Plenary Invited Speakers

Cosmology and Multi-Messenger Astrophysics with Gamma-Ray Bursts
Since their discovery in the late '60s, Gamma-Ray Bursts constitute one of the most fascinating and mysterious phenomena for modern science, with strong implications for several fields of astrophysics and fundamental physics. In this review, I will focus on the perspective key-role of GRBs for cosmology and multi-messenger astrophysics. Indeed, the huge luminosity, the redshift distribution extending at least up to z~10 and the association with the explosive death of very massive stars make long GRBs (i.e., those lasting up to a few minutes) potentially extremely powerful cosmological probes (early Universe, geometry and expansion rate of space-time, "dark energy" evolution). At the same time, short GRBs (lasting no more than ~1-2s) are the most prominent electromagnetic signature of gravitational-wave sources like NS-NS and NS-BH merging events, and both long/short GRBs are expected to be associated with neutrino emission. I will also report on the status of the THESEUS space mission project, aiming at fully exploiting these unique potentialities of the GRB phenomenon.
Ulugh Beg's Scientific School in Samarkand
Ulugh Beg was the grandson of Tamerlane who conquered a vast area in Transoxania and Iran around 1400. Mohammad Taraghay, best known as Ulugh Beg (lit. "Grand prince") was born in 1394 in Sultaniya (Zanjan, Iran). In 1409, he became the ruler of Samarkand where he founded a school in 1420 which is still well preserved there. Astronomy was the major subject taught in the school and Ulugh Beg gathered a group of astronomers there. He also founded an observatory in 1424 which was designed by the Iranian scholar Jamshid Kashani (al-Kashi) who upon Ulugh Beg's request, supervised the construction and operation of the observations made there. After Ulugh Beg's tragic murder arranged by his son in 1449, the observatory was destroyed and forgotten. Its remnants were rediscovered in 1908 near Samarkand. The main part of the observatory was a huge stone sextant more than 40 meters long. It measured the meridian transit of celestial bodies from which the declination of the ecliptic, the equinoxes and the geographical latitude of the locality could be determined accurately. The results of the observations were composed in a Persian treatise called Zij Ulugh Beg. Zij is a Persian word used for a collection of astronomical tables with explanations for using them in astronomical calculations. Several commentaries are written on this work and selections of it are translated into Arabic, English, French, Russian and Turkish. Ulugh Beg also devised a method for finding the sine of one degree for which he solved a cubic equation by an iterative method.


Dragging of inertial frames by matter and waves
We shall analyze three specific general-relativistic problems in which gravitomagnetism plays the important role: the dragging of magnetic fields around rotating black holes, dragging inside a collapsing slowly rotating spherical shell of dust, compared with the dragging by rotating gravitational waves (CQG 34, 205006 (2017), Phys. Rev. D 85 124003, (2012) etc). We shall also briefly show how "instantaneous Machian gauges" can be useful in the cosmological perturbation theory (Phys. Rev. D 76, 063501 (2007)). Finally, we shall mention the "Quantum Detection of Inertial Frame Dragging" (Phys. Rev. D 103, 024027 (2021)).
Frame-Dragging and its tests with Laser Relativity and Geodesy Satellites
Dragging of inertial frames, or frame-dragging, is an intriguing and fascinating phenomenon of Einstein's theory of General Relativity (GR) with relevant astrophysical implications. Some theories of gravitation, alternative to GR but in agreement with its post-Newtonian tests, predict a different result from GR for frame-dragging. However, frame-dragging tests, in agreement with GR, have been obtained with LARES (LAser RElativity Satellite), of the Italian Space Agency (ASI), successfully launched in February 2012, and with data from the LAGEOS (Laser Geodynamics Satellite), LAGEOS 2 and GRACE (Gravity Recovery and Climate Experiment) satellites. The accuracy of these tests reached a few parts in a hundred. The forthcoming ASI LARES 2 satellite, to be launched in 2021, together with data from the LAGEOS and GRACE Follow-On satellites, is aimed at frame-dragging tests with an accuracy of a few parts in a thousand.
