Theory

Supervisors: Archie Bott, Gianluca Gregori, Alexander Schekochihin

There are a number of possibilities within this project to design, take part in, and theorise about laboratory experiments employing laser-produced plasmas to model astrophysical phenomena and basic, fundamental physical processes in turbulent plasmas. Recent examples of our work in this field include turbulent generation of magnetic fields ("dynamo") [1,2], supersonic turbulence mimicking star-forming molecular clouds [3,4], diffusion and acceleration of particles by turbulence [5,6], suppression of thermal conduction in galaxy-cluster-like plasmas [7]. Our group has access to several facilities (including the National Ignition Facility, the largest laser system in the world). Students will also have access to a laser laboratory on campus, where initial experiments can be fielded. Depending on the student's inclinations, it is also possible to pursue a project focused on theory and/or numerical modelling of plasma phenomena in astrophysical and laboratory-astrophysical environments.

Background Reading

1. P. Tzeferacos et al., "Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma," Nature Comm. 9, 591 (2018)
2. A. F. A. Bott et al., "Time-resolved fast turbulent dynamo in a laser plasma," PNAS 118, e2015729118 (2021)
3. T. G. White et al.,"Supersonic plasma turbulence in the laboratory," Nature Comm. 10, 1758 (2019)
4. A. F. A. Bott et al., "Proton imaging of stochastic magnetic fields," J. Plasma Phys. 83, 905830614 (2017)
5. L. E. Chen et al., "Transport of high-energy charged particles through spatially intermittent turbulent magnetic fields," Astrophys. J. 892, 114 (2020)
6. J. C. Y. Chu et al., "Measurement of ion acceleration and diffusion in a laser-driven magnetized plasma," Nature Comm. 17, 3354 (2026)
7. J. Meinecke et al., "Strong suppression of heat conduction in a laboratory analogue of galaxy-cluster turbulent plasma," Science Adv. 8, eabj6799 (2022)

Supervisors: Bence Kocsis

Galactic nuclei are among the most extreme environments in the Universe. A supermassive black hole is surrounded by a dense cluster of stars and stellar-mass black holes, and, when active, by a luminous gaseous accretion disc. The gravitational interactions between these components drive complex dynamics and may produce black hole mergers, tidal disruption events, and distinctive electromagnetic signals. Understanding these processes is essential for interpreting observations of the Galactic centre, active galactic nuclei (AGN), and gravitational wave sources.

In this project, the student will work with Prof. Bence Kocsis to develop theoretical and numerical models of the dynamics and observational signatures of galactic nuclei. Several complementary research directions are possible. One is to model the coupled evolution of an AGN disc and the surrounding nuclear star cluster: stellar objects interact with the gas, some become embedded in the disc, and black holes can form binaries that eventually merge. Another is to investigate gravitational lensing by supermassive black holes and their binaries, including how the inhomogeneous stellar environment modifies the resulting magnification patterns and light curves. A third direction is to study how a massive perturber, such as an intermediate-mass black hole, changes the structure and long-term evolution of nuclear star clusters and stellar discs.

The project will combine analytical methods, stellar dynamics, statistical physics, and computational simulations. The precise research questions will be selected together with the student, allowing the project to develop according to their interests and the most promising new results.

Links to further reading

• Tagawa, Haiman & Kocsis (2026), ApJ, 1007, 67 — Properties of black hole mergers in disks of active galactic nuclei
• Rowan, Whitehead & Kocsis (2024) — Black Hole Merger Rates in AGN: contribution from gas-captured binaries
• Wang, Zumalacárregui & Kocsis (2026), PRL, 136, 061403 — Black Holes as Telescopes: Discovering Supermassive Binaries through Quasiperiodic Lensed Starlight
• Panamarev, Zou & Kocsis (2026), MNRAS — Stellar discs and intermediate-mass black holes in galactic nuclei I
• Panamarev & Kocsis (2026) — Stellar discs and intermediate-mass black holes in galactic nuclei II

