SIO360: How Gravitational Lensing Helps Us Understand the Universe

Transcript

This transcript was generated automatically and may contain errors — the audio is the record.

0:00:15 Thomas: Hello and welcome to Serious Inquiries Only. This is episode 360. Wow, 360. Done a total 360 from where I was when I started. No, that doesn't make any sense. Anyway, I'm Thomas Smith and instantly broke my promise of a Sunday night release. I know, I am a day late. Okay, here's the thing. This was a chunky episode. I thought I was going to have a two-parter here because it's just, it's very dense. There's a lot here. And so I was working a little slow. I was thinking, oh, okay, I'll, I'm a little late, but I'll give them two parts. That'll kind of make up for it. And then by the time I got done editing, it's not long enough to justify a two-parter. I didn't want to, I always feel so guilty. I don't want to do two parts if it's not long enough. So it's a one-parter, but it took a little bit extra to edit. So that's why I'm a little late but it's great. I have Dr. Bryan Gillis back and he's going to tell us about Gravitational Lensing, which is actually his specialty.

0:01:06 Thomas: And also not just gravitational lensing, but how it relates to dark matter, dark energy. What is dark matter? What is dark energy? The universe. What a concept. It's really cool. Some of the most fascinating stuff. And there's really useful, especially toward the end, make sure you listen all the way through because if you've ever wondered about some of these cosmological constants and the lambda and all this stuff, Bryan does a really good job breaking it down. I think it took me a little while to understand some of the stuff. And of course, I think that maybe means you'll be sure to understand it because I had to have them reiterate a few times on some of these. And as a result, I, I think it's super cool. It's once again, the goal of the show to be able to get lay people like me to understand really complicated science. Oh, and a reminder, please, please, pretty please support the show, patreon.com/seriouspod.

0:01:57 Thomas: I can't tell you how important it is, even though I try every time it's vitally important, this particular juncture in my life. Thank you so much to the patrons who have working on a lot of good stuff, as I've referenced already this month. And as a result, I have, there's going to be plenty in mind for a bonus content, even though, man, last month's bonus was really fun doing those ads with Heath.

I don't know how soon I'll be able to get him back, but I don't know, maybe I could do something similar again. Eh, we'll see. There's so many good options. So anyway, patreon.com/seriouspod to get all those good options and to avoid these auto ads. You don't want to hear ads. Come on, stop hearing ads, by going to patreon.com/seriouspod. And for those who already have, thank you so much for doing that. Really appreciate it. You're the best. We're best friends, actually. Is actually, that's how close I feel to you. All right, here we go. Dr. Bryan Gillis on gravitational lensing, dark energy, dark matter, black holes, just all kinds of cool stuff. The universe. And I'm once again joined by Dr. Bryan Gillis. How are you doing, Brian?

0:02:57 Bryan: Hi, Thomas, I'm doing well. Thank you very much. How are you today?

0:03:03 Thomas: Doing well. I assume you've been back in your observatory, in your royal chair, looking through your eyepiece, doing some science, and you're back to talk to us more about it. But we were talking about more topics to tackle, and I thought, why not just find something that's kind of near and dear to your heart, sort of how, so the average person probably has something they could give a lecture on just from their own experience. So but that, I think, is maybe gravitational lensing. So we're going to get a little lesson on gravitational lensing, which I am super pumped about. Out. I

0:03:33 Bryan: Yeah, so I thought to start off, we can talk about the basic science behind gravitational lensing, then go to the specific project I'm working on, which is the Euclid Space Telescope, what we plan to do with that, and if we have time, even get into some of the technology behind how it takes the images and how we analyze them.

0:03:54 Thomas: Prefer non-Euclidean space telescopes, but that's Just me, you know? Yeah.

