A stroboscope in a dark room picks out fleeting shadows from the crowd, shadows the eye cannot catch. A primordial black hole the size of a virus, passing in front of a distant star, briefly magnifies its light for a few seconds, like a magnifying glass held over a bulb. Until now, no one has caught such flashes: they are too short for ground-based telescopes, and in bright star fields, atmospheric jitter blurs them away.
The JWST telescope observes from space, where air does not interfere, but its standard images stack light over minutes, erasing rapid changes. The authors of the new study built the FLASH pipeline, which reads raw detector data and preserves each portion of light within an exposure. This turns the telescope into a stroboscope, snapping frames every 21.5 seconds.
To detect a subtle brightening, brightness must be measured to one percent accuracy. The JWST detector is nonlinear: the more light it receives, the more it "tires" and underestimates the signal. The scientists calibrated this fatigue using the data themselves, equalizing pixel sensitivity across thousands of frames. After this procedure, the precision improved enough to search for events with amplitudes of just a few percent.
The result was empty but meaningful. Over an hour of observations in each filter, no convincing flash was found. Calculations showed that if all dark matter consisted of primordial black holes with masses around one billionth of the Sun's mass, the expected number of events would be 0.03. This yields an upper limit: such objects can make up no more than about one hundred times the total dark matter.
The stroboscope has not yet caught any shadows, but it has proven it can see them. The current observations were not optimized for this purpose: the telescope often changed its pointing, and filters were chosen for other goals. If JWST were aimed at a single Andromeda field and observed it continuously for hours, the sensitivity would increase by tens of thousands of times. Then either the first flashes from primordial black holes would appear, or this dark matter candidate would finally fade into the shadows.
At a mass of $10^{-9}$ solar masses, a primordial black hole has a Schwarzschild radius of about 3 nanometers, comparable to the size of a virus, yet its gravitational field can brighten a star in the Andromeda galaxy by a few percent.
The idea of invisible compact objects distorting the light of distant stars echoes the novel 'Roadside Picnic' by the Strugatsky brothers, where anomalous zones are left by mysterious aliens, although here the focus is on natural dark matter candidates.