Supervisors: Alexander Schekochihin, Michael Barnes

In magnetised astrophysical plasmas, there is a turbulent cascade of electromagnetic fluctuations carrying free energy from large to small scales. The energy is typically extracted from large-scale sources (e.g., in the solar wind, the violent activity in the Sun's corona; in accretion discs, the Keplerian shear flow; in galaxy clusters, outbursts from active galactic nuclei) and deposited into heat---the internal energy of ions and electrons. In order for this dissipation of energy to happen, the energy must reach small scales---in weakly collisional plasmas, these are small scales in the 6D kinetic phase space, i.e., what emerges is large spatial gradients of electric and magnetic fields and large gradients of the particle distribution functions with respect to velocities. This prompts two fundamental questions: (1) how does the energy flow through the 6D phase space and what therefore is the structure of the fluctuations in this space: their spectra, phase-space correlation functions etc. [1,2,3] (these fluctuations are best observed in the solar wind, but we can measure density and magnetic fluctuations even in extragalactic plasmas, via X-ray and radio observations); (2) when turbulent fluctuations are dissipated into particle heat, how is their energy partitioned between various species of particles that populate the plasma: electrons, bulk ions, minority ions, fast non-thermal particles (e.g., cosmic rays) [4,5]. The latter question is particularly important for extragalactic plasmas because all we can observe is radiation from the particles and knowing where the internal energy of each species came from is key to constructing and verifying theories both of turbulence and of macroscale dynamics and thermodynamics. This project has an analytical and a numerical dimension (which of these will dominate depends on the student's inclinations). Analytically, we will work out a theory of phase-space cascade at spatial scales between the ion and electron Larmor scales. Numerically, we will simulate this cascade using "gyrokinetic" equations---an approach in which we average over the Larmor motion and calculate the distribution function of "Larmor rings of charge" rather than particles (this reduces the dimension of phase space to 5D, making theory more tractable and numerics more affordable).

With a theory of plasma turbulence in hand, it is possible to attack what is probably the most fundamental question of the field: in the absence of collisions, are there universal equilibria, or classes of equilibria, independent of initial conditions, that a turbulent plasma will want to converge to? There is some recent progress indicating that the answer is yes and that one can predict statistical-mechanically the emergence of universal power-law (in particle energy) distributions [6]---this is exciting both on its own merits and because of the astrophysical challenge of explaining theoretically power-law distributions that are observed for, e.g., cosmic rays or solar-wind electrons. How to construct a theory of that for a magnetised, turbulent plasma is an open and exciting question. Attempting to do this will again involve kinetic theory and/or kinetic simulations.  

Background Reading:

1. A. A. Schekochihin et al., "Astrophysical gyrokinetics: kinetic and fluid turbulent cascades in magnetized weakly collisional plasmas," Astrophys. J. Suppl. 182, 310 (2009)
2. A. A. Schekochihin et al., "Phase mixing vs. nonlinear advection in drift-kinetic plasma turbulence," J. Plasma Phys. 82, 905820212 (2016)
3. R. Meyrand et al., "Fluidization of collisionless plasma turbulence," PNAS 116, 1185 (2019)
4. Y. Kawazura et al., "Thermal disequilibration of ions and electrons by collisionless plasma turbulence," PNAS 116, 771 (2019)
5. J. Squire et al., "High-frequency heating of the solar wind triggered by low-frequency turbulence," Nature Astron. 6, 715 (2022)
6. R. J. Ewart et al., "Relaxation to universal non-Maxwellian equilibria in a collisionless plasma," PNAS 122, e2417813122 (2025); see also A. Schekochihin's MMathPhys Lecture Notes, sec. 12-14

Supervisors: Will Coulton

A wealth of cosmological observations are providing unprecedentedly precise measurements of the 3D structure of our Universe. Buried in these detailed maps of the cosmos are tiny statistical signatures that encode the physics of the processes that shaped the evolution of the Universe and, hopefully, smoking gun signatures from the Universe’s beginning. A key challenge is how to disentangle the primordial signals from the dominant, late-time signals to ensure our understanding of the primordial universe is robust. This PhD project will focus on developing new statistical tools to address that problem and applying them to new datasets.

The Oxford cosmology group has expertise in many on-going surveys, including the Simons Observatory and the Vera Rubin Observatory and this project will use a mixture of analytical, computational and statistical tools to search these datasets for primordial signals. The goal will be to drive forward our understanding of this key epoch in the history of the Universe.

References:
Coulton et al (2023)  https://arxiv.org/abs/2206.01619
Yap, Coulton, Iršič (2026)  https://arxiv.org/abs/2609.35713
Yap, Coulton, Hou, Gratton (2026) https://arxiv.org/abs/2610.08671

Supervisors: David Alonso, Matt Jarvis

The spatial distribution of galaxies and other tracers of the large-scale structure (LSS) are crucial observables in our quest to understand the origin and evolution of our Universe, as well as the complex physics that governs the formation of galaxies within the cosmic web of dark matter. Key open questions are:

1. The nature of dark matter and dark energy, and their impact in the evolution of cosmic structure.
2. The physics of feedback from supermassive black holes and supernovae, and their impact on the formation and evolution of stars and galaxies.
3. How these processes and galaxy properties are related to their dark matter halo environments 

In this project, we will combine state-of-the-art LSS datasets with novel theoretical and statistical approaches to address these questions. On the observational side, we will leverage Oxford's access to deep-field galaxy survey data, providing pristine measurements of clustering on small scales, with wide-field probes of the large-scale structure across multiple wavelengths. These will include the first galaxy and weak lensing data from the Vera C. Rubin Observatory and Euclid, and the first maps of the microwave sky from the Simons Observatory. On the theory side, we will combine analytical approaches describing the physics of the small-scale clustering of matter, gas, and galaxies, with simulation-based techniques providing an accurate connection with the large-scale cosmic web.

