Plenary Sessions

09:00-09:40Dark matter revealed by the first stars? (Rennan Barkana)
09:40-10:20High-energy cosmic rays: origins and connections (Martin Lemoine)
10:20-11:00Gamma-ray Astronomy (Ulisses Barres de Almeida)
11:30-12:10Open Problems in Cosmology (Joe Silk)
12:10-12:30Texas 2021 announcement / Final remarks and closure


Monday 16/12

Testing General Relativity with the First EHT Black Hole Images


Dimitrios Psaltis (Astronomy, University of Arizona)

The Event Horizon Telescope recently revealed the first picture of the black-hole shadow in the center of the M87 galaxy. I will discuss the technological and theoretical advances during the last decade that led to this result. I will then focus on how this picture allows us to accurately measure the mass of the black hole and test Einstein’s theory of General Relativity.

Supermassive black holes: A window onto the relativistic Universe


Christopher Reynolds (Institute of Astronomy, University of Cambridge)

Although it has been more than 50 years since the identification of the first active galactic nucleus (AGN), supermassive black holes (SMBHs) are still a focus of modern astrophysics. With developments in both X-ray astronomy and sub-mm Very Long Baseline Interferometry, SMBHs are finally yielding the long-promised window on strong gravitational physics. In this presentation, I shall review recent developments in SMBH studies, focusing on probes of strong gravitational physics (including black hole spin), the existence of event horizons, and the use of black holes to probe particle physics beyond the Standard Model. I shall end with a brief look at future prospects in this field.

On the role of magnetic fields in powering relativistic jets: a lesson from Galactic X-ray binaries


Elena Gallo (University of Michigan)

To fully understand the nature of exoplanets, their atmospheric evolution must be studied along with their orbital history: where did a planet form ? when did it migrate close to its star and start losing its atmosphere? The orbital architecture of planetary systems can be used to trace back in time their orbital evolution. Yet most measurements have been obtained for Jupiter-size planets around early-type stars, limiting our ability to assess the impact of migration on a large variety of planets. I will show how the development of a new technique opened the measurement of orbital architectures to new classes of planetary systems, and allowed us to unveil a possible interplay between long-term atmospheric and orbital evolution for planets at the edge of the desert.

High-energy astronomy with neutrinos: moving towards the planetary scale


Elisa Resconi (TU Munich)

In this presentation I will talk about the recent results obtained with the IceCube South Pole Neutrino Observatory and in particular about the first association between high energy neutrinos and an astronomical source. The recent progress motivates a future phase of IceCube as well as a new project, the Pacific Ocean Neutrino Experiment (P-ONE). P-ONE is a new initiative which aims to redevelop ocean-based neutrino telescopes by harnessing one of the largest comprehensive ocean observing infrastructures in the world, Ocean Networks Canada. A test-bed of sensitive optical instruments have been deployed and operated at ONC’s Cascadia Basin site, at a depth of 2.6km, providing the crucial initial site information needed to develop of a full-scale telescope. We present here the first conceptual study of P-ONE and its role within the planetary network of neutrino telescopes under development.

Tuesday 17/12

Two years of Observations of a neutron star merger across the electromagnetic spectrum


Raffaella Margutti (Physics and Astronomy/CIERA, Northwestern University)

GW170817 is the first cosmic event observed with gravitational-waves and light. I will provide an overview of the observations of the neutron-star merger event GW170817 across the electromagnetic spectrum during its 2.5 years of evolution, focusing on the most critical aspects of the interpretation.

Binary neutron stars: Einstein’s richest laboratory


Luciano Rezzolla (Institute for Theoretical Physics, Goethe University Frankfurt)

I will argue that if black holes represent one the most fascinating implications of Einstein’s theory of gravity, neutron stars in binary system are arguably its richest laboratory, where gravity blends with astrophysics and particle physics. I will discuss the rapid recent progress made in modelling these systems and show how the inspiral and merger of a binary system of neutron stars is more than a strong source of gravitational waves. Indeed, while the gravitational signal can provide tight constraints on the equation of state for matter at nuclear densities, the formation of a black-hole–torus system can explain much of the phenomenology of short gamma-ray bursts, while the the ejection of matter during the merger can shed light on the chemical enrichment of the universe.

