A supermassive black hole

Developments in the search for the axion

The imaginary particle that is impossible to understand.

The axion is a hypothetical subatomic particle, created in order to solve a contradiction called the strong CP problem. To very briefly summarise this contradiction: it has been calculated that the strong interactions between quarks shouldn’t be symmetrical when a reversal of charge (‘C’) or parity (the flip in sign of a spatial coordinate, a mirror image, ‘P’) occurs. Experimentally however, we have only seen symmetry remain, not once has a change in CP resulted in a different interaction between these strong forces.

The axion is also a particle that would solve the mystery of what dark matter is. It acts in a similar way to dark matter: barely interacting with baryonic matter, but still interacting with gravity. The axion’s utilisation in solving big questions doesn’t stop at the strong CP problem and dark matter however, it’s also been hypothesised to be the reason for the imbalance of matter and antimatter.

Understandably, if an axion barely interacts with normal matter, it is hard to detect and observe. However, with new imaging abilities concerning black holes, detecting them becomes more of a possibility. Theoretically, a black hole can produce a dense clumping of axion particles in the surrounding area. The picture in this article is from the supermassive black hole in galaxy M87, and it was thought to be the right size for creating ultra-lightweight axions. The picture, and the search for axions, was done through the use of the Event Horizon Telescope, or EHT, a network of radio telescopes across the earth. If the black hole did produce a cloud of axions, it would produce a wobble in the orientation, or polarisation, of light that the EHT could detect.

Unfortunately, after taking out background polarisation from the black hole itself, they did not find enough instability to suggest an axion cloud. However, this only rules out the possibility of the ultra-lightweight axions that this black hole would produce. Axions come in a range of different masses, and the bigger the black hole, the smaller the axion. Axions produced by the black hole in M87 would have ten billionths of a billionth of a billionth of an electron’s mass.

The same technique however can now, excitingly, be used to hunt for axions of different masses. There are plans to look at the black hole in the centre of our own galaxy, that would produce heavier axions, being 1000th of the mass of the galaxy in M87.

The search for these particles could result in solving the most important conundrums in astrophysics, leaping forwards our critical understanding of the topic, and certainly makes this a topic on which one should keep an eye on for updates.

Anika, VI