Mohamad Shalaby

Plasma Astrophysicist | HAPI Research Associate

The Sound of the Universe: Resonant Gravitational Instability

We discovered a new resonant gravitational instability driven by the relative drift between baryons and dark matter after decoupling. When the projected dark matter drift is subsonic, baryon sound waves resonate with Doppler‑shifted dark matter modes, producing exponential growth that exceeds the intrinsic dark matter growth rate. The instability operates on short timescales (years to Myr) across all scales – from planets to galaxy clusters – and naturally explains long‑standing puzzles: the persistence of spiral arms in galaxies, the heating of the intracluster medium (cooling flow problem), turbulence in molecular clouds, and high‑frequency solar oscillations. It also introduces a collisionless drag that may alleviate the Hubble and σ8 tensions and offers direct observational tests via seismic vibrations (Earth, Moon, ice sheets) and pulsar timing arrays.

Resonant Gravitational Instability

First comprehensive analysis of a new instability that alters the standard picture of gravitational structure formation, unifying subsonic enhancement and supersonic suppression of baryon perturbations.

arXiv:2604.22665 & Submitted to ApJ

Astrophysical & Cosmological Implications

From driving spiral arms in galaxies to heating the intracluster medium and seeding Population III stars – the instability provides a new lens on dark matter–baryon interactions across all scales.

Cosmic Ray Research

My research here focuses on understanding the microphysics of cosmic ray transport and their significant impact on astrophysical environments, from the interstellar medium to galaxy clusters.

Intermediate-Scale Instability

Discovered a novel CR-driven instability that grows significantly faster than previously known instabilities, with profound implications for CR transport and electron acceleration.

Paper 1 (ApJ) Paper 2 (JPP Letter)

Saturation mechanisms

Developing predictive frameworks for nonlinear saturation mechanisms of CR driven instabilities which dectate their transport.

Read Paper (ApJ)

CR Transport Modeling

Developing predictive frameworks for CR transport that move beyond quasi-linear theory limitations, incorporating nonlinear saturation effects in realistic astrophysical conditions.

Galactic and stellar Feedback

Investigating how CRs drive galactic winds, regulate star formation, and contribute to anomalous ionization in molecular clouds through advanced plasma simulations.

Electron Acceleration at Shocks

Investigating electron and ion acceleration mechanisms in astrophysical shocks, from supernova remnants to intracluster medium, with emphasis on the role of plasma instabilities.

Efficient electron acceleration via intermediate-scale instability

By changing the Alfvénic Mach number, MA, we show that the shocks where intermediate-scale instability operates have a much higher electron acceleration efficiency compared to high MA shocks, i.e., MA > 10.

Read Paper (ApJ)

Effect of ion-to-electron separation on energy dissipation at shocks

Study of how mass ratio variations affect shock dynamics and particle acceleration.

Read Paper (ApJL) View Simulation

Impact of Magnetization on the Formation and Dynamics of Parallel Electron-Ion Collisionless Shocks

Using particle-in-cell simulations, we investigated how ion magnetization (σi) affects parallel electron-ion collisionless shocks and found that strongly magnetized shocks (σi > 1) exhibit lower compression ratios and suppressed particle acceleration while maintaining stability, whereas weakly magnetized shocks drive instabilities and generate suprathermal particles through shock-drift acceleration.

Read Paper (ApJ) View Simulation

Shock Obliquity Dependence

Systematically studying how magnetic field orientation affects electron acceleration efficiency in non-relativistic shocks.

Low Mach Number Shocks

Investigating electron acceleration in low Mach number shocks relevant to structure formation in the intracluster medium.

SHARP Code Development

As the primary developer of the SHARP code, I've created a highly accurate plasma dynamics simulator that enables previously intractable simulations of astrophysical plasmas.

High-Order Accuracy

Implemented up to 5th-order spline interpolation for field gathering and particle scattering, achieving three-order-of-magnitude improvement in energy conservation.

Read Paper (ApJ)

Hybrid Framework

Integrated kinetic, ideal fluid, and Landau-fluid plasma descriptions in a unified framework for multi-scale astrophysical simulations.

Read Paper (JPP)

Beam-Plasma Instabilities

I investigate the nonlinear evolution of beam-plasma instabilities in diverse astrophysical environments, ranging from solar radio bursts to the intergalactic medium (IGM).

Specifically, within the IGM, these instabilities are critical for constraining the strength of intergalactic magnetic fields and may be responsible for substantial heating in the large, empty regions known as cosmic voids.

Nonlinear Saturation

Investigating the nonlinear saturation mechanisms of beam-plasma instabilities and their impact on cosmic magnetic field origins.

Paper 1

Inhomogeneous Background Effects

Demonstrated how density gradients can significantly enhance or suppress beam-plasma instabilities in realistic astrophysical environments.

Paper 1 Paper 2

Constraints on dark photon dark matter

Using PIC simulations, we re-examine and invalidate all constraints on dark photon dark matter from its resonant conversion to plasmons in the early universe.

Selected Publications

Complete list: Google Scholar

  • [2026] Shalaby, M. & Broderick, A., "The Sound of the Universe: A Resonant Gravitational Instability Driven by Baryon–Dark Matter Relative Drift", arXiv:2604.22665, submitted to ApJ
  • [2025] Shalaby, M., Bret, A., and Fraschetti, F., "Parallel Collisionless Shocks in strongly Magnetized Electron-Ion Plasma. I. Temperature anisotropies", ApJ 991 26
  • [2024] Shalaby, M., "Energy Dissipation in Strong Collisionless Shocks: The Crucial Role of Ion-to-electron Scale Separation in Particle-in-cell Simulations", ApJL 977 L43
  • [2023] Shalaby, M., Thomas, T., Pfrommer, C., Lemmerz, R., and Bresci, V., "Deciphering the physical basis of the intermediate-scale instability", JPP Letters 89.6
  • [2022] Shalaby, M., Lemmerz, R., Thomas, T., and Pfrommer, C., "The Mechanism of Efficient Electron Acceleration at Parallel Nonrelativistic Shocks", ApJ 932 86
  • [2021] Shalaby, M., Thomas, T., and Pfrommer, C., "A New Cosmic-Ray-driven Instability", ApJ 908 206