Is the Sun a Dark Matter Factory?

Hinode X-ray observation of a solar sigmoid (David McKenzie/Montana State University)
The hypothetical axion is a particle that might help scientists work out where the bulk of dark matter may be held in the Universe. So far, there has been much talk about the search for another type of hypothetical particle, the weakly interacting massive particle (WIMP), and little attention has been paid to the lowly axion. WIMPs are very appealing to scientists as proving they exist will help patch some holes in quantum theory. What’s more, WIMP detectors need to be huge, large volumes of underground caverns filled with hi-tech sensors and cleaning fluid – this makes for a cool funding proposal; think up and grand idea, explain that it will prove our understanding of the Universe and then receive a multi-billion $/£/€ cheque (it’s not quite as easy as that, but there are socioeconomic and political reasons for building such an awesome structure).

So how do you go about finding an axion? Surely this exotic particle will need an even bigger detector, especially as it has zero charge, very low mass and cannot interact via the strong and weak nuclear forces? Actually, a large WIMP-type detector would be useless for axion detection. Fortunately axions have a neat interaction with magnetic fields that can be detected with existing instrumentation. What produces the strongest magnetic field in the Solar System? This is where the Sun can help out…

The Universe is made up of a huge proportion of dark matter. We know it is out there as we can indirectly deduce its existence through its gravitational effect on visible objects like stars and galaxies. But what exactly is “dark matter”? Dark matter is theorized to be made up from baryonic matter and non-baryonic matter. “Baryonic dark matter” may be held in large astronomical objects that emit little radiation, such as neutron stars or black holes (usually known as massive astrophysical compact halo objects, or MACHOs for short). “Non-baryonic dark matter” is stuff that isn’t made up of baryons (neutrons or protons). WIMPs and axions are considered to be two major non-baryonic particles, as are more well-understood particles such as neutrinos.

Non-baryonic matter is notoriously difficult to detect, as often these particles will be of low mass and zero charge. As axions are so small and fast moving, they cannot use the strong or weak force to interact with baryonic matter. WIMPs on the other hand can interact via the weak nuclear force as they are more massive and slower moving. This is why, although they are still very difficult to detect, by building a large chamber filled with fluid, occasionally WIMPS should (theoretically) hit ordinary molecules, sending out a spark of Cherenkov radiation. Axions are less likely to do this as their interaction cross section is much smaller (as they are low mass) and they are thought to travel very close to the speed of light.

Axions do however have an interesting quality; they are thought to turn into photons when passing through a strong magnetic field.

Hugh Hudson of UC Berkeley thinks he has a method to detect these strange particles by analysing the light from the Sun. His theory goes a little like this: some X-ray photons generated in the solar core (at a temperature of around 10 million Kelvin) will experience the Sun’s magnetic field, turning into axions. As the axions are weakly interacting, they will slip straight through the core and to the solar surface as if there was nothing there. Usually, photons would undergo countless absorption/emission events through the solar body, eventually escaping hundreds of thousands of years later (the path of a solar photon from core to surface is commonly referred to as the “drunkards walk”). The axions will essentially take a shortcut.

As the axion energy has not been lost through interactions with any solar matter, it has pretty much the same energy it had when it was generated seconds earlier. As the axion encounters the powerful magnetism in the solar corona (the Sun’s atmosphere) it could change again, but this time into an X-ray photon. This is where Hudson hopes to detect the signature of this strange particle by using existing data gathered by solar observatories such as Yohkoh, the High Energy Solar Spectroscopic Imager (RHESSI) and Hinode. None have been detected so far, but the Berkeley group are working hard to separate axion signature from solar “noise”.

Looking for the signature of axions generated in the Sun doesn’t stop with space-based observatories. The CERN Axion Solar Telescope in Geneva has its own superconducting magnet to artificially change any intact axions that have travelled all the way to Earth back into an X-ray photon.

If that wasn’t extreme enough for you, astrophysicist Karl van Bibber and his team at the Axion Dark Matter Experiment, Lawrence Livermore National Laboratory in California, wants to create their own axions by generating a powerful magnetic field and then looking out for the microwave signal after an axion decays into a single, real photon. Bibber’s explanation for the process is sheer quantum joy:

The axion is so light that it doesn’t decay into two photons in free space. However, you can play a very remarkable trick. If I shoot a photon into a magnetic field (which you can think of as a sea of virtual photons), a real photon and a virtual photon [interact] to make an axion and vice versa.” – Karl van Bibber


Whether astrophysicists actually discover these ghostly particles is anyone’s guess, but it would seem that the axion hunt is beginning to catch up with all those WIMP detectors out there. Perhaps axions will be as common as neutrinos in a decade or so…

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6 thoughts on “Is the Sun a Dark Matter Factory?”