0:03:56 Bryan: Yeah. Gravitational lensing. So what do you know about gravitational lensings to start off with, Thomas? Uh,

0:04:03 Thomas: Let's see, let's see if it's something that's actually gravitational lensing or just something that is a thing that's obvious outside of that. I know that, uh, you know, gravity bends light. I think that was something that probably wasn't known for a while because it seems kind of counterintuitive. But, uh, and I've seen, you know, images, telescope images and so forth of like, hey, we've got a mirror kind of galaxy, for example, or a certain phenomenon where it's like, yeah, we have two images of this thing. One is, well, I was going to say the normal, I don't know if you could say for sure it's that, but like one is more direct, let's say, and one has been gravitationally lensed, warped around. And I think if I'm remembering right, I think there's often you can, you can detect like, yeah, there's a difference in some number of years here. Like this one is an older image, because it's been lensed around so much. I don't if I' exaggerating that, but is that the kind of gravitational lensing we're talking about?

0:04:53 Bryan: Yeah, that that's basically it. Uh, So whether or not gravity can attract light depended on the physics assumed at a time, but there were a lot of people who assumed light was just another type of particle. And if it was just another type of particle, they saw no reason it wouldn't be attracted by gravity. So if you look back in time, there were a lot of theories that would think that light would be affected by gravity, it was just assumed to not be scientifically useful at all, because the effect is very subtle in most cases.

0:04:55 Thomas: Cool. Oh, right. Yeah.

0:05:30 Thomas: Light being a particle. That's an interesting calculation to try to make.

0:05:33 Bryan: Well, you can actually get very close to the right answer if you assume light is a particle. So if you go back to pre-relativity, imagine you have a particle traveling very quickly past a source and just run through standard gravitational calculations on this, you get to almost the correct answer. You just have to multiply by a factor of two and you get what we now have as the correct Answer.

0:05:48 Thomas: Oh. Huh. Oh. I was assuming, we back then we wouldn't have known, like, so what if you're talking about it as a particle, it's affected by gravity. I know that, you know, the gravity calculations really require mass. How would you know what, what mass value to use back then, I guess is what I was thinking.

0:06:13 Bryan: Well, The thing is, the only mass you need is the mass of the attractor, everything else is attracted by a different force, but that force depends on that object's mass, and then when you want to go to acceleration you divide off that mass, so the net result is everything is accelerated the same. So when you're doing all the calculations, you just put in some unknown mass, and then when you divide it out later, it doesn't matter.

0:06:31 Thomas: Oh, right. Yeah. Gotcha.

0:06:40 Bryan: So you do need to know the speed of light for this, but they had, uh, good enough calculations of it, and you could figure out how much it was bent. Now, once Einstein's theory of relativity came along, this showed that some of the assumptions that went into it, particular, um, one that mattered here was the relationship between energy and momentum, were a bit different once you get up to the speed of light. And this means that it's actually twice as powerful as was previously believed.

0:07:08 Thomas: Hmm. So when you say twice as powerful, do you mean gravity bends light twice as well? Or the opposite of that?

0:07:14 Bryan: It bends light twice as well as it would if relativity weren't at play. The idea was there for a long time. It's just that it wasn't until very, very recently that people thought if you could do much useful with it. So if you think about how gravity bends light, let's imagine this in a very extreme case. Let's say you have one extremely massive object right in front of you, and far behind it is a light source. Let's say you've got a black hole in front of

0:07:16 Thomas: Oh, wow. Oh. Okay, right. Right. All

0:07:47 Bryan: You and A star behind it. Now, let's say that things are aligned just perfectly so that the light from the star, if it goes out to the side, let's say one degree, it probably wouldn't be as much as one degree in reality, but let's stay one degree. It gets bent by the black hole on the path and comes straight back to your eye and you see a star one degree off to the site. Now, since this is a perfectly symmetrical image, this means you're going to be seeing path of light from that star not only to the right, but directly up.

0:07:47 Thomas: Right. Oh, Would you see a ring?

0:08:23 Bryan: Yes, exactly.

0:08:24 Thomas: Oh, interesting.

0:08:25 Bryan: You see a ring, and we call this an Einstein ring when it happens. So I can't show you pictures of this, but if you go onto the internet and search for Einstein ring, you will see some phenomenal images of cases where we've found this setup.

0:08:25 Thomas: Okay. Oh that's cool.