Exploring the Neutron Star Interior through Transient Low-Mass X-Ray Binaries


Dany Page (Instituto de Astronomía, Universidad Nacional Autónoma de México)

I will present recent progress on our long-term study of the thermal response of neutron stars to long phase of accretion in low-mass X-ray binaries. During the accretion phase, the crust of the neutron star is strongly heated and most of this heat flows into the core. During the quiescence phase, the star relaxes back to thermal equilibrium and observation of this phase allows us to map the physical properties of the stellar crust. Long term evolution also gives information about the core properties as its neutrino emission efficiency and its specific heat. Evidence for very fast neutrino emission from a Direct Urca process has emerged in a few cases and recent constraints on the total stellar specific heat become comparable to theoretical expectations and may soon, with more data, provide relevant constraints on the nature of dense matter.

As a dessert I will show preliminary results of interpretation of the recent identification of the neutron star in the SN 1987A remnant.

Secrets of the Non-linear Universe


Tom Giblin (Physics, Kenyon College)

We have no direct evidence that general relativity is wrong; every precision test is a resounding confirmation of this elegant and powerful mathematical model. Trouble is: the greatest cosmological problems of our time (likely require) us to abandon general relativity. About 95% of the Universe remains a mystery whose solution evades our abilities. I will talk about how there may still be places in general relativity that have, until now, gone unexplored. Numerical simulations are a powerful tool that can model the complex non-linear issues of general relativity on cosmological scales. I will present progress that we have made toward modeling the late Universe in its full splendor and outline where there’s hope that we can start to tackle these great questions.

Signatures of Primordial Black Holes


Juan Garcia-Bellido (Institiuto de Fisica Teorica, Universidad Autonoma de Madrid)

I will give a review on Primordial Black Holes as Dark Matter and the present status of the Gravitational Waves and Electromagnetic signatures that we are likely to use to detect or constrain PBH in the Universe.

Wednesday 18/12

GW ground based detectors: results so far and perspectives


Jess McIver (University of British Columbia, Canada)

Four years have passed since the first direct detection of gravitational waves. In this talk we will overview the main physics results achieved by LIGO and Virgo so far, give an outlook on the evolution of the gravitational wave ground based detectors in the mid and long term and discuss the scientific perspectives of the “3rd generation” interferometers.

Probing the fundamental physics of matter with gravitational waves from binary inspirals


Tanja Hinderer (University of Amsterdam)

The recent detections of gravitational waves from merging black holes and neutron stars have established gravitational waves as a new cosmological messenger. Gravitational waves from binary systems further provide a unique window onto matter in unexplored regimes, ranging from the ultra-dense nuclear matter in neutron stars to the potential to discover exotic compact objects comprising scalar field condensates or possibly certain types of quantum gravity corrections to black holes, or dark matter or other ultralight particles from beyond standard model physics surrounding such binaries. Extracting this information from the observed signals requires a detailed theoretical understanding and accurate modeling of matter effects in relativistic binaries. During a binary inspiral, a relatively clean regime where matter effects are small but cumulative, there are a number of general effects associated with the presence of matter and having characteristic parameters that reflect its properties. I will discuss important examples of these phenomena and outline aspects that are generic versus specific to the kind of matter considered. I will conclude with an outlook onto the remaining challenges and exciting prospects for the next years as gravitational waves continue to move towards an era of precision physics.

Pushing frontiers with the Laser Interferometer Space Antenna, LISA


Elena Maria Rossi (Leiden Observatory, Leiden University)

The Laser Interferometer Space antenna (LISA) is a revolutionary large ESA mission for unprecedentedly detect gravitational wave in space, opening a completely unexplored frequency band. LISA’s launch is expected for 2032. In my talk, I will review the mission history and concept and the broad range of science that its data releases will enable ranging from astrophysics, to cosmology and fundamental physics. I will finish by describing the structure of the LISA consortium and in particular of the LISA Science Group, of which I am deputy team leader.

