Short answers, pulled from the story.
The Shiva hypothesis is a scientific theory concerning impact events that suggests gravitational disturbances occur when the Solar System crosses the plane of the Milky Way galaxy. William Napier and Victor Clube published this paper in the journal Nature during 1979 under the title A Theory of Terrestrial Catastrophism.
The hypothesis suggests these large impacts happen roughly every 30 million years. Evidence points to intervals near 30 million years between major impacts, with one such event marking the Cretaceous Paleogene extinction.
During the 1990s, Rampino and Bruce Haggerty renamed the theory after Shiva. This Hindu god represents destruction within ancient traditions and their new title became known as the Shiva hypothesis.
Critics argue that assuming all extinctions share one cause is flawed because evidence suggests most extinction events result from a variety of different causes. These diverse triggers are unlikely to be cyclically induced by a single mechanism according to geological records.
Some models suggest the Sun passes through higher density spiral arms instead of crossing the galactic plane while another theory proposes a brown dwarf named Nemesis drives cycles every 26 million years. Critics note these alternative explanations lack consistent observational support compared to the original proposal.