Axons, dark matter and neutron stars: How to find the world's most mysterious matter

The most fascinating astrophysical question at the moment is simple: what exactly is dark matter?

We know dark matter exists, and we know it is in every corner of the world. We know it will, and won't do some things ...... but somewhat embarrassingly, we don't know what it is.

Astrophysicists have some guesses, and are testing them. In a recent report, astronomers say they have focused their attention on a certain unique flavor of dark matter - light-weight axions - but they have not observed any light-weight axions. That's unfortunate, because these exotic subatomic particles likely make up dark matter. But this doesn't completely rule out the conjecture that axions are dark matter, and perhaps inspires new tests.

Astronomer Vera Rubin was the first to actually propose the existence of dark matter, which she used to try to explain the irregular features of spinning galaxies, versus what they look like when they are clustered. All these objects seem to get "extra" gravity beyond their mass, but if dark matter - something that doesn't reflect light and is thus unobservable to us - exists and surrounds these galaxies, it would be five to six times heavier than "normal" matter (e.g., protons, electrons).

Scientists have proposed many conjectures about dark matter, but the content of these conjectures can be explained by non-dark matter theories. In the last decade, these conjectures have been gradually denied, leaving only the more extreme ones, such as "very small black holes" (author: very unlikely, but very interesting), or whatever sub-atomic particles we still have not discovered.

Quantum physicists still have many theoretical questions about the behavior of the quantum universe, so they have proposed a particle that could explain these questions, a particle that has a very small mass, does not react with normal matter, does not emit light and is scattered throughout the universe. This particle is a perfect candidate for dark matter.

Astronomers have named this particle axion, a name derived from the clean new detergent.

But how do we observe something that doesn't emit light? After research, astronomers have found that on special occasions, dark matter can glow. Under a strong magnetic field, dark matter should transform into energy (remember E=mc2?). That means that matter and energy are two sides of the same coin and can be converted to each other.) We think that axions have very small masses and the energy they convert out should be in the radio wave region.

Top: Drawing showing the magnetic field lines of a neutron star (Source: Kathy Reed/Penn State)

Neutron stars are the remaining collapsed cores of stars that have exploded after their superstars. Neutron stars have very strong magnetic fields, which can be trillions of times stronger than Earth's, or even stronger. We can point radio telescopes at them, and if the axions decay into radio waves, we might observe them here.

Astronomers are making the above observations. Using the 100-meter Greenbank telescope and the radio telescope, astronomers have observed two neutron stars, as well as the center of the Milky Way, where thousands of neutron stars are clustered. They also dabbled in the neighboring Andromeda Galaxy, an angle from which scientists will be able to observe more neutron stars. Another object involved in this study is the globular cluster M54, which is closer to Earth and has about hundreds of thousands of stars, so there will also be many neutron stars in its core.

Above: Greenbank is a giant 100-meter-long telescope located in West Virginia and has a full-range manipulable radio wave dish that receives radio-length electromagnetic waves from all corners of the universe.

What scientists are looking for are radio waves of several specific wavelengths that are directly related to the mass of the axion. Although we don't know the exact mass of the axions, we can narrow it down. So astronomers have tried to find these specific wavelengths ...... but found nothing.

This is somewhat disappointing, but not why we should abandon the axion theory. Not all the energy released by axions can be ranked one by one, scientists can only observe about half of the wavelengths, and radio astronomical telescopes do not intercept the electromagnetic waves released by slightly heavier axions. We will continue to explore axions in the future and plan to examine all wavelengths.

At the beginning of the universe, dark matter formed a large network, as imaged in the computer simulation in the figure below. Galaxies then formed along the network.

Perhaps there aren't enough axions around neutron stars to form electromagnetic waves, so that any radio signal is too weak. Or perhaps there are other external forces that prevent axion electromagnetic waves from being detected. We also cannot rule out the possibility that axions do not emit radio waves, or even that axions are not the dark matter we are looking for. Perhaps dark matter is massive weakly interacting particles (WIMP for short).

But combined with recent technological advances in particle exploration and astronomical telescopes, I believe we will soon achieve a better understanding of dark matter.

So despite the disappointing latest results, I remain hopeful for the exploration of dark matter.

The universe is unimaginably large, hundreds of trillions of kilometers wide! (And that's just the part we can see; the universe continues to stretch farther and farther, and perhaps it doesn't end there.) Even the fifth cubic meter of a million is only considered longer in the universe.

In order to understand the universe, we need to study the very small and the very large matter, one without the other. And we humans, who are neither very small nor very large, are just right for such a job.