My main research interests focus on the study of gravitational waves from the early Universe and the study of primordial magnetic fields.
Gravitational waves from the early Universe
Collaboration publications:
- [LGWA Collaboration] (incl. A. Roper Pol), The Lunar Gravitational-wave Antenna: Mission studies and science case, J. Cosmol. Astropart. Phys., 01, 108 (2025), arXiv: 2404.09181.
- [LISA Cosmology Working Group], C. Caprini, R. Jinno, M. Lewicki, E. Madge, M. Merchand, G. Nardini, M. Pieroni, A. Roper Pol, V. Vaskonen, Gravitational waves from first-order phase transitions in LISA: Reconstruction pipeline and physics interpretation, J. Cosmol. Astropart. Phys., 10, 020 (2024), arXiv: 2403.03723.
- [EPTA and InPTA Collaborations] (incl. A. Roper Pol), The second data release from the European Pulsar Timing Array IV: Implications for massive black holes, dark matter and the early Universe, Astron. Astrophys., 685, A94 (2024), arXiv: 2306.16227.
- [LISA Cosmology Working Group] (incl. A. Roper Pol), Cosmology with the Laser Interferometer Space Antenna, Living Rev. Relativ., 26, 5 (2023), arXiv: 2204.05434.
Gravitational waves from first-order phase transitions
Gravitational waves are produced during a first-order phase transition due to the collision of expanding bubbles of the symmetry-broken phase. As they expand, these bubbles drag the expanding primordial plasma leading to the formation of fluid shells around them. When these bubbles collide, they lead to the production of gravitational waves and can be a promising source for current pulsar timing array (PTA) observations and for the planned space-based detector LISA. Both experiments can be used to probe the early Universe at the QCD and the electroweak scale, respectively.
The main sources of gravitational waves from first-order phase transitions:
- Sound waves: compressional fluid motion in the linearized regime (small fluid perturbations)
- Turbulence: compressional (acoustic turbulence) and vortical motion of the fluid perturbations in the linear regime
- Bubble collisions: gradients of the scalar field
Gravitational waves from sound waves
The sound-shell model assumes that the fluid perturbations are a linearized superposition of sound waves produced from each of the bubbles after they have collided. This model allows to reproduce the gravitational wave signal for weak phase transitions when the fluid perturbations are small.
Main collaborators
Publications:
- A. Roper Pol, S. Procacci, C. Caprini, Characterization of the gravitational wave spectrum from sound waves within the sound shell model, Phys. Rev. D, 109, 063531 (2024), arXiv: 2308.12943.
Higgsless simulations
Under the Higgsless approach, the fluid motion induced by the phase transitions can be studied by integrating out the dynamics of the scalar field, which occur within the bubble wall thickness, usually many orders of magnitude smaller than the fluid scales. This method allows to reduce the separation of scales between the fluid and the scalar field and capture most of the dynamics of the plasma in a first-order phase transition.
Main collaborators
Publications:
- I. Stomberg, A. Roper Pol, Gravitational wave spectra for cosmological phase transitions with non-linear decay of the fluid motion, Contribution to "Proceedings of the 59th Rencontres de Moriond on Gravitation" (2025), arXiv: 2508.04263.
- C. Caprini, R. Jinno, T. Konstandin, A. Roper Pol, H. Rubira, I. Stomberg, Gravitational waves from first-order phase transitions: from weak to strong, J. High Energy Phys., 07, 217 (2025), arXiv: 2409.03651.
Gravitational waves from MHD turbulence
Main collaborators
Publications:
- A. Roper Pol, S. Mandal, A. Brandenburg, T. Kahniashvili, Polarization of gravitational waves from helical MHD turbulent sources, J. Cosmol. Astropart. Phys., 04, 019 (2022), arXiv: 2107.05356.
- A. Roper Pol, Gravitational radiation from MHD turbulence in the early universe, Contribution to "Proceedings of the 55th Rencontres de Moriond on Gravitation" (2021), arXiv: 2105.08287.
