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Last week, we gave you an update in particle physics. This week it’s time to see what’s new in the world of dark matter. Spoiler alert, we still have no idea what it is, but maybe a few more ideas for what it isn’t.
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This episode is sponsored by: RxBar
Show Notes
Dark Matter
Gravitational Lensing
Cosmic Microwave Background
?CDM (Lambda cold dark matter) or Lambda-CDM model
Cold Dark Matter
NGC-1052 – the galaxy with no dark matter
Entropic gravity
Transcript
Podcast Transcription provided by GMR Transcription
Female Announcer: Today’s show is brought to you by RXBAR. For 25 percent off your first order, visit rxbar.com/astro and enter promo code “astro” during checkout.
Fraser: Astronomy Cast, Episode 487: Dark Matter Update. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, the Director of Technology and Citizen Science at the Astronomical Society of the Pacific and the Director of CosmoQuest. And Pamela, you are actually at the Astronomical Society of the Pacific, which I think is awesome.
Pamela: I am. I am in San Francisco. As soon as I’m done with the workday, I’m heading up to Sacramento, and yeah, it’s great to actually be at the headquarters of the company I work for and get to see some of my best friends, which I made strictly because of this show.
Fraser: Right on. And a huge thank you to the Astronomical Society of the Pacific for their support of all of the educational work that we do through CosmoQuest and Astrosphere, and we couldn’t have a better partner in being able to get all this educational information out and be able to get all the Citizen Science done. They’ve been a real treat to work with, and I can’t wait that I get a chance to come down and visit them. So, one just big, big piece of news which is – I think I’ve mentioned briefly – Dave Dickenson and I are working on a book, The Universe Today: Ultimate Guide to the Cosmos I think. I forget the exact name. We’re still working it out.
The gist is it is a how to be an amateur astronomer, and just all of the sky charts, and what to see in the night sky, and how to pick a telescope, and just that really comprehensive book. And Dave’s been writing his heart out and delivered the manuscript last month, and then we just wrapped up all of the photographs, and that’s the part that I’m most proud of is that we’ve got, I think, 60-plus, 70 astrophotographers who have contributed photographs to this book, and they’re all gonna be pictures that people have never seen before. They are amateur astrophotography of all of the planets, and the sun, and a lot of the deep sky objects, and the Milky Way, and really great artistic work.
And I’m just so proud and so thankful for all of the photographers who participated, and we’ll give you more information as we get closer, but wow, what a load off to put this thing off to the publishers. So, I said I’d never write another book, and I didn’t because Dave did it, but it still is a tremendous amount of work, and I’m really, really proud of what’s come together, and Dave’s just done a great job. So, stay tuned as we get closer to the actual book, and then maybe you’ll have to interview us for Astronomy Cast or something.
Alright, let’s move on. So, last week, we gave you an update in particle physics. This week, it’s time to see what’s new in the world of dark matter. Spoiler alert: we still have no idea what it is, but maybe a few more ideas for what it isn’t.
Pamela: This is true. Now, this is one of those great updates we get to do where we go from knowing definitively that dark matter is a thing and not a modification to gravity, which we had I think in our first season. And now, we can actually say with even more certainty that it is totally, totally a thing with lots more lines of evidence. So, just in the time that we’ve been doing this show, we have gone from “We don’t know what dark matter is” to “Dark matter is stuff” to “Dark matter is definitely stuff, and here are three lines of evidence.”
Fraser: Right. And before we get into that, I think it’s funny, now that we’ve been doing this show for 10 years, we’re doing Episode 487, we’re closing in on Episode 500, and people are always like, “What are you gonna do for Episode 100? Are you gonna look back at all the stuff that’s changed? What are you gonna do for Episode…” you know. “And what are you gonna do for Episode 500?” I’m really glad. It’s almost like we’re doing update season as opposed to necessarily we try to go through one episode. I think this was genius of you to put these episodes into the queue, and I wouldn’t be surprised if this runs us through up to 500, but you know.
