Copernicus Revisited: is the Earth special?

Nearly 500 years ago, Nicolas Copernicus published his disruptive theory that Earth is not the center of the universe. This "Copernican demotion" has held fast over the centuries, as astronomers have learned that there is nothing particularly remarkable about Earth or even the Milky Way. In the last two decades, however, a new test of the Copernican Principle has emerged -- the discovery of an abundance of planets orbiting other stars. These discoveries allow us to put Earth in context and evaluate whether the formation, architecture, and present-day characteristics of our Solar System are in fact typical. Thanks to the newly launched James Webb Space Telescope (JWST), we are finally able to study other Earth-size planets in detail, and in particular search for and characterize their atmospheres. In this talk, I will give a status report on JWST observations of rocky planet. I will cover the latest results for the iconic TRAPPIST-1 system, the study of the surface of the airless planet LHS 3844b, the search for atmospheres on lava worlds, and observations of planets in the radius valley at the boundary of rocky and gaseous worlds. Taken together, these results provide a first picture of the building blocks available for the origin of life on terrestrial planets beyond the Solar System, providing essential context for how special Earth really is.

The Climates of Other Worlds: Searching for the Next Habitable Planet.

The Climates of Other Worlds: Searching for the Next Habitable Planet
The discovery of numerous relatively close planets orbiting low-mass stars signals a major
planetary population that may be the primary environment explored in the search for life
beyond the Solar System. The environments of these types of stars are unique, and their
climatic impacts, along with the effects of the many other factors and processes that can
strongly influence planetary climate and habitability, must be understood to accurately
determine a planet's habitability potential. Professor Shields will describe the methods used
by her research group, the Shields Center for Exoplanet Climate and Interdisciplinary
Education, to quantify the effects on planetary climate of a range of factors currently
unconstrained by observations. She will share recent results from this work using a hierarchy
of computer models, and discuss the implications of these results for planetary habitability
and the identification of those planets most capable of supporting life.
 

A case for Mars terraforming research.

What we cannot create, we do not understand." Can we understand enough about climate and ecosystems to build them on other planets (and exoplanets)? A necessary, but insufficient, first step for making Mars's surface suitable for life would be to warm the planet. Motivated by our collaboration's recent work on engineered-aerosol warming of Mars (e.g. Ansari et al. Science Advances 2024), which is >5,000x more mass-effective than optimized greenhouse-gas mixes, I will discuss what we know about Mars, what we think we know about Mars terraforming (including alternative approaches), and the case for more research. Before we can assess whether warming Mars is worthwhile, relative to the alternative of leaving Mars as a pristine wilderness, we must confront practical requirements, cost, and possible risks. Further research is needed on both Mars and warming options before committing to any plan for Mars' future.

The Tale of the Multiply-imaged Standard Candle Supernova "H0pe" That Yielded a Value for the Current Expansion Rate of the Universe
 

Abstract: We present results from the James Webb Space Telescope (JWST) Prime Extragalactic Areas and Reionization and Lensing Science (PEARLS) program in the direction of galaxy clusters, from the spectacle of "first light" observations to the scene of vast panoramas of gravitationally-lensed arcs. One point-source was discovered in the galaxy cluster field PLCK G165.7+67.0 (G165) that was bright and appeared in three locations as a result of strong gravitational lensing. Follow-up observations confirmed this source to be a normal Type Ia supernova (SN) at a redshift of 1.783 that we call "SN H0pe." The SN host galaxy is centrally-situated in a high star-forming galaxy group. A delay in the arrival times of the SN light into each image arises from their different geometric light paths, which together with a gravitational lensing model, returns a value for the Hubble-Lemaitre constant "H0."  Under double-blinded protocols, the two time delays were measured by a photometric approach and a rare spectroscopic approach, and seven lens models were constructed. The time delays were augmented by measurements of the absolute magnifications, summing up to a set of five observables. By a scaled fit of the lens model-predicted observables to the measured set, a best-fit value was inferred of H0 = 75.7(+8.1)(-5.5) km/s/Mpc. This is only the second time H0 has been measured by this method, and the first time using a standard candle. Regular monitoring of G165 and galaxy cluster fields with similar properties may be well-rewarded by the detection of additional strongly-lensed SNe.

Simulating Black Hole Feasts, Burps, and Fireworks
 

Based on latest state of the art general relativistic magnetohydrodynamic simulations, I will discuss how black holes feast on the surrounding gas, burp away their surroundings, and produce bright emission in a wide range of contexts, ranging from stellar-mass black holes in the context of binary mergers and collapsing massive stars to supermassive black holes at the centers of active galaxies.

Is machine learning good or bad for astrophysics?

Machine learning methods are having a huge impact in astrophysics. However, ML has a strong ontology — in which only the data exist — and a strong epistemology — in which a model is considered good if and only if it performs well on held-out training data. These philosophies are in strong conflict with our goals, practices, and philosophy in astrophysics. I identify locations in astrophysics practice at which the philosophy of ML is valuable (for example: in causal separations of foregrounds). I also show that there are contexts in which the use of ML methods introduce strong, unwanted, and un-correctable biases. The answer to the question posed in my title is "both." Work in collaboration with Soledad Villar (JHU).

