Justin L. Ripley

Current position

I will be starting a postdoctoral research position at Cambridge University (DAMTP) in the fall of 2020.

Graduate School

I obtained my PhD from the Department of Physics, Princeton University. My adviser was Frans Pretorius. A pdf of my thesis can be found here.

Research Interests

The main goal of most of my research is directed towards testing General Relativity (GR) in the strong field, dynamical regime. Below I describe some of my current and past research projects.

Nonlinear strong field dynamics of modified gravity theories

I am currently working on numerically solving for the equations of motion of a fairly wide class of modified gravity theories that involve adding a new scalar field to the Einstein equations, which couples to the gravitational (metric) field in such as way so that all the equations of motion are second order in derivatives (``Horndeski theories''). Horndeski theories have been invoked to model a wide variety of gravitational phenomena, from dark energy to dark matter, to new ``exotic compact object'' solutions. Many Horndeski theories have been strongly constrained by gravitational wave observations, but at least from some variants of the theory the strongest constraints (or perhaps the most unsuspected surprises!) will come from comparing the predictions of these theories to those of GR in the strong field dynamical regime, such as near the moment of merger of two black holes. Horndeski theories have recently been shown to admit a well-posed initial value problem, which opens up the possibility of actually solving for the dynamics of these theories in that regime.

Black hole perturbation theory

I am currently working on understanding the leading nonlinear dynamics of gravitational wave interactions near Kerr black holes. More specifically, I am working on developing second order perturbation theory for Kerr black holes in a computationally feasible way. This work is motivated by a desire to understand the regime of applicability of linear perturbation theory in describing the ringdown of a Kerr black hole formed after the merger of two black holes/compact objects. This project should provide a quantitative estimate of how well linear perturbation theory does, and provide a means to explore phenomena such as the proposed onset of ``gravitational wave turbulence'' around rapidly rotating (near extremal) Kerr black holes.

Permanent email address:

lloydripley [at] gmail [dot] com


My CV (in .pdf format)


My GitHub account


Links to my publications:

InSpire HEP

Google Scholar


Orcid ID

ORCID iD iconhttps://orcid.org/0000-0001-7192-0021


Slides (in .pdf format) of my PhD defense, which was about modified gravity, with a particular focus on Einstein dilaton Gauss-Bonnet gravity (July 1, 2020; given remotely)

Slides (in .pdf format) for a talk on Einstein dilaton Gauss-Bonnet gravity. Given at the Perimeter Institute (April 9, 2020; given remotely)