Scientists studying a mysterious signal from far-off galaxies didn't find dark matter as they'd hoped. But the inventive new technique they used to detect this strange signal, which uses our own galaxy to hunt for dark matter, could elevate the hunt for the elusive material.
For decades, scientists have been searching for dark matter, an invisible material that doesn't interact with light but which permeates our entire universe. And a signal coming from a nearby galaxy spotted in a 2014 study gave scientists hope that this was the long-sought evidence for dark matter.
Some current models predict that dark matter particles slowly decay into ordinary matter, a process that would produce faint photon emissions that X-ray telescopes could detect. And in 2014, scientists spotted an X-ray emission from a galaxy in a dark matter hunt, as it's known that dark matter collects around galaxies.
Related: The 11 biggest unanswered questions about dark matter
Researchers think that the emission, known as the "3.5 keV line" (keV stands for kilo-electronvolts), is likely made of sterile neutrinos, which have long been thought of as a candidate for dark matter, study co-author Chris Dessert, of the University of Michigan, told Space.com.
Sterile neutrinos are hypothetical particles that are a close relative of the neutrino, a neutral subatomic particle with a mass very close to zero. They are released in nuclear reactions like those in nuclear plants on Earth and in the sun. Because the tiny amount of mass in neutrinos can't be explained by the Standard Model of particle physics, some think that sterile neutrinos could make up this mystery mass that is actually dark matter.
But in this new study of objects in the Milky Way, which analyzed a mountain of raw data over the past 20 years from the XMM-Newton space X-ray telescope, researchers found evidence that this signal seen in the 2014 study wasn't coming from dark matter. In fact, in searching for dark matter with their new technique, they didn't see the signal at all. However, this doesn't rule out sterile neutrinos as a strong candidate for dark matter, the researchers said.
To come to this conclusion, researchers looked for the 3.5 keV line in the sky. Since we live in the Milky Way's dark matter halo, any observation made through the halo must have dark matter in it.
So when the team found no trace of a 3.5 keV line in the data, they determined that "the 3.5 keV line isn't due to dark matter," Dessert said.
Now, while the 3.5 keV signature is caused most likely by sterile neutrinos, this might seem to rule out the hypothetical particle as a candidate for dark matter. But it's still possible that different mass sterile neutrinos, which wouldn't put out the same signal, could explain the elusive material.
"Even if you find this evidence compelling, that that 3.5 keV line is not necessarily there or is not necessarily dark matter, that does not rule out sterile neutrinos as a dark matter candidate," Kerstin Perez, an assistant professor of physics at the Massachusetts Institute of Technology who was not involved in this study, told Space.com. There are "still a lot of different masses that sterile neutrinos could have and it could still constitute all or some of the dark matter in the universe."
New dark matter hunting techniques
While Dessert admitted it was fairly disappointing that the researchers didn't observe a 3.5 keV line, the technique they developed could further the search for the elusive material.
"While this work does, unfortunately, throw cold water on what looked like what might have been the first evidence for the microscopic nature of dark matter, it does open up a whole new approach to looking for dark matter, which could lead to a discovery in the near future," co-author Ben Safdi, an assistant professor of physics at the University of Michigan, said in a statement.
"In the past, people have said, 'Well, let's look at a part of the sky that has a huge amount of dark matter in it and let's see if we see [dark matter] there,'" Perez said.
But, with this team's technique, which is similar to a technique that Perez uses in her own work, they use our place in the universe to their advantage because, "if this signal really is dark matter it should be all over the sky with some varying intensity because we live within the halo of dark matter."
"I think that that is a really exciting way to think about these searches because it allows you to use essentially the full sky," Perez added. "Previously we were kind of taking snapshots of the sky and looking at them kind of separately."
While looking through the Milky Way's dark halo for this signature helped the team to determine that the signal didn't come from dark matter, it did have additional benefits. "Looking through the dark matter halo in the Milky Way, you're not actually losing any sensitivity," Dessert said.
"The previous techniques are basically you point your X-ray telescope at a cluster of galaxies or just a galaxy that has a dark matter halo, and you look for the dark matter decay signal which is going to show up as a line," Dessert continued. He added that, with their technique in which they look through our galaxy's dark matter halo, they are able to get better results in their search.
"The dark matter halo around our galaxy is much closer to us, and that means that you're more likely to get the photons resulting from dark matter decay in our galaxy than you are if you're looking at some cluster far away."
Dessert added, "This technique we've developed can be used in other searches so, for example, this 3.5 keV line."
This work was published today (March 26) in the journal Science.
