Cosmological gravitational waves and their interaction with large-scale magnetic fields

by Mr Yutong He (Stockholm University)

Wednesday 10 Apr 2024, 10:30 → 11:30 Europe/Stockholm

FC61 (AlbaNova Main Building)

Gravitational waves (GWs) and magnetic fields are two essential topics of cosmology. They interact in different ways in the early and late Universe.

In the radiation era preceding the Cosmic Microwave Background (CMB), stochastic GW backgrounds (SGWBs) can be produced by magnetohydrodynamic (MHD) turbulence after the phase transitions at the electroweak (EW) and quantum chromodynamic (QCD) scales. We develop a general framework to study the SGWB spectrum in general relativity (GR) and modified gravity (MG) theories, with massive gravity and scalar-tensor theories as two examples. We then apply the framework to the MHD-GW system at the EW and QCD phase transitions to provide concrete constraints on effective MG parameters in light of ongoing and upcoming GW experiments, such as the Laser Interferometer Space Antenna (LISA) in the mHz band, and various pulsar timing array (PTA) collaborations in the nHz band.

In the post-CMB Universe, high-frequency GWs (HFGWs) propagating through large-scale magnetic fields have a probability of converting into photons of the same frequency. This conversion contributes to the distortion of blackbody CMB. Using the reported excess radio signals over the CMB, we place upper limits on the HFGW amplitude. We also forecast the capability of the Square Kilometer Array (SKA) and proposed future CMB surveys to significantly tighten the existing HFGW upper bounds in the MHz to THz frequency bands.

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