Archive for the ‘Dark matter’ Category
Dummies on dark matter 3: all these neutrinos…
wateva
Canto: So, look up neutrinos and dark matter together on any bona fide sciency website, such as Astronomy magazine, or Nature, and you’ll get apparently contradictory claims – ‘neutrinos cannot constitute dark matter’ and ‘neutrinos may solve the mystery of dark matter’, so what’s a dummy to think?
Jacinta: It’s ongoing, and exciting, we must suppose. Dark matter is often given another adjective – cold dark matter – and neutrinos are too ‘hot’, which is to say they travel close to light speed. The clumpy nature dark matter is believed to have – remember they’re believed to clump around the outskirts of galaxies, explaining the observed higher velocity of outer galaxy stars – that clumpy nature isn’t consistent with zippy neutrinos.
Canto: Yes – neutrinos are kind of slight and speedy whereas dark matter is fat and lumpy?
Jacinta: Well that’s one way of putting it, but if it was fat it’d be visible, but it appears to be ‘transparent’ as Hossenfelder describes it.
Canto: That’s funny, a lot of fat people would prefer to be invisible, maybe dark matter has worked out a way… But if this matter is transparent or invisible, how can they detect it, or know that it’s clumpy? It seems to be just a placeholder to explain the gravitational behaviour of galaxies – doesn’t it?
Jacinta: Obviously I can’t answer that. Mathematics, however, may find a way…
Canto: I was hoping you wouldn’t mention that word.
Jacinta: Well it’s a return to neutrinos – sterile neutrinos. They only interact via gravity, but they are heavy, as needs to be the case. It’s all about missing mass after all.
Canto: Sounds like a similar profile to WIMPs but I think WIMPs, which are just postulates, I think, only interact through the weak nuclear force, an interaction that brings about nuclear radioactive decay. But I don’t think gravitational interactions have been ruled out for them.
Jacinta: WIMPs have gone off the boil recently, I think. It’s all such groping in the dark stuff at the moment, and if you have virtually no mathematics, it’s deadly. I’ve just been reading a dialogue between a physicist and a mathematician on neutrinos and dark matter, which after various increasingly heated exchanges of equations and talk of Minkowski spacetime, Lagrangians, anti-commuting spinor-valued fields, Weyl spinors and the like, it got to the point of pistols at dawn and aim for the heart. But the equations did look impressive.
Canto: Time to get back to basics. Remember we know about three types of neutrinos, also called flavours – tau, muon and electron. And remember they’re called leptons because they’re elementary particles and not very interactive….
Jacinta: That doesn’t explain why they’re called leptons, though, does it? Actually, when I try googling that very question, all I get is what leptons are, or what physicists thank they are.
Canto: You didn’t frame the question well enough:
Lepton was first used by physicist Léon Rosenfeld in 1948: ‘Following a suggestion of Prof. C. Møller, I adopt—as a pendant to “nucleon”—the denomination “lepton” (from λεπτός, small, thin, delicate) to denote a particle of small mass’.
Jacinta: Okay, all Greek to me. And by the way there are six lepton types, let’s get this clear – the three neutrinos and the particles they’re connected with, the electrons, muons and tauons. But I don’t know how or why they’re connected.
Canto: It seems that the three neutrino types are electrically neutral versions of, or sisters of, the negatively charged electrons, the also negatively charged muons – which have a half-spin, apparently – and the tauon or tau particle, which is also negatively charged with a half-spin. How can they tell them apart you ask? Well, according to the US Department of Energy, ‘Muons are similar to electrons but weigh more than 207 times as much’. Which is a bit like saying I’m similar to my neighbour but she weighs more than 15,000 kgs.
Jacinta: Ah yes, I’ve met her. A gentle giant, but a bit negative.
Canto: Well, multiply my neighbour’s mass – I mean a muon – by 17 and you have the mass of a tau particle. You’d think they’d be unmissable, but the first lepton to be discovered was by far the smallest, the electron. That was in 1897, and the rest are 20th century discoveries. And there are anti-leptons, of course.