De Mink
Planck and the H0 tension
The Hubble constant tension
An important and unresolved question in cosmology today is whether there is new physics that is missing from our current standard Lambda Cold Dark Matter (LCDM) model. A current discrepancy in the measurement of the Hubble constant, Ho, could be signaling a new physical property of the universe or, more mundanely, unrecognized measurement uncertainties. I will discuss two of our most precise methods for measuring distances in the local universe: Cepheids and the Tip of the Red Giant Branch (TRGB). I will present new results from the Carnegie-Chicago Hubble Program (CCHP. Using the Hubble Space Telescope Advanced Camera for Surveys, we are using the TRGB to calibrate Type Ia supernovae out into the Hubble flow to provide an independent measurement of Ho. I will address the uncertainties, discuss the current tension in Ho, and whether there is need for additional physics beyond the standard LCDM model.
Christopher Lee
Probes of the Progenitors, Engines and Physics behind Stellar Collapse
Understanding the nature of the stellar collapse plays an important role in a wide range of astrophysics. Stellar collapse produce the compact remnants (neutron stars and black holes) that make up a menagerie of exotic astrophysics objects from pulsars and X-ray binaries to the merging compact objects detected in gravitational waves. Stellar collapse plays an important role in most gamma-ray burst engines and their supernovae disseminate many of the heavy elements into the universe. But, to understand the true impact of stellar collapse on these phenomena, we must understand the engine driving supernova explosions. Despite recent successes supporting the convection-enhanced, neutrino-driven engine driving these explosions, we are far from a complete picture of these explosions. For example, uncertainties in stellar evolution prior to collapse and the engine itself make quantitative predictions from stellar collapse. Here we review our current understanding of stellar collapse (both the engine and its progenitor stars) and the observations (both current and proposed) to help us improve this understanding.
IceCube: Cosmic Neutrinos and Multimessenger Astronomy
IceCube detects more than 100,000 neutrinos per year in the GeV to 10 PeV energy range. Among those, we have isolated a flux of high-energy neutrinos of cosmic origin, with an energy density in the extreme universe similar to that of high-energy photons and cosmic rays. We identified their first source: on September 22, 2017, following an IceCube neutrino alert, observations by other astronomical telescopes pinpointed a flaring active galaxy, powered by a supermassive black hole, as the source of a cosmic neutrino with an energy of 290 TeV. We will review recent progress in measuring the cosmic neutrino spectrum and in identifying its origin.
The neutrino in stellar evolution and in the Sun
Motivated in part by the recent measurements of the Borexino Collaboration, I will describe the current status of solar neutrino physics, including the impact of measurements on our knowledge of neutrino properties. Questions remain about the sun's metallicity and the equivalence of its weak and electromagnetic luminosities - topics relevantly, respectively, to early solar evolution and to possible "new physics" tests involving solar neutrinos. Additional aspects of the flavor mixing first identified through solar and atmospheric neutrino experiments arise in explosive astrophysical environments, such as supernova cores, neutron star mergers, and the Big Bang. I will describe some of the associated open issues and how they affect current efforts to better characterize such environments through "multi-messenger" observations and analysis.
Laser interferometry in space
In the last decade, laser interferometry in space has advanced from planning on ground to an established technique for gravitational physics, both for the detection of gravitational waves (LISA project) as well as for global observation of the Earth gravity field (GRACE Follow-On). I will summarize the past and planned missions including LISA Pathfinder, GRACE Follow-On, Pathfinder missons in China and LISA.
James Anthony
The Southern Wide-field Gamma-ray Observatory
The Southern Wide-field Gamma-ray Observatory (SWGO), is a new project for gamma-ray astrophysics in the energy range from a few hundred GeV up to a PeV. SWGO will be a steradian field of view, 100% duty-cycle detector, surveying the southern sky and monitoring for transient phenomena, as such it complements very well the planned flagship facility the Cherenkov Telescope Array. As the first instrument of this type in the Southern Hemisphere, SWGO will ideally suited to map out the diffuse emission of the inner galaxy, the Fermi bubbles, and search for WIMP annihilation in the halo of the Milky Way. In this talk I will present the status of SWGO detector design and site search is well as give an overview of the scientific prospects.
The Hubble tension
In recent years, a determination of the Hubble constant from supernovae has become increasingly discrepant with that inferred from the cosmic microwave background. This "Hubble tension" is not easily attributable to any known systematic artifacts in either measurement and may thus be indicating some new physics beyond that in the standard cosmological model. Easy fixes based on late-time modifications to the expansion rate are elusive as they require violations of the strong energy principle and even then introduce new discrepancies. One possible explanation involves a modification to the early-time expansion history of the Universe. I will discuss the Hubble tension, the difficulties with late-time solutions, these new "early dark energy" models, and their current status.
