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‘Rainbow gravity’ theory could shed light on black holes’ endgame

Published online 10 June 2014

Zeeya Merali

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‘Rainbow gravity’ — an alternative theory of cosmology — could resolve a decades-long mystery about the fate of black holes, according to a new analysis published in Physical Review D1.

Black holes were first predicted to exist by Einstein’s theory of gravity, general relativity. They have such an immense gravitational pull that they swallow all matter, and even light itself.

In the 1970s, British physicist Stephen Hawking uncovered a paradox about black holes. He proved that black holes slowly radiate energy, gradually growing smaller, until they vanish2. The question was, when they disappear, where does the information about the swallowed matter go? This information seems to vanish from the Universe, but the laws of physics state that this is impossible because information cannot be destroyed.

Now, Ahmed Farag Ali, a physicist at Zewail City of Science and Technology in Egypt, has a solution. He re-analysed the evolution of black holes using an alternative ‘rainbow gravity’ proposed ten years ago3. In Einstein’s theory, all radiation follows the same set of paths when escaping from a black hole, no matter what the energy of the radiation is. By contrast, the path taken by radiation escaping from so-called ‘rainbow black holes’ depends on its energy. 

In rainbow models, Ali discovered, the black hole’s temperature reaches a peak when the black hole has shrunk down to a certain small size. After that, the black hole’s mass will decrease slightly more, but its temperature rapidly drops, until it reaches absolute zero. At this point, the black hole stops radiating, leaving behind a tiny “black hole remnant,” says Ali. “There is no longer a mystery about what happens to the information  it stays in this remnant forever.”


  1. Ali, A. F. Phys. Rev. D 89, 104040 (2014)
  2. Hawking, S. W. Nature 248, 30–31 (1974)
  3. Magueijo J. & Smolin L. Classical and Quantum Gravity 21: 1725–1736 (2004)