0:08:41 Thomas: That's interesting. So thinking about that. So we're doing kind of a hypothetical where we're saying, like, we're looking directly, a star is like directly behind a black hole. So we would see, because it's bending it around in all the kind of the directions, 360 degrees, you're saying we'd see a ring. And now would the circumference of that ring, would it be the event horizon, basically? Is that how big it would be?

0:09:00 Bryan: It It would be much bigger than the event horizon. The event horizon is the limit for light escaping. For light bending around it, it could actually be much, much larger. And the exact distance out depends on how far the star and black hole are. Yeah.

0:09:04 Thomas: Oh. Oh. Okay. So I was thinking of it when I said that, I was speaking of it of just like the black hole is literally blocking your view, like so pretend there's no gravitational lensing, I was saying oh you'd see a spot, you know, the size of the event horizon because that's what the light can't get through. But okay, so you're saying since it bends so much more than that, you're actually, it's way way bigger because it's pulling more light around it.

0:09:37 Bryan: Well, I think let's make this clear. So imagine first that it is blocking your view.

0:09:37 Thomas: Okay. All.

0:09:43 Bryan: Let's say the star, the apparent size of the star is smaller than the apparent size of a black hole's event horizon. So that if it weren't for gravitational lensing, you wouldn't see the star at all. They'll still be some distance out to the side of the star where light gets bent just right to bend it back towards you and you'll see a ring for this, but that distance is going to be larger than the event horizon. It also depends on both the distance of the black hole and the distance to the star. So imagine the star is much further away.

0:09:54 Thomas: Right. Right. Right, right, good point.

0:10:17 Bryan: Then the light has to be bent by a much shallower angle to get to you.

0:10:17 Thomas: Yeah. I See, yeah, if the star's like right behind the black hole, obviously you'd have to bend it way more, just, just like, you know, if you're looking at an everyday object, if something's really really far away from the thing that's blocking your view, it's going to be way easier to like kind of move a little to the side and see it.

0:10:27 Bryan: Yeah.

0:10:37 Bryan: So this is an effect we call strong lensing, where it's very obvious looking at an image that something is lensing it, and it isn't just when it happens to be an Einstein ring. Like, you can imagine in this case, if you move the star just off, off to the side, the image of the ring doesn't just vanish. It turns into an arc where you're seeing it most of the way around the black hole, but there's a bit of a gap on the side. And the further you move off of this point, the more that gap widens and it turns into less of an arc and then a streak. And then finally, as you pull it off to this side, you get basically just the image of the star with only minor distortions, and then further and further off, and it's just the original image of the star.

0:11:23 Thomas: Mm-hmm.

0:11:24 Bryan: So basically anywhere in that regime where it's obvious by eye that it's been distorted is what we call strong gravitational Lensing. So what I work with is what we call weak gravitational lens. This is where the object is in the regime where it is mathematically being distorted by the presence of mass in between it and us. We just can't tell by eye that it's happening. Most of the time when we work with this, I've given the star black hole example, but when we're trying to do science with this we're looking at things much further off. We're looking at distant galaxies as the light sources and other galaxies or even clusters of galaxies as the lenses, which is what we call the sources of mass which are causing the gravitational acceleration to bend the light. So if you think of images you have of distant distant galaxies.

Unlike stars, you don't expect these to be circular in the first place. Even if they aren't being lensed, you're going to have some interesting shape to the galaxy. A lot of them that we call elliptical galaxies appear as kind of just a smudge that's stretched out in some random direction.

0:11:31 Thomas: Mm-huh.

0:11:48 Thomas: Mm-hm.

0:12:42 Bryan: And then there are disk galaxies, which might have a big bulge in the center and then a thin disc around it and if

0:12:48 Thomas: Just to clarify, are you talking about just regular pre-lensing the different shapes of galaxies?

0:12:49 Bryan: Yes. Pre-lensing.

0:12:53 Thomas: Okay, so they come in lots of different shapes and sizes just before you're even talking about distorting the vision of it,

0:12:54 Bryan: Yes. Exactly, so these are cases where we could believe almost any orientation that we see as being an unlensed image of it.