Observations of Fast Radio Bursts
Fast Radio Bursts (FRBs) are few-millisecond bursts of radio waves coming from far outside the Milky Way. Some repeat.
Their origin is presently unknown as is whether they represent a single class of object or multiple classes.
Recently there has been tremendous observational progress on understanding. FRBs thanks to a variety of new instruments designed for their study. In this talk I review what is known about FRB observational properties, including population property distributions of both repeaters and apparent non-repeaters, as well as what is known about their host galaxies and environments.
Roy Patrick
Singularity theorems in spinning black holes
There are three regions in the Kerr spinning black hole metric, separated by the two event horizons. The outer two are probably good approximations to the corresponding regions as a real black hole forms, but the inner Kerr is not. It has to have something to generate the gravitational field outside, and that can only be a singularity since it is by definition matter free. However, even after 58 years there is no proof that singularities form inside real collapsing bodies. I believe this is because they are singularity free!
New results from testing relativistic gravity with radio pulsars
We experience a golden era in testing and exploring relativistic gravity. Whether it is results from gravitational wave detectors, satellite or lab experiments, radio astronomy plays an important complementary role. Here one can mention the cosmic microwave background, black hole imaging and, obviously, binary pulsars. This talk will concentrate on the latter and new results from studies of strongly self-gravitating bodies with unrivalled precision. I compare the results to other methods, discuss implications for other areas of relativistic astrophysics and will give an outlook of what we can expect from new instruments in the near future.
Einstein confirmed: New high precision tests of General Relativity
General Relativity (GR) is a consequence of the Einstein Equivalence Principle. Accordingly, tests of GR are either test of its foundation or test of consequences of GR. In general, tests of the foundations are zero tests. Test of predictions of GR rely on certain notions like standard clocks or nonrotating frames which can be defined within GR and which are basic in the prediction of certain numerical values for particular effects. We outline the structure of these tests and report on recent high precision laboratory tests of foundations and of consequeces of GR. At the end the importance of quantum tests of GR emphasized and the importance of fundamental tests for practical applications is outlined.
Numerical Relativity and the Interpretation of Gravitational Wave Observations
Numerical relativity simulations of compact-object binary coalescences have played an important role in the detection of gravitational wave observations and the characterization of the sources. As current detectors increase their sensitivity and future detectors join the effort, the role of numerical relativity will become more prevalent. I will provide an overview of the current status of compact-object binary simulations and discuss the challenges that numerical relativity will face in the near future imposed by gravitational wave observations.
Evolution of close binary stars and their role in the most powerful stellar explosions
The majority of massive stars is born in close binary stars, and their evolution is strongly altered by their companion star. We discuss the main mechanisms of close binary interaction, and their relevance for understanding the diversity of core collapse supernovae. Binary interaction also affects, and sometimes enables, extreme events, like hypernovae, long-duration gamma-ray bursts, superluminous supernovae, and compact object merger. We will explore the capabilities and problems of binary evolution models in predicting these events and their discrete progenitor states, and the consequences thereof.
The Transient Sky viewed through the Five-hundred-meter Aperture Spherical radio Telescope (FAST)
Inspired by the visionary efforts of building Arecibo, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) was formally established in 2007; its construction commenced in 2011; achieved first light in 2016; started normal operation in 2020. The first internationally open call-for-proposal was released in March 2021. I report here a few science highlights so far, particularly from the Commensal Radio Astronomy FAST Survey (CRAFTS), which is an unprecedented large-scale commensal radio survey enabled by a several novel techniques. CRAFTS simultaneously records pulsar, Galactic HI, extra-galactic HI, and transient data streams. CRAFTS has discovered more than 150 new pulsars, including more than 40 MSPs, more than 20 binaries, and at least one DNS system. We have imaged about 5% of the full sky in HI, including the Lockman hole, the Orion region, etc. CRAFTS has also resulted in 6 new FRBs, including one high DM repeater that has since been localized and is shown to be the FRB with the largest fraction of local host DM. Other dedicated programs have provided the stringiest limit on the radio flux of the Galactic FRB source, the first evidence of 3D alignment between a pulsar's spin axis and spatial velocity, the most radio faint pulsar through a search of unassociated Fermi sources, etc.