Because we went after a lot of that low-hanging fruit when we first started the show. We’re like, “Extra solar planets. Dark matter. Dark energy. Black holes.” All of this stuff that have just been puzzles that have plagued astronomers. I think dark matter was our third episode, dark energy was our fourth, right? So, we knew that these were gigantic mysteries, and we would’ve thought that there would’ve been, if you’d asked me 10 years down the road, would we know what dark matter is, I probably would’ve gone, “Yeah, probably.” So, here we are.
You mentioned that we have three lines of evidence, but can you just sort of set the stage for what we thought dark matter could be and then how we got to the point now where we’ve been able to knock off some of those possibilities?
Pamela: So, to give a really quick roundup of the history of dark matter, it was first discovered before you or I were born as some weird thing that was causing stuff in galaxies to orbit faster than it should, and that was deeply mysterious. And then we started looking at galaxy clusters, and it was seen that there was the exact same problem of stuff in galaxy clusters orbiting faster than it should. And the amount that things were orbiting too fast was a function of how far the stuff was from the center of the galaxy, from the center of the galaxy cluster, and so there seemed to be this diffusion of – huge exclamation mark, question mark – we didn’t know what.
And it had different distributions in different galaxies and different distributions in different galaxy clusters. So, it was theorized that either there is this stuff that we didn’t know what was that existed in different proportions and densities in different environments or there was something about gravity that required us to modify the equation for gravity at great distances. And pretty much cannon for the professional astronomy community was it’s a thing, and we shall call this thing dark matter because we don’t know what it is and didn’t know what it is at the level of didn’t know if it was a distribution of black holes and cold, dark white dwarfs, and other high-mass compact objects.
Fraser: Neutrinos.
Pamela: Yeah, we didn’t know if neutrinos had mass or not, we didn’t know how much mass they had because they refused to interact in a logical manner, so all we knew was dark matter was probably stuff that you can actually account for the dark matter in our galaxy by putting one acme brick per solar system volume of space. And so, it could be like rogue asteroids – it could’ve been anything. All we knew was there was stuff that was exerting a gravitational influence that was acting like on aggregate, there was a bigger mass in the center of the galaxy.
Fraser: I just wanna note that you said “stuff” in a very Phil Plait kind of way. Like, I don’t know if anyone else caught that, but wow, that was the way Phil Plait would’ve said “stuff”. But anyway, so this whole idea of MND, this modified Newtonian dynamics –
Pamela: Gravity.
Fraser: Gravity? Yeah.
Pamela: Yeah, dynamics.
Fraser: Yeah, is out, but now we have these three lines of evidence which are really helping us at least nail down some of the characteristics of dark matter.
Pamela: And one of the early shows that we did in which I screwed up throughout the entire show – I kept saying it was the Ballet Cluster and it was actually the Bullet Cluster.
Fraser: Oh, no.
Pamela: Yeah. And no one caught me until the episode was recorded and posted.
Fraser: See, and this is the thing – I would’ve caught it now, but I wouldn’t have caught it then.
Pamela: Because it was brand-new research, and what the research on the Bullet Cluster that I renamed in our past episode, what we learned was there were these two galaxy clusters in the process of colliding. And folks used gravitational microlensing to look at how various background objects had their shapes distorted because with galaxies, if you take a whole bunch of galaxies and image them; they should average out because of the random orientations of galaxies. They should average out to be perfect circles, but they were actually averaging out to be more like teardrops and other distorted shapes.
And you can work backwards from the distorted shapes to figure out the gravitational distribution that is altering all of these shapes. And they mapped out with this technique where all of the dark matter was in this emerging cluster. And what they found was according to the distributions, the dark matter had pretty much just passed through each other with no interactions while the luminous matter – the stars that illuminated the gas, the normal stuff – had all gotten distorted and compressed as the luminous stuff collided and distorted the galaxies. And this was the first hint that dark matter is stuff – still don’t know what it is – but we could add the word non-collisional stuff.
Fraser: Right. And I even remember you brought up this term of this small cross-section, right? And that was actually a term that I don’t think I was familiar with, just this idea that the particles won’t bonk into each other as clouds of them come past each other.
