Christmas is a time for unexplained stars, so I thought I would talk about one of the most down to Earth and yet oddest anomalies I've come across: sonoluminescence, meaning light from sound or more poetically: a underwater star. To make sonoluminescence in the lab you fill a spherical glass bulb with degassed water and emit sound waves within it at the resonant frequency of the sphere. This causes a bubble in the centre of the sphere, which then collapses repeatedly. The interesting thing is that just after the bubble reaches its minimum size of about 0.1 micrometres it emits flashes of em radiation lasting 20 ps, like a little star flashing with amazing regularity, and whose Planck spectrum indicates that the temperature in the bubble is 10,000 Kelvin, hotter than the Sun's photosphere (the bit we can see) which has made some question whether fusion might be possible on this small scale..

I've been interested in this phenomenon even since I read of it, since I'm always looking for high acceleration experiments that might demonstrate MiHsC. This is relevant because in a MiHsC-cosmology paper that I finally published this year after many years of trying (McCulloch, 2014) I showed that MiHsC predicts that if you have a 'universe' of width W, then the background temperature in it must be greater than

T > 0.2hc/2kW (1)

where h is Planck's constant, c is the speed of light and k is Boltzmann's constant. This formula, for example, predicts a Cosmic Microwave Background (CMB) for the small early universe. MiHsC does this by ensuring that the Planck wavelength of all the heat emitted in the universe must be shorter than the size of the universe otherwise it would be unobservable. It is interesting that if it is assumed that the sonoluminescent bubble is a little universe, of width 0.1 micrometres, then the temperature predicted by MiHsC at the minimum size of the bubble is

T > 14,340 K

This agrees with the temperature that is inferred from the radiation that comes off the bubble (10,000K). Of course, there are lots of other possible explanations of sonoluminescence. The popular ideas are the compression of gas within the bubble or the formation of a plasma in the centre that leads to Brehmsstrahlung, but arguments against these are the lack of observed warming of the water and the quickness of the flash (Eberlein, 1996). Julian Schwinger in his last years suggested using the dynamical Casimir effect and this idea was developed by Eberlein
(see the reference below). As always, to decide between all these suggestions, more data will be needed and it's tricky in this case because water absorbs a lot of the spectrum of the radiation emitted. A possible connection to MiHsC could be tested by looking at how the frequency of the light emitted depends on the minimum size of the bubble: using equation (1).

**References**

Eberlein, C., 1996. Sonoluminescence as quantum vacuum radiation. Phys.Rev.Lett., 76: 3842-3845. http://arxiv.org/abs/quant-ph/9506023

McCulloch, M.E., 2014. A toy cosmology from a Hubble-scale Casimir effect. http://www.mdpi.com/2075-4434/2/1/81