Jacinta: Of course. So for completeness’ sake, and for our education, there are leptons, mesons and hadrons. Oh, and fermions. I’m just throwing those names out there. And gluons, and quarks, and bosons… and that might be it.
Canto: Well considering that we can account for only 4 or 5 percent of universal mass-energy – unless something’s very wrong with our accounting – we might be adding a few more possibly speculative particles in future. Is it really exciting or is it just a mess?
Jacinta: You want me to answer that?
Canto: Rhetorical, rhetorical. But it’s no wonder that respected physicists like Neil Turok is finding that we’ve complicated the field way too much. As he says, the LHC, the most touted experimental device in physics in the last 40 years, has discovered nothing but the Higgs boson, which of course was a really important discovery, but…
Jacinta: He says the dark matter is probably a right-handed neutrino, which, whatever it means, sounds simple enough. And that the universe is a kind of flat space, with nice and simple geometry…
Canto: Okay, a right-handed neutrino, let’s follow that up. The first thing I would think would be – it’d have to be heavy, and non-interactive, which means very difficult/impossible to detect. And then – if there are right-handed neutrinos there must surely be left-handed ones. These terms relate to spin, and the Standard Model, I think, gives neutrinos a left-handed spin, with a ‘helicity’ of -1, and these are paired with right-handed anti-neutrinos, with a helicity of +1.
Jacinta: So Turok is out on a limb here?
Canto: How would I know? It starts to get into mathematics and if-then speculations very quickly, and I get lost. But Turok feels that there must be more simple solutions to the big dark matter-dark energy conundrums without positing all these new particles. I know he seems to be positing one himself, but it’s just a variant of the neutrinos that’ve been proven to exist.
Jacinta: Helicity, by the way, is ‘the projection of the spin onto the direction of momentum’. Just another head-scratcher for dummies. Helicity, at any rate, is conserved. It doesn’t change over time. And here for, what it’s worth to the likes of me, is what one commentator says about Turok’s hypothesis:
For a heavy neutrino to serve as dark matter, it needs to be quite stable. Apparently this is tough if it interacts with the Higgs—how true is that, exactly? But a neutrino that’s its own antiparticle can have a mass without interacting with the Higgs: a so-called ‘Majorana mass’.
In Turok’s theory all the neutrinos have Majorana masses, described by a mass matrix. To make the heaviest right-handed neutrino stable, a bunch of matrix entries must vanish—and this makes the lightest left-handed neutrino massless!
Canto: Yeah, ain’t mathematics magical.
Jacinta: Hmmm, I’m wondering if we should leave all this dark stuff behind us for a while. Leave it to the Dark Lords to work out.
Canto: Haha, not very female supremacist of you…
References
https://www.nature.com/articles/d44151-022-00024-6
https://golem.ph.utexas.edu/category/2022/12/neutrino_dark_matter.html
Dummies on dark matter 2: there are problems…
from Forbes website, see below
Canto: So there are candidates for dark matter, and there are also those who think that, though there is a serious problem in cosmology, to do with mass and energy, ‘dark matter’ won’t be the fix.
Jacinta: By the way, I was chatting with another dummy on this topic recently, who had the excuse of being much much younger than myself, and she asked if dark matter had anything to do with black holes. I wasn’t able to give a very effective answer, but Sabine Hossenfelder, one of our heroes, says in a video linked below that black holes and brown dwarfs (whatever they are), and other such exotic objects ‘would make too many gravitational lenses, which have not been seen’, to be candidates. Also black holes are so called because they ‘swallow’ light, that’s to say, light-emitting particles. So they really are black, in a sense, whereas ‘dark’ matter is more transparent than anything, according to Hossenfelder.