Pamela: Right. And we know that this is a thing that happens because of neutrinos. Neutrinos are like, “I don’t care. I’m going to keep going. I just don’t care. I can’t be bothered.”
Fraser: A lightyear of lead, no problem.
Pamela: Right. And it appears that dark matter has the same “can’t be bothered to collide” mentality as neutrinos. And so, when we’d look at colliding galaxies, colliding galaxy clusters, we can use this gravitational microlensing to figure out the distribution of dark matter. And another episode that we did was on the cosmos project where they were able to map out the distribution of dark matter in a fairly large column of space. What they did was they looked at the distribution and distortions to galaxies at one epoch, at the next epoch, at the next epoch. And the furthest galaxies are going to have their light distorted by everything.
The slightly nearer galaxies are only going to be distorted by the stuff between them and us. The nearest galaxies are also only done by the stuff between them and us. And because the distortions change with distance, you can use that changing distortion to map out the galaxy’s dark matter in the intervening space. So, we have this amazing map now not just from cosmos but multiple different projects that shows that galaxies are inside these vast halos of dark matter, and some of them have bigger halos, some of them have smaller halos, and sometimes, we see dark matter that doesn’t have any luminous matter in the center.
Fraser: And the opposite as well. I mean, just a couple of weeks ago, they announced that they found a galaxy that had no dark matter in it whatsoever, and the way they had determined this was by calculating the motions of the galaxy clusters moving through the galaxy. And they determined that their motions were only due to the matter that they could see alone, and so this was a galaxy that had no dark matter in it whatsoever. And as you said, there are galaxies that are just the opposite, that they’re essentially only dark matter. I love this idea with dark matter that we don’t know what it is, but we can use it as a telescope.
Pamela: Yes. And what’s super amazing about this galaxy without dark matter – its name is NGC-1052 – it’s what’s called an ultra-diffuse galaxy. This is a galaxy where the starts are so separate that you can look through the galaxy. And by measuring the rates that these starts are orbiting as a function of distance because they can see these individual stars, they were able to figure out that all of these motions could be accounted for just from the luminous matter. This was the galaxy we expected to see back when Vera Rubin first started doing her work studying motions in galaxies and discovered dark matter.
So, here we are, finding this holy grail of astronomy essentially, the galaxy that acts like we originally predicted, which means it has no dark matter. And if you can have a galaxy that has no dark matter, and you can have a dark matter halo with no galaxy in the center, and you can find everything in between these two points, that makes it hands down, no questions asked, dark matter is stuff because it is stuff that can gather in large amounts, and it is stuff that can simply not show up.
And if we actually required a modification to gravity, we would never have a galaxy that has no dark matter because there is always this lingering “is there maybe a small-term that’s there that we just can’t separate out and we do still need to modify gravity”? And the answer is no. We understand gravity. And that’s fabulous. There’s no longer a question mark.
Fraser: But I think if you talk to a lot of cosmologists, that line of evidence, although compelling, is probably not the best one. That’s not the smoking gun that says for absolute certain there is dark matter. But I think – and I’m assuming you’re gonna get to this in a second – the one is our old friend, the cosmic microwave background radiation gives us the definitive “There is absolutely dark matter”.
Pamela: The cosmic microwave background, actually, what we see in it doesn’t help tell us if gravity might not have an extra term. It tells us that there is definitely a substance or an extra term to gravity. So, in order to say dark matter is stuff and not a failure of understanding of gravity, this is where that not having it actually is the “Yes, gravity is fine, and dark matter is stuff”. So, cosmic microwave background is just one more “Huh, the universe has a larger density than we knew about”, and it tells us that it’s either a larger mass density or gravity doesn’t behave the way we thought it did at large distances. We can’t differentiate those two.
Fraser: Right, but from what I understand, there’s oscillations in the CMB, and if there was dark matter, they would have one which matches the observations, and if there wasn’t, they would have a different representation that we don’t see. And so, it’s like the model for dark matter as a particle is well-represented by what we see in the oscillations in the cosmic microwave background. So, anyway, whenever I have these conversations with cosmologists, they always wanna sort of just remind me that the Bullet Cluster, for example, are great evidence and these great maps are wonderful, but boy, is the evidence there in the cosmic microwave background.