- A. Brandenburg, G. Gogoberidze, T. Kahniashvili, S. Mandal, A. Roper Pol, N. Shenoy, The scalar, vector, and tensor modes in gravitational wave turbulence simulations, Class. Quantum Grav., 38, 145002 (2021), arXiv: 2103.0114.
- T. Kahniashvili, A. Brandenburg, G. Gogoberidze, S. Mandal, A. Roper Pol, Circular polarization of gravitational waves from early-universe helical turbulence, Phys. Rev. Res., 3, 013193 (2021), arXiv: 2011.05556.
- A. Roper Pol, S. Mandal, A. Brandenburg, T. Kahniashvili, A. Kosowsky, Numerical simulations of gravitational waves from early-universe turbulence, Phys. Rev. D, 102, 083512 (2020), arXiv: 1903.08585.
- A. Roper Pol, A. Brandenburg, T. Kahniashvili, A. Kosowsky, S. Mandal, The timestep constraint in solving the gravitational wave equations sourced by hydromagnetic turbulence, Geophys. Astrophys. Fluid Dyn., 114, 130 (2020), arXiv: 1807.05479.
Propagation of gravitational waves in modified theories of gravity
Main collaborators
Publications:
- Y. He, A. Roper Pol, A. Brandenburg, Modified propagation of gravitational waves from the early radiation era, J. Cosmol. Astropart. Phys., 06, 025 (2023), arXiv: 2212.06082.
Primordial magnetic fields
Multi-messenger searches of primordial magnetic fields and GWs with LISA and PTA
Main collaborators
Publications:
- A. Roper Pol, A. Neronov, C. Caprini, T. Boyer, D. Semikoz, LISA and γ-ray telescopes as multi-messenger probes of a first-order cosmological phase transition, submitted to Astron. Astrophys. (2023), arXiv: 2307.10744.
- A. Roper Pol, Gravitational waves from MHD turbulence at the QCD phase transition as a source for Pulsar Timing Arrays, Contribution to "Proceedings of the 56th Rencontres de Moriond on Gravitation" (2022), arXiv: 2205.09261.
- A. Roper Pol, C. Caprini, A. Neronov, and D. Semikoz, Gravitational wave signal from primordial magnetic fields in the Pulsar Timing Array frequency band, Phys. Rev. D, 105, 123502 (2022), arXiv: 2201.0563.
- A. Neronov, A. Roper Pol, C. Caprini, D. Semikoz, NANOGrav signal from MHD turbulence at the QCD phase transition in the early universe, Phys. Rev. D, 103, L041302 (2021), arXiv: 2009.14174.
Evolution of primordial magnetic fields in the early Universe
Main collaborators
Publications:
- A. Roper Pol, A. S. Midiri, Relativistic magnetohydrodynamics in the early Universe, submitted to Rep. Prog. Phys. (2025), arXiv: 2501.05732.
- T. Kahniashvili, A. Brandenburg, A. Kosowsky, S. Mandal, A. Roper Pol, Magnetism in the Early Universe, Contribution to "Proceedings of the IAU, FM8: New Insights in Extragalactic Magnetic Fields" (2019), arXiv: 1810.11876.
- A. Brandenburg, T. Kahniashvili, S. Mandal, A. Roper Pol, A.G. Tevzadze, T. Vachaspati, The dynamo effect in decaying helical turbulence, Phys. Rev. Fluids, 4, 024608 (2019), arXiv: 1710.01628.
- A. Brandenburg, T. Kahniashvili, S. Mandal, A. Roper Pol, A.G. Tevzadze, T. Vachaspati, Evolution of hydromagnetic turbulence from the electroweak phase transition, Phys. Rev. D, 96, 123528 (2017), arXiv: 1711.03804.
Primordial magnetic fields from chiral effect
Main collaborators
Publications:
- M. Gurgenidze, A. J. Long, A. Roper Pol, A. Brandenburg, T. Kahniashvili, Primordial magnetic field from chiral plasma instability with sourcing, submitted to Phys. Rev. D (2025), arXiv: 2512.09177.