0:13:07 Thomas: Okay, that's why it makes it harder to tell, probably, that it's being lensed, the fact that there's so much variation. Now, is that because just random events, like a black hole whips through an entire galaxy and stirs it up in a weird way, or

0:13:21 Bryan: That that can happen, you could have galaxy mergers, or it could be that it's just devolving on its own with nothing particularly interesting. Yeah.

0:13:21 Thomas: Yeah. Disney buys all the galaxies, so they own, have merged and they own all the galaxies, you know, and so they, if you want to watch one of the galaxies, like, oh, they own that now, so it's, you know, that kind of thing. Okay, gotcha, corporate greed strikes again.

0:13:34 Bryan: Yeah. Yeah. A lot of those Or, it could be a galaxy that's just evolved on its own with nothing interesting happening, but it just happens to be oriented a certain way relative to us.

0:13:40 Thomas: Yeah. Oh.

0:13:50 Thomas: That's one of my favorite things about all of sci-fi is that it tends to just stick to two dimensions, like there's just tends to, everything stays in the same plane for some reason, I guess it's just too hard for us to handle, like, going down for a long long time, I, I wish, I'm sure some show has done that, but I always thought it'd be interesting if it's like, hey, in this particular battle, just have them like go, like, we're, we're gonna go this way, there's, you get to go all directions in space, right?

0:13:51 Bryan: Yeah. Well.

0:14:14 Bryan: They did that in, in Star Trek, The Wrath of Khan, the enemy Khan, eventually Spock realizes that Khan is thinking two-dimensionally. And so, oh, why don't we just go down and come up at him? You'll never see that coming.

0:14:14 Thomas: There's no He supposed to be a super genius, why would he be thinking two-dimensionally? Weird, I well, one thing that's interesting thing is, there, um, is obviously in our solar system there is a bit of a plane that kind of, you know, for the most part the planets are on, and I wonder if that is one of the reasons why people still tend to think of things in kind of that sense in space, like there's still like a, and, and that seems to be true of a given, of, like, certain individual galaxies and systems, right? Does it tend to be because, like, the way they're formed, but relative to, like, a mix of galaxies relative to each other, there's no reason why they're on like the same plane, exactly, right?

0:14:29 Bryan: Yeah. Yep. Yeah. Yeah, Locally, you do tend to get strong correlations. This comes from the angular momentum they have. Basically, you can imagine that in the primordial universe, there's just a whole bunch of stuff going random directions. And then you take a snapshot of it, find some region that's slightly over dense and say, okay, all of this is eventually going to collapse together and form a galaxy. And that galaxy will have many stars in it, many of those with their own solar systems. And when you look at all of the stuff in there, there will be on average some net spin to it, some direction it tends to be spinning in. So you can imagine, um, figure skater on ice, when they hold their arms out, um, they'll be spinning slowly, but when they pull them in, conservation of angular momentum means that they have to be spinning much faster in order for the angular momentum to be conserved.

0:16:02 Bryan: So this happens as well with galaxy collapsing, as we call it, that even though at the beginning there's only a very slight tendency, it builds up as things fall together. And then within this, things start to break apart and fragment, and you get solar systems, solar which, because of the overall momentum, will tend to be correlated strongly with this. So in the Milky Way galaxy, it's a disk, and it has its own rotation, and most star systems within it are going to be the same way. And Then within each of those systems, most planets that form will be rotating approximately the same same way, which is also something we see in our solar system, that most planets rotate around roughly the same axis.

0:16:34 Thomas: Right.

0:16:45 Thomas: And that's why our toilets swirl the one direction, it's the same, no, no, I'm just kidding, the Bernoulli effect, um, but so a quick question, uh, chance for pedantry, I know I said like other solar systems, is that a pedant thing where, like, ours is the only solar system, but other things are star systems? Or am I remembering that wrong?

0:16:50 Bryan: There Yeah.