Canto: Well, getting back to Peter Fisher’s Royal Institution talk, he talks about the 1980s as a time of confusion and excitement in theoretical physics and cosmology when so many things weren’t adding up. At the same time a concept called super-symmetry was being mooted. It ‘predicted all kinds of heavier particles that we wouldn’t have observed in accelerators because they weren’t powerful enough’, according to Fisher. He also presented, what I’ve heard before, a conjecture that there must be this ginormous halo of dark matter surrounding galaxies to make up the missing mass and to account for the behaviour of visible matter at the edge of galaxies. In other words, this dark matter must have a gravitational effect on the outer arms of these galaxies.
Jacinta: I know that Hossenfelder is no great fan of bigger and more expensive accelerator-colliders in the hope of discovering more teensy-tiny but ultra-ultra numerous particles to fit the dark matter bill, but Fisher also goes on to talk about the Standard Model and how effective it has been, without dark matter screwing it up…
Canto: Yes it’s been very effective for accounting for some 4% of the mass-energy of the universe. Anyway Fisher helped to debunk a theory regarding ‘heavy neutrinos’ as a candidate for dark matter in the late eighties, which seems to this dilettante like an absurdity – neutrinos being near-massless, which presumably helps them to pass through planets as if they’re not there.
Jacinta: I think this heavy neutrino thing might’ve morphed, in theory, into the idea of weakly interacting massive particles, or WIMPs, and they’re still looking for em, for example on the International Space Station. They’re also looking for theoretical particles called axions, using special detectors. No luck so far for WIMPs or axions.
Canto: Fisher describes another source, black holes, via work done by Stephen Hawking, but I found it difficult to follow, so I’ll try roughly quoting:
In the beginning of the universe there was all this mass around, it’s very dense, and in a regime where quantum mechanics is very important, so the density is… fluctuating and changing, and [Hawking] thought, could it be that the density would be high enough to form a black hole? He did some rough calculations and found that, yes they could collapse into a black hole, and there’d be a lot of black holes, but there must be a way of getting rid of them, because we don’t see them. Over time, he invented a mechanism by which black holes radiate light at a very low level, a concept now called Hawking radiation, a remarkable notion, as it suggests the only connection we currently have between gravity and quantum mechanics.
The connection with dark matter is that there still may be ‘primordial’ black holes with a lot of mass but tiny in size. No luck in finding them either, needless to say.
Jacinta: So now let’s focus on Sabine Hossenfelder’s RI talk. It seems to me she goes into a lot more detail about the anomalies in what we observe, ascribed to the missing matter. For example, structure formation in the universe – which I remember being fascinated by when Carl Sagan presented it image-wise in his Cosmos series. Here are two points she makes on structure formation:
- Dark matter cannot build up radiation pressure and therefore starts forming structures sooner than normal matter
- normal matter on its own does not produce sufficient structures on short scales to be compatible with observation
And I have no idea what they mean. She mentions things that we can see, such as ‘galactic filaments, and so on and forth’. So, thinks me, wtf are galactic filaments? Well, Wikipedia calls them galaxy filaments, and they’re the largest known structures in the universe…
Canto: This is actually exciting – how could I have lived so long without knowing about these things?
Jacinta: Haha well Wikipedia is pretty good on this stuff:
In cosmology, galaxy filaments are the largest known structures in the universe, consisting of walls of galactic superclusters. These massive, thread-like formations can commonly reach 50/h to 80/h Megaparsecs (160 to 260 megalight-years) — with the largest found to date being the Hercules-Corona Borealis Great Wall at around 3 gigaparsecs (9.8 Gly) in length — and form the boundaries between voids. Due to the accelerating expansion of the universe, the individual clusters of gravitationally bound galaxies that make up galaxy filaments are moving away from each other at an accelerated rate; in the far future they will dissolve.
Galaxy filaments form the cosmic web and define the overall structure of the observable universe.
Canto: Great. What’s a void?
