Scientific Series

Compact objects like neutron stars and black holes host extreme conditions that cannot be replicated in terrestrial laboratories or even by the most ambitious future particle colliders. These conditions make them unparalleled natural laboratories for exploring new physics. Neutron stars stand out due to their incredibly strong electromagnetic fields, some of the most intense in the universe. This makes them ideal for testing the limits of electromagnetism and for probing physics beyond the Standard Model, including axions and dark photons, which are among the leading dark matter candidates. In this talk, I will highlight recent advancements in using pulsars to detect axions and discuss ongoing efforts to probe axions using magnetars, ultra-magnetic neutron stars that are the most magnetic objects in the Universe.

The calm of the quiescent sky is regularly interrupted by bursts of light that can outshine the entire galaxy. These powerful explosions are associated with mergers of compact objects, the collapse of stars, and rapidly accreting black holes, where gravitational binding energy is released and efficiently converted into radiation. The advent of electromagnetic time-domain surveys, high-energy telescopes, and gravitational-wave detectors is unveiling how matter works at these extreme conditions, offering insight into fundamental questions such as: How do compact objects form and acquire their magnetic fields? Which mechanisms launch outflows and jets? What determines the nature of the remnants they leave behind?  What are the main sites of heavy-element production?

The good books on GR agree on certain aspects of the theory that we could consider to be its “core” features. Beyond the core, however, we do not know exactly what GR should allow or not allow physically. For example, should we consider spacetimes with closed causal loops as part of physical GR or not? Are spacetimes with certain types of boundaries or singularities part of GR?  Most people expect that the answer to these sorts of questions will be answered by quantum gravity when we have it.  We can start to address them within different quantum gravity approaches even at their current partial state of their development. Within the framework of the path integral for quantum gravity and a Feynmanian heuristic for the recovery of the classical approximation, I will sketch out how answers may arise. I will use the example of causal set quantum gravity but I suggest that the ideas are applicable to other path integral approaches.

Pulsar timing arrays (PTAs) consist of a set of regularly monitored millisecond pulsars with extremely stable rotational periods that are used as precise cosmic clocks. The arrival time of pulses can be altered by the passage of gravitational waves (GWs) between them and the Earth, thus serving as a galaxy-wide GW detector. Evidence for low-frequency (~nHz) gravitational waves has recently been reported for the first time across multiple PTA collaborations, opening a new observational window into the Universe. I will discuss how pulsar observations are used to measure GWs, what we are currently learning by mapping the nanohertz GW sky, and what lies ahead following a first detection.

The AdS/CFT duality has been extremely useful in understanding quantum gravity. Tensor networks are a toy model of holography that have provided valuable lessons that have been imported to AdS/CFT. In particular, random tensor networks have a quantitative connection with fixed-area states in the gravitational path integral. This has allowed for a translation of various tensor network results to gravity that I will discuss, including an understanding of the Renyi entropy, entanglement negativity, entanglement wedges for bulk regions and resolving entropic paradoxes using wormholes.

 In this talk I'll explain how to take a well-defined flat-space limit of brane models of AdS coupled to a non-gravitating bath. In the dual BCFT this amounts to a triple-scaling limit where both the number of boundary degrees of freedom and the boundary coupling are taken to infinity while the BCFT boundary piecewise approaches a lightcone. The resulting construction offers a new approach to study the relation between the flat limit of AdS and Carrollian/Celestial CFT, as well as the potential to carry over ideas from AdS/CFT to flat space. I will discuss some implications for entanglement entropy in flat space, in particular the fact that two different notions of entanglement entropy arise on scri, depending on whether one traces out modes which have left through scri.

Galaxy clusters are visible across the electromagnetic spectrum. Observations of their structure, abundance, and evolution provide constraints to cosmology. We are in a golden age of statistical power for galaxy clusters, where observations will provide multiwavelength data for tens of thousands of galaxy clusters. However, our ability to maximize the use of clusters as cosmology probes is limited by how well we measure cluster masses and quantify systematic effects in galaxy cluster detection and characterization. I will discuss modeling efforts that enable us to test for systematics that arise from both astrophysical and observational effects.