Pamela: And the unfortunate thing is just like there are cosmologists who claim that through cosmology and particle physics, you can prove the existence of God or you can prove the not-existence of God, there are also cosmologists out there that are able to create equations for gravity working in different ways in different conditions and come up with a theory, a modified Newtonian dynamics theory that allows you to erase dark matter as a particle.
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Fraser: So, how many of your lines of evidence did you get through? Did you get through two?
Pamela: So, the fact that you have halos with no luminous matter inside of them, the fact that you have luminous with no dark matter, and the fact that you can use microlensing to trace out the distribution are the three lines of evidence.
Fraser: Right, and I’m gonna throw the CMB in there as well. But okay, so let’s talk a bit about then the search for dark matter. We now assume that it’s some kind of particle, it has a cross-section although very small, so it doesn’t, as I mentioned, use the scientific term “bonk into each other”. What attempts have been made to try and figure out what it is, or at least what it isn’t?
Pamela: There are two major kinds of experiments that are being done. The first is you take a particle accelerator, whether it be Fermi National Labs, CERN, or any of the other colliders in the world. And you collide different things together and look what falls out, and you hope for some particle that has a high mass and other characteristics that make it not work and play well with the electromagnetic force. You hope for that to fall out of the collision, so you can go, “I have found a particle that is not a neutrino that has all of the characteristics we need,” or you use the same kinds of detectors that we use to find neutrinos.
These are large tanks built deep underground, usually in some sort of an old mine, and you fill them with highly reactive fluid. It’s actually for one kind of reaction; it’s a heavy water experiment. In another kind, it’s very much the kind of fluid that you actually use to dry clean clothes. And these different kinds of fluids are highly reactive. You add just the smallest [inaudible] [00:21:51] of energy, and they emit photons. And with this highly reactive fluid, you put it underground, so you don’t have to worry about just your random cosmic ray triggering a reaction, and the neutrinos which really don’t care what they’re passing through will in small percentages actually interact with these fluids.
And different kinds of neutrinos interact in different situations. The hope is that dark matter will also, but at a much lower percentage than even neutrinos do, react with this kind of fluid, and so we’ll see reactions that don’t have the correct energy to be one of the three flavors of neutrinos, and we’ll have the same lack of interaction characteristics, and we’ll say, “Here be dark matter.”
Fraser: Right. And so, that same idea, as you said, neutrinos don’t have much of a cross-section. They’ll gladly pass through almost a lightyear of lead, and so if you generate or have a ton of neutrinos moving through this liquid, every now and then, one is gonna interact, and you’re gonna get this cascade of particles, and it’s gonna allow you to detect it. So, have any of these – because I know there’s one in Canada, it’s Sudbury. There’s a great one in Japan that you always see those wonderful pictures of –
Pamela: Yes, Kamiokande.
Fraser: – with the people in the boats with these crazy gold hemispheres.
Pamela: Photomultiplier tubes.
Fraser: Yeah, it’s such a cool place. I’d love to be in that. And there are others. Has anything detected anything?
Pamela: There have been some inconclusive detections. And by inconclusive, I mean they saw once this weird thing, but it had too little signal to noise to actually say, “Huh, yes, we definitely found something.” It’s like the earliest detections of the Higgs boson. And the earliest detections of the Higgs boson, people were like, “We think we see a signal. We think we see a signal at the correct energy. We’re not entirely sure, but we’re gonna say we saw something at low signal to noise that is not a confirmation, but we want to make sure that we have a placeholder paper to make sure.”
Fraser: Right, for when the Nobel Prize comes later on.
Pamela: And none of the placeholder papers got the Nobel Prize, but they wanted to stomp all over their energy level and call it mine. And so, we’re starting to see some of those early low-sigma, “maybe we saw something, maybe we didn’t” detections – nothing definitive.
