Photon rest mass from localized fast radio bursts with improved distribution of dispersion measure from extragalactic gas

TL;DR

This work tests the photon rest mass using fast radio burst dispersion measures by deriving a mass-induced dispersion component $DM_\gamma \propto m_\gamma^2$ that depends on the cosmological history via $H_\gamma(z)$. It introduces a cosmology-aware extragalactic DM distribution with redshift-evolving parameters $(\alpha, \beta, \sigma_{\mathrm{cosmic}})$ and a corrected normalization for $P_{\mathrm{cosmic}}(\Delta)$, calibrated against mock catalogs. A joint analysis of 104 well-localized FRBs together with SN Ia Pantheon+, Planck 2018 CMB, and DESI DR2 BAO provides 1σ upper limits on the photon mass of $m_\gamma \lesssim (4.7-4.9)\times 10^{-51}$ kg across $\Lambda$CDM, $w$CDM, and $w_0w_a$CDM, showing only weak dependence on the cosmological model. The results reinforce the massless nature of the photon and highlight the robustness of FRB-based constraints when the DM distribution is treated with cosmology-aware normalization and redshift evolution. The study also clarifies biases in earlier FRB analyses that used the missing normalization, improving reliability for fundamental physics tests with FRBs.

Abstract

The assumption that photons are massless is a foundational postulate of modern physics, yet it remains subject to experimental verification. Fast radio bursts (FRBs), with their cosmological distances and precisely measured dispersion, offer an excellent laboratory for testing this hypothesis. In this work, we propose an improved distribution function for the dispersion measure arising from extragalactic gas and demonstrate that it provides an excellent fit to mock data. We then apply this distribution to constrain the photon rest mass under the $Λ$CDM, $w$CDM, and $w_{0}w_{a}$CDM cosmological models, the last of which is favored by recent DESI baryon acoustic oscillation observations. The corresponding 1$σ$ upper limits on the photon mass are found to be $4.83\times10^{-51}\,\mathrm{kg}$, $4.71\times10^{-51}\,\mathrm{kg}$, and $4.86\times10^{-51}\,\mathrm{kg}$, respectively, which are the most stringent constraints derived from FRBs to date. These results indicate that the choice of cosmological model has only a minor impact on photon-mass bounds, demonstrate that FRBs provide robust and reliable constraints, and offer strong empirical support for the massless nature of the photon.

Figures (6)

• Figure 1: 1D marginalized posterior distribution and 2D 1-2$\sigma$ contour regions for totally parameters constrained by a combination of FRB, CMB, BAO and SN Ia in the $\Lambda \mathrm{CDM}$ model.
• Figure 2: Constraints on photon mass and free parameters in the $w \mathrm{CDM}$ model.
• Figure 3: Constraints on photon mass and free parameters in the $w_{0}w_{a}\mathrm{CDM}$ model.
• Figure 4: The upper limits on the photon rest mass from FRBs, derived from both previous works and the current study, are presented. Cyan points represent constraints obtained from previous studies, while the three green ones correspond to our results within the framework of three different cosmological models. The red dashed line indicates the average upper limit on $m_\gamma$ across the three cosmological models considered in this work.
• Figure 5: Variations of $\alpha$, $\beta$ and $\sigma_\mathrm{cosmic}$ with redshift $z$.