The project itself can evolve in various directions depending on the interests of the student, with the overarching aim to bring together a deeper understanding of galaxy formation and evolution to gain a more precise understanding of structure formation at all scales and is ideally suited for students with an interest in combining both theory and observations. It will provide experience in multiple areas of data analysis, from images and maps to science-ready observables, and it will introduce the student to the latest theoretical tools to model the formation of cosmic structures and the physics of galaxies and the inter-galactic medium.

Supervisors: Julien Devriendt, Adrianne Slyz

Stars in the Universe form predominantly in clusters within giant galactic molecular clouds. This clustered mode of star formation has profound implications for the formation and evolution of galaxies, as it concentrates stellar feedback in both space and time. Ionising radiation bubbles and supernova explosions emanating from massive stars within clusters overlap, changing how stellar feedback energy couples to the interstellar medium and driving powerful, mass-loaded gas outflows.

Whilst observations of star clusters have traditionally been the preserve of the local Universe, JWST, combined with gravitational lensing, now allows us to probe their formation at cosmic dawn. Magnifications of tens to hundreds resolve individual star clusters in galaxies from z=6 to beyond z=10. In the Cosmic Gems arc at z=10.2 for example, Adamo et al (2024) find five clusters younger than 50 Myr with masses around 10⁶ M₈ (see Fig 1.). These objects appear unusually bright for their host galaxies. Why the first clusters should be so luminous is an open question. They are also so dense that the stars at their very centre are believed to collide with one another, eventually fusing together to form intermediate mass black holes.

For this project, the DPhil student will perform numerical studies of the initial cluster mass function (ICMF) within the explicit framework of a cold dark matter dominated Universe, using the adaptive mesh refinement radiative hydrodynamics code RAMSES-RT (Rosdahl et al 2013). At first, they will be tasked 

with developing a stellar wind feedback module, building on the previous star cluster modelling work carried out within the group (Thompson et al 2026; see Fig. 2).

They will then move on to designing and running their own high-resolution simulations of galaxy formation, building on the newly developed module and radiative hydrodynamics, to incorporate both magnetic fields (Fromang et al 2006, Martin-Alvarez et al 2018) and cosmic rays (Dubois & Commercon 2015; Rodriguez-Montero et al. 2024). Scientific goals include assessing which feedback processes most strongly influence the ICMF, and in which way; investigating how these processes shape molecular clouds at both local and galactic scales particularly through their impact on the cloud mass function; assessing whether simulated ICMFs produce clusters bright enough to account for the JWST observations; and exploring how these ICMFs impact the initial black hole mass function predicted by stellar collisions within these clusters, as well as the resulting gravitational wave signals and rates of tidal disruption events.

The DPhil student will be part of an international collaboration (US, France, South Korea, UK) developing and exploiting state-of-the-art, high-resolution, and physics rich cosmological simulations.

Supervisors: Pedro Ferreira, Tariq Yasin, Harry Desmond

One of the open questions in physics is whether General Relativity and collisionless dark matter are really the whole story. Dark matter detectors have steadily closed off much of the parameter space for the most popular particle candidates without finding anything, and the cosmic expansion history on its own cannot distinguish between the many theories proposed to explain cosmic acceleration. Both fields have been looking under the same few lampposts for a while now, and it is time to look under new ones. Galaxies are an extremely promising hunting ground. Each occupies a different gravitational environment, ambient dark matter density and velocity field, providing a population of natural experiments in regimes inaccessible in the laboratory. For dark matter in particular, the phenomenology is rich. In the standard picture, a galaxy's stars, gas, dark matter and central black hole all fall together, but if dark matter interacts with itself, it can pull them apart. A halo dragged back as its galaxy moves through the surrounding dark matter can shift the stars off-centre, bend the disk into a warp, thicken it, or leave it lopsided. Each of these distortions points in a direction set by the galaxy's environment. The catch is that galaxies are messy systems, and ordinary astrophysics can affect the same observables. The project will tackle this with what we call "Messy Bayesian Forward Modelling": reconstructing each galaxy's latent environment and properties, predicting the effect of the dark matter interaction object by object, and marginalising over baryonic physics, halo structure, selection and measurement uncertainty. Rather than committing to a specific theory, we will develop general, phenomenological prescriptions for the effect of novel dark-sector physics, allowing us to cast our net as widely as possible in the search for the particle nature of dark matter.

Related works:

• SIDM constraints from galaxy warps: - https://arxiv.org/abs/1911.04497
• Messy Bayesian Forward Modelling - https://arxiv.org/abs/2009.08743v2
• Effective description of dark matter microphysics for structure formation: https://arxiv.org/abs/1512.05344
• Review of astrophysical tests of SIDM - https://arxiv.org/abs/2207.10638