm, NPB 2165]

### Title: Gravitational Waves and General Relativity

### Abstract:

The gravitational wave detectors LIGO/Virgo have discovered the signals generated by the coalescence of compact binary systems (made of black holes or neutron stars) at astronomical distances. The theoretical and numerical works on the two-body problem in general relativity play a crucial role when deciphering and interpreting the gravitational wave signals. In this talk, after a general review on gravitational waves and the problem of motion, we shall present the state-of-the-art on approximation methods in general relativity, such as the famous post-Newtonian expansion, which is an expansion when the orbital velocity of the compact objects is small with respect to the speed of light. We shall show how important is this approximation for the GW detections, notably the recent binary neutron star event, and discuss the latest developments in the field.

#### February 7: Special High Energy Seminar

#### [2:00pm, NPB 2165]

#### February 14: No Seminar

#### February 21: No Seminar

#### February 26: Special Colloquium -- Vera Gluscevic (IAS)

#### [4:00pm, NPB 2205]

### Title: Cornering dark matter

### Abstract:

On galactic scales, structure is gravitationally held by dark matter, while stars and gas present merely an "icing on the cake." In spite of their dominance over the matter budget in the universe, particle constituents of dark matter have eluded detection and characterization to the present day. This talk will focus on a variety of promising new techniques that will help corner dark matter particles in the coming decade of synergistic searches through precision measurements of direct detection experiments and cosmological surveys.

#### February 27: Vera Gluscevic (IAS)

#### [2:00pm, NPB 2165]

### Title: Statistical anisotropy in the cosmological sky as signature of new physics

### Abstract:

Under the standard models of cosmology and particle physics, the temperature and polarization patterns of the primary cosmic microwave background (CMB) are captured by a single set of power spectra for the entire sky. In other words, searches for statistical anisotropy can reveal new physics. As an example, I will describe a technique I developed to search for new cosmic scalar fields (that could constitute dark energy), using parity-violating correlations in the CMB. I will then discuss a similar technique that will enable searches for primordial magnetic fields with future tomographic 21-cm surveys, with sensitivity that surpasses the best current bounds by ten orders of magnitude. I will argue that many statistical methods used in precision analyses of the CMB will also prove essential in the era of 21-cm cosmology in the coming decade.

#### March 7: No Seminar [Spring Break]

#### March 14: Davide Gerosa (Caltech)

#### [12:30pm, NPB 2165]

### Title: Runaways: phenomenology and detectability of black-hole recoils

### Abstract:

Black-hole recoils are arguably the strong-gravity phenomena with most striking astrophysical consequences. In the late inspiral and final coalescence of black-hole binaries, anisotropic emission of gravitational waves causes significant linear momentum loss. The remnant black hole, therefore, recoils in the opposite direction. This final kicks can reach magnitudes up to 5000 km/s ("superkicks"), larger than the escape speed of even the most massive galaxies, thus opening the possibility of black hole ejections. In this talk, I summarize recent advances in modeling black-hole recoils and their astrophysical environments. I will address the relevance of disk-assisted spin alignments, new approaches to model black-hole recoils with waveform approximants and prospects to directly detect superkicks with future space- and ground-based gravitational-wave detectors.

#### March 21: [Seminar cancelled]

#### March 28: Rebecca Nevin (CU Boulder)

#### [12:30pm, NPB 2165]

### Title: A New Imaging and Kinematic Technique to Identify Merging Galaxies from Hydrodynamics Simulations

### Abstract:

Both galaxies and supermassive black holes grow and evolve over cosmic time. My work investigates some key processes that drive this evolution: AGN (Active Galactic Nuclei) feedback and galaxy mergers. I will first discuss my work modeling radiatively-driven outflows from AGN and the results of how these biconical outflows affect their host galaxies. Then, I will discuss merging galaxies, and how progress in our understanding of galaxy evolution is slowed by the difficulty of making accurate galaxy merger identifications. Mergers are typically identified using imaging alone, which has its limitations and biases. With the growing popularity of integral field spectroscopy (IFS), it is now possible to use kinematic signatures to improve galaxy merger identifications. I use GADGET-3 N-body/hydrodynamics simulations of merging galaxies coupled with SUNRISE dust radiative transfer simulations to apply the same analysis to simulations and observations. From these simulations, I create mockup IFS and images and from the mockup galaxies, I have developed the first merging galaxy classification scheme that is based on kinematics and imaging. Utilizing a Linear Discriminant Analysis tool, I have determined which kinematic and imaging predictors are most useful for identifying various types of merging galaxies (mergers of different mass ratios, gas fractions, and merger stages). I will discuss the strengths and limitations of the classification technique and then my plans to apply the classification to the >10,000 observed galaxies in the MaNGA (Mapping Nearby Galaxies at Apache Point) IFS survey. Through accurate identification of merging galaxies in the MaNGA survey, I will advance our understanding of supermassive black hole growth in galaxy mergers and other open questions related to galaxy evolution.

