Recent Publications

Galison, Peter, Chyld King, and Michael D. Johnson. Light at the Edge of the Universe: The Black Hole Explorer. USA, 2024.Abstract

Around the horizon of a black hole, an edge of the universe, light is captured, spun into orbit by the black hole’s powerful gravitational pull. Lying within the orange donut in the famous first image of a black hole, this “photon ring” would be a prize to measure—it would reveal the nature of spacetime itself, directly, near the horizon. Indeed, the shape of this pure ring of light tells everything about the black hole. With the stakes this high, a new collaboration—physicists, astronomers, engineers from around the world—has formed to loft a spacecraft that capture the photon ring. We are at the beginning of what is probably a ten-year effort—this is a film about the start of that adventure.

A film by Peter Galison, Michael Johnson, and Chyld King

Dream of a Shadow
Galison, Peter. Dream of a Shadow. USA, 2023.Abstract
My world was jolted by two shadow images: one, thrilling, the other terrifying.  After a years-long effort—with 200 other scientists—we made the first image of a black hole, its shadow of no return.  Then I fell into debilitating pain. A deadly shadow blot appeared on an MRI of my spine.  Faced with emergency surgery and no assurance of success, I sought comfort in memory images from the past and, from collaborative work that engrossed me: images of the whole visible universe could be stored in light circling a black hole.  It was a universal memory. An experimental back and forth between the innermost-personal and the astronomical, where shadows and consolation cross.
Visualisation as a Political Act
Galison, Peter, and Moritz Neumüller. “Visualisation as a Political Act.” In The Routledge Companion to Photography, Representation and Social Justice, edited by Moritz Neumüller, 249–258. London: Routledge, 2023.
Selective Dynamical Imaging of Interferometric Data
Farah, Joseph, Peter Galison, Kazunori Akiyama, Katherine L. Bouman, Geoffrey C. Bower, Andrew Chael, Antonio Fuentes, and et al. “Selective Dynamical Imaging of Interferometric Data.” The Astrophysical Journal Letters 930 (2022): L18. Publisher's VersionAbstract
Recent developments in very long baseline interferometry (VLBI) have made it possible for the Event Horizon Telescope (EHT) to resolve the innermost accretion flows of the largest supermassive black holes on the sky. The sparse nature of the EHT’s (u, v)-coverage presents a challenge when attempting to resolve highly time-variable sources. We demonstrate that the changing (u, v)-coverage of the EHT can contain regions of time over the course of a single observation that facilitate dynamical imaging. These optimal time regions typically have projected baseline distributions that are approximately angularly isotropic and radially homogeneous. We derive a metric of coverage quality based on baseline isotropy and density that is capable of ranking array configurations by their ability to produce accurate dynamical reconstructions. We compare this metric to existing metrics in the literature and investigate their utility by performing dynamical reconstructions on synthetic data from simulated EHT observations of sources with simple orbital variability. We then use these results to make recommendations for imaging the 2017 EHT Sgr A* data set.
Measuring Photon Rings with the ngEHT
Tiede, Paul, Michael D. Johnson, Dominic W. Pesce, Daniel C. M. Palumbo, Dominic O. Chang, and Peter Galison. “Measuring Photon Rings with the ngEHT.” Galaxies 10, no. 6 (2022): 111. Publisher's VersionAbstract
General relativity predicts that images of optically thin accretion flows around black holes should generically have a “photon ring”, composed of a series of increasingly sharp subrings that correspond to increasingly strongly lensed emission near the black hole. Because the effects of lensing are determined by the spacetime curvature, the photon ring provides a pathway to precise measurements of the black hole properties and tests of the Kerr metric. We explore the prospects for detecting and measuring the photon ring using very long baseline interferometry (VLBI) with the Event Horizon Telescope (EHT) and the next-generation EHT (ngEHT). We present a series of tests using idealized self-fits to simple geometrical models and show that the EHT observations in 2017 and 2022 lack the angular resolution and sensitivity to detect the photon ring, while the improved coverage and angular resolution of ngEHT at 230 GHz and 345 GHz is sufficient for these models. We then analyze detection prospects using more realistic images from general relativistic magnetohydrodynamic simulations by applying “hybrid imaging”, which simultaneously models two components: a flexible raster image (to capture the direct emission) and a ring component. Using the Bayesian VLBI modeling package Comrade.jl, we show that the results of hybrid imaging must be interpreted with extreme caution for both photon ring detection and measurement—hybrid imaging readily produces false positives for a photon ring, and its ring measurements do not directly correspond to the properties of the photon ring.
First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
Collaboration, Event Horizon Telescope. “First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way.” The Astrophysical Journal Letters 930 (2022): L12. Publisher's VersionAbstract
We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 ± 2.3 μas (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 × 106 M, which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination (i > 50°), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of 103–105 gravitational radii to event-horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87* shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass.
Shattering Stars
Galison, Peter. Shattering Stars. USA, 2021.Abstract
1930, Madras, India. On a steamship to England, 19-year-old Indian physics prodigy, Subrahmanyan Chandrasekhar makes a paradigm-shattering discovery about the life cycle of stars: a discipline pioneered by Sir Arthur Eddington, the world’s most famous astronomer and Chandra’s new advisor. After years of work under his mentor's watchful eye, Chandra presents at a public forum, only for Eddington to denounce his protégé and his work. A half-century later, after a more conservative career, Chandra is awarded the Nobel Prize for the work begun on board that ship to England. Grounded in his recorded memories, “Shattering Stars” alternates between realistic and imaginary animation.
Publications

Black Holes | The Edge of All We Know