  1. Originally posited to account for the apparent anomalous shape of galaxy rotation curves, the search for dark matter, whether in the form of WIMPS, Axion’s, SUSY, Unparticles, etc. has absorbed a huge amount of both time and money. While other phenomena, like gravitational lensing and cluster dynamics seems to support dark matters existence I suspect we may be introducing unnecessary complexity into our models of the physical universe. It may become increasingly difficult to reconcile dark matter with other observations. Recent work on galaxy rotation curves appears to suggest that neither non-baryonic matter, nor Milgrom’s MOND are necessary to correctly model these data: see, for example, arXiv:0806.1513v1 [astro-ph] 9 Jun 2008 “Galaxy rotation curves without non-baryonic dark matter and modifications to gravity: effect of the Ampere force” and “Newtonian mechanics & gravity fully model disk galaxy rotation curves without dark matter” by Dilip G. Banhatti (School of Physics, Madurai-Kamaraj University, Madurai 625021, India) who presents Kenneth F Nicholson’s approach to the problem in arXiv: astro-ph/0309823 : “Errors in equations for galaxy rotation speeds.” which uses only Newtonian dynamics & gravity to arrive at apparently valid galaxy rotation curves.

    While I have not examined these papers in detail (that much math I reserve for long dark winter days) I suspect that it may well be that, if we find some evidence for dark matter, in whatever form, we may pose an even greater puzzle. If dark matter indeed exists in sufficient quantity to account for all observed phenomena, then why are approaches like Tsiklauri’s and Nicholson’s so successful?

  2. Okay, simple stupid question… In the proposed test above how do they propose to know when an axion “flip flops” into something else like an x-ray, or a uv ray or a turkey dinner? And how do they know there was an axion there to begin with to “flip” into one of the aforementioned entities, the latter being rather improbable? And how would they know it came from the core or from outer space, or any other place? We don’t have detectors in the center of the sun. For all we know the sun could be hollow and only the outer surface is a tufted plasma. We can’t see past its “surface.” This seems like a similar problem to the issue of neutrinos flipping flavors willy nilly at the behest of experimenters who can’t seem to get the expected neutrino fluxes right. They then claim that somehow as some point along the way between the “core of the sun” and our detectors they’ve flip-flopped. Honestly, how do they know? We have no neutrino detector at the center of the sun, none between the sun and the Earth. So, how can a neutrino detected AT EARTH tell us anything about its state somewhere else, let alone that it was in a different state we never observed?

    This has been a confounding question for me, personally, in try to accept the various (in my opinion nonsensical) press releases coming down the pike alleging to have “resolved the issue of the neutrino deficit.”

    Simple thought experiment: A train leaves New York, New York and travels to Portland, Oregon. In Portland, we receive 13,000 gallons of wine. Can we claim that we know for certain that the 13,000 gallons of wine were 13,000 gallons of grape juice before it left New York and changed into wine during the intervening couple of days simply by viewing the wine at its destination?

    How do we know that it was not pre-packaged wine that was wine in New York, wine in Boise and still wine in Portland? How do we know that the contents of the box cars have or haven’t been altered on their way to Portland? It could be that the box car wasn’t touched the entire way and the original contents are the same as the final contents. Or, it could be that some boxes have been swapped out along the way.

    Without a station in New York (and a starting manifest), a station in Boise (checking the manifest) as well as the station in Portland checking the manifest against the original manifest and the previously checked manifest, how can we know anything about the prior states the box car and its contents have been in (pun intended)? That’s what doesn’t make sense to me and a number of people I have occasional correspondence with.

    My 2c. Albeit probably controversial.

  3. Last use of the Sun i’d heard was to use it as a gravitational lens for a long focal length telescope. Was it 500+ AU, or 1500+ AU?, i forget. Not quite an interstellar mission, but non-trivial. And i wondered how bad the light pollution would be. The Sun’s atmosphere goes out quite a ways, and the Sun itself is pretty bright.

    Of late, there’s a pretty good idea on how to use the Milky Way Galaxy as a telescope. First, find a whole bunch of millisecond pulsars, in different directions. Then watch them for minor variations, that could be due to gravity waves. I think we’re going to need a telescope that big for gravity waves. This is seriously hard stuff. I think i see how we can subtract the movement of the Earth, the Sun, and so on to get at the real data. Do local vibrations matter? Probably. But not if the frequencies differ dramatically.

    The “how do we know it’s not some other noise” question is often a very good one. I asked an astronomer how we know that we’re seeing extrasolar planets and not just pulses in the parent stars. I mean other than the odd transit. I got treated to how it’s done for twenty minutes. Fantastic. Still, i’m glad we get the odd transit predicted by radial velocity data. And, better, it was first done with a 6″ scope. Hey, i’ve got a 10″ scope. I could do that, right? What, i need a tracking mount and a camera? Shoot. Maybe i should get a 6″.

    I’m still waiting to hear from Hawking on why and under what circumstances information can escape a black hole. I have a vague notion that by information, he means things like mass, charge and maybe momentum or spin or something. I mean, if you fall down a black hole, you still turn into spaghetti. But he hasn’t said that if you point your electron gun (cathode ray tube) at an isolated black hole such that you put alot (highly technical term) of electrons into the black hole so that the black hole becomes measurably charged, does it spit it back out as Hawking Radiation, or do you have to wait for the black hole to completely evaporate? I’m sure he has some idea. It cost him a set of encyclopedias. Very strange.

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