Fraser: Now, when you mention that low-sigma, you’re talking about the level –
Pamela: Compared to background noise.
Fraser: – of certainty, right?
Pamela: Yeah.
Fraser: And so, when the Higgs boson was confirmed, and that was Nobel Prizes all around, they had detected it to a level of certainty that you could say for sure that particle exists there at that level of certainty. And I’ve even seen some recent potential discoveries of dark matter that as more experiments are done, it was like, “Oh, nope. It was background noise.”
Pamela: No, retracted. Yeah, and we saw a lot of retractions come out of early detections of what people thought were the supersymmetric particles at CERN. So, the issue is – have you ever been in a really dark room, and closed your eyes, and you still see this noise pattern?
Fraser: Mm-hmm. Mm-hmm.
Pamela: So, that’s actually in some cases caused by cosmic rays interacting with the retina in your eyeball, so –
Fraser: Apparently, for astronauts, that’s actually pretty distracting because they see them a lot more. And so, they close their eyes, and they just see a lightshow which is kinda scary.
Pamela: Yeah, and it’s super creepy. And the human brain being what it is will start hallucinating things in this background noise. So, that’s the background noise of your eyeball. And you can imagine if you’re in a completely dark room with your eyes shut, and someone rapidly turns on and off a laser beam that is pointed at your eyelids – a low-power one – you might think you saw something, but because it’s a low-power laser beam through your eyelids, you’re not entirely sure because it might’ve just been a particularly bright cosmic ray hitting your retina.
That’s a low signal to noise detection, but at the same time, if your eyeballs are closed, and your significant other, or your friend, or whatever comes into the room and turns on the lights, it’s going to wake you up because you can totally detect that bright light through your eyeballs, and that’s a high signal to noise detection.
Fraser: Now, there’s sort of two ways to go about it, right? In the case of, say, these dark matter detectors that we were talking about, this is you trying to essentially capture naturally forming dark matter particles that happen to be moving through the earth. The other way is to generate them with something like the Large Hadron Collider. How’s that going?
Pamela: Well, it was actually the exact same discovery. They thought they had found some supersymmetric particles but also account for dark matter because that is one of the great hopes of a lot of these particle physicists that all of these supersymmetric particles will actually be dark matter. And we’re just not finding them. This is what we talked about in the last episode where we were like, “The Standard Model seems to be totally good by itself. We don’t seem to need underlying physics. It’s just good.”
So, while Kepler’s laws of planetary motion needed Newton’s theory of gravity to explain the how behind the observable, with the Standard Model, we have no how and why. People came up with supersymmetric particles, they went looking for them, but CERN is only, as it stretches its energy to higher and higher levels of collisions, they’re only really starting to hit the mainstream particle theories for supersymmetric particles. So, what they’ve ruled out is some of the lower energy, not most talked about –
Fraser: Right, some of the extreme ones.
Pamela: – theories.
Fraser: I mean, I love this idea, right? You’ve got some theorist, and they’ve written a paper, and they’ve said, “Crank up the LHC to 11, and then at that energy level, you should see this cascade of particles in all of your detectors, and that means you’ve found dark matter.” And then the folks at the LHC, they crank up their proton beams to 11, and they listen, and they don’t see that. And so, then the poor theorist has to just take their theory and just go, “Well, that’s in the garbage,” and then who’s next?
Pamela: What’s hilarious is a lot of these theorists will say, “No, no, no, no, no. I actually meant 16.” Among observational astronomers, one of the things that I learned in grad school as a form of mocking theorists is if you lock five theorists in a room, they will come out with nine mutually exclusive theories.
Fraser: Right. Alright, well, I guess we need to come back in another 10 years, another 500 episodes, and we’ll give you guys another update on how the search for dark matter is going. What’s next week? You wanna come back around to dark energy?
Pamela: Next week is dark energy.
Fraser: Right on. Alright, that sounds great.
Pamela: So, we’re just gonna keep on updating you.
Fraser: Alright, we’ll see you then, Pamela.
Pamela: Bye-bye.
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Duration: 31 minutes
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