Scientific Series

Gravitational wave observations have unveiled the population of merging black holes and neutron stars, providing direct access to strong-field gravity and dense nuclear matter. These compact binaries emit gravitational radiation as they inspiral and merge, producing waveforms that encode: the masses and spins of the constituents, neutron star's equation of state, the properties of gravity in extreme regimes, and details of their astrophysical environments. Extracting this information requires precise theoretical predictions for comparison with observations. Next-generation detectors, both ground and space-based, are expected to observe 10^3-10^5+ cycles of a given binary merger, with potentially many thousands of events per year across disparate regimes of masses, eccentricities, etc. At this scale, numerical relativity simulations alone become computationally prohibitive, making precision analytical calculations essential.

In this talk I will present new analytical approaches to the gravitational two-body problem that combine effective field theory techniques from particle physics together with tools from General Relativity. I will discuss methods for computing the dynamics of isolated binaries at high perturbative orders, as well as methods for describing binaries embedded in gaseous or dark matter environments where environmental effects modify the orbital evolution and gravitational wave signal.

I will present a complex matrix model (CMM) for the computation of protected two-point and three-point correlation functions of 1/2-BPS operators. The model can be efficiently studied in large N limit for huge operators (with dimensions ~N^2). An advantage of this approach is the direct identification of the distribution of eigenvalues of complex matrix with the shapes of droplets in Lin-Lunin-Maldacena (LLM) bubbling geometry. For correlators of two huge operators with a small SO(4) symmetric "probe" operator (with dimension N^0) we established a perfect correspondence with old computations of Skenderis-Taylor in dual LLM gravity. CMM allows efficient analytic study of certain two- and three-point correlators of huge operators of exponential and coherent state types and relates these problems to the Laplacian growth and 2d quantum gravity. We also compute explicitly the correlators of two huge operators with giant magnon operator. In a much more difficult case of 1/4- and 1/8-BPS operators we make a curious observation of equivalence of a pair correlator of such coherent state operators to the quenched Eguchi-Kawai reduction of the principal chiral model.

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.