0:17:04 Bryan: Yeah, I think you're supposed to say star systems. But I do that myself, I, I don't worry about that

0:17:07 Thomas: Okay. Well, yeah. Well I can take, I can take the blame for that one. We'll say star systems and we're, we're the only solar system. Right. Is that right? Whatever. All

0:17:13 Bryan: Yeah. I, I think, um the sun refers specifically to our sun, as does solar. And for other ones, it's stellar. But I'm not an expert in star science, so don't quote me on that.

0:17:28 Thomas: Right. Well we don't want to get any other alien life forms mad at us. So we'll try to stick to that. So star systems.

0:17:33 Bryan: Yeah. All of this is to say, if you go far outside our own galaxy and look at something far away, which we do, that correlation decreases to the point of it no longer matters, that we don't expect to see things aligned with how we are. Despite how every sci-fi show illustrates it. We'll have things off at random orientations. So when we think, well, this is probably lensed somehow, but we don't know what the original orientation is, we've got a bit of a problem to solve.

0:18:05 Thomas: Yeah. Yeah. If every, if every single galaxy was just a clone of ours. It'd be super easy to be like, well, okay, that's easy math. Like here's what it should look like. Here's what It does look like now let's plug in. I should say easy, but easy for smart people like you to be like, all right, well here's what gravity would have to be doing to it, given that, you know, you know the distance and roughly where everything is, I guess, but because you know you're looking out and you don't know what you could be looking at in true terms, like, it with what the actual form is, that makes it a lot harder to do the a detective work of, like, is, is this just like a British monarchy galaxy that's, you know, bred with a few related galaxies too much, or is this being gravitationally lensed?

0:18:19 Bryan: We We call that intrinsic alignment scientifically, that there are things that tend to be correlated with each other and have similar alignments when they're actually close in space. But, yeah, it's hard to know, and we can only get it statistically. Let's think a bit about, okay, we know this lensing is going on, but what question are we actually trying to answer with it? And most of the time, the question we're trying to ask is, how much mass is there that's lensing it in some form or another? Other. So imagine we have a cluster that's acting as a lens, and we want to use the effect of lensing to measure how massive the cluster it is. Obviously, the more massive it is, the more it attracts light, the greater the effects of lensing. So this is one way to weigh clusters, and it allows us to do a lot of interesting science with it.

0:19:40 Bryan: Primarily, it allows us to get insight into dark matter, which by definition we can't see. We can see all the stars and we know how much light they're emitting and it's pretty straightforward to do the math on that to figure out how much mass there is in stars. The gas is a bit harder, but doable.

Other things like rocky matter that doesn't emit light on its own, sometimes you can get indications of reflected light from that, but as it goes on that all gets lumped into dark matter. So historically speaking, when astrophysicists refer to dark matter, we aren't just talking about this one particular unknown substance which we now call dark matter.

We're talking about anything that can't be seen, and there was a long history of figuring out, well, maybe all of this dark matter is just planets and rocky material that doesn't emit its own light, and once you do a lot of calculations and science around that, you, you can estimate how much there is of it, and we figure that there wasn't nearly enough to explain some of the mass measurements we were doing, well, you can't really get into gravitational lensing without talking about dark matter, as

0:20:44 Thomas: Yeah. Well, dark matter episode probably needs to be its own thing. Good

0:20:53 Bryan: It's kind of the point of why we're doing so much with gravitational, gravitational lensing, is to weigh all of this dark matter. To avoid getting too into the details, we initially knew it was there from rotation curves. That is, looking at the orbits of things, they seem to be attracted by more mass than we could account for by stars. And rotation curves are one way to calculate this, but there are others, of which there's gravitational lensing. And there are some explanations for the rotation curves that don't apply to gravitational lensing. And by looking at other ways calculate to this, we can determine between these explanations. So there was a long while where people were wondering, okay, is this really extra matter we can't see? Or is it that on these very long cosmological scales, something different is going on with gravity, which, you know, Einstein had to revise gravity before from the Newtonian analysis.

0:20:58 Thomas: Okay.

