The Universe is expanding, but at what speed? Astronomers have been arguing for decades about the precise value of the Hubble constant. Two classic methods give slightly different results. Now there's a third — using mysterious radio bursts.
The method is akin to measuring the depth of a well by dropping a handful of sand: heavier grains hit the bottom faster than lighter ones. Here, instead of sand grains, we have radio waves of different frequencies, and instead of air, clouds of electrons in space. Low frequencies are slowed more and lag behind high frequencies. From the delay, scientists calculated the distance to the source, and then the expansion rate. After analyzing more than 2,100 bursts from the CHIME telescope catalog, they arrived at a result: about 74 km/s per every 3.26 million light-years. The uncertainty is still 18%, but
Amazingly, individual bursts emit more energy in a thousandth of a second than the Sun does in 100 years. An independent measurement of the Hubble constant will help settle the dispute, while also refining the age and fate of the Universe.
as written by the authors arXiv:2604.03769
We present constraints on the Hubble constant ($$H_0$$) derived from the observed dispersion measure (DM) distribution of unlocalized fast radio bursts (FRBs). While localized FRBs with redshift measurements have been used to investigate the Hubble tension, their sample remains limited. Here we demonstrate that unlocalized FRBs -- which are far more numerous -- can independently constrain $$H_0$$ without requiring redshift information, as cosmic expansion imprints itself on their DM distribution. Analyzing a selected sample of 2124 unlocalized FRBs from the CHIME Catalog II, we obtain $$H_0 = 73.8^{+14.0}_{-12.3}~\mathrm{km\,s^{-1}\,Mpc^{-1}}$$ at the $$1\sigma$$ confidence level, corresponding to an uncertainty of about 18%. Breaking the degeneracy between $$H_0$$ and the characteristic cutoff energy $$E_*$$ of the FRB isotropic energy distribution would reduce this uncertainty to 9%. This work constitutes the first $$H_0$$ measurement derived solely from the DM distribution of unlocalized FRBs, highlighting their potential as a new cosmological probe. Future joint analyses with localized FRBs promise even tighter constraints.
The most powerful radio burst on record released as much energy in milliseconds as our Sun produces in an entire century.