New Method For Tracing Dark Matter Reveals Its Location, Abundance As Never Before
“By measuring the distorted light from distant galaxies behind a galaxy cluster, scientists can reconstruct the total cluster mass. In every galaxy cluster, the majority of the mass is outside of the galaxies: there is a huge dark matter halo. The intracluster gas, however, may be distributed differently, as normal matter can collide and heat up, emitting X-rays. But individual stars, ejected from galaxies, should trace the same path as the dark matter. In a cosmic first, scientists measured this intracluster light, and found it traces out the dark matter perfectly.”
If you want to know where the dark matter is located in the Universe, you had to infer its presence and abundance by measuring the gravitational effects it had on space. When it comes to large-scale structures, like galaxy clusters, this often involved exceedingly difficult reconstructions involving gravitational lensing, and relied on serendipitous alignments of observable background structures. But a new study has concocted an alternative method that works extremely well: just measure the intracluster light from stars that have been ejected from the component galaxies.
Well, with the first two clusters down, we have a verdict: it’s the best dark matter-tracer of all time. Come get the remarkable story today!
This Is How Mastering Dark Matter Could Take Us To The Stars
“Because dark matter is everywhere, we wouldn’t even need to carry it with us as we traversed the Universe. As far as we understand it — and admittedly, we need to understand it a lot farther — dark matter could truly deliver our dream of the ultimate fuel. It’s abundant all throughout our galaxy and beyond; it should have a non-zero annihilation cross-section with itself; and when it does annihilate, it should produce energy with 100% efficiency.
Perhaps, then, most of us have been thinking about experiments seeking to directly detect dark matter all wrong. Yes, we want to know what makes up the Universe, and what the physical properties of its various abundant components truly are. But there’s a science-fiction dream that could come true if nature is kind to us: unlimited, free energy just waiting there for us to harness, no matter where in the galaxy we go.
Mastering dark matter is the endeavor that just might make it so.”
When we talk about our dreams of traveling to the stars, it normally involves a mythical, futuristic form of travel that goes beyond the known laws of physics. Why’s that? Because even if you increase the efficiency of your rocket fuel far beyond the limitations of any chemical-based reaction we know of, you’d still be limited in how far you could go by the mass of your spacecraft and the fuel you were able to take with you on board. You’d still have to accelerate (and decelerate) all the fuel you brought with you, until you ran out. If only there were a 100%-efficient fuel source that was ubiquitous all throughout the galaxy and beyond.
There is: dark matter. Here’s why it’s so important to study, understand, and eventually, fulfill the dream of harnessing it!
This Is Why Every Galaxy Doesn’t Have The Same Amount Of Dark Matter
“It isn’t the properties of one or two galaxies that will be the ultimate test of dark matter, however. Whether these galaxies are generic dwarf galaxies or our first examples of dark matter-free galaxies isn’t the point; the point is that there are hundreds of billions of these dwarf galaxies out there that are presently below the limits of what’s observable, detectable, or having their properties measured. When we get there, especially in the distant Universe and in post-interaction environments, we can fully expect to truly find this yet-unconfirmed population of galaxies.
If dark matter is real, it must be separable from normal matter, and that works both ways. We’ve already found the dark matter-rich galaxies out there, as well as isolated intergalactic plasma. But dark matter-free galaxies? They might be right around the corner, and this is why everybody is so excited!”
When the Universe was first born, everything was uniform. There was dark matter and normal matter everywhere, in the same 5-to-1 ratio in all structures. But then the Universe had to go and get messy. It formed stars and galaxies of different masses and sizes, and that’s where the trouble started. In large, massive galaxies, even cataclysms like supernovae or active supermassive black holes don’t eject very much normal matter. But in small galaxies, significant amounts of normal matter can get ejected, upping that ratio to dozens or event hundreds to one. That ejected matter doesn’t just go away, but can itself, at least in theory, form dark matter-free galaxies. Where are we in our understanding of galaxies, dark matter, and gravitation?
It’s just a small piece of the puzzle, but this explains why not every galaxy has the same 5-to-1 ratio you might naively expect!
Ask Ethan: What’s The Real Story Behind This Dark Matter-Free Galaxy?
“I read a study that said the mystery of a galaxy with no dark matter has been solved. But I thought that this anomalous galaxy was previously touted as evidence FOR dark matter? What’s really going on here, Ethan?”
