Why does a collapsing bubble in water flash with light (sonoluminescence)?

Partly settled. Some of it is understood, some is still argued about.

First observed as multi-bubble 'sonoluminescence' in the 1930s and isolated as a stable, repeatable single-bubble effect in 1989 (Gaitan, Crum, Church, Roy), the phenomenon involves a micron-scale gas bubble in water, driven by an acoustic standing wave, collapsing so violently that it emits a picosecond-scale flash of light with an effective temperature of many thousands of kelvin. The leading 'standard model,' reviewed by Brenner, Hilgenfeldt, and Lohse in their comprehensive 2002 Reviews of Modern Physics article, attributes the light to adiabatic compressional heating of the gas, possibly aided by an imploding shock wave, causing partial ionization and thermal (bremsstrahlung-like) emission -- but rival proposals (chemiluminescence from chemical reactions in the heated gas, electrical breakdown, and more exotic quantum effects) have each had periods of serious consideration, and even recent studies have found cases where strong shock waves are not accompanied by bright light emission and vice versa, suggesting the shock-heating and light-emission processes may be only loosely coupled rather than one simply causing the other.

Slamming a sound wave into water can squeeze a bubble hard enough to briefly recreate temperatures found on the surface of a star, and nobody has fully nailed down exactly how.

Explanations that circulated

• The light is simple thermal blackbody radiation from a uniformly heated gas, fully accounting for the observed spectrum and flash timing
• The effect is caused by an exotic quantum vacuum (dynamical Casimir) process, an idea that gained attention but has been largely discounted by follow-up analysis

References

• Single-bubble sonoluminescence, M. P. Brenner, S. Hilgenfeldt, D. Lohse, Reviews of Modern Physics, 2002