Can Stars Form 50-100 Solar Mass BHs?


Raphael Hirschi (School of Physical and Chemical Sciences, Keele University)

Black holes (BHs) observed in HMXBs are in the mass range circa 5 to 20 solar masses. LIGO and Virgo Gravitational Wave observatories increased the observed BH mass distribution up to about 50 solar masses (masses before merger). An upper limit around 50 solar masses is also predicted from stellar evolution modelling including the pulsation-pair instability. The discovery of LB-1, with an inferred BH mass of 68 (+11/-13) solar masses, was thus a major surprise and it questions theoretical predictions. In this talk, I will first review how very massive star evolve and the standard theoretical predictions. I will then discuss the key uncertainties in their modelling and whether or not stars can form BHs in the 50-100 solar mass range.

Cosmic Concordance and Tensions


Antony Lewis (University of Sussex)

Cosmological measurements from the cosmic microwave background, large-scale structure, lensing, supernovae and other data are now able to constrain multiple cosmological parameters to percent-level precision within in the context of the standard Lambda-CDM cosmology. Disagreements between these measurements assuming Lambda-CDM could provide strong evidence for beyond-Lambda-CDM physics. I review the status of current measurements and their agreement (or otherwise) within the standard cosmological model. I’ll mention some possible types of model extensions that could help to resolve tensions and how new physics might be pinned down by forthcoming data.

Thursday 19/12

A new symmetry principle for quantum spacetime


Astrid Eichhorn (CP3-Origins (U of Southern Denmark) & Heidelberg University)

Scale-symmetry is providing a new guiding principle in the understanding of the quantum structure of spacetime, most importantly in the asymptotic-safety approach to quantum gravity. I will review the key idea of the approach, the evidence for its viability and highlight open questions, before presenting work aimed at bridging the gap between the quantum-gravity regime and phenomenology, both in particle physics as well as for black holes.

Testing gravity theories using gravitational waves


Takahiro Tanaka (Department of Physics, Kyoto University)

Verification of general relativity in strong gravitational fields around black holes and neutron stars became possible by observations of gravitational waves. Since the test of general relativity is one of the major target of gravitational wave observation, it is already studied by by the LIGO/Virgo collaboration. However, since the gravitational wave signal is weak, it is possible to conduct a more precise test by conducting analysis dedicated to particular types of deviations from general relativity. I would like to introduce some results of such data analysis. Also, the possible detection of gravitation wave echo after merger has been reported. We studied the expected gravitational wave waveform when we assume the reflecting boundary outside the horizon and reanalyzed the data, finding no significant signal of echoes.

The speed of Gravity


Claudia de Rham (Physics, Imperial College London)

The recent direct detection of gravitational waves marks the beginning of a new era for physics and astronomy with an opportunity the probe gravity at its most fundamental level and have already been used to successfully constrain or rule out many effective field theories relevant for cosmology. I will discuss the strengths and limitations of these constraints and explore other complementary approaches in segregating between various effective field theories.

Cosmology with Large Galaxy Surveys


Elisabeth Krause (University of Arizona)

The accelerated expansion of the Universe is the most surprising cosmological discovery in decades. It has inspired a newgeneration of ambitious surveys to determine the fundamental nature of this acceleration. I will introduce the different measurement techniques used by today’s cosmologists, describe the landscape of current and near future wide-field galaxysurveys, and present cosmology constraints from the Dark Energy Survey. This analysis constrains the composition andevolution of the Universe through a combination of galaxy clustering, galaxy-galaxy lensing, and cosmic shear. These threemeasurements yield consistent cosmological results, and in combination they provide some of the most stringent constraints oncosmological parameters. I will describe the validation of measurements and modeling from pixels to cosmology and I will give anoutlook on cosmology analysis plans and challenges for future, much larger experiments such as LSST and WFIRST.

