Cosmology-Independent Photon Mass Limits from Localized Fast Radio Bursts by Using Artificial Neural Networks

A hypothetical photon mass m_γ can produce a frequency-dependent vacuum dispersion of light, which leads to an additional time delay between photons with different frequencies when they propagate through a fixed distance. The dispersion measure and redshift measurements of fast radio bursts (FRBs) have been widely used to constrain the rest mass of the photon. However, all current studies analyzed the effect of the frequency-dependent dispersion for massive photons in the standard ΛCDM cosmological context. In order to alleviate the circularity problem induced by the presumption of a specific cosmological model based on the fundamental postulate of the masslessness of photons, here we employ a new model-independent smoothing technique, artificial neural network (ANN), to reconstruct the Hubble parameter H(z) function from 34 cosmic-chronometer measurements. By combining observations of 32 well-localized FRBs and the H(z) function reconstructed by ANN, we obtain an upper limit of m_γ ≤ 3.5 × 10⁻⁵¹ kg, or equivalently m_γ ≤ 2.0 × 10⁻¹⁵ eV/c² (m_γ ≤ 6.5 × 10⁻⁵¹ kg, or equivalently m_γ ≤ 3.6 × 10⁻¹⁵ eV/c²) at the 1σ (2σ) confidence level. This is the first cosmology-independent photon mass limit derived from extragalactic sources.