The Gaia binary star revolution 

By precisely measuring the motions stars on the sky over time, the Gaia mission is conducting a comprehensive census of the Milky Way's binary stars. These data are transformative both for population modeling and for discovery of rare objects. I will describe our emerging view of the populations of black holes, neutron stars, and white dwarfs in au-scale binaries, focusing in particular on their mass, period, and eccentricity distributions. Compared to previous surveys, Gaia is revealing post-interaction binaries in wider orbits, whose properties are difficult to explain with standard binary evolution models. I will discuss how the Gaia catalogs can be leveraged for statistical inference, despite their complex selection function, and how they can discriminate between competing formation models.

Learnings about, and from, fast radio bursts

The origins of the millisecond-duration, energetic (>10^39 erg) fast radio bursts (FRBs) at extragalactic distances remain shrouded in mystery. Although FRBs are likely associated with neutron stars, they appear to occur in a remarkable diversity of environments. Understanding the formation of FRB sources is thus intertwined with problems in neutron-star formation. FRBs additionally form exquisite tracers of the contents and physical conditions of the otherwise "missing" baryons along their sightlines. I have led the development of the Deep Synoptic Array (DSA-110) radio telescope, which is discovering and pinpointing FRBs to host galaxies at a world-leading rate. The host-galaxy and radio-burst properties of the DSA-110 sample shed new and arguably conclusive light on the origins of FRBs. DSA-110 discoveries have, for the first time, sensitively determined the split between the cosmic baryon contents around and in between galaxies, and are enabling direct measurements of the suppression of the matter power spectrum on 0.1—10 Mpc scales. 

The detection of GW170817 enabled us to track down and watch the cataclysm event in multiple wavelengths of light, allowing us to scrutinize the source of these cosmic ripples for the first time. This discovery provided the first solid evidence that neutron-star smashups are the source of much of the Universe's gold, platinum and other heavy elements in the Universe. With a single event, we were able to answer fundamental questions in general relativity, cosmology, nuclear physics, and astrophysics. However, other parts of the story told by these events are still shrouded in mystery. For astronomers and physicists across disciplines, this is an extremely exciting time to be alive.

Cosmology is undergoing a data revolution. Surveys such as the imminent Legacy Survey of Space and Time (LSST) to be conducted by the Vera C. Rubin Observatory will deliver huge galaxy catalogues that provide critical tools for understanding the nature of dark matter and dark energy. However, in order to obtain accurate cosmological constraints from these enormous datasets, we need reliable ways of estimating galaxy properties using only photometry. I will present pop-cosmos: a forward modelling framework for photometric galaxy survey data, where galaxies are modelled as draws from a population prior distribution over redshift, mass, dust properties, metallicity, and star formation history. These properties are mapped to photometry using an emulator for stellar population synthesis, followed by the application of a learned model for a survey's noise properties. Application of selection cuts enables the generation of mock galaxy catalogues. This enables us to use simulation-based inference to solve the inverse problem of calibrating the population-level prior on a deep multiwavelength catalogue, COSMOS2020. We use a diffusion model as a flexible population-level prior, and optimise its parameters by minimising the Wasserstein distance between forward-simulated photometry and the real COSMOS2020 survey data. The resulting model can then be used to derive accurate redshift distributions for upcoming photometric surveys, to facilitate weak lensing and clustering science. I will show applications of this framework, demonstrating how we are able to extract redshift distributions, and make inferences about galaxy evolution. I will also discuss the use of pop-cosmos as a prior for performing inference on individual galaxies in a highly scaleable manner, as well as ongoing work to analyse data from the Kilo-Degree Survey (KiDS) in preparation for LSST.
 

The Cosmic Quest for Neutrino Mass

Neutrinos are the second most abundant particle in the known Universe yet they remain mysterious. While they played an important role in the early Universe, today they contribute at most 1-2% of the energy budget and leave only faint signatures. A major goal of the current decade of experiments is to measure the neutrino contribution to the energy budget at high significance, thereby determining the neutrino mass scale and perhaps even the ordering of masses. I will describe the physical effects of neutrino mass, highlighting the distinct roles played geometry and structure on constraints, and how some dataset combinations give a peculiar preference for “negative neutrino mass,” while others hint at a detection. I will also discuss novel astrophysical probes of neutrino mass that may ultimately help resolve current conundrums and contribute to validating any detection of relic neutrinos in the late Universe. 

Life on the edge: reconnection-powered emission at the boundaries of black hole jets
 

The boundaries of relativistic black hole jets—at the interface between the jet and the disk wind—lie at the core of our recent understanding of jet-powered phenomena. They harbor intense velocity and magnetic shears, which provide the free energy needed to power a number of observational signatures. We demonstrate that magnetic reconnection—a process by which opposite field lines annihilate, releasing their energy to the plasma—ultimately governs dissipation of the free energy at jet boundaries. Reconnection resulting from the nonlinear evolution of Kelvin-Helmholtz-type vortices can naturally explain the limb-brightened radio emission of AGN jets, such as M87. Inverse Compton scattering within the chain of magnetic islands / flux ropes self-consistently created by reconnection at the jet boundary can power the mysterious hard X-ray “coronal” emission of X-ray binaries. We also argue that reconnection-driven hadronic acceleration in the coronal regions of NGC 1068 may be the source of the TeV neutrinos recently detected by IceCube. 

