Sometimes in fantasy fiction, you don’t want to explain something that seems inexplicable, so you throw your hands up and say, “A wizard did it.” Sometimes in astronomy, instead of a wizard, the answer is dark matter (DM). If you are interested in astronomy, you’ve probably heard that dark matter solves the problem of the “missing mass” to explain galactic light curves, and the motion of galaxies in clusters.
Now [Pedro De la Torre Luque] and others are proposing that DM can solve another pair of long-standing galactic mysteries: ionization of the central molecular zone (CMZ) in our galaxy, and mysterious 511 keV gamma-rays.
The Central Molecular Zone is a region near the heart of the Milky Way that has a very high density of interstellar gases– around sixty million times the mass of our sun, in a volume 1600 to 1900 light years across. It happens to be more ionized than it ought to be, and ionized in a very even manner across its volume. As astronomers cannot identify (or at least agree on) the mechanism to explain this ionization, the CMZ ionization is mystery number one.
Mystery number two is a diffuse glow of gamma rays seen in the same part of the sky as the CMZ, which we know as the constellation Sagittarius. The emissions correspond to an energy of 515 keV, which is a very interesting number– it’s what you get when an electron annihilates with the antimatter version of itself. Again, there’s no universally accepted explanation for these emissions.
So [Pedro De la Torre Luque] and team asked themselves: “What if a wizard did it?” And set about trying to solve the mystery using dark matter. As it turns out, computer models including a form of light dark matter (called sub-GeV DM in the paper, for the particle’s rest masses) can explain both phenomena within the bounds of error.
In the model, the DM particles annihilate to form electron-positron pairs. In the dense interstellar gas of the CMZ, those positrons quickly form electrons to produce the 511 keV gamma rays observed. The energy released from this annihilation results in enough energy to produce the observed ionization, and even replicate the very flat ionization profile seen across the CMZ. (Any other proposed ionization source tends to radiate out from its source, producing an uneven profile.) Even better, this sort of light dark matter is consistent with cosmological observations and has not been ruled out by Earth-side dark matter detectors, unlike some heavier particles.
Further observations will help confirm or deny these findings, but it seems dark matter is truly the gift that keeps on giving for astrophysicists. We eagerly await what other unsolved questions in astronomy can be answered by it next, but it leaves us wondering how lazy the universe’s game master is if the answer to all our questions is: “A wizard did it.”
We can’t talk about dark matter without remembering [Vera Rubin].
There always seems to be a bit of deus ex machina when people bring dark matter into the discussion. It’s reminiscent of the Luminiferous Aether theory of the 19th century. It’s going to be interesting to see what actually shakes out of this.
511 keV gammas are not unusual at all: they are (as stated) a normal consequence of an electron meeting a positron: they annihilate each other, giving up their mass in the form of the equivalent gamma ray energy, just like E=mc^2 says.
Even making the positrons and electrons is not unusual: that’s a normal way a higher-energy gamma ray loses energy: When a gamma ray with an energy higher than 1022 keV passes by a nucleus, it has some probability to convert some of its energy into a matching anti-pair of electron and positron. This process is called pair production, of course.
That electron and positron will wander away at some speed. After a while, after they bounce around a bit and slow down enough, they will eventually run into and orbit (usually) another anti-particle, dance for a bit, then annihilate again, producing a pair of those 511 keV gammas.
So seeing them is not at all mystery — dark matter is not at all required to explain their production. The question is why so much of them, and why does their source seem to be smoothly-distributed.
And this particular Dark Matter variant is one of the possible “Why?”s that has 1) not been invalidated experimentally (e.g unlike WIMPs) and 2) makes consistent predictions that match observations thus far.
Dark Matter has the annoying property that attempts to disprove it keep failing. Alternatives like MOND proposed thus far also fail to make predictions that match observations (e.g. MOND predicts galactic spin differences, but predicts one consistent spin difference whereas observed galaxies have a whole range of behaviours). ‘Simple’ solutions like slightly modified GR have long been ruled out.
Whilst there may potentially be an alternative explanation for observed phenomena that does not involve Dark Matter, it would have to be consistent with all observations that are consistent with Dark Matter, so functionally would have the same properties as Dark Matter (i.e. is gravitationally interacting but with very weak to no interaction with the other 3 forces, is not homogenous across space, is not a universal factor, etc).
“So seeing them is not at all mystery — dark matter is not at all required to explain their production. The question is why so much of them, and why does their source seem to be smoothly-distributed.”
The 511 keV emission itself isn’t a good candidate for probing dark matter, because there are lots of ways to produce positrons (you don’t have to produce them from pair production, you can also get quite a bit from beta decay). The article tries to suggest that this is a “mystery” but it’s… not really that big of one. It’s just that there are a lot of possibilities. In fact we know it’s not all dark matter, because we know there are other sources that have to be there.
That’s why the paper’s talking about the ionization level of the CMZ, and specifically pointing out that you could imagine looking for spatial correlations between the two.
It comes from effects in higher order forces. Edward E. Smith, PhD wrote a series of excellent texts on the subject beginning with “The Skylark of Space”.
