BOUNCING BLACK HOLES

Image: Illustration of a dusty supermassive black hole. Credit: ESA/NASA, the AVO project and Paolo Padovani

Black holes could be bouncing stars as a consequence of quantum gravity: when the density of matter becomes high enough, quantum gravity effects generate sufficient pressure to compensate the matter's weight, the collapse ends, and matter bounces out. In a black hole, matter's collapse could stop before the central singularity is formed.



The standard event horizon of the black hole can be replaced by an apparent horizon which is locally equivalent to an event horizon, but from which matter can eventually bounce out.

It was shown there that primordial black holes could generate a signal in the 100 MeV range, possibly compatible with very fast gamma-ray bursts.

The scenario was developed with the discovery of an explicit metric satisfying Einstein’s equations everywhere outside the quantum region. The model describes a quantum tunneling from a classical in-falling black hole to a classical emerging white hole. The process is seen in extreme "slow motion" from the outside because of the huge time dilatation inside the gravitational potential: this is why massive black holes would appear to us as long living black holes. Only light black holes -as primordial black holes- are expected to yield observational signatures of this model because the time required for the bounce to occur can then be smaller that the current age of the Universe.

After a sufficiently long time, the black hole can tunnel to a white hole. This phenomenon is similar to the cosmological bounce studied in loop quantum cosmology where a contracting universe tunnels into an expanding one.
The white hole "fireworks" emerging from the bouncing black hole takes place where the black hole is.

In a recent paper (Barrau et al 2016) the authors investigate the astrophysical signals implied by this hypothesis, focusing on primordial black holes. They consider different possible bounce times and study the integrated diffuse emission.

They suggest that the shape of the signal might be used as an observational signature of its specific origin in the high energy case. It looks indeed like a slightly distorted blackbody law that is not to be expected from any other known astrophysical effect. In the low energy case, the situation is less clear as the accurate shape (in particular width) on the signal is unknown but, still, quite generic features do appear in the figures, leading to some hope for detection.

• Barrau et al. 2016 - Phenomenology of bouncing black holes in quantum gravity: a closer look (arXiv)