Cosmology from DESI's First Year of Large-Scale Structure Measurements

Over a five-year period, the Dark Energy Spectroscopic Instrument (DESI) will spectroscopically classify nearly 40 million galaxies and quasars over 1/3 of the sky and to redshifts z < 3.5. The DESI collaboration has completed measurements of the baryon acoustic oscillation (BAO) feature and more generally, of large-scale structure, using data from the first year of observation. In this talk, I will present those measurements and their implications for our understanding of the cosmological model. In particular, I will discuss the constraints on the Hubble Constant, dark energy equation of state, and summed mass of the three neutrino mass eigenstates. In doing so, I will discuss the new and future DESI measurements with respect to the hints of tension that have been reported in the Hubble Constant and with LCDM in general. 

Stars Disrupted, Destroyed and Coalesced

Supernovae, tidal disruption events and merging neutron stars can provide a wealth of physical insights from nuclear physics to cosmology; they can teach us about the creation of the elements, the formation of compact objects, accretion processes, feedback, galaxy evolution, and more. I will present how my research group is investigating the nature of tidal disruption events and their surprising emission properties; how we are using supernovae to constrain late-stage stellar evolution and explosion, and asteroseismology to constrain the inner structure of massive stars; and how we are hoping to find more gravitational-wave counterparts through multi-observatory coordination. 

An Early JWST View of Quasars at Cosmic Dawn 

Quasars at cosmic dawn are powerful probes to the formation and growth of early supermassive black holes in the universe, their connections to high-redshift galaxy and structure formation, and the evolution of the intergalactic medium at the epoch of reionization. I will first review the progress in surveys of the most distant quasars, which have discovered hundreds of luminous quasars within the first billion years of cosmic history, with the highest redshift at z~7.6. They are powered by billion solar mass black holes, possible only by a combination of massive early black hole seeds with highly efficient and sustained accretion. I will then present the latest results of studies of early quasars and their environments using JWST. While rapid early black hole growth is accompanied by intense star formation and feedback in their host galaxies, the diverse quasar environment unveiled by these observations suggests a complex interplay between black hole accretion, galaxy assembly, the physics of reionization and the emergence of early large scale structure. I will conclude with a look into the future, as the era of the first supermassive black holes is finally within the horizon of our research efforts.

Date

 Speaker

 Institution

1/30/2024

Jonathan Fortney

UCSC

New views of planetary and brown dwarf atmospheric physics and chemistry with JWST

JWST has enabled high-fidelity infrared spectroscopy for a wide range of transiting planets, directly imaged planets, and brown dwarfs.  In the past year, for transiting planets, observers have detected several molecules for the first time in an exoplanet.  In this talk I will describe observations of several planets from the MANATEE collaboration, which focuses on Jupiter- and Neptune-class planets in a new temperature regime, below 1000 K, where a range of chemical transitions are expected to occur.  These new observations require a range of new modeling efforts, and in my work I seek to make connections between a diverse range of atmospheres.  These impressive observations enable us to break new ground on previously highly uncertain atmospheric process.  These include the physics of mixing in (isolated) brown dwarfs and in (strongly irradiated) transiting planets, as well as the role of the atmosphere/interior connection, far below the visible atmosphere, in helping to dictate visible atmospheric abundances.

2/6/2024

Erin Kara

MIT

Black hole accretion in the TDAMM Era

Most of the power from an Active Galactic Nucleus is released close to the black hole, and thus studying accretion at event horizon scales—at the intersection of inflow and outflow—is essential for understanding how much matter accretes and grows the black hole vs. how much matter is ejected, thus effecting the black hole’s large-scale environments. In the past decade, we have had a breakthrough in how we probe the inner accretion flow, through the discovery of X-ray Reverberation Mapping, where X-rays produced close to the black hole reverberate off inflowing gas. By measuring reverberation time delays, we can quantify the effects of strongly curved space time and measure black hole spin, which is key for understanding how efficiently energy can be tapped from the accretion process. In this talk, I will give an overview of this field, and will show how extending these spectral-timing techniques to extreme, transient (and possibly multi-messenger) accretion events like Tidal Disruption Events and Quasi Periodic Eruptions can help us understand the growth and impact of black holes in galactic centers. 

2/13/2024

Daniel Scolnic

Duke University

Measurements of the Expansion Rate of the Universe and the Lingering Hubble Tension

The standard model of cosmology has passed every test over the last twenty years.  Yet it remains unsatisfactory, with 95% of the universe being dark components, whose nature we did not understand.  Now, there are possible ‘cracks’ in the model, as recent observations of the local expansion rate of the universe, parameterized by the Hubble constant, do not match predictions using data from the Cosmic Microwave Background and our standard model.  This is the best end-to-end test of our cosmological model, and currently, we do not pass the test.  I will discuss my team’s Pantheon+SH0ES measurements on the local side, and review the numerous crosschecks and tests on our data and analysis.  I will show new data from the James Webb Space Telescope and explain how it strengthens the current tension.  Finally, I will talk about how the community is moving forward, with different probes of the early and late universe, and what new theoretical ideas show the most promise.  

