Trans-Planckian origin and laboratory analogs of Hawking radiation

6 problems

• How does Hawking emission change the analog horizon that produces it?

  Partially resolved

  A real black hole should shrink as it radiates; in a condensate the emitted sound should likewise change the flow that makes the horizon. Theory now predicts how much the flow is changed, but no experiment has measured the change.

• Independent replication of spontaneous Hawking radiation in an atomic condensate

  Open

  One laboratory has reported that a flowing Bose-Einstein condensate (a gas cooled until its atoms move in lockstep) emits Hawking-like sound from quantum fluctuations alone. The question is whether a second, independent group sees the same emission with the predicted temperature.

• Certify vacuum origin and entanglement of analog cosmological pair creation

  Open

  In an expanding universe, empty space should produce pairs of particles; ramping or expanding a condensate produces pairs of sound waves in the same way. Experiments see the pairs, but have not yet shown that they come from quantum vacuum fluctuations and are entangled rather than amplified heat.

• How accurately does Hawking emission survive modified short-distance dispersion?

  Partially resolved

  In a fluid, sound waves stop behaving like ordinary sound at very short wavelengths, just as light might at the tiny Planck length. The question is to put an exact bound on how much this changes the thermal glow from a sonic horizon.

• Are analog Hawking quanta quantum entangled with their partners?

  Contested

  Hawking particles should be born in pairs that are entangled, meaning linked more strongly than any classical correlation allows, with one member escaping and one falling in. Experiments so far infer this link from density measurements alone, and it is disputed whether that is enough.

• Where do Hawking quanta of a real black hole originate?

  Contested

  Traced backwards in time, each Hawking particle came from a wave squeezed to a wavelength far below the Planck length (about $1.6 \times 10^{-33}$ cm, the scale where gravity itself should behave quantum mechanically), where known physics fails. Which short-distance physics actually supplies these particles, and does the answer change the predicted temperature?