We've all heard of black holes. But what about white holes?
A black hole forms when a massive star at the end of its life shrinks catastrophically under its own gravity down to an infinitesimally small point, or singularity. All that is left behind from the stellar collapse is a grossly warped region of space, so where does the star’s matter actually go? Surely it must go somewhere?
One possibility is that it explodes into another universe as a white hole. So far, white holes are purely hypothetical objects, but astronomers are contemplating how they could form in reality.
The white hole has to be in another universe, because the defining characteristic of a black hole is that the event horizon, which hides the singularity from view, is one-way. In other words, things that fall through it can never emerge again in our Universe.
It is postulated that for matter to pass between universes, the black hole and white hole must be connected by what's called a wormhole - a tunnel through spacetime. Some scientists suggest super-energetic objects such as quasars are actually white holes. And, perhaps most intriguing of all, is that the Big Bang was a gigantic version of this.
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