2/20/2024

Anna Ho

Cornell University

Finding Relativistic Stellar Explosions as Fast Optical Transients. 

For the last half-century, relativistic outflows accompanying the final collapse of massive stars have predominantly been detected via high-energy emission, as long-duration gamma-ray bursts (GRBs). Yet, it has long been hypothesized that GRBs are the tip of the iceberg of relativistic stellar explosions, because the conditions required to produce and detect a GRB are contrived. I will present results from a search for relativistic stellar explosions using optical time-domain surveys. The emerging zoo includes afterglows at cosmological distances with no detected GRB, supernovae with luminous X-ray and radio emission, and a mysterious class of "fast blue optical transients" with minute-timescale optical flares at supernova-like luminosities. An understanding of the origin of these events and their relation to GRBs will be enabled by upcoming time-domain surveys in other bands, including X-ray, UV, and submillimeter.

2/27/2024

Kumiko Kotera

IAP

Towards EeV neutrino astronomy with GRAND

We are living exciting times: we are now able to probe the most violent events of the Universe with diverse messengers (cosmic rays, neutrinos, photons and gravitational waves). One challenge to complete the multi-messenger picture resides in the highest energies, as no ultra-high energy neutrinos, with energy > 10^17 eV, have been observed yet. This challenge could be undertaken by the GRAND (Giant Radio Array for Neutrino Detection) project. GRAND is a proposal for a large-scale array of self-triggered radio antennas. It stands out today as a unique experiment which plans to reach ambitious sensitivity and sub-degree angular resolution to launch multi-messenger astronomy at ultra-high-energies. A design has been proposed for the GRAND detector, and the instrumentation is being tested and optimized with 3 small-scale prototypes: GRAND@Nançay, GRAND@Auger, and GRANDProto300. Based on these pathfinders, the GRAND Collaboration is starting to explore improved GRAND technical designs for the next large-scale phase of the project. This will consist in two arrays of 10'000 km2 each, in the Northern and Southern hemispheres to be deployed from 2028. In this talk, we will present the status of the GRAND prototypes, the preliminary designs and simulation results for the next stages, and the rich research program that these will enable.

3/5/2024

Mike Brown

Caltech

Planet Nine from Outer Space

Astronomers have been predicting and searching for planets beyond Neptune for almost 180 years. In all previous cases the predictions were based on bad data, bad physics, or both, and the predictions turned out to be wrong. In 2016, we joined this  inglorious group and declared that orbital alignments of the most distant objects in the solar system demand the presence of a distant giant planet on an eccentric and inclined orbit. I'll describe the mounting evidence for this planet, discuss the counter-proposals, and talk about the ongoing search for what would be the fifth largest planet of our solar system.

3/12/2024

 PU Spring Break – No Colloquium

3/19/2024

Adam Leroy

Ohio State University

A "Cloud-Scale" View of the Matter Cycle in Galaxies

The gas-star formation-feedback "matter cycle" acts in many ways as the engine of galaxy evolution. Over the last decade, the PHANGS surveys have carried out surveys across the electromagnetic spectrum aimed at resolving galaxies into the fundamental units of this cycle: molecular clouds, HII regions, and star clusters. I will describe some of the key results from these surveys, emphasizing our new, dynamic view of molecular clouds from the PHANGS-ALMA CO survey and the new view of dust revealed by PHANGS-JWST. Based on these surveys, we have our first "big picture" view of the demographics of star-forming molecular clouds, which show a clear link to their larger scale environment. The properties of these clouds, and the contrast between tracers of gas and recent star formation reveal the efficiencies and timescales for star formation, highlighting some tension between the data and popular turbulent models of star formation. The key role of stellar feedback is also visible on multiple scales from these data. Feedback helps maintain the large scale dynamical equilibrium in galaxy disks, rapidly reshapes molecular clouds, and carves pervasive shells and bubbles through the ISM. In all of these areas the revolutionary resolution and sensitivity of JWST through the near and mid-infrared promise to enable the next advances. In this area, I will highlight the PHANGS Cycle 1 and 2 JWST Treasuries, which provide high quality, immediately public near- and mid-IR imaging of a representative sample of z=0 star forming galaxies. There will be pretty pictures of galaxies from across the spectrum!

3/26/2024

Evan Kirby

University of Notre Dame

Early r-process enrichment in globular clusters

Stars in nearly all globular clusters show complex relations among the abundances of light elements (up to Na).  Many also show anti-correlations of Mg and Al.  Until now, only one cluster (M15) conclusively showed any star-to-star variation in neutron-capture elements, like Eu.  Using a trick of stellar evolution, I show that these variations are primordial, not caused by external pollution.  Then, I show that M92 also has variations in barium and lanthanide abundances (but not first-peak r-process) among the first generation of stars but not the second.  The evidence points to a rare source of the "main" r-process that happened right at the start of the formation of the cluster.

4/2/2024

Jamie Tayar

University of Florida

How well can we estimate stellar ages?

