My research focuses on the origin of ultra-high-energy cosmic rays, specifically on their acceleration at large-scale accretion shocks. I work across the fields of astrophysics, astroparticle physics, and plasma physics, including the subfields of cosmic rays, galaxy clusters, supernova remnants, multiwavelength observations, and large-scale simulations.
My recent work was featured by the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) in a research highlight titled “Nature’s Ultimate Particle Accelerators.” The piece gives a non-technical overview of how large-scale cosmic structures—such as galaxy clusters and filaments—can accelerate particles to energies far beyond those achievable on Earth.
→ Read the KIPAC research highlight
Main interest
Ultra-high-energy cosmic rays (UHECRs) are one of the biggest puzzles in astrophysics, though they don’t get as much attention as dark matter or black holes. Cosmic rays are energetic nuclei from space with energies ranging from a few hundred MeV to over 200 EeV, or \( 2 \times 10^{20} \) eV. That’s over 30 million times the energy humans have achieved after decades of effort and billions of dollars. And yet, Nature makes 200 EeV cosmic rays routinely. But we don’t know exactly how or where.
The most popular ideas typically involve either supermassive black holes or neutron stars, the most extreme objects in the Universe. Those are the usual suspects for extreme astrophysics. But I am trying to revive a lesser-known idea that the largest shocks in the Universe—those associated with large-scale structure formation—accelerate UHECRs through a widely known process called diffusive shock acceleration.
Building on my PhD research with Roger Blandford, my collaborators and I have developed a hierarchical model in which a series of shocks accelerates cosmic rays in stages, from low energies within galaxies to the highest energies at extragalactic shocks. To test this model, we draw on observations of radio waves, gamma rays, neutrinos, and cosmic rays themselves. The figure here is a temperature map from a cosmological hydrodynamic simulation by Kirk Barrow.
Other interests
Astrophysics is a web of interconnected phenomena, so my interests spread out into other phenomena. I’ve investigated the origins of lower-energy cosmic rays, which still have very high energy compared to particles accelerated on Earth by humans. The study of cosmic rays naturally involves plasma physics and the many instabilities that arise is astrophysics. My work on UHECRs has developed an interest in the large-scale structure of the universe, namely the filamentary structure that forms as matter collapses into galaxy clusters, filaments, and sheets.
Early in my career, I briefly worked in experimental and theoretical particle physics, and I haven’t lost my interest in these fields. I have my eye on the field of quantum foundations as an exciting frontier, but I haven’t worked on it yet.