ICTS

For the first time in history, we have captured images of black holes, allowing us to study their event horizons—the very boundaries beyond which nothing escapes. This achievement, realized through the Event Horizon Telescope (EHT), was the result of two decades of innovation, collaboration, and technological advancements. By linking radio telescopes across the globe, we created an Earth-sized observatory capable of resolving the shadows cast by supermassive black holes. These images have confirmed Einstein’s predictions, tested gravity in extreme conditions, and provided unprecedented insights into black hole physics. But this is just the beginning. Future advancements in imaging techniques, additional telescopes, and space-based interferometry will sharpen our view and allow us to explore black holes across the universe. As we push the boundaries of observation, we will uncover even more mysteries about these enigmatic cosmic giants and their role in shaping galaxies.

Readings:
Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole, GRAVITY collaboration, https://www.aanda.org/articles/aa/pdf/2020/04/aa37813-20.pdf
Testing the No-hair Theorem with Observations in the Electromagnetic Spectrum. III. Quasi-periodic Variability, by Johannsen & Psaltis, https://iopscience.iop.org/article/10.1088/0004-637X/726/1/11/pdf
Testing the No-hair Theorem with Observations in the Electromagnetic Spectrum. IV. Relativistically Broadened Iron Lines, by Johannsen & Psaltis, https://iopscience.iop.org/article/10.1088/0004-637X/773/1/57/pdf
Testing Gravity with Black Hole X-Ray Data, by C. Bambi, https://arxiv.org/pdf/2210.05322

Stationarity & vacuum versus real universe

Matter around BHs: circular motion, stability, unstable light rings and their decay time. Cross-section for absorption

Spinning black holes: the static limit and the ergoregion, energy extraction

Testing the Cosmic Censor

Forming black holes: the Hoop conjecture

Gravitational wave signals from merging compact binary systems provide unique avenues for testing the general theory of relativity (GR) predictions in the strong-field, dynamic, and relativistic regimes. As the gravitational wave (GW) transient catalogue grows in number and source diversity, designing and implementing tests for these predictions in model-agnostic ways and for specific alternative theory predictions is mandatory. In this presentation, I will introduce the routinely employed model-agnostic tests of the nature of compact objects using the observed binary population and implications of such constraints to alternative models. Then, I will describe theory-specific tests of the nature of compact objects, focusing on spinning black holes in modified GR theories and discuss the possible constraints on such models from current and future GW observations.

- BH mimickers: classifications and open problems.
- Buchdhal's theorem and how to evade it
- Maximum compactness in GR and beyond
- Models of BH mimickers*:
    - Boson stars
    - Anisotropic/elastic stars
    - Fuzzballs/BH microstates

- BHs vs mimickers: no-hair theorems, breaking of axial/equatorial symmetry
- GW modelling*

* only basic aspects, room for tutorials. The goal would be to see how the quadrupole moment enters the waveform of a binary

- Tidal Love numbers
- Tidal heating
- Modeling tidal effects for BH mimickers
- GW modelling*

* only basic aspects, room for tutorials. The goal would be to see how the Love number enters the waveform of a binary

- Perturbation theory and role of the boundary conditions
- Echoes: theory and modelling
- EMRIs
- Ringdown tests with GWs: present and future
- Ergoregion instability