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There is more to the Universe than meets the eye, at least, if you’re an astrophysicist. For some time, these guys have been scratching their heads over the puzzling lack of mass out there in the cosmos.

The trouble is that galaxies do not contain enough visible mass to generate the gravity necessary to hold them together. Without this extra mass, galaxies ought to fly apart.

This is where the idea of dark matter comes from. Some kind of force must be holding galaxies together. So astrophysicists imagine that galaxies must be filled with invisible, dark matter that provides the necessary extra tug.

But to generate enough gravity, dark matter must be vastly more common than the stuff we can see. We must be swimming in it but unable to detect it for some reason. At least not easily.

When theoretical physicists attempt to create mathematical models of this kind of universe, they find something interesting. Dark matter particles must interact with each other in some way and the mediator of this kind of exchange would be a photon-like particle. So the universe must also be streaming with so-called hidden photons.

There doesn’t seem much prospect of ever seeing hidden photons but there is a way we might see their effects because they ought to interact very weakly with ordinary photons. And by the probabilistic laws of quantum mechanics, ordinary photons must sometimes become hidden ones and vice versa, oscillating back and forth, albeit very rarely.

Experiments are already afoot to spot this kind of oscillation. The idea is to shine a powerful laser at a wall with a photon detector on the other side. Since ordinary photons cannot pass through walls, any that do pass through must have turned briefly into hidden photons and back again. We looked at one these experiments called ALPS last year.

The oscillations are so rare, however, that you need to generate huge numbers of photons. ALPS creates 10^19 per second and even that looks like too few.

Today, Davide Cadamuro and Javier Redondo from the Max Planck-Institute fur Physik in Munich, Germany, say there is another option. Why not just stare at the Sun, they point out. But from a behind a wall, obviously.

It turns out that the Sun may well be a rich source of hidden photons. The creation of ordinary and hidden photons both generates neutrinos, which we can also measure. Cadamuro and Redondo point out that the number of neutrinos match the number ordinary photons only to within 10 per cent. So that leaves plenty of scope for the difference to be made up by hidden photons.

But calculating how many of these hidden photons would get to us is tricky because the oscillation into ordinary photons is very sensitive to conditions inside the Sun. It may be that the switch to ordinary photons is highly unlikely to occur at the distance of Earth’s orbit

Nevertheless, various groups, such as the solar hidden photon search (SHIPS) at Hamburg Observatory, have started wall watching experiments in the hope of spotting the odd anomalous photon that gets through.

Good luck to them–staring at a wall can’t be an easy job. But it’ll certainly be worthwhile if they find any.

Ref: arxiv.org/abs/1010.4689: Hidden Photons from the Sun

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