Good estimations of stellar ages are important for a wide range of science cases, from understanding the evolution and habitability of exoplanetary systems to tracking back the cosmic history of our own and other galaxies. These ages, however, are not directly measured, but rather inferred from theoretical models or empirical calibrations using a variety of techniques. I will discuss the current precision and accuracy of isochrone-based, rotation-based, and asteroseismically inferred ages, as well as some of the current challenges of each method. Finally, I will discuss some of the recent and ongoing work in my group to increase the number of stars with well-measured ages, as well as some of the interesting stellar physics puzzles we've run into along the way.

4/9/2024

Aparna Venkatesan

University of San Francisco

The Cost of Brightening Night Skies on Ground-Based Astronomy

Dramatic rises in ground-based light pollution in recent years as well
as increasingly congested low-Earth orbits are leading to brightening
night skies worldwide, with consequences reaching far beyond this
decade. I share calculations of the potentially large future rise in
global sky brightness from space objects in low Earth orbit (LEO),
including qualitative and quantitative assessments of how ground-based
astronomical observations may be affected. Debris proliferation is
especially a concern: all log-decades in debris size may contribute
approximately the same amount of night sky radiance, and given the
rising risk of debris-generating events in LEO, this could lead to rapid
rises in global night sky brightness. This will in turn lead to
increased loss of astronomical data and diminished opportunities for
ground-based discoveries as faint astrophysical signals become
increasingly "lost in the noise". This will affect many constituencies
beyond professional astronomy that are reliant on dark and quiet skies,
including Indigenous communities' sky traditions, animal/bird migratory
patterns, amateur astronomy, astrotourism, human/animal circadian
rhythms, and the seasonal and pollination cycles of plants. Globally
coordinated regulatory policies and mitigation strategies are urgently
needed to protect the shared environment and intangible heritage of
space and dark skies for future generations.

4/16/2024

Jack Hare

MIT

Radiatively-cooled Magnetic Reconnection Experiments

In extreme astrophysical systems, such as black hole coronae and pulsar magnetospheres, the process of magnetic reconnection is significantly modified by strong radiative cooling. This cooling removes internal energy faster than it is injected by the reconnection process, triggering a radiative collapse in which runaway cooling and compression leads to a cold, thin, and dense reconnection layer. The high-energy X-ray radiation released is, in turn, often the only signature of reconnection in these remote astrophysical environments. To study this process, we have conducted a series of radiatively cooled reconnection experiments driven by the Z Machine at Sandia National Laboratories, the world's largest pulsed-power facility. I will present observations of the formation of brightly-emitting and fast-moving hotspots within the reconnection layer, which we interpret as plasmoids formed by the tearing instability. These hotspots emit the majority of the high energy X-rays, localising the radiation signature in space and time.

4/23/2024

Tim Bedding

University of Sydney

A Golden Age of Asteroseismology with Kepler and TESS

Asteroseismology uses the natural oscillation modes of stars to study their interiors. The wonderfully precise measurements by NASA's Kepler and TESS missions are ideal data sources for the technique. These space telescopes have been monitoring the brightness of hundreds of thousands of stars, with the main goal of discovering extra-solar planets as they transit their parent stars.  At the same time, observations of stellar oscillations have led to a revolution in asteroseismology. I will discuss some of the key results, including the use of gravity modes to probe the cores of red giant stars, the characterization of stars found to host exoplanets, and the measurement of ages for young stellar associations.

4/30/2024

Spitzer Lecturer: Brant Robertson

UCSC

Date

Name

Institution

Area of Expertise

9/5/2023

Ting Li

University of Toronto

MW observation, galactic dynamics, stream detection and characterization

Title:
The Power of Milky Way's Stellar Streams Enabled by Multi-Object Spectroscopic Surveys

Abstract:
We are entering an extremely data-rich era in the next decade, with full 6D+chemistry information on dozens of stellar streams, to shape our understanding on the chemo-dynamical evolution of the Milky Way, as well as the nature of the dark matter. In this talk, I will discuss two ongoing spectroscopic programs to study the stellar streams in our Milky Way and highlight a few latest scientific results from these two programs. The Southern Stellar Stream Spectroscopic Survey (S5), started in 2018, is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere. The science results from S5 include a homogeneous study of the kinematic and chemical properties of dozen streams in our Milky Way, the finding of a stream at ~30 kpc possibly perturbed by the dark matter subhalo, the constraints on the mass of the Milky Way and the Large Magellanic Cloud with stellar streams, and the discovery of the fastest hyper velocity stars ejected from Galactic center that can be used to study the shape of the Milky Way halo. The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI), on the other hand, is a recently started 5-yr spectroscopic program in the Northern Hemisphere. With just the first year of data collected in 2021-2022, 4 million unique stars have been observed by DESI, including many stars in the streams of the northern sky (e.g. GD-1) and showing some interesting features as well as the streams beyond the Milky Way.