#### April 4: Kohta Murase (Penn State)

#### [12:30pm, NPB 2165]

### Title: High-Energy Cosmic Particle Mysteries

### Abstract:

New frontiers of particle astrophysics have been opened by IceCube's discovery of high-energy cosmic neutrinos. Their origin is a new mystery in the field, and solving this problem may enable us not only to understand the physics of astrophysical sources but also to obtain important clues about old mystery, the origin of cosmic rays, and to utilize neutrinos as probes of neutrino properties, dark matter, and fundamental physics. In this talk, I summarize the latest results of IceCube observations, and emphasize the relevance of multi-messenger approaches. I will review recent basic ideas on the origin of high-energy cosmic neutrinos, including the grand-unified model we recently proposed, and some implications for particle physics properties.

#### April 11: Smadar Naoz (UCLA)

#### [12:30pm, NPB 2165]

### Title: Triples are Here and There, Triples are Everywhere (Triples in Planets, Stars, and Black holes)

### Abstract:

Many observed triple systems in our Universe are in a hierarchical configuration: two objects orbit each other in a relatively tight inner binary while the third object is on a much wider orbit. In this case, the secular approximation (i.e., phase-averaged, long-term evolution) can be applied, where the interactions between two non-resonant orbits are equivalent to treating the two orbits as massive wires. Thus, the orbits may change shape and orientation, on timescales longer than the orbital periods, but the semi-major axes are constant. This approximation has been proven to be very useful in many astrophysical contexts, from planetary to triple-star systems and even black holes. I will discuss recent developments in that field and will show that hierarchical triple systems are richer and far more exciting than considered of before. In particular, the tight orbit can reach extremely high eccentricities and undergo chaotic flips of its orientation. This behavior has important implications for the evolution of many systems, and I will present some seminal examples, such as retrograde hot Jupiters, blue stragglers, and black-hole binaries.

#### April 18: Luke Kelley (Harvard/CfA)

#### [12:30pm, NPB 2165]

### Title: Massive Black Hole Binary Mergers: Dynamics, Environments & Expected Detections

### Abstract:

Pulsar timing arrays around the world are using the incredible consistency of millisecond pulsars to measure low-frequency gravitational waves (GW) from massive black hole (MBH) binaries. We use comprehensive MBH merger models based on cosmological hydrodynamic simulations to predict the expected signals from these coalescing behemoths. Excitingly, we find that both a stochastic gravitational wave background and resolvable individual-binaries are likely to be detected in roughly a decade. The GW background encodes a wealth of information about MBH binary populations and their galactic environments, while single sources exhibit a marked robustness to severe pulsar red-noise. We will also discuss recently identified tension between MBH binary candidates identified in optical AGN surveys and the most recent pulsar timing array upper-limits on the GW background. Finally, we will present new models for producing periodic-variability in binary-AGN systems and their prospects for detection with current and future surveys.

#### April 25: Tamara Bogdanovic (Georgia Tech)

#### [12:30pm, NPB 2165]

### Title: When galaxies merge, what happens to their supermassive black holes?

### Abstract:

Gravitationally bound supermassive black hole binaries are thought to be a natural product of galactic mergers and growth of the large scale structure in the universe. They however remain observationally elusive, thus raising a question about characteristic observational signatures associated with these systems. In my talk I will discuss current theoretical understanding and latest advances made in observational searches for supermassive black hole binaries.

## Fall 2017 Schedule

#### September 6: Shun-Pei Miao (NCKU)

### Title: Electrodynamic Effects of Inflationary Gravitons

### Abstract:

I begin by describing the origin of big IR effects during primordial inflation. I then review a series of works on EM + GR from flat space to de Sitter, commenting in particular on the issue of gauge dependence. Finally, I make a conjecture for resolving the issue, and present the latest work which has been done to test this conjecture.

#### September 27: Special Event [NPB Rm. 2205]

### Title: Academic Jobs Panel

#### October 4: Richard Woodard (UF Physics)

### Title: MOND Cosmology

### Abstract:

MOND is a phenomenological model which modifies the extreme weak field regime of Newtonian gravity so as to explain galactic rotation curves without dark matter. If correct, it must be the non-relativistic, static limit of some relativistic modified gravity theory. I show how the only possible metric-based modification of gravity is nonlocal, and I construct the action using the Tully-Fisher relation and weak lensing. Then I explore the consequences of this model for cosmology. This talk is based on three arXiv papers: 1106.4984, 1405.0393 and 1608.07858.

#### October 11: Adrian Hamers (IAS)

### Title: Dynamics around supermassive black holes: from mpc to kpc

### Abstract:

We consider orbital dynamics at various scales around supermassive black holes (MBHs) with masses comparable to the MBH in the Milky Way Galactic Center (GC). First, we address the relativistic regime within a few mpc from the MBH. In this regime, orbital diffusion is affected by relativistic precession which has implications for the evolution of stars like the S-stars, and for the rates of inspirals of compact objects onto the MBH (i.e., EMRIs). Second, we consider the X-ray and near-IR flares that are observed from the MBH in the GC on an approximately daily basis. A proposed origin for these flares is the tidal disruption of large (> 10 km radius) planetesimals, and we study the dynamics of such planetesimals in the GC at the parsec scale, giving clues into their origin. Lastly, we venture to larger scales of up to a kpc from the MBH and consider relaxation of objects, in particular binary stars, due to transient nuclear spiral arms. We show how this relaxation process can enhance the disruption rates of stellar binaries, with implications for the orientation of hypervelocity stars originating from the GC.

