Dennis Hartmann

Astrophysics PhD at Johns Hopkins University - Graduate research assistant at STScI

Dennis in front of a door that has Steven Weinberg's nameplate.

Whitmore Lab 322 - Institute for Gravitation and the Cosmos

State College, Pennsylvania 16803

This website is in need of updating — this represents my work prior to joining JHU in the summer of 2025. I will be updating it soon.

I am a research assistant in the Department of Astronomy and Astrophysics as well as in the Institute for Gravitation and the Cosmos (IGC). Much of my undergraduate work was focused on trying to develop a better understanding of gravity and how it might behave at a quantum scale. For the most part, I am still working on this post-graduation, but have also joined the Penn State High Energy Astrophysics Detector Instrumentation (HEADI) Lab, which is focused on detector characterization for the upcoming BlackCAT (Black Hole Coded Aperture Telescope) CubeSat mission in collaboration with NASA.

My main interests for my theoretical work concern studies of compact object formation in Emergent Modified Gravity, a theory in which gravity effectively emerges from a canonical phase space. I have studied various forms of black holes and their formation mechanisms; my earliest work concerned a type of black hole in which spacetime was divided into a Lorentzian region and a Euclidean region. I am now studying models for dust collapse and a method by which a massless scalar field can also form black holes. I also work on two different approaches to quantum gravity — one called CDT, in which spacetime is divided into triangles called simplexes, and the other being Asymptotic Safety, which attempts to characterize what a quantum field theory of gravity should look like. You can read more about my theory work on my Theory page.

I currently work in the HEADI on the NASA BlackCAT mission, which is a wide-field X-ray imaging telescope placed on a 6U CubeSat satellite to observe gamma-ray bursts and other high-energy transient and flaring sources using novel X-ray hybrid CMOS detectors. My work largely revolves around the calibration and assembly of the payload that is scheduled to launch October 2025. I am also working on software to synthesize images using RML techniques to enhance the resolution of detector data. To learn more about my astronomy work, check out my Astro page.

Selected Publications

  1. New type of large-scale signature change in emergent modified gravity

    Bojowald, Martin and Duque, Erick I. and Hartmann, Dennis
    Phys. Rev. D, 2024
    David Bohm award
    This paper factored in heavily to the combination of the David Bohm award and the Brickwedde award.
    Abstract
    Emergent modified gravity presents a new class of gravitational theories in which the structure of space-time with Riemannian geometry of a certain signature is not presupposed. Relying on crucial features of a canonical formulation, the geometry of space-time is instead derived from the underlying dynamical equations for phase-space degrees of freedom together with a covariance condition. Here, a large class of spherically symmetric models is solved analytically for Schwarzschild-type black hole configurations with generic modification functions, using a variety of slicings that explicitly demonstrate general covariance. For some choices of the modification functions, a new type of signature change is found and evaluated. In contrast to previous versions discussed for instance in models of loop quantum gravity, signature change happens on timelike hypersurfaces in the exterior region of a black hole where it is not covered by a horizon. A large region between the horizon and the signature-change hypersurface may nevertheless be nearly classical, such that the presence of a signature-change boundary around Lorentzian space-time, or a Euclidean wall around the Universe, is consistent with observations provided signature change happens sufficiently far from the black hole.
    Cite (BibTeX) 
    @article{Bojowald:2023vvo,
    title = "{New type of large-scale signature change in emergent modified gravity}",
    author = "Bojowald, Martin and Duque, Erick I. and Hartmann, Dennis",
    eprint = "2312.09217",
    archiveprefix = "arXiv",
    primaryclass = "gr-qc",
    doi = "10.1103/PhysRevD.109.084001",
    journal = "Phys. Rev. D",
    volume = "109",
    number = "8",
    pages = "084001",
    year = "2024"
}

  2. Covariant Lemaitre-Tolman-Bondi collapse in models of loop quantum gravity

    Bojowald, Martin and Duque, Erick I. and Hartmann, Dennis
    Phys. Rev. D, 2024
    Abstract
    Models of gravitational collapse provide important means to test whether non-classical space-time effects motivated for instance by quantum gravity can be realized in generic ways in physically relevant situations. Here, a detailed analysis of marginally bound Lemaitre-Tolman-Bondi space-times is given in emergent modified gravity, which in particular includes a covariant formulation of holonomy modifications usually considered in models of loop quantum gravity. As a result, generic collapse in this setting is shown to imply a physical singularity that removes the bouncing behavior seen in vacuum space-times with the same type of modifications.
    Cite (BibTeX) 
    @article{BojowaldLT,
    title = "{Covariant Lemaitre-Tolman-Bondi collapse in models of loop quantum gravity}",
    author = "Bojowald, Martin and Duque, Erick I. and Hartmann, Dennis",
    eprint = "2412.18054",
    arxiv = "2412.18054",
    archiveprefix = "arXiv",
    primaryclass = "gr-qc",
    doi = "https://journals.aps.org/prd/abstract/10.1103/PhysRevD.111.064002",
    journal = "Phys. Rev. D",
    volume = "111",
    number = "6",
    pages = "064002",
    year = "2024"
}