9/12/2023

Will Farr

CCA / Stony Brook

Gravitational waves, compact object evolution, gravitational dynamics of exoplanets and dense stellar systems

Title: Cosmology and Fundamental Physics from Stellar Mass Binary Black Holes

Abstract: The first three observing runs of the LIGO, Virgo, and KAGRA gravitational wave detectors produced a wealth of "firsts," including the first observation of multiple "ringdown" modes from a black hole---the remnant of the first-ever binary black hole merger, GW150914.  Just like an excited atom, a recently-formed black hole is expected to emit a characteristic spectrum of gravitational radiation as it settles down to its equilibrium state---in general relativity, a Kerr spacetime.  "Black hole spectroscopy" thus provides the opportunity to directly probe these extreme spacetimes and test fundamental gravitational theories, just as atomic spectroscopy provided an ideal testbed for the new quantum mechanics of the early 20th century.  I will review the 50+ year effort toward black hole spectroscopy, including the recent detection of multiple ringdown modes in GW150914, and describe the near future of this field as LIGO, Virgo, and KAGRA's fourth observing run begins (last month!).  Along the way, I will review the rapidly changing landscape of gravitational wave astronomy, and describe some of the other exciting recent firsts in this new field, particularly the possibility of using binary black hole mergers for intrinsically-calibrated distance measurements to trace cosmic expansion.

9/19/2023

Maura McLaughlin

WVU

PTAs, NANOGrav

Title: Pulsar Timing Arrays: A New Window on the Gravitational Wave Universe

Abstract: Millisecond pulsars are rapidly rotating neutron stars with phenomenal rotational stability. Pulsar timing arrays world-wide monitor over 100 of these cosmic clocks in order to search for perturbations due to gravitational waves at nanohertz frequencies. The tell-tale sign of a stochastic background of gravitational waves in pulsar timing data is the presence of quadrupolar spatial correlations. Recently, and for the first time ever, pulsar timing array collaborations have found evidence of these spatial correlations in multiple independent pulsar datasets.  The signal is consistent with that expected from an ensemble of supermassive black hole binaries, but could also be attributable to more exotic sources, such as cosmic strings or early universe inflation. I will describe these experiments and the most recent results, concentrating on those from the NANOGrav collaboration, and will discuss the increases in sensitivity expected from the combination of data observed with new and existing telescopes across the globe.

9/26/2023

Alessandra Corsi

Texas Tech

Multi-messenger time-domain astronomy, relativistic radio transients, gravitational wave physics

Title: Multi-messenger observations of cosmic collisions and explosions: progenitors, relativistic ejecta, and remnants

Abstract: The births and mergers of neutron stars and black holes, the most exotic objects in the universe, can launch the fastest cosmic jets (gamma-ray bursts; GRBs) and shake the very fabric of space-time with gravitational waves. GW170817, the merger of two neutron stars witnessed through both its gravitational wave siren and its glow at all wavelengths of light, marked the beginning of a golden age in multi-messenger astrophysics. While the GW siren of GW170817 has directly confirmed that at least some short GRBs originate from NS-NS mergers, the smoking gun linking long GRBs to core collapses i.e., the association with stripped-envelope core-collapse supernovae, has been challenged by several electromagnetic (EM) observations. In this context, I will discuss how observations at radio wavelengths can probe the ejecta and environments of compact binary mergers and extreme core collapses and help unveil their progenitors and remnants. I will then highlight opportunities and challenges ahead, as new observational facilities will transform a trickle of multi-messenger discoveries into a flood.

10/3/2023

Ralph Schoenrich

UCL

Galactic dynamics, Gaia

Title: Spinning into darkness: the slowing bar and Galactic structure

Abstract: 

In the inner Milky Way stars trek together in a bar-shaped, rotating overdensity: the central bar, which dominates the inner disc out half-way to the Sun. The rotation velocity/pattern speed of this bar has been intensely debated. The best insight into the bar's motion are gained from its resonant effects on the Milky Way disc: Similar to Jupiter holding in resonant motion Solar System asteroids like Greeks and Trojans, some stars near the Sun are locked in resonant motion, e.g. in corotation resonance circling the Lagrange points on the bar's minor axis. When the bar slows, these resonances move outwards through the Galaxy dragging bound stars along. Along their path they can pick up new stars. The corotation resonance is enhanced by this process, explaining the high contrast of the observed Hercules stream. My group has used this to measure for the first time the slowing of the bar's rotation. We have also resolved long-term issues with bar pattern speed measurements. This also promises new insights into the Milky Way's history. Most importantly, the bar's slowing can only be explained with angular momentum loss to the Milky Way's dark matter halo, thereby proving its inertial mass.

10/10/2023

Kevin Schlaufman

JHU

Planet formation, stellar populations

10/17/2023

Fall Break

10/24/2023

Karen Masters

Haverford College

Galaxy observation, spiral structure, bars

10/31/2023

Nikhil Padmanabhan

Yale

Inflation, dark energy, fuzzy dark matter

Title: Baryon Acoustic Oscillations with Galaxy Surveys: Present State and Some Future Prospects

Abstract: I will weave three separate threads. The first will be to describe recent and ongoing results from the Dark Energy Spectroscopic Survey. I will present the BAO results from the early DESI data, and some of the preparatory work for the Year 1 data. I will then present a new approach to reconstruction based on optimal transport, highlighting some recent results and ideas for future applications. I will end by discussing merging perturbative reconstruction ideas with convolutional neural networks, and a possible new application to primordial non-gaussianity.

