Surajit Kalita (Warsaw University Observatory)
Fast Radio Bursts (FRBs) are millisecond radio transients with high dispersion measures, making them powerful tracers of ionized matter across cosmological distances. In this talk, I present two complementary approaches, Bayesian analysis and machine learning, applied to a set of localized FRBs to rigorously test the consistency of the $\Lambda$CDM model at late cosmic epoch. Our results reveal a redshift-dependent variation in the inferred Hubble constant, a behavior that stands in contradiction to the core postulate standard cosmology. I will further show that this discrepancy can be resolved for alternate cosmological models. These findings suggest a fundamental inadequacy in the standard cosmological framework and necessitate a deeper revision of the theoretical underpinnings of cosmology to resolve the Hubble tension.
Mirosław Kicia (CAMK PAN, Warsaw)
Pratik Dabhade (National Center for Nuclear Research (NCBJ), Warsaw)
Anabella Araudo (Institute of Physics, Czech Academy of Sciences)
Fast Radio Bursts (FRBs) are transient episodes of intense, coherent radio emission lasting from microseconds to milliseconds. While the origin of FRBs remains uncertain, most are detected at extragalactic distances. Notably, the repeating FRB 200428 has been associated with the Galactic magnetar SGR 1935+2154. We propose a new model for FRB emission from SGR 1935+2154, where streaming instabilities in a baryon-loaded expanding fireball with different electron and ion temperatures forms density cavities filled with electrostatic fields. Using one-dimensional particle-in-cell kinetic simulations, we constrain the size of these plasma cavities and characterize the electrostatic fields. The resulting FRB emission originates from the coherent Bremsstrahlung of relativistic particle bunches accelerated within the cavities. Our model reproduces the observed radio fluxes of FRBs from SGR 1935+2154 with a small coherence parameter. The relationship between the wave coherence scale and the electric field amplitudes indicates that harmonic emission is several orders of magnitude weaker than the fundamental emission. Moreover, unlike previous models that attribute cavity formation to Langmuir collapse in pair plasmas, our results show that these structures can continue to generate ion acoustic waves after Langmuir saturation. Detecting harmonics in FRB observations, or placing upper limits on their luminosity, can help discriminate among emission mechanisms and constrain the electron–ion or electron–positron composition of magnetar environments during such events.
