In December 2019, astronomers detected a brief, one-hour brightening of a star in the Large Magellanic Cloud — a classic gravitational microlensing event. Such events occur when a compact object passes in front of a distant star, bending its light. The object responsible, dubbed Phoebe, has a mass roughly three times that of our Moon. That mass is far too small for a stellar black hole, but aligns with predictions for a primordial black hole formed moments after the Big Bang.
Primordial black holes are theoretical remnants from the early universe, created not by collapsing stars but by extreme density fluctuations in the first instants after the Big Bang. Unlike their stellar counterparts, they can be tiny in mass — even asteroidal or lunar-scale. The detection of one such object would provide direct evidence for these elusive bodies and open a new window into early-universe physics.
This candidate emerged from a single microlensing event, but the data cannot yet confirm its true nature. Other possible explanations exist, such as a free-floating exoplanet or a brown dwarf — though those would typically have higher masses. Follow-up observations or additional microlensing events with similar mass signatures will be needed to strengthen or refute the primordial black hole hypothesis.
The finding was reported by Universe Today and highlights the growing power of microlensing surveys to probe the dark, low-mass population of the cosmos.
Critics caution that without multiple observational signatures — such as an absence of thermal emission or parallax signals — Phoebe remains a compelling but unconfirmed candidate. The broader search for primordial black holes continues, with upcoming surveys like the Vera Rubin Observatory expected to spot many more microlensing events in the next decade.