Jacinta: Cosmic voids, doncha know, are those vast spaces between filaments that contain few or no galaxies. But to return to Hossenfelder, who’s a theoretical physicist, and a lot more proficient in maths than we are, as we’ll see. She’s also something of a dark matter skeptic, it seems. She highlights four problems that dark matter doesn’t solve, and we should try to understand them:
- the brightness of galaxies is strongly correlated with the (asymptotic) rotational velocity (‘Tully-Fisher Law’). Dark matter doesn’t explain this
- dark matter leads to density peaks in galactic centres which badly fits with observations (‘galaxy cusps’)
- dark matter predicts too many dwarf galaxies
Canto: Okay let’s start with asymptotic rotational velocity. Asymptotic analysis, in mathematics, is about describing behaviour of functions as they approach a limit, such as infinity. So galactic velocity presumably has some sort of limit, which can be calculated mathematically. The Tully-Fisher Law, or Relation, from Wikipedia:
is a widely verified empirical relationship between the mass or intrinsic luminosity of a spiral galaxy and its asymptotic rotation velocity or emission line width. Since luminosity is distance-dependent, the relationship can be used to estimate distances to galaxies from measurements of their rotational velocity.
So the point Hossenfelder makes here, I think, is that rotational velocity correlates well with brightness, which correlates with distance, as measured from Earth. Dark matter appears to be irrelevant to these calculations. Of course I may be getting this all wrong.
Jacinta: I wouldn’t know. But it seems that dark matter and its supposed halo should be interfering with orbital velocities and so interfering with calculations, but it isn’t?
Canto: Hmmm.. Anyway, second point. Galaxy cusps takes me to the ‘cuspy halo problem’, which I’ll try to explain in my own words. It’s also called the core-cusp problem, which, broadly speaking, is a discrepancy found in small, low-mass galaxies, according to different measurement systems or predictions. ‘Cuspy’ represents an energy-mass distribution which is denser at small radii, whereas most dwarf galaxies have a more flat ‘core’ distribution.
Jacinta: These are all just the beginnings of our explorations of this topic. In a hundred years or so we’ll be fully conversant with the issues. As to dark matter predicting too many dwarf galaxies, aka the dwarf galaxy problem, apparently we’ve observed and identified some 38 of these dwarf galaxies in our Local Group (a dumbbell-shaped group of galaxies with the Milky Way and Andromeda forming the two lobes), instead of the 500 or so predicted by dark matter simulations, and that’s just around the Milky Way.
Canto: Okay, we’re doing well, sort of. Final problem – the alignment of satellite galaxies. Essentially, they form a disk rather than a halo. Perhaps surprisingly, the Forbes website has what seems to me an excellent article on the subject. Dark matter simulations produce halos merging together in spiral formations surrounded by sub-halos in a variety of orientations. But that’s not what we see – we see satellite galaxies in the same orientation as their ‘hosts’, and co-rotating with them. This has been observed for the Milky Way, Andromeda and most satellite galaxies observed, such as Centaurus A. So what accounts for these discrepancies?
Jacinta: You don’t know? Well, we’ll have to look at that next time, or not. I suspect there might end up being hundreds of these dark matter posts. We might even have to learn some maths….
References
Is Dark Matter Real? – with Sabine Hossenfelder (Royal Institution video)
What is dark matter? – with Peter Fisher (Royal Institution video)
https://en.wikipedia.org/wiki/Galaxy_filament
https://en.wikipedia.org/wiki/Void_(astronomy)
https://en.wikipedia.org/wiki/Tully–Fisher_relation
https://en.wikipedia.org/wiki/Cuspy_halo_problem
https://en.wikipedia.org/wiki/Local_Group
Journey into dark matter , iTelescope Webinars, with Dr Maggie Lieu
Dummies on dark matter 1 – the missing galactic masses
Jacinta: So here’s where we try to educate ourselves on dark matter, just for the fun of it.