11/7/2023

Meredith MacGregor

JHU

Multi-wavelength observations of debris disks, planetary formation, habitability

Title: How to Form a Habitable Planet

Abstract: Planets form from disks of dust and gas surrounding young stars.  As they grow, these new planets inherit their chemical composition from the surrounding material and then sculpt it through gravitational interactions to form gaps and other asymmetric structures.  In the last decade, the Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionized our ability to study planet formation, allowing us to examine this process in high resolution.  I will present highlights from ongoing work using ALMA and other facilities that explores how planetary systems form and evolve by (1) connecting disk structure to sculpting planets and (2) understanding the impact of stellar flares on planetary habitability.  Together these results provide an exciting foundation to investigate the evolution of planetary systems as a whole through multi-wavelength observations.  In the future, new facilities, specifically in the far-infrared, will help complete our understanding by tracing the chemistry of water and other volatiles critical for life.

11/14/2023

Fabian Schmidt

MPA

Cosmology

Title: How much (robust) cosmological information can we obtain from galaxy clustering?

Abstract: All large-scale structure cosmologists are faced with the question: how do we robustly extract cosmological information, such as on dark energy, gravity, and inflation, from observed tracers such as galaxies whose astrophysics is extremely complex and incompletely understood? I will describe why guaranteeing this robustness is so difficult, and how a perturbative effective-field-theory (EFT) approach offers such a guarantee when focusing on galaxy clustering on large scales. The natural next question then is: how much cosmological information is left on these large scales if we marginalize over all the free parameters introduced in the EFT? To answer this question, I will introduce our implementation of the EFT on a lattice as an explicit field-level forward model, which can be used both for full Bayesian inference at the field level and for likelihood-free inference based on summary statistics. One crucial advantage of this forward model is the non-perturbative treatment of the displacement from initial positions to observed coordinates, with ramifications for BAO reconstruction and redshift-space distortions.

11/21/2023

Masahiro Takada

IPMU

Cosmology, LSS, weak lensing

Title: Subaru Hyper Suprime-Cam Year 3 Cosmology results: S8 tension?

Abstract: We used more than 25 million galaxies in the Subaru Hyper Suprime-Cam (HSC) shear catalog in the redshift range up to z~1.5 to measure weak lensing distortion effects due to large-scale structures. We used the measured weak lensing signals to perform a blinded cosmology analysis to measure the cosmological parameters of the flat LambdaCDM model. To obtain a robust constraint on the cosmological parameters, we employed an uninformative flat prior on a possible residual systematic error in the mean redshift for  HSC galaxies at z>1. As a result, we were able to measure the “S8” parameter at a 4% accuracy (sigma(S8)~0.04), but the central value exhibits about 2.5sigma tension with the Planck inferred S8 value. Our results indicate a non-zero residual error in the mean source redshift compared to the photometric redshift estimates for the HSC galaxies at z>1. In this talk, I will discuss the HSC cosmology results, and, if time is allowed, present the current status of the upcoming Subaru Prime Focus Spectrograph project, which promises significantly improvement of the HSC cosmology results. 

11/28/2023

Janice Lee

STScI

ISM, star formation, PHANGS-JWST

12/5/2023

Michael Boylan-Kolchin

UT Austin

Galaxy formation

How magnetized turbulent plasmas can accelerate charged particles to high energies represents a long-standing question with far-reaching implications for high-energy and multi-messenger astrophysics. It indeed goes back to the seminal works of Enrico Fermi (1949, 1954) and nowadays, it is commonly invoked to model the generation of non-thermal particle spectra in a broad variety of astrophysical sites, including extreme, relativistic sources. In particular, it has recently been considered as a possible origin for the high-energy neutrinos seen by Ice Cube in the direction of nearby active galactic nuclei. 
Our understanding of particle acceleration in turbulent plasmas has known substantial progress in recent years, mostly spurred by large-scale, kinetic numerical simulations. This talk will address those developments and discuss a theoretical picture to describe the physics at play, based on non-resonant interactions between particles and velocity structures. This model, which can be seen as a modern implementation of the original Fermi scenario, appears supported by recent numerical simulations of turbulence in the semi- and fully-relativistic regime. It also brings to light an interesting connection between the properties of intermittency of the turbulence and the spectrum of accelerated particles. I will discuss those features then conclude with some possible applications and extensions.

Searching for Cosmological Concordance with New Physics in the Dark
Sector: Hints and Challenges

I will discuss recent and ongoing work focused on attempts to restore concordance amongst cosmological data sets, motivated by discrepancies amongst some inferences of the cosmic expansion rate (H_0) and the matter clustering amplitude (S_8).  I will explain why the most viable models to resolve the H_0 problem invoke new physics at or prior to the last scattering epoch.  Such models include modified recombination scenarios, quasi-accelerating early dark energy (EDE) models (and extensions thereof, featuring EDE-dark matter interactions), or scenarios featuring new light particles with non-trivial interactions. 

I will present constraints on such scenarios derived using data from the Atacama Cosmology Telescope (ACT), the Planck satellite, and large-scale structure surveys.  I will highlight newly obtained constraints on EDE models derived from Lyman-alpha forest data, which severely hinder the ability of this scenario to resolve the H_0 problem.  I will conclude with a look ahead to forthcoming CMB analyses from ACT, which will provide a powerful test of these scenarios in the low-noise, high-resolution regime.

