Roman Berens

Bio

I am a theoretical physicist interested in general relativity and relativistic astrophysics, with a particular focus on black holes. I have been a postdoctoral researcher at Vanderbilt University as part of the Initiative for Gravity, Waves, and Fluids since 2023. I received my Ph.D. from Columbia University in 2022 and my undergraduate degree from Harvard University in 2016.

You can read more about my research here.

Research

I work in general relativity, with an emphasis on black hole perturbation theory. My program has three strands: (i) reconstruction of gravitational perturbations, (ii) exploration of new symmetries, and (iii) lensing and photon shell dynamics. Together, these threads reveal how black holes respond to disturbances and connect theory to predictions for gravitational-wave dectectors and horizon-scale images.

Metric Reconstruction

Metric reconstruction is the problem of recovering the perturbation of the metric from the perturbation of the Weyl scalars, i.e., turning curvature data back into data about the metric itself. We carried out the complete reconstruction of metric perturbations on spinning black holes, giving explicit formulas for the metric components. This work was featured in the Wolfram Community. We then extended this framework to black hole spacetimes with nonzero cosmological constant, preserving the essential structure while handling the added complexity. Ongoing work will apply this procedure to extremal (rapidly spinning) black holes and investigate new classes of perturbations in this regime. These results provide tools for numerical studies and for extending black hole perturbation theory beyond first order.

Symmetries and Tidal Response

Tides quantify how an object deforms in an external gravitational field. Black holes have a surprising property: for static external fields, they exhibit no tidal response, i.e., their "Love numbers" vanish. I study the symmetries behind this behavior and how those symmetries constrain perturbations across spins and dimensions.

For scalar (spin-0) perturbations of non-spinning black holes, we uncovered ladder symmetries with a clear geometric origin that account for the vanishing Love numbers. We used the same methods to clarify the structure of quasinormal modes in de Sitter space. Building on that, we identified additional symmetries for electromagnetic (spin-1) and gravitational (spin-2) perturbations that likewise exclude a static tidal response. These ideas further extend to higher dimensional black holes, where they predict exactly which Love numbers vanish (not all).

Lensing and Photon Shell Dynamics

The photon shell is the region around a spinning black hole where light can orbit (unstably) without escaping or being captured. It strongly shapes the lensing of light by the black hole, most notably by producing the photon ring, a bright, thin ring in black hole images formed by light rays that linger near the black hole for many orbits before escaping. To study this region and its effects, I have helped guide the development of two tools with Vanderbilt graduate student Trevor Gravely. The first tool is a JavaScript ray tracer for light rays around spinning black holes. Some of the animations we made can be viewed here. We used this tool to develop visualizations of the spacetime around a black hole as part of the Black Hole Explorer initiative, which seeks to image the photon ring around supermassive black holes. The second tool is an interactive app called Black Hole Vision, now available on iOS, which simulates real-time gravitational lensing by a rotating black hole using the device's front and rear cameras.

More recent work studies how small perturbations to the spacetime affect the photon shell, the region most sensitive to such changes. When the spacetime is perturbed, the crisp boundary between light that escapes to infinity and light that falls into the black hole wrinkles into a fractal, signaling the onset of chaos. We quantify this instability by measuring how quickly nearby light rays diverge (calculating the Lyapunov exponent).

Prior Work

During my Ph.D., I studied a model of black hole formation in a de Sitter background in an extension of general relativity known as massive gravity. I also explored the dynamics of a black hole moving through a uniform magnetic field, exploring via simulations the chaotic trajectories of charged particles and the possibility of such a black hole acquiring an electric charge.

An up-to-date list of my publications can be found on INSPIRE.
My CV can be viewed here.

Contact

I can be reached at roman dot berens at vanderbilt dot edu.

• GitHub

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