0:21:50 Bryan: It's perfectly plausible we might need to revise it again. So weak lensing eventually gave us a way to answer the question there, and we were able to find examples where it was very obvious that there was invisible matter going on and you couldn't explain it.

0:22:07 Thomas: Oh, really? Oh, I wasn't sure if that was still an open question or not. So So just to kind of sum up and make sure me and the audience are following along. Yeah, I kind of remember that in stuff I've seen about dark matter that, a possible explanation you're saying is, well, maybe our theory of gravity is a little bit off. And so it wouldn't really be, you dark matter. It could just be normal matter. But we're accounting for the gravity incorrectly because of our theories. So you're saying that's been kind of solidly answered as a no, though.

0:22:33 Bryan: Pretty much. There are still holdouts. Of course, people get very committed to one view of it. They build up their own theories about alternative gravity. And then even when the evidence comes in against it, they're already so solidly in one position, they don't really accept it. But from the overall community perspective, I'd say that it's pretty solidly of the theory that dark matter does exist. It's some unknown thing, which we don't know what it is. It's not anything else that we know and can put a name to. And there doesn't seem to be a need at present to modify our theories of gravity in any way, at least in these ways. There theories in other ways that we might need to figure out gravity on a quantum mechanical level, but that's a completely different question.

0:22:39 Thomas: Right. Hmm.

0:23:23 Thomas: And if we did that, you're saying the consensus is, even if we did that and it changed our understanding, it still wouldn't explain dark matter. So we'll go with that for the purpose of this show. Okay. So, we're all on board with that consensus for the purposes of this lesson.

0:23:30 Bryan: Yeah. Yeah. We should actually go into the why do we know this? How can we use gravitational lensing to figure this out? So the example we have of where we use gravitational lensing to prove that there's dark matter is in a phenomenon that we call the Bullet Cluster. So this is, despite what the name may have you believe, it's not a single cluster of galaxies, but two different clusters of galaxies that basically collided. They happen to be going at an extreme relative velocity to each other, and the smaller one slams straight through the middle of the other one.

0:23:53 Thomas: Okay.

0:24:13 Thomas: So you're seeing this is like a specific of thingies that we're looking at. Okay, gotcha.

0:24:17 Bryan: Just one event that we found. So in this case, are made up of, you can think of it as three components. There are the stars in it, the gas, and the dark matter. The stars, despite every illustration you see in sci-fi, the stars are very far apart and the density of stars in space is extremely low.

0:24:19 Thomas: Okay, mm-hmm. Yeah. Depressingly so.

0:24:40 Bryan: Yeah, similarly with the densityof galaxies within clusters is also very low. So when two clusters collide and one goes through the other, you'll have basically zero times where, yeah, actual collisions of stars or galaxies. You'll have a few, but in the billions of stars, or hundreds or thousands of galaxies.

0:24:54 Thomas: Like actual collisions. Ah, interesting. Boy, that's bad luck, though. Yeah. Of all the gin joints and all the solar systems, star systems in all this galaxy, we had to run into another one is what I would say. I'd be on the one planet that's like, damn it. How do we run into each other? So much room. Just go around.

0:25:17 Bryan: Yeah. Exactly.

0:25:20 Thomas: Galaxies don't know how to zipper merge.

0:25:23 Bryan: So when you have these two clusters moving, basically what happens is the galaxies just pass right by each other and they go out the other side. The gas, on the other hand, actually takes up space. And so the gas from one cluster is going to hit the gas from the other cluster, and you get a big collision of.

0:25:44 Thomas: When you say it goes through each other, though, doesn't, so maybe stuff doesn't collide, but doesn't the gravity get all messed up? Like wouldn't orbits and stuff be all kind of thrown up? Gotcha.

0:25:49 Bryan: Yeah, it does. It definitely gets slowed down, but it still goes through each other and will come out the other side and be slowed down by it. Although, actually, it's initially accelerated as they start going together, and then afterwards slowed down. So it will still go through each another at a pretty good velocity and probably even keep going in this case and keep going out and maybe never re-merge, maybe only re-emerge in billions of years.

