Stephon Alexander

Academic life

Alexander was a postdoctoral researcher at Imperial College London from 2000 to 2002 and subsequently at the Stanford Linear Accelerator Center from 2002 to 2005. He joined Pennsylvania State University as an assistant professor of physics in 2005 and moved to Haverford College in 2008 as an associate professor. In 2012, he became the Ernest Everett 1907 Associate Professor of Natural Sciences at Dartmouth College.

Alexander returned to Brown University in 2016 as a professor of physics. He has also held visiting positions at Princeton University, the California Institute of Technology, ETH Zurich, the Perimeter Institute for Theoretical Physics, Columbia University, and the Center for Computational Astrophysics of the Simons Foundation. Brown lists his current research specialization as the interface between cosmology, particle physics, and quantum gravity, including string theory and loop quantum gravity.

He has served on the editorial board of the Journal of Cosmology and Astroparticle Physics and has taught courses in general relativity and cosmology, astrophysics and cosmology, quantum field theory, and quantum mechanics. He also advocates for historically under-represented groups in the sciences. He is a member of the editorial board of Universe.

String cosmology and inflation

One of Alexander's early research areas was string cosmology, particularly the use of ideas from string theory to construct models of cosmic inflation. In a 2001 paper, Inflation from D-\bar{D} brane annihilation, he proposed a scenario in which the annihilation of D5 and anti-D5 branes produces a vortex localized on a three-brane, with its false-vacuum energy driving an inflationary phase. The model also provided a mechanism for ending inflation and localizing matter on the resulting brane.

His work in this area contributed to the development of models connecting higher-dimensional string theory with the dynamics of the early universe. Alexander is a co-inventor of a string-theory-based model of cosmic inflation developed in the early 2000s.

Quantum gravity and parity violation

A recurring theme in Alexander's research has been the possibility that quantum-gravitational effects could produce observable departures from general relativity. His work has examined parity violation—the distinction between physical processes under spatial reflection and its possible consequences for cosmology and gravitational phenomena.

In 2007, Alexander studied whether parity-violating modifications of general relativity could account for anomalies in the cosmic microwave background, including suppression of power at large angular scales and possible alignments in the low multipole moments of the WMAP data. The work also considered connections between parity violation and leptogenesis in the early universe.

His later research developed these ideas through Chern–Simons modifications of gravity. In work beginning more than a decade ago, he developed what has been termed dynamical Chern–Simons gravity, an extension of general relativity involving a coupling between a scalar field and the gravitational Chern–Simons term. The theory provides a framework for investigating possible deviations from general relativity in strong-gravity environments and has applications to black holes, cosmology and gravitational waves.

Alexander has subsequently studied the propagation of gravitational waves in parity-violating theories. A 2020 paper examined gravitational waves in parity-violating Copernican universes, while a 2024 Physical Review D paper with Tucker Manton investigated gravitational parity violation involving the Kalb–Ramond field and considered constraints from gravitational-wave observations, including GW170817 and the GWTC-3 catalog.

Dark matter, dark energy and the early universe

Alexander's research has also addressed the nature of the dark sector, including theoretical approaches to dark matter and dark energy. His work considers these problems in the context of the limitations of general relativity and the Standard Model. His research group has used theoretical, computational and conceptual methods, including machine learning, to investigate these questions.

He has also studied cosmic inflation, leptogenesis and the generation of matter-antimatter asymmetry in the early universe. His research has examined how parity-violating interactions could produce an imbalance between matter and antimatter and how such effects might leave signatures in cosmological observations.

Gravitational waves and cosmology

Alexander has investigated whether observations of gravitational waves can provide evidence for physics beyond general relativity. His research examines how parity-violating interactions could affect the polarization and propagation of gravitational waves and whether these effects could be distinguished through astrophysical observations. His work with collaborators has considered constraints from existing gravitational-wave data and possible future tests.

A 2021 Simons Foundation grant supported Alexander's research into modifications of general relativity, particularly approaches intended to address singularities associated with black holes and the Big Bang. The project examined whether an extension of Einstein's theory could provide a framework for understanding phenomena that remain unresolved within classical general relativity.

Interdisciplinary research

Alexander has also applied mathematical methods from theoretical physics to problems outside conventional cosmology. His research interests have included machine learning, signal processing, computational algorithms, and mathematical approaches to musical perception. At Brown, he has participated in STEMJazz, an interdisciplinary initiative connecting science, mathematics, engineering and music.

Media

In February 2013, Alexander wrote in The New York Times about the need for black academics to set a positive trend for the next generation. In the article, he called upon a number of personal experiences from his own education and life.

Alexander has been interviewed or quoted in media sources such as the Tavis Smiley Show, Forbes Magazine, NPR, Brian Lehrer Show, Science Salon/Skeptic Society, Downbeat Magazine, and Mercury News.

Music

His album with Rioux was Here Comes Now. Alexander and bassist Melvin Gibbs formed a group that they named God Particle. His first book, The Jazz of Physics: The Secret Link Between Music and the Structure of the Universe, was published by Basic Books in 2016. The book examines historical and conceptual connections between music, mathematics and physics, including the work of figures such as Pythagoras, Johannes Kepler, Albert Einstein and John Coltrane.

His second book, Fear of a Black Universe: An Outsider's Guide to the Future of Physics, was published by Basic Books in 2021. It discusses open problems in modern cosmology, including dark matter and dark energy, alongside Alexander's experiences as a physicist and his views on scientific creativity and diversity.

Alexander is also the author of The Jazz of Physics, a book that discusses the link between music and the structure of the universe.

On a Nova documentary, Alexander was featured discussing his life as a jazz saxophonist, while also working as a physicist during the day.