Imagine you looked at the Universe, and saw a galaxy unlike any other. Whereas every other galaxy we’ve ever looked at exhibited a large discrepancy between the amount of matter that’s present in stars and the total amount of gravitational mass we’d infer, this new galaxy appears to have no dark matter at all. What would you do? If you’re being a responsible scientist, you’d try to knock down this galaxy by any scrupulous means possible. You’d wonder if you had mis-estimated one of its properties. You’d try to re-confirm the measurements with different instruments and techniques. And you’d wonder if there weren’t an alternative explanation for what we were seeing.
Well, if you read that the galaxy has dark matter after all, and the mystery has been resolved, you should definitely read this instead. The story is far from over, and even if the new team’s results hold up, there’s still a mystery at play here.
Scientists Discover Space’s Largest Intergalactic Bridge, Solving A Huge Dark Matter Puzzle
“Lo and behold, these shocks are some of the first things you notice if you look at the Chandra images of the Bullet cluster on their own! The fact that we’ve identified relativistic charged particles in the presence of a large-scale magnetic field in one pair of colliding clusters is strongly suggestive of the same effects existing in other clusters. If this same type of structure that exists between Abell 0399 and Abell 0401 also exists between other colliding clusters, it could solve this minor anomaly of the Bullet cluster, leaving dark matter as the sole unchallenged explanation for the displacement of gravitational effects from the presence of normal matter.
It’s always an enormous step forward when we can identify a new phenomenon. But by combining theory, simulations, and the observations of other colliding galaxy clusters, we can push the needle forward when it comes to understanding our Universe as a whole. It’s another spectacular victory for dark matter, and another mystery of the Universe that might finally be solved by modern astrophysics. What a time to be alive.”
When two galaxy clusters collide, the normal matter heats up and emits X-rays, experiencing shocks and separating from the gravitational effects of the clusters they originated from. But as compelling as this evidence is for dark matter, a few of the colliding clusters we’ve found, such as the original (the Bullet cluster), appear to be moving faster than theory predicts. Either dark matter is incomplete, our observations were wrong, or something else is working besides gravity to accelerate the matter.
Guess what? We’ve just found something else between galaxy clusters that accelerates matter: enormous magnetic fields and relativistic electrons! Come get the full story here, and learn what it means and why!
This Is Why It’s Meaningless That Dark Matter Experiments Haven’t Found Anything
“To date, the direct detection efforts having to do with dark matter have come up empty. There are no interaction signals we’ve observed that require dark matter to explain them, or that aren’t consistent with Standard Model-only particles in our Universe. Direct detection efforts can disfavor or constrain specific dark matter particles or scenarios, but does not affect the enormous suite of indirect, astrophysical evidence that leaves dark matter as the only viable explanation.
Many people are working tirelessly on alternatives, but unless they’re misrepresenting the facts about dark matter (and some do exactly that), they have an enormous suite of evidence they’re required to explain. When it comes to looking for the great cosmic unknowns, we might get lucky, and that’s why we try. But absence of evidence is not evidence of absence. When it comes to dark matter, don’t let yourself be fooled.”
If dark matter is so successful, then why haven’t we directly detected the particles that make it up yet? Doesn’t the failure of all these experiments attempting to directly detect dark matter point to a failure of the dark matter hypothesis.
Not at all, and if you think that, you’d better learn the difference between model-dependent and model-independent tests. Here’s where you’ll want to start.
Dark Matter Search Discovers A Spectacular Bonus: The Longest-Lived Unstable Element Ever
“Whenever you build an experiment that can take you beyond your previous sensitivity limits, you open yourself up to the possibility of discovery. In robustly detecting this extraordinarily rare decay with a longer lifetime than any other we’ve ever seen, the XENON collaboration has demonstrated how capable their apparatus is. Although it was designed to search for dark matter, it’s also sensitive to a number of other possibilities which might herald rare or even entirely new physics.
While the direct detection of the longest-lived unstable decay is an incredible feat, its implications go far beyond a simple discovery. It’s a demonstration of XENON’s sensitivity, and its ability to tease out even a tiny signal against a well-understood, low-magnitude background. It gives us every reason to be hopeful that, if nature is kind, XENON may reveal some of its even more profound secrets.”
