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In 1939, Albert Einstein declared the non-existence of black holes in nature. Fast forward a quarter-century, and the discovery of quasars challenged this notion. These enigmatic point-like sources of intense light at cosmological distances were later attributed to supermassive black holes at the centers of galaxies. Could it be that one such black hole influenced the evolution of life on Earth?
When gas flows toward a black hole, it swirls like water down a drain, heating up and glowing brightly in the process. This accretion disk can outshine an entire galaxy. Quasars, the visible manifestations of these feeding frenzies, can be seen from the edge of the visible universe. Yet, almost every galaxy hosts a supermassive black hole at its center, mostly starved but occasionally bursting into activity.
The Nobel Prize in Physics was awarded this year to Andrea Goetz and Reinhard Genzel for confirming the presence of a black hole at the center of our own Milky Way. This "monster" is currently dormant, but it once glowed as the radio source Sagittarius A*. Could this black hole have played a role in Earth's history?
Clues suggest that Sagittarius A experienced episodes of vigorous feeding. Stars and gas clouds near the galactic center could have been tidally disrupted, triggering quasar-like flares. The presence of young stars in preferred planes around Sagittarius A indicates that they formed from planar gas disks, similar to our own solar system.
Could these flares have implications for life on Earth? In collaboration with John Forbes, it was shown in 2018 that the XUV radiation emitted during such flares could evaporate the atmospheres of Mars or Earth if our solar system had been closer to the galactic center. Even at greater distances, the radiation could suppress the growth of complex life.
Recent studies suggest that the Sun's birthplace might have been closer to the galactic center. If true, the Sun's migration away from Sagittarius A* could have exposed early Earth to harmful XUV flares, potentially shaping the rise of oxygen levels in our atmosphere. This migration might be the reason complex life on Earth flourished only after the Sun moved to its current location.
Traditionally, the Sun was seen as the sole astronomical influence on Earth's life. However, the possibility that Sagittarius A* played a crucial role in shaping our planet's history is a fascinating revelation. Just as a stranger might influence your family history before you were born, so too might a cosmic giant have shaped the story of life on Earth.
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