A test exists for a leading explanation for the nature of dark matter, but it relies on catching a supernova in the moment of explosion. Although we now do that quite frequently, the supernova needs to be closer than any we have seen for decades, and we have to have a gamma-ray telescope pointing in the right direction when it happens. A team of scientists is using this scenario to argue for launching more gamma-ray satellites, so we can have the sky covered when the event occurs.
Strong evidence for the existence of dark matter was seen a century ago, and it has kept coming since. Nevertheless, attempts to identify more than a tiny fraction have failed. The long unsuccessful quest to find most of the universe’s matter is starting to be used by the enemies of science as evidence for its failures, and alternative explanations are gaining more coverage, even while few scientists find them plausible.
Perhaps, as in all the best mystery stories, the detectives need a lucky break.
One explanation for what we have, and have not, seen, is that dark matter is composed of axions, theoretical particles whose tiny mass is compensated by their astonishing abundance. QCD Axions, favored by string physicists, differ from other proposed dark matter particles in that they interact with the other fundamental forces besides gravity, but have a maximum mass 32 times smaller than an electron.
Unfortunately, QCD axions would be at the edge of our capacity to detect them with existing instruments, thanks to the exceptional weakness of their interactions with the three non-gravitational forces. Efforts have been made for 40 years using a variety of approaches, but like so many other dark matter searches, have yet to succeed.
We expect that if axions are real, core collapse supernovae (all types other than Ia) produce immense amounts of them in their first few seconds. That’s not very useful if we can’t detect them, but models of axion behavior propose their interactions with the electromagnetic force would convert those passing through strong magnetic fields into gamma rays.
The neutron stars produced by most core collapse supernovae often have magnetic fields of the required strength, Dr Benjamin Safdi of the University of California, Berkeley and colleagues argue. Not only can we detect these gamma rays, they propose, but we can also distinguish them from those released by supernovae directly.
There are just two flies in this ointment. The first is that to detect enough ex-axion gamma rays to be able to be convincing, the supernova needs to be nearby. Ideally, it would be inside the Milky Way, but the majority of explosions in one of the satellite galaxies that surround ours would also be suitable, Safdi and co-authors calculate.
That means we could be waiting a while. We haven’t seen a supernova inside the Milky Way for more than 400 years, although some may have occurred but been hidden by dust. The abundance of enormous stars in the Large Magellanic Cloud (LMC) creates a lot of supernova prospects, but the last one there was still 37 years ago, in the form of SN 1987a. Best not to hold your breath.
“If we were to see a supernova, like supernova 1987A, with a modern gamma-ray telescope, we would be able to detect or rule out this QCD axion, this most interesting axion, across much of its parameter space—essentially the entire parameter space that cannot be probed in the laboratory, and much of the parameter space that can be probed in the laboratory, too,” Safdi said in a statement.
Schematic showing an axion produced in a supernova core collapse being converted to a gamma ray by the strong magnetic field, and reaching an Earth better suited for gamma-ray satellites.
Image credit: Benjamin Safdi, UC Berkeley
We can’t do anything about the supernova drought, but we can address the other potential impediment. The anticipated surge of gamma rays is thought to last just 10 seconds or so. That’s not sufficient for a gamma ray-detection instrument to be alerted by a telescope operating elsewhere in the spectrum and turn its attention to the right spot.
Consequently, we need to have a gamma-ray eye on the right part of the sky for the project to succeed. If gamma-ray telescopes were like optical ones, with very small fields of view, the prospects might be dim. The Large Area Telescope, one of the Fermi Gamma-ray Space Telescope’s two instruments, is the one currently operating instrument Safdi and co-authors think is up to the job. It views about a fifth of the sky at a time.
“It would be a real shame if a supernova went off tomorrow and we missed an opportunity to detect the axion,” Safdi added. “It might not come back for another 50 years.”
“The best-case scenario for axions is Fermi catches a supernova. It’s just that the chance of that is small,” Safdi said. “But if Fermi saw it, we’d be able to measure its mass. We’d be able to measure its interaction strength. We’d be able to determine everything we need to know about the axion, and we’d be incredibly confident in the signal because there’s no ordinary matter which could create such an event.”
The alternative is to expand our capacity in this part of the spectrum. Safdi and colleagues are promoting an idea for a full-sky gamma ray watch. They even have a name for it; GALactic AXion Instrument for Supernova (GALAXIS).
Much as scientists want to find dark matter, we don’t envy someone trying to persuade funding bodies to pay for a network of gamma-ray telescopes to wait for an event that may not occur for centuries. Fortunately, just as Fermi has been a rich source of other scientific data, GALAXIS could do plenty else while it waits.
Nevertheless, the whole idea relies not only on axions being real, but that they also have the specific characteristics predicted for them by string theory. “It seems almost impossible to have a consistent theory of gravity combined with quantum mechanics that does not have particles like the axion,” Safdi said.
However, with a substantial pool of physicists deeply suspicious of string theory, not everyone agrees. GALAXIS, plus a suitable supernova, might settle that debate, but first agreement must be found to launch the instruments.
The study is published in Physical Review Letters.
Source Link: A Supernova In A Nearby Galaxy Could Reveal Dark Matter – If We’re Lucky