As physicist Cumrun Vafa once told me, puzzles in science often come in pairs, each serving as the other’s solution. Basically, you never know what concept in physics might end up useful for explaining unexpected phenomena—especially since there’s so much we’ve yet to understand about the universe.
Astrophysicists from the University of British Columbia in Canada believe that could be the case for axions—a hypothetical particle and a leading candidate for dark matter—in relation to white dwarfs. To be clear, the arXiv preprint, which has yet to be peer-reviewed, didn’t find evidence for axions. However, the analysis presents a compelling account of a white dwarf’s death in terms of axion physics, opening exciting opportunities for future research.
Just a bit of background
Initially, axions were meant to be a solution to a problem involving an imbalance between matter and antimatter in the quantum realm. That was back in 1977, and the idea slowly receded as detection attempts failed to spot the theoretical particle, which was hypothesized to be weakly interacting with other matter and have a low mass.
Now, consider dark matter. Physicists believe some 85% of the universe consists of dark matter, and there’s ample evidence to suggest it exists. True to its name, dark matter is “dark” in the sense that it rarely interacts with anything we can see and is presumably lightweight—similar to axions, if they exist. Given the parallels, physicists have long considered axions to be a good candidate for dark matter. That said, scientists have yet to actually find any real sign of axions—or, for that matter, any dark matter candidate.
And then there are white dwarfs—the dense, cold, generally inactive stellar cores left over from the death of a star. Technically speaking, at a certain point, these half-dead stars are so dense that they should collapse from too much gravitational pressure.
But they don’t, thanks to something called electron degeneracy pressure. To put it simply, electrons in the quantum realm can’t share the same energy state. So the electrons spiraling in and out of the star travel faster and faster, eventually generating enough pressure to keep the white dwarf from falling apart.
A stellar resurrection
This odd electron movement was what made white dwarfs popular among physicists searching for axions or axion-like particles, according to the study. Specifically, some theoretical models propose that axions could be formed by fast-moving electrons.
In addition, astrophysical observations had revealed that occasionally, white dwarfs would cool off way faster than expected. If these half-dead stars were actively producing axions, that energy loss would make more sense, the researchers explained in the paper, as axions escaping the star would siphon whatever energy was left over in it.
To test their hypothesis, the researchers took archival data from the Hubble Space Telescope and ran multiple simulations on how and whether the presence of axions influenced the activity of white dwarfs. The experiments helped them develop several predictions about the temperature and age of a white dwarf, with and without the extra cooling from axions.
After they completed their experiments, they compared their calculations with actual data from 47 Tucanae, a globular cluster populated by white dwarfs. To their disappointment, however, they found that their model failed to turn up any evidence of axion cooling.
Okay, but hear me out
As crazy as this may sound, the researchers concluded that the exercise still found a new limit on the ability for electrons to produce axions: around once every trillion chances. F better or worse, when looking for something as elusive as dark matter, learning what doesn’t work is often what brings scientists closer to what does.
“This result doesn’t rule out axions entirely, but it does say it’s unlikely that electrons and axions directly interact with each other,” Paul Sutter, an astrophysicist at Johns Hopkins who wasn’t involved in the study, wrote in a commentary for Space.com. “So, if we’re going to keep searching for axions, we’re going to have to find even more clever ways to look.”
Read the full article here