Canto: But we’re serious. Our blog’s motto is ‘Rise above yourself and grasp the world’, supposedly from the wisdom of Archimedes, and he meant the universe, or would’ve.
Jacinta: Well, smarter guys than us are trying to grasp dark matter, with limited success it seems.
Canto: Yeah but they’ve been smart enough to recognise that there’s this missing matter, when they look at galaxies and find they’re spinner faster than their observed mass suggests they should, because physical laws apparently tell us that the greater the mass, the greater the gravitational effect, which would cause a greater spin towards the black hole sun, I think. But when astrophysicists measure a galaxy’s mass – which must surely be a tricky process, but I think it’s about measuring light spectra – different molecules give off different electromagnetic waves, though how they manage to measure all that is beyond me – anyway when they measure the galaxy’s mass and its spin, the numbers are off by orders of magnitude. I think.
Jacinta: Galaxies contain hundreds of billions of stars, I hear, so they must’ve built some impressive measuring technology. Okay, first research – from Physics LibreTexts:
The mass of [our] Galaxy can be determined by measuring the orbital velocities of stars and interstellar matter.
The article I’m quoting from focuses on our galaxy as a more or less typical example. Our sun is orbiting the galaxy’s centre (a black hole presumably?) at 200 kilometres per second, and it’s calculated that it takes about 225 million years for a full orbit.
Canto: Orbital velocity sounds so much more legitimate than spin, methinks.
Jacinta: So I want to put this explanation of the Milky Way’s proposed mass in my own terms to try to understand it better. Imagine the Sun’s roughly circular orbit around the galaxy, which is shaped somewhere between a sphere and a more or less two-dimensional disc. As Newton worked out, in the case of a sphere like Earth, gravity acts to pull everything towards the centre. Our Sun, which lies in an outer arm of the galaxy, gets whipped around at great speed due to the large mass between it and the galaxy’s centre (some 26,000 light years away). Kepler’s third law comes into play here…
Canto: His third planetary law (from the NASA Science website):
The squares of the orbital periods of the planets are directly proportional to the cubes of the semi-major axes of their orbits. Kepler’s Third Law implies that the period for a planet to orbit the Sun increases rapidly with the radius of its orbit.
This was presumably simply an observational measurement from Kepler, but Newton and others found that, the further out from the Sun, the faster the planet moved, due to the greater accumulated mass in between. I think.
Jacinta: Uhh no. The period increases rapidly, not the velocity. The word ‘rapidly’ has led you astray. Naughty NASA Science website, it should have said that the period becomes longer, and not just because there’s a greater distance to travel but because the planet itself is revolving more slowly. Don’t forget that’s there’s relatively little, in fact very very very little mass, relatively speaking, added between Jupiter and the Sun than there is between Earth and the Sun. And here’s some figures: Earth is revolving round the Sun at about 107,000 km/h, Jupiter at around 47,051 km/h and Mercury at 170,500 km/h.
Canto: Okay, so galaxies are quite different from solar systems. So in the centuries after Kepler and Newton, astrophysicists used their theories, along with further developments, to measure the Milky Way’s mass, generally maintaining the theory that the vast majority of that mass lay between the Sun and the centre. This was assumed to be reasonable because:
The number of bright stars and the amount of luminous matter (meaning any material from which we can detect electromagnetic radiation) both drop off dramatically at distances of more than about 30,000 light-years from the galactic center.
But this assumption has turned out to be wrong, because we now know that there’s lots more matter well out from the centre, but it happens to be invisible – to us at least, and maybe to our instruments, but then if it’s undetectable how do we know it exists?
Jacinta: Well, Physics LibreTexts gives a good explanation:
We can understand how astronomers detected this invisible matter by remembering that according to Kepler’s third law, objects orbiting at large distances from a massive object will move more slowly than objects that are closer to that central mass. In the case of the solar system, for example, the outer planets move more slowly in their orbits than the planets close to the Sun. There are a few objects, including globular clusters and some nearby small satellite galaxies, that lie well outside the luminous boundary of the Milky Way. If most of the mass of our Galaxy were concentrated within the luminous region, then these very distant objects should travel around their galactic orbits at lower speeds than, for example, the Sun does. It turns out, however, that the few objects seen at large distances from the luminous boundary of the Milky Way Galaxy are not moving more slowly than the Sun.