It’s not the same as ether at all. Ether was an attempt at a theory. Dark matter is a set of observations.
All the armchair experts will no doubt jump into these comments so I’ll strongly encourage everyone to watch Dr. Collier’s video on the topic. And remember- dark matter is not a theory, it is a set of observations. We know it exists, we only wonder about the exact nature and origin of it.
People should also watch this video because she specifically addresses all the things that laypeople incorrectly think about it, along with all the armchair “expert” ideas about it. It should be mandatory viewing before commenting on articles like this.
https://youtu.be/PbmJkMhmrVI?si=GT34wg5GTpjMaXS2
…but it leaves us wondering how lazy the universe’s game master is if the answer to all our questions is: “A wizard did it.”
um, the game master is the wizard
Personally I’ve often wondered if the answer is topological, the expansion rate changing over time because the universe is progressing over a curved surface. Although we may find that those virtual particles that appear to pop in and out of existence in the vacuum add up on average, perhaps mass pulls, to a lesser extent, on things in adjacent time too, so objects act upon themselves. Superimposed concurrent iterations of space time would not only give rise to things like inertia, gravity and magnetism as emergent properties, but also provide a physical mechanism for things like entanglement. Just speculation of course, if anything, it’ll make a good scifi novel one day.
“Personally I’ve often wondered if the answer is topological, the expansion rate changing over time”
It can’t really be something global, that’s what the Bullet Cluster data shows. Two galaxies collided, most of the mass of the galaxies just passed straight through each other without any interaction. That’s why theories like MOND had to become super-strained and crazy after the Bullet Cluster data – seriously, the dude who proposed MOND eventually added small amounts of dark matter to more recent models to help explain stuff like the Bullet Cluster (and somewhere, Occam’s Razor cried).
Something like dark energy, that could be something global.
ah, the comedy of ‘light dark matter’ – light can mean low-mass or bright.
if the dark matter is interacting in such a way as to produce positrons then it isn’t dark. the power of dark matter as an explanatory concept is that it can’t be observed except gravitationally. once you find a non-gravitational way to observe it, it stops being dark matter and a new metaphor is needed.
this exotic matter that can interact non-gravitationally but hasn’t been observed in a lab raises more questions than it answers. hard to rule it out but
“the power of dark matter as an explanatory concept is that it can’t be observed except gravitationally.”
The ‘dark’ here originally meant “does not glow” (as opposed to stars) when Zwicky proposed it and then became “does not couple significantly to photons” after the CMB results.
It never meant “can’t be observed except gravitationally.” At one point it was possible that you could have like, tons of compact non-stellar objects (like brown dwarfs) that’d make up dark matter, but eventually the lensing results ruled that out and the CMB results blew them up.
And neutrinos would’ve been really nice dark matter for a long time, except we can measure enough now to know there’s not heavy enough (or enough of them).
The CMB results tell you there’s a significant amount of ‘stuff’ that 1) compresses normal matter (has gravity) and 2) doesn’t experience significant pressure when there’s a high photon temperature.
It could be totally inert, but there’s no need for it to be and it does make things a bit more complicated in the earlier Universe because dark matter would freeze out (decouple) earlier.
i think you don’t understand what i meant. dark matter doesn’t have any properties because it doesn’t exist. things get labeled dark matter when you can’t prove that they don’t exist because you postulate that they aren’t observable except gravitationally. it’s a social phenomenon that i’m talking about, where scientists use this word while scratching their head and saying “something ain’t right.” one says it, then the other says it, and before you know it they’re all saying “yeah dark matter.” a mantra of frustration, not an explanation.
the moment it has posited interactions, it doesn’t fit that model anymore because it becomes falsifiable and soon it goes the way of the neutrino as ‘dark matter candidates’ go. dark matter is one of those things where once we know what it really is, we won’t use that phrase anymore. if neutrinos had turned out to be dark matter, no one would ever call it dark matter anymore, we would call it neutrinos.
“things get labeled dark matter when you can’t prove that they don’t exist”
Yeah, no, this is not true. It’s not a ‘social phenomenon’. It’s an empirical model. I mean, this is how science works: you have a set of observations you can’t explain, so you start parameterizing what features would explain them. Then you use that model to explore other things which you can then observe and feed back into the model to constrain the parameters. That’s what lambda-CDM is – it’s an empirical model to explain observations using a constant cosmological constant (lambda) and ‘cold’ dark matter.
Dark matter (as a model) is definitively not “only interacts gravitationally” – over time it has moved towards that but that’s literally because of the feedback from the observables onto the model. You had tons of different empirical dark matter models, all of which were slowly excluded based on observables.
“the moment it has posited interactions, it doesn’t fit that model anymore”
Again, this also isn’t true, you can integrate generic interactions into the model without knowing how they work. This is how you determine the parameters of the model you’re working with. Saying “dark matter annihilates into e+/e-” doesn’t actually mean it’s any more falsifiable than “the dark matter density is X” because it’s just a model parameter. The interaction cross-section or branching ratio can smoothly go down to zero, for instance.
Using an empirical model to constrain where the parameters of the actual underlying physical model must lie is how modern science works. The difficulty with dark matter is that it turns out that the underlying physical model is a royal freaking bastard to pin down. But there’s nothing that says that the Universe had to be nice to us!
