The night of March 4, 2024. The MeerKAT radio telescope in South Africa catches a millisecond pulse: a fast radio burst coming from a depth of 17.4 gigaparsecs. This is light that traveled 10.5 billion years to reach Earth. At that moment, the universe was only three billion years old, a youth when galaxies were birthing stars at a furious rate. Such a distant burst is like a lighthouse flashing in the fog of ancient cosmos: it not only announces itself but also illuminates the invisible web of gas and magnetic fields along the entire path of the beam.
This burst is named FRB 20240304B. Its dispersion measure, the delay of the radio signal in ionized plasma, is 2458.20 ± 0.01 pc/cm³. The more plasma along the path, the more low frequencies lag behind high ones. Astronomers use this delay as a scale: it allows weighing all the ionized matter between us and the source. But for the scale to work, the distance must be known. And that is where the hunt began.
Ground-based telescopes Keck and MMT did not see the host galaxy: it was too faint. Then the James Webb Space Telescope stepped in. Its NIRCam camera found a tiny galaxy 0.3 arcseconds from the burst position, and the NIRSpec spectrograph measured the redshift: z = 2.148 ± 0.001. The luminosity distance is 17.4 Gpc. This is twice as far as any previously localized fast radio burst. The galaxy turned out to be a dwarf: its stellar mass is only about $10^7$ solar masses, and the star formation rate is 0.2 solar masses per year. For such a small galaxy, this is a storm: the specific star formation rate is above the main sequence, and the entire stellar mass formed in 52 million years. The burst is strongly linearly polarized (49%), and the rotation measure in the rest frame reaches −550 rad/m². This means the burst was born in dense magnetized plasma, as expected for a young magnetar, a neutron star with an extremely strong magnetic field.
Previously, it was unknown whether fast radio bursts could occur at redshifts greater than two. Now it is clear: they can, and they are born in the era of cosmic noon, the peak of star formation. The properties of the host galaxy indicate a short delay between the starburst and the appearance of the burst. This rules out scenarios with long delays, such as neutron star mergers, for most FRBs. The practical benefit is enormous: such bursts become probes for tomography of the cosmic web and measurement of magnetic fields on scales of billions of light-years. With the commissioning of SKA-Mid and new space telescopes, the number of FRBs at z>2 will multiply, and then the Macquart relation, the connection between dispersion measure and redshift, can be tested in the distant universe, refining the Hubble constant and even peering into the epoch of reionization. The beacon from the universe's youth did not just flash: it illuminated the way.
The light from FRB 20240304B traveled to Earth for 10.5 billion years, about three-quarters of the age of the universe. During that time, galaxies, stars, and planets formed on Earth, and radio telescopes appeared that caught this millisecond pulse.