As the hunt for dark matter, the universe’s most common yet most mysterious “stuff,” continues, scientists are understandably eager to get the most powerful space telescope in on the action.
Space.com spoke to three scientists, cosmic detectives who are hot on the trail of dark matter candidates using the James Webb Space Telescope (JWST).
The question is how, even with its incredibly sensitive infrared eye, the JWST could possibly search for something that is itself effectively invisible in all wavelengths of light.
If dark matter is composed of hypothetical particles called axions, then there is a chance that these particles could “decay,” breaking down into other particles. This process could release photons, the particles that make up light, which the JWST could then detect.
Related: What is dark matter?
“A successful discovery of this light would be groundbreaking — truly the discovery of the century,” theoretical astroparticle physicist Elena Pinetti told Space.com. “Finding dark matter would unlock an entirely new chapter in our understanding of the universe.
“It’s a bit like cosmic detective work, helping us sift through the noise of the universe to spot something truly extraordinary.”
The trouble with dark matter
Dark matter is the name scientists have given to the mysterious substance that accounts for around 85% of the universe’s total matter content.
That means all the stuff we see around us on a day-to-day basis, from the most massive stars to the tiniest bacteria and beyond, accounts for just 15% of the “stuff” in the cosmos.
Yet, despite its ubiquity, dark matter is frustratingly elusive, because whatever comprises it doesn’t interact with light or “ordinary matter.” Either that or dark matter interacts so weakly and so rarely that it is effectively invisible.
Up until now, the only way astronomers have been able to infer the presence of dark matter is via its interaction with gravity and the way that then influences light and visible “everyday” matter.
The fact that dark matter seems to interact with light very weakly, if at all, means it can’t be composed of the particles that make up atoms —electrons, protons and neutrons. That’s because those particles. which serve as the building blocks of everything we see around us, strongly interact with light.
This has led scientists to search for potential dark matter particles beyond the Standard Model of Particle Physics, the framework that currently explains everything we know about particles and forces.
Currently, the leading suspect for dark matter particles are axions, which have remained frustratingly hypothetical.
Related: We still don’t know what dark matter is, but here’s what it’s not
“Axions are unstable particles, meaning that they can spontaneously turn into, or decay, into other particles. This is very similar to neutrons, which decay into protons in about 15 minutes unless they’re bound inside a nucleus of an atom,” Christopher Dessert, Flatiron Research Fellow at the Center for Computational Astrophysics, told Space.com.
“Axions will decay into two photons (the particles that make up light), and each of those photons has an energy equal to half the mass of the axion through Einstein’s matter/energy relationship E=mc2,” Dessert added.
He explained that if the mass of the axion is about 1 electronvolt (eV)/c2 (where “c” is the speed of light), then those emitted photons are in the infrared wavelength range, and JWST could see them. If it’s that simple, you may wonder why other infrared telescopes have failed to detect axion decay. There’s another hitch, unfortunately.
“Even though axions decay, if they make up dark matter, then their lifetime has to be much larger than the age of the universe because we know dark matter was around in the early stages of the universe, and we know it’s still around today,” Dessert said. “So we’re looking for a very rare process. But if axions make up the dark matter, then there are about 1077 (that’s 10 followed by 76 zeroes) axions in the Milky Way, so the very rare process still happens pretty often!”
Elisa Todarello, a theoretical astrophysicist from the University of Nottingham in England, explained why the JWST is the right instrument to hunt for light from axion decay.
“The JWST can see extremely faint objects and can distinguish different frequencies of infrared light very accurately because it has very high spectral resolution,” Todarello said. “These are desirable characteristics for an instrument to detect the electromagnetic radiation produced by the decay of a dark matter particle.”
Todarello further explained that, if axion decay is concentrated at a specific frequency, equal to about half the axion mass, it would create a narrow “spectral line” that allows it to be differentiated from light coming from sources that emit a smooth spectrum over a large range of frequencies.
“It would be more difficult to differentiate it from spectral lines of another origin — for example, atomic transitions,” Todarello said. “If the axion mass happens to be such that the spectral line due to axion decay coincides with that of an atomic transition, it would be very difficult to disentangle the two.”
The shape of the spectral line created by axion decay could give scientists important information about the distribution of dark matter in our galaxy, according to Todarello.
“Since we have a good idea of how dark matter is spread across the Milky Way, we can predict how much should appear in different parts of the sky,” Pinetti added. “Astrophysical emissions — signals from stars, gas, or other cosmic objects — vary depending on where we look. By comparing observations from different regions, we can try to tell the difference between ordinary cosmic noise and a potential dark matter signal.”
Related: Could dark matter have been forged in a ‘Dark Big Bang?’
Where to hunt for dark matter
That covers how these dark matter detectives intend to look for dark matter signals and what they intend to look for. With the ubiquity of this mysterious stuff, the remaining question is where to look, to have the best possible chance of a dark matter detection.
Dessert and Pinetti explore different possible “dark matter crime scenes” in their respective research.
“We originally were not sure what kind of targets would be more sensitive probes of axion decays. In our work, we forecasted both sensitivity to axions decaying in the Milky Way’s dark matter halo and those in smaller sub-halos around dwarf galaxies,” Dessert said. “We found that the Milky Way axions are more sensitive because JWST will collect more data looking at those, but this wasn’t obvious a priori.
“We’re also looking into obtaining data on JWST looking at axions decaying inside a dwarf galaxy, which could provide an additional cross-check on those results.”
Pinetti explained that, in her research, the team searched for a dark matter signal in so-called “blank-sky fields.”
Blank-sky fields are off-target observations that astronomers use to remove background noise from their data.
“For astronomers using the JWST, these blank-sky observations are not terribly interesting,” Pinetti said. “But here, we’ve found an entirely different use for these blank-sky observations — the lack of bright astronomical sources in them makes them especially useful for dark matter searches.”
Pinetti added that this approach is particularly exciting because every JWST observation, regardless of the target, requires these blank-sky fields to be produced as part of its observing program.
“So our data set is going to keep on growing as long as JWST operates, allowing us to probe deeper and deeper into the dark sector,” Pinetti said. “And all of this comes for free since we can make use of nearly any kind of JWST observation!”
If the JWST does not turn up a signal from axion decay, this would not completely eliminate these hypothetical particles as dark matter suspects.
That’s because the JWST’s investigation focuses on the decay of particles with masses between 0.1 eV and 4 eV. Failure to make a detection could mean that the particles that comprise dark matter are outside this mass range.
“It’s always good to investigate many different possibilities, but a non-detection of axion dark matter by JWST would not mean that such a dark matter candidate is now disfavored,” Todarello said. “It would simply mean that the mass of the axion is in a different range, or that the coupling constant to photons is smaller than the JSWT can detect.”
For Pinetti, this kind of investigation drives her passion for science.
“Astroparticle physics and space science never cease to amaze me —there’s always something new and unexpected!” Pinetti concluded. “In fact, there are some suggestively mysterious signals out there in JWST data, and we’re not yet sure where they come from. It’s definitely worth digging deeper — every mystery brings us one step closer to understanding the cosmos.
“This discovery would completely transform what we know about the cosmos and possibly reveal mysteries we haven’t even imagined yet.”
Both Pinetti’s and Dessert’s latest JWST dark matter research was published on Feb. 18 in the Physical Review Letters.
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