On any given day, somewhere in the observable universe, a source of incomprehensible violence releases as much energy in a millisecond as the Sun puts out in three days — all of it in radio waves, all of it compressed into a burst so brief it can be shorter than the blink of a human eye. These are fast radio bursts, and while astronomers have now detected thousands of them, the field has an uncomfortable confession to make: we still don't know, with any certainty, what makes the vast majority of them.
That's not a failure of observation. It's a consequence of success. The Canadian Hydrogen Intensity Mapping Experiment — CHIME — a stationary radio telescope in the mountains of British Columbia that looks like four half-pipe skateboarding ramps pointed at the sky, has transformed FRB science from a field defined by scarcity into one defined by abundance. Since it began full operations in 2018, CHIME has detected thousands of bursts, catalogued their dispersion measures, tracked their sky positions, and identified dozens of sources that repeat. What it has not done, and what no instrument yet has done, is answer the central question: what are these things?
A signal with too many suspects
The first fast radio burst was discovered not in real time but in archival data. Duncan Lorimer and his student David Narkevic were combing through pulsar observations taken by the Parkes radio telescope in Australia when they found a 5-millisecond burst in 2007 data from a survey of the Magellanic Clouds. The signal's dispersion — the degree to which its lower-frequency components lagged behind its higher-frequency ones, an effect caused by intervening free electrons — was far too high to be explained by anything in our own galaxy. It had clearly traveled an intergalactic distance. This was the Lorimer Burst, and it opened a door that has never closed.
The physics of the dispersion measure is one of the more elegant tools radio astronomers use. As radio waves travel through the diffuse plasma that permeates intergalactic space, lower frequencies slow down more than higher ones. The integrated electron density along the line of sight — the dispersion measure, in units of parsecs per cubic centimeter — encodes the distance the signal has traveled. For the brightest known repeating FRB source, FRB 20121102A (also called FRB 121102), the dispersion measure put it at a redshift of roughly 0.19, placing it about 3 billion light-years away. That was confirmed by optical follow-up identifying the host galaxy: a low-metallicity dwarf galaxy in Auriga. The burst energy implied was staggering.
The leading theoretical explanation for most FRBs involves magnetars — neutron stars with magnetic fields trillions of times stronger than Earth's, capable of cracking their own crusts and unleashing electromagnetic fireworks. This picture got a dramatic boost in 2020 when astronomers detected FRB 200428, a burst that came from within the Milky Way itself, from a known magnetar designated SGR 1935+2154. Two independent observatories — CHIME and the STARE2 array in the US — detected the burst simultaneously. For the first time, there was a confirmed FRB source with an identified progenitor. It was a magnetar. Case closed, right?
Not quite. The galactic FRB was orders of magnitude less energetic than the cosmological ones, and magnetars alone may not be enough to explain the most luminous events or the peculiar timing patterns seen in some repeaters. The mystery had a partial answer, not a complete one.
The repeater problem
What makes the CHIME catalog particularly vexing is the sharp divide it has revealed between FRBs that repeat and those that, so far as anyone can tell, do not. Of the thousands of detected bursts, only a few percent come from confirmed repeating sources. The rest appear to be one-off events. Whether that distinction is intrinsic — meaning there genuinely are two populations with different physical mechanisms — or observational (the non-repeaters simply haven't been watched long enough, or repeat on timescales too long to catch) is one of the field's most contested questions.
Among the confirmed repeaters, the behavior is sometimes eerily structured. FRB 20180916B repeats on a 16.35-day cycle: active for roughly four days, then quiet for twelve. FRB 20121102A has its own apparent periodicity, around 157 days, though with less regularity. These sub-second bursts from billions of light-years away are somehow synchronized to timescales of weeks — and no one fully agrees on why. Leading candidates include orbital precession in a compact binary system, precession of the neutron star itself, or geometric beaming effects that sweep a beam in and out of our line of sight like a lighthouse. Each explanation has adherents and difficulties.
The burst morphology adds further texture. Some FRBs show drift in their sub-burst frequencies over time, a pattern called the "sad trombone" effect, where successive peaks in a burst slide to lower frequencies. This is seen consistently in repeaters and less often in apparent non-repeaters, and it may point to different emission geometries. Some bursts show fine temporal structure — sub-millisecond microstructure — that constrains the emission region to sizes smaller than a kilometer. Whatever is happening, it's happening in an extraordinarily compact space.
What CHIME changed, and what comes next
Before CHIME, the catalog of known FRBs was measured in dozens. The instrument's design — a fixed cylindrical reflector that sees the entire overhead sky as the Earth rotates, rather than pointing at individual targets — made it a firehose. Its first major catalog, published in 2021 and covering roughly one year of data, contained 536 bursts from 492 distinct sources. Updated catalogs have continued to expand that number. The statistical properties of the population — their distribution across the sky, their dispersion measures, the fluence distribution — are now beginning to constrain cosmological models in their own right.
Because intergalactic dispersion measure encodes the integrated electron density between source and observer, FRBs offer a way to probe the diffuse baryonic matter that fills the cosmic web — matter that is genuinely hard to observe by any other means. The "missing baryon problem," the longstanding puzzle of where the baryons predicted by Big Bang nucleosynthesis actually reside, may be partially resolved by FRB dispersion statistics. Several studies have already used FRB samples to constrain the baryon density of the intergalactic medium with competitive precision.
CHIME is not working alone. The Australian Square Kilometre Array Pathfinder (ASKAP) has been essential for precise localizations, catching bursts in real time and pinpointing them to specific host galaxies. The Deep Synoptic Array-110 (DSA-110) in California has pushed localizations further. The forthcoming Canadian Hydrogen Intensity Mapping Experiment/Fast Radio Bursts project (CHIME/FRB Outriggers) will use VLBI baselines spanning thousands of kilometers to localize bursts to arcsecond precision, enabling routine host galaxy identification at cosmological distances.
What that precision buys is context. When you know which galaxy an FRB came from, you can ask whether it lives in a star-forming region, an old stellar population, a spiral arm, a galactic center. The emerging picture is that FRBs don't all live in the same neighborhoods: some appear in young, star-forming dwarfs, others in massive spiral galaxies with older stellar populations. If all FRBs came from young magnetars formed in core-collapse supernovae, you'd expect them to trace star formation. Some do. Some don't. The universe, as usual, is not making this easy.
The field is poised at one of those productive moments in science where the data is outrunning the theory. The CHIME catalog has made FRBs a statistical population rather than a curiosity, revealed structure in their behavior that must mean something, and opened cosmological applications no one fully anticipated when the Lorimer Burst was found in old Parkes data in 2007. What it hasn't delivered yet is a clean, unified account of what produces them. That account is coming. The bursts keep arriving, millisecond by millisecond, from sources the universe hasn't named for us yet.