Lía Corrales (Virtual)
University of Michigan

New Frontiers of Short Wavelength Exploration: From Astromineralogy to Exoplanet

View presentation here.

Uncovering the physics of galaxy evolution has been a longstanding problem for astronomers.  Physical galaxy properties like the star formation rate, stellar mass, and metallicity can tell us not only how these properties change over generations of galaxies, but they also give us a window into the conditions of the universe at the time of galaxy formation.  Galaxy surveys, which are predominantly used to measure these properties, only observe the brightest galaxies, which are a biased sample.  Line intensity mapping (LIM) observes the aggregate emission from brighter and fainter galaxies over much larger volumes.  The EXCLAIM survey is a pathfinder for this technique, and has the potential to aid in constructing a full census of galaxy emission while minimizing sample variance.  In this talk I will present how we can use LIM surveys to construct a galaxy property census.  Specifically, I will discuss currently planned methods using LIM to uncover galaxy properties like molecular hydrogen density, star formation, and metallicity properties, as well as their limitations.  I will also discuss potential ways we could get around these hurdles using hydrodynamic simulations along with semi-analytic star formation models.  Finally we will discuss how these methods could be applied to EXCLAIM and other upcoming LIM surveys.

A Galaxy Property Census with Line Intensity Mapping

View presentation here.

Uncovering the physics of galaxy evolution has been a longstanding problem for astronomers.  Physical galaxy properties like the star formation rate, stellar mass, and metallicity can tell us not only how these properties change over generations of galaxies, but they also give us a window into the conditions of the universe at the time of galaxy formation.  Galaxy surveys, which are predominantly used to measure these properties, only observe the brightest galaxies, which are a biased sample.  Line intensity mapping (LIM) observes the aggregate emission from brighter and fainter galaxies over much larger volumes.  The EXCLAIM survey is a pathfinder for this technique, and has the potential to aid in constructing a full census of galaxy emission while minimizing sample variance.  In this talk I will present how we can use LIM surveys to construct a galaxy property census.  Specifically, I will discuss currently planned methods using LIM to uncover galaxy properties like molecular hydrogen density, star formation, and metallicity properties, as well as their limitations.  I will also discuss potential ways we could get around these hurdles using hydrodynamic simulations along with semi-analytic star formation models.  Finally we will discuss how these methods could be applied to EXCLAIM and other upcoming LIM surveys.

The Chiral Universe

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Among a handful of mysteries in the LambdaCDM paradigm of cosmology, I focus on three: Dark Matter, Baryogenesis and the origin of structure. I then provide a pedagogical introduction to Chiral Gravity and show how these three mysteries may be interconnected. I also discuss some observational windows including the inevitability of Superfluid Dark matter.

Learning Symbolic Equations with Deep Learning

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We develop a general approach to "interpret" what a network has learned by introducing strong inductive biases. In particular, we focus on Graph Neural Networks. The technique works as follows: we first encourage sparse latent representations when we train a GNN in a supervised setting, then we apply symbolic regression to components of the learned model to extract explicit physical relations. The symbolic expressions extracted from the GNN using our technique also generalized to out-of-distribution data better than the GNN itself. Our approach offers alternative directions for interpreting neural networks and discovering novel physical principles from the representations they learn. In particular, we will show examples of recovery of newton's law and masses of solar system bodies with real ephemeris data and recovery of navier-stokes equations with turbulence dataset. We will speculate what one can do with this new tool.

Satellite Swarms vs. Astronomy and the Night Sky

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New technology and a lack of regulation are allowing massive swarms, or "constellations", of low-Earth communications satellites such as SpaceX's Starlinks to be launched at relatively low cost, leading to a dramatic rise in the number of satellites at altitudes 300-1200 km already in orbit: over 2000 new satellites in the last 2 years, with more than 100,000 planned by 2030. The large number and the apparent brightness (by reflected sunlight) of these satellites pose serious and possibly catastrophic challenges to ground-based and even space-based astronomy and the appearance of the starry night sky even to casual observers. Radio interference due to intended and unintended emission from LEO satellite swarms further threatens to overwhelm sensitive radio telescopes including those investigating the CMB. There are national and international efforts underway to understand and try to control and respond to these major new challenges; meanwhile, rockets are launching every few days, each with 60+ more satellites, observatories and skywatchers are reporting increasing interference from satellite streaks, and the view of the night sky has already been changed.

The birth of the first massive galaxies and black holes

We are the first generation of human beings able to directly observe and study the cosmic era when the first galaxies and black holes formed. Quasars are among the most luminous sources known and can be studied in detail even during the first billion years of the Universe (at redshifts z>6). I will summarize my team's efforts to search for and characterize the most distant quasars. This has led to the discovery of the largest number of bright quasars at z>6, including the most distant radio-source known at z~7, and the three most distant quasars known at z>7.5. These distant quasars provide important clues about the build-up of the first massive galaxies and black holes, as well as the epoch of reionization. I will review the diverse range of physical properties of these quasars on different scales, including follow-up studies from X-rays to radio wavelengths.