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The space.com report says "Researchers think that the emission, known as the "3.5 keV line" (keV stands for kilo-electronvolts), is likely made of sterile neutrinos, which have long been thought of as a candidate for dark matter, study co-author Chris Dessert, of the University of Michigan, told Space.com. "
The report indicates the search did not confirm the candidate for DM. There are many other candidates too like WIMPS, axions, axion like particles or ALP, etc. Scientific American published on this latest DM search effort and noted :
"...Existential Crisis Ultimately, scientists are left scratching their head at the extremely odd behavior of 85 percent of the mass in the universe. Do the new studies discrediting the supposed signals of dark matter in our galaxy make them doubt dark matter exists? “No,” Abazajian says, “particle dark matter is so consistent with what’s been observed, from the subgalaxy scale out to the horizon of the cosmos, that it is, basically, without a doubt, there.” Even though their faith in the existence of dark matter is unshaken, scientists’ hope of finding it may be diminished. Not only is astrophysical evidence elusive, but direct detection experiments aiming to capture the particles responsible have so far failed...", ref - Milky Way Dark Matter Signals in Doubt after Controversial New Papers
The challenge sterile neutrinos pose is that if they exist, and the existence of Neutrino flavor oscillation says they either *must exist* or otherwise the standard model must be wrong, then they by definition can *never* be detected by the strong, weak or electromagnetic forces.
This is because by definition the weak nuclear force only acts on left handed particles and the weak force is the only force neutrinos can interact by. According to the Standard Model Sterile Neutrinos *must exist* because Neutrino flavor oscillation has been observed. i.e. the only way for the sterile or right handed neutrino not to exist is for all neutrinos to effectively be massless and thus identical.
Thus the only way a sterile neutrino could ever be detected is by decaying into a detectable particle with nonzero charge or color charge.
So the only question left is whether these right handed neutrinos are stable or at least long lived or whether they decay almost instantly. This is where the ambiguity resides because while flavor oscillation among left handed neutrinos tells us that all 3 flavors of neutrinos must have different masses it doesn't tell us what those masses are only that they are all different from each other. If all 3 types of left handed neutrinos have mass then sterile neutrinos would be unstable to at least some degree as their stability depends on this mass.
So while the existence of heavy right handed neutrinos is confirmed we can't know if they are stable or not without directly measuring neutrino masses which is hard.
Work has shown that if one of the 3 left handed neutrinos is stable then its counterpart right handed neutrino must be stable as well so we can not assume that right handed neutrinos are unstable.
The problem of course is that if sterile neutrinos are stable i.e. don't decay they will forever remain undetectable as sterile neutrinos according to the standard model can not interact via the strong or electroweak forces and interact by gravity.
This means the only way they can be confirmed is by directly observing neutrino masses which are ridiculously tiny. This is the same as measuring quantum level gravity and thus is currently beyond our reach.
However theorists show that if Sterile Neutrinos are stable i.e. 1 neutrino flavor is massless then they exist in about the right amounts to account for dark matter and will never be able to be detected by any theoretical means short of creating sterile neutrinos that is measuring the missing energy and momentum.
The continued non detection of alternative types of dark matter proposed to get around the fundamental undetectability required by the standard model by each and every mechanism we try only further strengthens the case for sterile neutrinos this work only weakens the case for unstable sterile neutrinos.
WIMPs the dream child of the unsubstantiated supersymmetry model never had any evidence that it should be correct aside from pure wishful thinking. Supersymmetry has been effectively ruled out by experiments in the LHC kept "alive" by the sunken cost fallacy and the addition of correction terms in terms of orthogonal series functions (like the famous Fourier series) which are directly analogous to the epicycles that allowed the Polemic model of the universe to remain consistent with observations. This model also fails to explain Neutrino flavor oscillation so should have been ruled out last century when epicycles started to get added on top of epicycles.
Looking back towards the one thing that has been affirmed again and again at these scales the Standard Model we only have any theoretical leeway because we don't know what the Neutrino masses are. In principal right handed Neutrinos should be able to be created if they exist thus we either should see their decay or not depending on their stability.
The lack of detections says that if they are being created, and we have no reason to believe they wouldn't be created in the modern universe since left handed neutrinos are created, that they do not have a short lifetime.
This either tells us one of 3 things must be true.
1)Right handed neutrinos exist but they decay instantly or nearly so that the reactions that make them are so improbably rare that they can't be observed within the observable universe since the big bang and thus they can't account for all of dark matter.
2) Right handed Neutrinos exist and the reactions that make them are not super improbably rare and sterile neutrinos are stable or long lived and thus can naturally fully account for all of dark matter.
3) the standard model is wrong despite no evidence for the theory being incorrect and any and all alternatives failing to match experimental evidence.
Of these Occam's razor strongly suggests option 2 is most likely to apply for our universe as we have no reason why the reactions for these should be so absurdly rare that we can detect left handed neutrinos but not right handed ones and option 3 is equivalent to throwing our hands up in the air and saying everything we know is wrong because we don't like what the theory is telling us.
Every non detection only further strengthens this case it is possible axions exist since they emerge from within the less understood strong force components of the standard model that has yet to be combined together with electroweak theory. Either way their existence doesn't matter for the case of sterile neutrinos as we have observed Neutrino flavor oscillation. Axions thus would just be a bonus that could account for dark matter allowing more variability in the amount of sterile neutrinos out there.
My observation. Continued failure to identify DM with repeated null record query results, could be reason to reevaluate the entire DM observations that are indirect and require vast distances from Earth to *see*.