Looking for Axions in Neutron Stars

[jedishrfu]

TL;DR Summary

Axions are hypothetical particles that have never been detected. However, it is theorized that they may be found near spinning Neutron stars.

https://www.sciencealert.com/we-fin...-for-the-universes-most-sought-after-particle

Neutron stars with a penchant for extreme spinning could be churning out one of the most sought-after particles in the Universe.

These elementary particles are called axions, and to date they are purely hypothetical. If we did manage to find them, though, we could solve some of the biggest problems in the cosmos, including the identity of at least one kind of dark matter.

 

[Hornbein]

I was about to post about that but you beat me to it. I believe the popular science article you recommended is somewhat inaccurate, but instead of criticizing it I'd rather introduce this instead. He cites three articles in the notes for the video, all of which I took a look at. They say that young neutron stars would produce axions at a prodigious rate due to collisions of their hot nucleons. Unlike heat, the axions escape the star easily. If axions were heavy they would cool the star too rapidly, so their mass has to be very small. They say that the neutron star's great gravity should trap some of these axions, which would then orbit for a long time resulting in a very dense axion cloud around the star. The intense magnetic field of the star can convert some axions to hard X rays. They say that these have been detected from seven isolated neutron stars and that there is no better explanation.

Axions could then be a significant part of dark matter.

 

[ohwilleke]

It is worth recognizing that the argument being made is something of a "god of the gaps" argument. In other words, our models of neutron stars aren't terribly precise (e.g. predictions of the maximum neutron star mass based upon QCD a.k.a. quantum chromodynamics which is the Standard Model theory of the strong force, and nuclear physics, and general relativity (GR) have relative uncertainties on the order of 10-20%).

So, any "new physics" that would impact the structure and evolution of neutron stars that has an effect smaller than this uncertainty can't be ruled out, even though there is no positive evidence that axions are present in neutron stars or even exist. An argument that "new physics" exists because we can't rule it out, even though we have really no positive evidence for it, is a weak argument.

One of the more clever ways proposed to indirectly identify the existence of dark matter, but not its exact type, is to look for systemic differences in the properties of neutron stars between the vicinity of our solar system near the rim of the Milky Way and the galactic center where the proportion of dark matter in neutron stars should be greater.

Efforts to estimate optimal parameters for "axion-like particles" that are ultralight typically come up with values for their mass more than ten orders of magnitude smaller than the mass that a QCD-axion is expected to have. (See, e.g., here at Section 3.5)

QCD-axions have some features that make them desirable dark matter candidates, nonetheless, because particles much below a keV mass can't be created via thermal freeze-out in the early universe, because otherwise they would have too high a mean velocity and would constitute "hot dark matter" which is inconsistent with the amount of large scale structure observed in the universe. Instead, lighter particles that constitute dark matter must be created and destroyed in equilibrium amounts over the course of the universe's life with fairly low (non-relativistic) momentum. QCD-axions are a candidate that would clearly fit that requirement.

On the other hand, numerous experiments aimed at directly detecting QCD-axions have come up empty. There has been lots of scholarly work on this hypothesis, but nothing so far to give it a decisive advantage over other possibilities.

 

[Hornbein]

One of the more clever ways proposed to indirectly identify the existence of dark matter,

"after a DM particle interacts and looses energy it is very improbable it can escape"

Ouch.

Then again

-- e.g., here at Section 3.5)

"we do not known either the mass not the cross section of such particles"

Oof.

 

[Hornbein]

https://arxiv.org/pdf/2410.10424

There is yet no proof of the existence of axions, and the most recent measurements that their mass cannot exceed 10−6𝑒𝑉 (one millionth of electronvolt), too small to have cosmological consequences, does not however rule out the possibility that these particles could still be relevant for the dynamics of galaxies. Other very light hypothetical particles, akin to axions, have been proposed as a possible explanations of cold dark matter. They have been dubbed Axion-Like-Particles (ALP). Thei crucial characteristics is that they are assumed to be self-interacting. This property is described by nonlinear equations that allow for the existence of localised stable configurations, similar to electromagnetic solitons in nonlinear media, or even to solitons observed on the surface of shallow waters [41]. Galactic dark matter halos could be ALP solitons. Experiments devised to find ALPs consists in trying generating them through the Primakoff effect [42], according to which photon beams that cross intense electromagnetic fields could, among other things, be converted into axions and/or ALPs and vice versa. Based on these processes are experiments likeADMX (Axion Dark Matter eXperiment) at the University of Washington in Seattle and XENONnT at the Gran Sasso Laboratories, Italy.

Encouraging results for ALPs come from the study of the gravitational lensing effect of the galactic system HS 0810-2554 [43]. This object is an elliptical galaxy at𝑧 = 0.89 that gravitationally lenses a background quasar at 𝑧 = 1.51, generating four distorted images of the latter. The structure of the dark matter halo of the foreground galaxy, inferred from the distorsion of the images of the background object, shows more dense and less dense regions that can be reasonably interpreted as the result ofan interference pattern generated by a quantum mechanical wave soliton of a large aggregate of ALP particles with masses expected between 10−22 and 10−20𝑒𝑉. This example, which is supported by theoretical models, seems to favour dark matter halos formed by ALP against those formed by WIMPs.

 

[jedishrfu]

We have to start somewhere, and having some research groups looking at neutron stars is a good start.

 

[ohwilleke]

We have to start somewhere, and having some research groups looking at neutron stars is a good start.

I don't really disagree. These papers are legitimate, if somewhat speculative, scientific papers.

They make predictions that, someday in the indeterminate future, when we can make more exact calculations of the properties of neutron stars with and without axion dark matter, could be checked against astronomy observations, when our ability to observe neutron stars with better telescopes and similar devices improves.

It just may be decades from now before that day comes.