Naturally-occurring xenon is made up of nine isotopes. While only one of them was known to be unstable, transmuting into barium through double beta decay, some of the isotopes should theoretically be unstable to decays through an even rarer pathway: double electron capture. For the first time, double electron capture has now been observed in the element xenon, thanks to the incredible work and the unprecedented detector sensitivities provided by the experiment XENON1T. Xenon-124 has now broken the record, and is officially the longest-lived unstable isotope ever to have its decay measured.
This discovery showcases the extreme versatility of the XENON collaboration’s detector, and should make us hopeful for an even greater prize. Here’s why.
How Much Of The Dark Matter Could Neutrinos Be?
“If we restrict ourselves to the Standard Model alone, we simply cannot account for the dark matter that must be present in our Universe. None of the particles we know of have the right behavior to explain all of the observations. We can imagine a Universe where neutrinos have relatively large amounts of mass, and that would result in a Universe with significant quantities of dark matter. The only problem is that dark matter would be hot, and lead to an observably different Universe than the one we see today.
Still, the neutrinos we know of do behave like dark matter, although it only makes up about 1% of the total dark matter out there. That’s not totally insignificant; it equals the mass of all the stars in our Universe! And most excitingly, if there truly is a sterile neutrino species out there, a series of upcoming experiments ought to reveal it over the next few years. Dark matter might be one of the greatest mysteries out there, but thanks to neutrinos, we have a chance at understanding it at least a little bit.”
Dark matter is a form of matter that gravitates, but neither absorbs nor emits light, and has been frustratingly difficult to pin down and directly detect. There’s a known particle that has exactly those same properties: the neutrino! You might wonder, then, if perhaps neutrinos had the right value of mass and number, if they could make up the dark matter? And if not all of it, could they at least make up part of it? This is a question that astronomers and physicists have pondered for decades, and we might be closer than ever to the actual answer.
How much of the dark matter can neutrinos actually be? Find out today!
The ‘WIMP Miracle’ Hope For Dark Matter Is Dead
“Our hunt for dark matter in the lab, through direct detection efforts, continues to place important constraints on what physics may be present beyond the Standard Model. For those wedded to miracles, though, any positive results now appear increasingly unlikely. That search is now reminiscent of the drunk looking for his lost keys beneath the lamppost. He knows they’re not there, but it’s the only place where the light enabling him to look shines.
The WIMP miracle may be dead and gone, as particles interacting through the weak force at the electroweak scale have been disfavored by both colliders and direct detection. The idea of WIMP dark matter, however, lives on. We just have to remember, when you hear WIMP, we include dark matter that’s weaker and wimpier than even the weak interactions will allow. There is undoubtedly something new out there in the Universe, waiting to be discovered.
But the WIMP miracle is over. But we still might get the best miracle of all: if these experiments turn up something beyond a null result. The only way to know is to look.”
We have all sorts of enormous experiments built on the hope that dark matter would be a very specific type of particle: a WIMP. If it has a mass of around the electroweak scale, and doesn’t interact with the strong or electromagnetic forces but does have a nuclear recoil, these experiments should see it. This was well-motivated by a slew of theories of new physics at the electroweak scale. But with the limits we presently have, recently improved by LUX and XENON1T, those WIMPs, motivated by a theoretical scenario called the “WIMP Miracle,” are now ruled out.
Dark matter may be wimpier than the weak interaction, but our original motivation for building these detectors is dead and gone.
Cold Dark Matter Is Heated Up By Stars, Even Though It Cannot ‘Feel’ Them
“This effect is what’s known as “dark matter heating.” It isn’t that any of the radiation from the stars or any of the heat from the normal matter is getting transferred to the dark matter itself; it doesn’t involve temperature or energy transfer directly.
Instead, what’s happening is that the additional energy imparted to the normal matter is expelling it from where it was previously the most concentrated: in the galactic center. Once that normal matter is removed from the galactic center, there’s less mass there to hold the dark matter in place, and it, too, has to move to a higher, less-tightly-bound orbit. Because the dark matter gets pushed out and bumped to a higher, more energetic orbit, it has the same effects as though the dark matter was given an extra burst of energy. It’s not actually hotter than it was previously, but the effects are identical.”
Dark matter isn’t supposed to interact with anything. Not with normal matter, not with itself, not with radiation. So how is it, then, that cold dark matter can be heated up by the formation of new stars? Why should that be the solution to the odd distribution of mass in dwarf galaxies?
It actually makes sense, if you reason your way through the physics. Come take that journey, and learn how it actually happens!