Could Neutron Stars Be the Answer to Finding Dark Matter?

 Quick recap:

What is Dark Matter?

Dark matter is matter that doesn’t interact with light, meaning that it doesn’t emit, absorb, or reflect light. This also means that we can’t see them. You might wonder if we can’t see them, how do we know that they’re there? In 1933 Fritz Zwicky came up with the term “dark matter” and said that it could be a possible explanation for the rotational speeds of galaxies. Then in 1978 Vera Rubin confirmed this in her observations of spiral galaxies. Essentially, we know dark matter exists based on its gravitational effect on visible matter.

Because of its properties, figuring out what it's made of is challenging. There’s different types of hypothetical particles that are candidates for dark matter. For this post, we’re primarily focused on axions, because it’s believed that when exposed to strong magnetic fields, axions could decay into photons.

 

What are neutron stars?

Neutron stars are the phase of a star’s life after Supernova – a crazy bright explosion that happens when the core of a star collapses. The star becomes much smaller than before, about the average size of a US city. Its mass will decrease too, to be just less than two times the mass of our Sun. The newly formed Neutron star will also become very dense. Some neutron stars emit light – we call these pulsars. Neutron stars also just happen to have incredibly strong magnetic fields, which is what we need for axions to convert to photons.

Pictured is J0167, a neutron star inside of IC 443, a supernova remnant (Gaensler, Leahy, et.al, 2006).

  

Could Neutron Stars Be the Answer to Finding Dark Matter?

Scientists have recently found that axions may occur in clouds around neutron stars. Published on the 17th of last month, Dr. Dion Noordhuis, Dr. Anirudh Prabhu, Dr. Christopher Weniger, and Dr. Samuel White states in their article “Axions Clouds around Neutron Stars” that neutron stars can create axions that are gravitationally bound to the star. This creates a dense cloud of axions around it. They believe that the density of this cloud and the gradual increase of its density matches the proposed overall density of dark matter. They assert that axion clouds around neutron stars have to be the answer to what is dark matter and how it works. This also means that this concept could be applied to all neutron stars, including pulsars. Neutron stars are good hosts for axion clouds because they have lots of gravity and strong magnetic fields, two things that axions need to eventually create a cloud.

Dr. Noordhuis also states that once an axion cloud gets big enough, it could be detected in telescopes after a while. In this case we’re talking about pulsars, a type of neutron star that gives off light and rapidly rotates so fast that it looks like a flickering light in the distance. If there was an axion cloud surrounding a pulsar, it would dim the light coming from the pulsar. That’s how it could one day be detected in telescopes (once the cloud has accumulated enough axions).

While this hasn’t been observed yet, one of the authors of the research paper Axion Clouds in Neutron Stars is following up on this discovery with further research.

 

 

 

 

Sources cited

CERN. “Dark Matter.” CERN, 2024, home.cern/science/physics/dark-matter. Accessed 09 Nov. 2024.

Department of Energy. “Doe Explains...Dark Matter | Department of Energy.” Energy.Gov, www.energy.gov/science/doe-explainsdark-matter. Accessed 10 Nov. 2024.

Gaensler, B., and Et. al. J0617 in IC 443. 1 June 2006.

Lea, Robert. “Dark Matter Might Live in a Dense Haze Around Stellar Corpses.” Space.Com, 31 Oct. 2024, www.space.com/neutron-stars-axions-dark-matter. Accessed 09 Nov. 2024.

Lea, Robert. “What Are Pulsars?” Space.Com, 22 Apr. 2016, www.space.com/32661-pulsars.html. Accessed 09 Nov. 2024.

Noordhuis, Dion, et al. “Axion Clouds Around Neutron Stars.” Physical Review X, vol. 14, no. 4, 17 Oct. 2024, doi:10.1103/physrevx.14.041015.

Science Daily. “Neutron Stars May Be Shrouded in Axions.” ScienceDaily, 18 Oct. 2024, www.sciencedaily.com/releases/2024/10/241018131244.htm. Accessed 09 Nov. 2024.

Spergel, Dave. “Axions.” Princeton University, The Trustees of Princeton University, 1996, www.astro.princeton.edu/~dns/MAP/Bahcall/node16.html. Accessed 09 Nov. 2024.

U of Amsterdam. “Physicists Show That Neutron Stars May Be Shrouded in Clouds of Axions.” Physicists Show That Neutron Stars May Be Shrouded In Clouds of Axions, 14 Oct. 2024, phys.org/news/2024-10-physicists-neutron-stars-shrouded-clouds.html. Accessed 10 Nov. 2024.