The Latest Progress of PandaX - a deep underground liquid xenon observatory
The dark matter and neutrinos are keys to the formation and evolution of the universe. Yet we do not know what is dark matter, and we do not fully understand the fundamental properties of neutrinos. PandaX is an underground xenon-based observatory located in the world deepest China Jinping Underground Laboratory in Sichuan, China. The current phase of PandaX consists of a 4-tonne scale dual-phase xenon time-project-chamber detector, and we plan to carry out a wide range of studies in dark matter searches, Majorana neutrinos, astrophysical neutrinos, etc. In this talk, I will present the latest progress of PandaX, and give an outlook of its future.
Testing LCDM on small scales with cluster lenses
The key elusive cosmic constituents - dark matter, dark energy and black holes - play a fundamental role in shaping the visible universe. In this talk, I will discuss the current status of our understanding of the distribution of dark matter on small-scales in LCDM and the key open questions. Gravitational lensing by clusters of galaxies offers a powerful way to map dark matter and the high quality of current data permits detailed comparison with simulations of structure formation. Deep Hubble Space Telescope data in combination with ground-based follow-up spectroscopy permit the construction of high-resolution lensing derived maps of dark matter that can be used to stress-test the LCDM model. I present results from recent work that reveal tensions between the predictions of the standard cold dark matter theory and observations of cluster lenses.
New Determination of the Hubble Constant with Gaia EDR3, Further Evidence of Excess Expansion
The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. A streamlined distance ladder constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide <2% precision and offer the means to do much better. By steadily improving the precision and accuracy of the Hubble constant, we now see evidence for significant deviations from the standard model, referred to as LambdaCDM, and thus the exciting chance, if true, of discovering new fundamental physics such as exotic dark energy, a new relativistic particle, or a small curvature to name a few possibilities. I will review recent and expected progress, most recently based on measurements from Gaia EDR3 released in December, 2020.
Makoto S.
Martin C.
Exploring the dynamic X-ray universe with the Einstein Probe mission
Time-domain astrophysics has been revolutionised by the advent of the multi-wavelength and multi-messenger era. The Einstein Probe (EP) is a space mission designed to discover and characterise high-energy transients and to monitor source variability in the X-ray band. Its large field-of-view telescope equipped with the micro-pore optic will carry out high-cadence all-sky monitoring survey with unprecedented sensitivity in the previously poorly monitored soft X-ray band. It has also the capability of quick and deep onboard follow-up observations and good source localization in X-ray. Currently in the development phase, EP is a project of the Chinese Academy of Science (CAS) with the participation of European Space Agency and Max Planck Institute for extraterrestrial Physics. In this talk I will introduce the Einstein Probe mission, and discuss its main science goals in the field of cosmic high-energy transients.
The Physical Mechanisms of Fast Radio Bursts
Fast radio bursts (FRBs) are cosmological millisecond-duration bursts in the radio band. The recent detection of the Galactic FRB 200428 suggests that magnetars can produce FRBs. In this talk, I will review the current understanding of the physical mechanisms of FRBs in reference of two related astrophysical phenomena, namely, radio pulsars and gamma-ray bursts. I will discuss the observational evidence in favor of FRB emission involving a neutron star magnetosphere. Some ideas and issues of various radiation mechanisms for FRBs and the associated X-ray bursts within the magnetar framework will be critically discussed. Several open questions in the field regarding repeaters vs. non-repeaters and whether there are engines other than magnetars will be presented.
Highlights of Insight-HXMT and perspectives of eXTP
Insight-HXMT is China's first X-ray astronomy satellite and was successfully launched on June 15th, 2017. It carries three sets of collimated X-ray instruments with large effective areas in 1-250 keV. In addition, it can also serve as a nearly all-sky monitor for high energy sources between 0.2 to 3 MeV, such as bright pulsars and gamma-ray bursts. I will review some highlights of the scientific results of Insight-HXMT. I will also briefly introduce the enhanced X-ray Timing and Polarimetry (eXTP) mission, a large China-Europe collaboration, currently in Phase B and planned for launch in around 2027. eXTP will open a new era in exploring the extreme universe.