“In the model, the DM particles annihilate to form electron-positron pairs.” So the Phlogiston molecules just sit around on the front porch all day just annihilating at whim? You can’t disprove it, nanny nanny boo boo. “Let’s just assume there’s something that explains everything and we’re done in time for lunch. Physics is easy!”
“You can’t disprove it, nanny nanny boo boo.”
Dark matter is a name for an empirical model which fits a large collection of observations. It’s not a theory that can be proven or disproven. It’s just an empirical model. It’s not real. You propose actual theories which replace that model, and those can be proven or disproven.
Even if you end up finding some modification of basic laws or whatever which end up explaining all those observables, you must be able to factor out a portion of that modification that you can represent as that model. And then it will still be convenient in some cases to represent whatever the changes are in a first-order approximation as that same model anyway.
The exact same thing happens in particle physics, where the actual underlying physics are way more complicated but in the end it’s easier to just represent the action as something simple like pion exchange or something.
A lot of dark matter candidates the particle theorists have tossed out are Majorana particles, which are a theoretical type of fermion which is its own antiparticle. Neutrinos might be majorana fermions, but the double-beta-decay experiments that would prove (or disprove) that assertion are still ongoing.
Just because it seems pat doesn’t mean it’s not true; truth is stranger than fiction, and sometimes good science looks like bad writing.
I’m going to be “that guy” and mention “volume 1600 to 1900 light years across”. Huh, that’s a volume ? The wiki linked to does a better job, saying the diameter of the CMZ is the aforementioned, though leaving the reader to calculate it’s volume from the galactic longitudes and latitudes and said diameter.
I luv being That Guy, but it’s a volume and is 1600 to 1900 light years across [presumably] its long axis. Volumes have lengths and widths. Stet.
Huh. My decades old learnin’ is woefully out of date, I remember when volumes had lengths, widths, and depths.
But on topic, I noticed the crazy choice of units in the article (and probably the paper as well).
You use metaphoric units of measurement so that humans can get a grasp of what you’re talking about. Saying a T. Rex is 6 meters tall and his head is 2 meters wide isn’t very evocative, but saying that he could look into the 2nd story window but his head wouldn’t fit because it’s too wide is very evocative, and gives you a mental model that you can use for imagination.
For example, was T. Rex bothered by lightning? (Hint: trees were significantly taller at the time, much taller than the dinosaur).
Saying “sixty million times the mass of our sun, in a volume 1600 to 1900 light years across” isn’t very evocative. It’s obfuscating the concept, and people just skip over it.
Saying “the gas is hundreds of times more dense than the average density of interstellar space” would be a bit better.
You can generate matter from information as much as you can generate information from matter, because at the quantum level there is no difference. At the quantum level, matter and information are fundamentally equivalent and interconvertible, with each transformation governed by established physical laws of energy, entropy, and quantum mechanics.
Well transporters and replicators are still a dream away.
Actually many of the so-called unsolved questions in Astronomy HAVE been solved, it’s just that the answers have been ignored.
If one takes into account that the detectable Universe is >99% plasma, which is generally accepted, then one should also understand that as plasma contains a high proportion of charged particles and, by definition, when these are moving they create an electric current, this means that the Universe is full of electric currents.
These currents, in turn, will then create a magnetic field around them, which will attract more particles, building up larger streams of plasma, hence forming Birkeland currents, like those connecting the Sun to the planets, and which, when energetic enough, can produce visible light in the form of the auroras on Earth and other planets.
For a more mathematical treatment of these do a search on Dr Donald Scott’s papers.
These currents are orders of magnitude more powerful at attracting matter than gravity, and hence are obviously the most likely explanation for how stars, galaxies and even larger structures are formed. When one takes this into account there is no need for “dark matter”, and many of the other “unsolved questions” no longer remain unsolved, including the “great walls” (which according to conventional theory would have been impossible to form in the commonly accepted age of the Universe), solar system formation (which under a gravity-only model only results in pebbles), the spiral galaxy “winding problem” and much more.
Similarly there is no need for “black holes” to be powering galaxies (especially the magical black hole like at the centre of our galaxy that somehow allows huge nebulae to pass through it) – plasmoids can explain the observations.
Ditto neutron stars and pulsars that are theoretically spinning so fast they can’t possibly remain in one piece – a cloud of plasma around a normal star that creates and electric circuit that spikes regularly can explain this.
Even the mysterious properties of the Sun can be explained with an electric circuit model, where fusion occurs on the surface, not in the centre that is supposedly millions of degrees and atmospheres of pressure, which contradicts what we can see with modern equipment looking into dark sunspots.
So, whilst all of the magical and undetectable objects that science entertainers constantly present as settled truths make for great science fiction books and movies, the simplest and most obvious explanation is that they are all the result of electromagnetism.
That may not sell a lot of media, but once this is actually accepted and taught the implications are enormous in terms of the technology we can create based on this knowledge, rather than continuing to waste enormous amounts of money, e.g. in the pursuit of hot high-pressure fusion, which is always (at least) 10 years off.