Coupling the atmospheric and orbital evolution of exoplanets: a journey across the desert


Vincent Bourrier (Department of Astronomy, University of Geneva)

More than 4000 exoplanets have been found, half of which orbit in less than 10 days around their star. These close-in planets range from small rocky objects to gas giants larger than Jupiter, putting in perspective the formation of the Solar system. Addressing the diversity of planetary systems is tied to the study of the “hot Neptunes desert”, a lack of Neptune-size planets on very short orbits whose origins remain uncertain.
The intense irradiation that close-in planets receive from their star can lead to a dramatic expansion of their upper atmosphere, and its ‘evaporation’ into space. Ultraviolet observations reveal the extended exospheres formed by the escaping gas as it occults the star, and I will show how the discovery of giant exospheres around warm Neptunes at the edge of the desert changed our view of atmospheric escape. The development of new tracers of evaporation at optical/infrared wavelengths opens thrilling perspectives to understand this mechanism and its role in shaping the desert.
To fully understand the nature of exoplanets, their atmospheric evolution must be studied along with their orbital history: where did a planet form ? when did it migrate close to its star and start losing its atmosphere? The orbital architecture of planetary systems can be used to trace back in time their orbital evolution. Yet most measurements have been obtained for Jupiter-size planets around early-type stars, limiting our ability to assess the impact of migration on a large variety of planets. I will show how the development of a new technique opened the measurement of orbital architectures to new classes of planetary systems, and allowed us to unveil a possible interplay between long-term atmospheric and orbital evolution for planets at the edge of the desert.
These new approaches to the characterization of atmospheric and orbital properties, combined with the servicing of new-generation instruments, will allow our community to explore with unprecedented details the processes that shaped the hot Neptune desert and the exoplanet population.

Friday 20/12

Dark matter revealed by the first stars?


Rennan Barkana (School of Physics and Astronomy, Tel Aviv University)

The cosmic radio spectrum is expected to show a strong absorption signal around redshift 20 that corresponds to the rise of the first stars; specifically, the stellar radiation turns on 21-cm absorption by atomic hydrogen. The EDGES global 21-cm experiment has detected the first such signal, finding a stronger absorption than the maximum expected. This absorption can be explained by invoking excess cooling of the cosmic gas induced by an interaction with dark matter. This would have far reaching consequences, including an upper limit on the mass of dark matter particles that conflicts with the expectations for WIMPs. Specific particle physics models are highly constrained, but observations will decide. We will discuss possible tests and alternative explanations.

High-energy cosmic rays: origins and connections


Martin Lemoine (Institut d’Astrophysique de Paris)

The acceleration of charged particles in powerful astrophysical sources, up to the highest energies, represents a central question in modern high-energy, multi-messenger, and relativistic plasma astrophysics. Those accelerated particles can indeed populate the cosmic ray spectrum, but they may also interact with ambient fields to produce secondary electromagnetic radiation and neutrinos. All three messengers offer unique probes of the very high energy Universe, and all three are being actively pursued by large-scale experiments and observatories. This presentation will address recent progress on the theoretical modeling of particle acceleration in extreme conditions and its astrophysical consequences.

Gamma-Ray Astronomy


Ulisses Barres de Almeida (High Energy Physics Division, Brazilian Center for Physics Research (CBPF))

This year marks the 30th anniversary of very-high-energy gamma-ray astronomy, following the long-sought detection of the first Teraelectronvolt astrophysical source — the Crab nebula. The commemoration could hardly happen under better circumstances, as we prepare for the start of activities of the Cherenkov Telescope Array (CTA), that will mark an important transition in the field as its first open observatory. The scientific context of the field, also, is richer as seldom before. Gamma-ray astronomy finds itself right at the heart of the multi-messenger revolution, and the ground-based instruments have finally reached term in their quest for a Holy Grail of the VHE domain: the first ground-based detections of gamma-ray bursts. In this talk I plan to present a review of the latest major results in the field, trying also to sketch where future directions will lie with the new instrumentation of the coming decade. They promise to be some “Roaring Twenties” for the field.

Open Questions in Cosmology


Joseph Silk (Institute d’astrophysique de Paris, CNRS)

I will address some of the open questions in cosmology, spanning the nature of dark matter and the quest for understanding dark energy. Phenomenological models establish a robust framework around which a number of fundamental issues remain unresolved. In order to make further progress, what is our optimal choice of future strategy?