Do all spiral galaxies have black holes at their centre? © Getty Images

What is a white hole?

Asked by: Dylan Evans, Cardiff

A white hole is a bizarre cosmic object which is intensely bright, and from which matter gushes rather than disappears. In other words, it’s the exact opposite of a black hole. But unlike black holes, there’s no consensus about whether white holes exist, or how they’d be formed.

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They are predicted by Einstein’s theory of gravity, and are most often mentioned in the context of ‘wormholes’, in which a black hole acts as the entry point to a tunnel through space and time, ending in a white hole somewhere else in the Universe. But this is deeply controversial, because Einstein’s theory predicts the existence of a so-called singularity at the centre of black holes – a state of infinite gravity which would prevent anything from passing through to the white hole on the other side.

However, some theorists think that a combination of Einstein’s theory and quantum theory points to a new way of thinking about white holes. Instead of being the ‘exit’ from a wormhole, they may be a slow-motion replay of the formation of the original black hole.

What is a white hole?

The process starts when an old massive star collapses under its own weight and forms a black hole (see diagram, above). But then, quantum effects occurring around the surface of the black hole halt further collapse to a singularity, and instead begin to gradually turn the black hole into a white hole that’s spewing out the original star matter again. The process is mind-bendingly slow, though, so we may be in for a very long wait to find out if white holes really exist.

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