#### October 17: Special Event [12:45pm, Bryant Space Center Rm. 217]

### Title: Discussion of LIGO results

### Abstract:

Steve Eikenberry will lead a discussion about the discovery of a binary neutron star merger by LIGO. (*Note the unusual time and location.*)

#### October 25: Cole Miller (Univ. of Maryland)

### Title: When Neutron Stars Collide

### Abstract:

On August 17, gravitational waves and gamma rays from a merger of two neutron stars reached the Earth. Over the next several weeks, astronomers studied this event across the electromagnetic spectrum, leading to remarkable confirmation of predictions and also some fascinating new puzzles to consider. I will share some perspectives about the importance of the observations in many subjects: from tests of theories of strong gravity to the structure of neutron stars to the nature of short gamma-ray bursts and the production of heavy elements. I will also speculate about the return we can expect from gravitational wave observing runs in the coming years.

#### November 1: Leo Stein (Caltech)

### Title: Probing strong-field gravity: Black holes and mergers in general relativity and beyond

### Abstract:

General relativity-Einstein's theory of gravitation-has been studied for more than 100 years. Over the past century, we have learned that the theory agrees with all available experimental and observational tests. At the same time we know that the theory is incomplete, as it leads to inconsistencies when coupled with quantum mechanics. The strong-field regime is our best hope to study GR, both observationally and theoretically, and thus understand how to correct its shortcoming. In this talk, I will discuss investigations in the strong-field, including black holes and neutron stars, in GR and theories beyond GR. The main focus will be predicting gravitational waves from merging black holes beyond GR. These predictions will allow for the most rigorous testing of general relativity, using LIGO, in the dynamical strong-field regime.

#### November 8: Steffen Aksteiner (Princeton)

### Title: Black Hole Perturbations. A complete set of local gauge invariants

### Abstract:

Pertubation theory on black hole spacetimes is commonly treated by wave-like equations for certain decoupled, gauge invariant curvature components known as Teukolsky equations. In this talk we present additional (locally independent) gauge invariants, discuss some of their properties and discuss in which sense they form a complete set. This is joint work with contributions by Lars Andersson, Thomas Backdahl, Igor Khavkine, Abhay Shah and Bernard Whiting.

#### November 15: Desika Narayanan (UF Astronomy)

### Title: Tracing the (Baryonic) Dark Sector of Galaxies

### Abstract:

Stars form in giant clouds comprised of molecular (H2) gas in galaxies. Tracing this gas, however, is a complicated mess. The radiation field from nearby stars, cosmic rays from supernovae, and collisions with energetic dust can all dictate the thermal, radiative, and chemical properties of this gas. In this talk, I go through pedagogical exercises aimed at non-experts in the physics of the star-forming interstellar medium, and provide evidence that up to half the gas in galaxies may be hitherto undetected.

#### November 29: Jack Hewitt (Univ. of North Florida)

### Title: Galactic Accelerators: Understanding the Sources and Propagation of Cosmic Rays

### Abstract:

Observations with the Fermi Large Area Telescope have raised new questions about particle accelerators within the Galaxy, including supernova remnants, pulsar wind nebulae, and star forming regions. Recent surveys with Fermi have detected more than 50 spatially extended sources, which is the most certain means of identifying these source classes. Joint studies with TeV observatories - HAWC, HESS, MAGIC and VERITAS - allow us to measure cosmic ray propagation and acceleration (or re-acceleration) in these sources. Continued studies promise to resolve whether we have identified the long-sought PeVatrons that produce Galactic cosmic rays.

#### December 6: Anna Heffernan (UF Physics)

### Title: The self-force: Who? What? How? Why? When? Gravity? Florida?

### Abstract:

The University of Florida historically boasts one of the biggest, most productive AND most famous self-force / gravity groups in the world - it's why I came here. After almost 2 years of chats, I've realised not everybody here is fully aware of the major contributions that were born here at Florida, nor the exact concept of the self-force and how it fits in the grander scheme of general relativity, gravitational wave detection and the very exciting future space based gravitational wave detector, LISA. This talk will serve to explain the basic concepts of the self-force, how it has progressed since it's birth, and the major current themes of research. I will depict how the Florida gravity group fits into all of the above, including highlights of my own research. Spoiler: I'll be talking about black hole binaries and their role in the LISA mission.

## Other Events

- Physics Colloquium
- Astronomy Colloquium
- High Energy Seminar
- Condensed Matter Seminar
- Student Seminar

Students may receive credit for attending the Astrophysics Seminar by registering for PHY 6391.