In 2019, something surprising happened in the cosmos. Astronomers spotted a star that brightened unexpectedly for about an hour. This change didn’t match any typical star behavior. It was too prolonged for a flare, too brief for a supernova, and too smooth to be a known stellar fluctuation.
After careful examination, a team led by Renee Key from Swinburne University of Technology in Australia proposed a unique explanation. They believed the event could signify a tiny primordial black hole, just three times the mass of our Moon. If it exists, this black hole, named Phoebe, would have an event horizon about the size of a period at the end of a sentence. The researchers pointed out that Phoebe could hint at a population of small, compact objects tied to dark matter in the Milky Way, potentially reshaping our understanding of cosmic inflation.
Typically, we think of black holes as enormous objects, often formed from the collapse of massive stars. However, primordial black holes might have emerged shortly after the Big Bang due to quantum fluctuations in space-time. These could collapse like stellar cores but would be much smaller—an Earth-mass black hole would measure only about 1.8 centimeters across.
While such small black holes are hard to detect using current instruments, they can reveal themselves in other ways. They bend the light of objects behind them, causing what’s known as microlensing. This was the case when the Dark Energy Camera observed a mysterious brightness increase from a star in the Large Magellanic Cloud in December 2019. The light flared for about 60 minutes, while surrounding stars remained dim.
Microlensing events are rare but can also be caused by rogue planets or stellar-mass black holes. In a thorough study, the team ruled out standard interference and modeled various scenarios, concluding that Phoebe is more likely a primordial black hole within the Milky Way’s dark matter halo than anything else.
The idea of primordial black holes has been the subject of heated discussion in the astronomical community. A recent analysis of data from the Subaru Telescope identified several microlensing candidates near the Andromeda galaxy, stirring debate on whether they indicate primordial black holes or just known stars. Key and her team believe their findings support the hypothesis of primordial black holes and emphasize the need for advanced telescopes like the planned Roman and Vera C. Rubin Observatory to further explore these cosmic mysteries.
The implications of discovering something like Phoebe are significant. Beyond reshaping our understanding of dark matter, it could provide new insights into fundamental physics, linking early universe conditions with current cosmic structure.
For those intrigued by the potential of primordial black holes, a detailed exploration can be found in their paper here.
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