E&M

Photon Rings

The optical appearance of black holes has a long history (see, for instance, [Luminet, 1979; Darwin, 1959; Bardeen,1973; Ohanian, 1987; Falcke, Melia, & Agol, 2000]) and the potential to be used to test general relativity (see, for instance, Refs. [Takahashi, 2005; Johannsen & Psaltis, 2010; Gralla, Lupsasca, & Marrone, 2020] and references therein) in the strong and non-dynamical regime. General relativity predicts that a black hole image should consists of a sequence of individual rings, ``photon rings,'' encircling the central brightness depression. Each ring consists of a lensed image of the main emission indexed by the number n of photon half-orbits executed around the black hole.

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Simulated data based on general relativistic magnetohydrodynamic (GRMHD) simulations have shown that the photon rings are persistent sharp features that should dominate time-averaged interferometric observations on long baselines [Johnson et al., 2020].

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In Ref. [Gralla, Lupsasca, & Marrone, 2020], a first step toward addressing the concept and viability of such a consistency test with the photon rings was taken. I am currently exploring these ideas! I am currently developing numerical tools (AART) to tackle this exciting and ambitious research agenda of understanding strong-gravitational phenomena.
Our Adaptive Analytical Ray-Tracing (AART) code is a numerical framework that exploits the integrability properties of the Kerr spacetime to compute high-resolution black hole images and their visibility amplitude on long interferometric baselines. We have used this code to show that the first photon ring does admit a precise angle-dependent diameter in visibility space, for which the Kerr metric predicts a specific functional form that tracks the critical curve [Cárdenas-Avendaño & Lupsasca, 2023]. We found that a measurement of this interferometric ring diameter is possible for most astrophysical profiles, paving the way for precision tests of strong-field general relativity via near-future observations of the first photon ring.

GW

Extreme mass-ratio inspirals

The complex dynamics of black holes present a profound challenge to our models and understanding of the Universe. Supported by a plethora of measurements and experiments [Will, 2014], most of these models assume Einstein’s general relativity (GR) as the underlying theory of gravity to describe the geometry around them. Despite these agreements, there is a consensus on a need to modify GR for observational (e.g., the late-time acceleration of the universe) and theoretical (e.g., its incompatibility with quantum mechanics) reasons [Berti et al., 2015]. If nature does deviate from GR, how much systematic error accrues by assuming GR is correct?

With the Laser Interferometer Space Antenna (LISA), a future space-based gravitational-wave detector, we will have an unprecedented opportunity to perform precision tests of GR [Arun et al. (incl. A. Cardenas-Avendano as a first-tier author), 2022], by carefully monitoring the phase of inspiraling compact objects, obtained from the gravitational waves (GWs) emitted when a small black hole falls into a supermassive or intermediate-mass black hole, in an extreme mass-ratio inspiral (EMRI).

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My research focuses on developing analytical tools for analyzing and interpreting deviations from GR with the LISA data.