So I added some italics to help us. If all the mass, or a vast majority, was located at the galaxy’s centre, as our sun with its massive mass is located at the centre of our local system, then this handful of luminous objects that are clearly part of our galaxy but further, sometimes much further, from the centre, should be travelling more slowly than the Sun, but instead they’re travelling faster.
Canto: Which presumably means unaccounted for mass, something to do with e = mc², more mass provides more energy, giving more velocity…
Jacinta: Yeah, something like. But light is a kind of energy, like electromagnetic energy, which presumably dark matter doesn’t have, but it must have energy if it has mass. Or must it?
Canto: Andrew Pontzen, in a Royal Institution lecture, tells us that, according to current calculations there should be five times more dark matter than visible matter, and it’s streaming through the planet, and our bodies, as we write…
Jacinta: Like neutrinos? Are neutrinos a kind of dark matter?
Canto: Ha, well that’s a good thought, maybe. Here’s a quote from an article in Nature India in early 2022:
Physicists have developed a mathematical model that may shed light on the identity of dark matter, the mysterious substance that far outweighs visible matter in the universe. Dark matter is not made of atoms or other known fundamental particles and doesn’t interact with any form of light or electromagnetic radiation, making it difficult to detect. The new model showed that a non-interacting or sterile neutrino is probably a dark matter particle and contributes to the mass of dark matter.
Jacinta: A sterile neutrino?
Canto: Yes, aka an inert neutrino – currently hypothetical, so outside of the Standard Model. The Wikipedia article on this is pretty comprehensive and complex… I’ll quote from the very beginning of the article, and then we can discuss it, hahaha:
Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that interact only via gravity and not via any of the other fundamental interactions of the Standard Model. The term sterile neutrino is used to distinguish them from the known, ordinary active neutrinos in the Standard Model, which carry an isospin charge of ±+1/ 2 and engage in the weak interaction. The term typically refers to neutrinos with right-handed chirality (see right-handed neutrino), which may be inserted into the Standard Model.
Jacinta: Well, just to complicate this apparently dire situation of masses of matter or stuff we know nothing about, there’s also dark energy, which according to another Royal Institution presentation, this time by Peter Fisher in 2022, makes up even more of the unknown stuff in the universe than dark matter, leaving us in the dark about 96% of universal stuff. How can we be so incompetent?
Canto: Well, it’s an increasingly informed incompetence I’m sure. We’ve learned, as you say, that all the particulate matter-energy that we know about makes up only 4% of all there is, so now we (I mean you and I) need to learn how we (the astrophysics community) learned this.
Jacinta: About 73% of the unknown stuff is dark energy, 23% is dark matter. So let’s try to regurgitate Fisher’s talk in our own words, so that we can sound more informed.
Canto: Right so we start with the ‘big bang’ 13.7 billion years ago, followed by ‘inflation’, a very rapid expansion over a relatively short period, resulting in the cosmic background afterglow, after which the universe slows down markedly in its expansion thenceforward, with first stars and then galaxies developing. But then, after about half the life of the universe, it starts to expand slightly more rapidly, apparently due to dark energy.
Jacinta: Yes, according to an illustration I’ve seen before, and which we’ll post here, dark matter dominates in the first half of universe’s history, and dark energy dominates the second half.
Canto: And why might that be? That is presumably an unsolved mystery. Which we will continue to explore…
References
https://science.nasa.gov/resource/orbits-and-keplers-laws/
https://en.wikipedia.org/wiki/Sterile_neutrino
What is dark matter? With Peter Fisher (Royal Institution lecture, 2022)