0:26:20 Bryan: So, the galaxies within the clusters, um, may never come back together. This

0:26:20 Thomas: Do you mean that like the two galaxies as a whole, is that what you're talking about? Oh, this is, so we're not even talking galaxies merging. We're talking a cluster of galaxies. Oh, so that's even, oh, gotcha. Okay. Sorry. I think I was on the individual galaxy. Oh, wow. Okay, so yeah, then nothing's going to hit each other. Yeah, okay, I gotcha, But we're talking. So the gas is, though, different story because it's just more ubiquitous, I guess. Right.

0:26:28 Bryan: Yes, Yeah. Yeah. Yeah. It fills out space pretty smoothly, and there's enough going on that the cloud from one cluster, what we call the intracluster medium, does significantly interact with that of the other. So the gas gets caught in the middle while the stars from each cluster pop out of either side. So let's take a couple snapshots in time. First time, right before a collision, you've got one cluster on on the left, its stars and its gas in it. One cluster on the right, it stars and gas in the same place. Then they all collide in the center. All four of those components light up in the same place, then final snapshot, the stars and galaxies pass through each other, come out the other side at basically the same velocity, as almost though nothing had happened, just a bit slowed down by gravity, but the gas is significantly slowed down and gets caught in the center and comes out much more slowly.

0:26:50 Thomas: Mm hmm.

0:27:45 Bryan: Question then is, what does the dark matter do? And to a large extent, this depends on what we think dark matter is. The common belief is that dark matter is something that doesn't tend to interact with anything very much outside of through gravity. And so if you assume that dark matter doesn't interact with anything, it's going to act just like the stars and galaxies do and just go through and even end up at roughly the same place as all of those galaxies. This is our state. We've got, on the left side, stars and dark matter. In the center, two globs of gas. And on the right side, more stars and darker matter. Now, we can do calculations of how much the gas should weigh versus the stars, and there are ways to see the gas.

0:28:37 Bryan: It doesn't typically appear luminous in the visible spectrum, but in the x-rays, we can get an idea of where the gas is, and we can use that to measure the mass of the gas. And for cases like this, it typically turns out that the gas weighs a lot more than the stars. So if there were no dark matter, we would expect that most of the mass of this cluster system after the collision would be roughly where the gas is, with a bit of it out in the outskirts where all of the stars are.

0:29:10 Thomas: So even though obviously on a density basis or a one-to-one basis, you think, well, gas is just this light thing that's going to whatever and a rock, a planet or whatever it might be, that seems pretty dense. But because the gas is kind kind of everywhere. And the space between everything is so much bigger than the actual things. The total, by total mass, the gas is just way more than, than the rocks and the whatever else, the other types of matter. Right.

0:29:30 Bryan: But, Yeah, Yeah.

0:29:38 Thomas: Okay. And so question I have about kind of all this is, I know we've, we established in relativity, there's no one moment of time exactly, but like, Like, when are we seeing this? Like, are depending on where you're seeing this, you're talking about it before and after and what it looks like and all that. We're seeing, I assume, unless there's some crazy gravitational lensing, you now, distorting time or something, we're seeing basically a snapshot, right? And then we're doing detective work as to how it got there and what is was and what isn't.

0:29:38 Bryan: That's basically it.

0:30:03 Bryan: Yeah. Pretty much So in the case of the Bullet Cluster, we're seeing a snapshot of the aftermath, and we're just imagining going going backwards through time, how it got to this. And this cluster, if you look at the images of it, is very dramatic, that you actually see one of the clusters, which is smaller, the gas from it has been deformed into this almost cone-like shape as it got pushed through the gas of the other one. So it's very obvious what happened there, that one one ofthe clusters went right through the center of the other.

0:30:11 Thomas: Mm-hmm. Mm.

0:30:39 Thomas: Interesting.

0:30:40 Bryan: And as for when, this is stuff that is very distant out there and happened billions of years ago. So as for what it looks like, quote-unquote, now, it's going to be even more different.

0:30:45 Thomas: Mm-mm. Right.

0:30:52 Bryan: And, yeah, this is one of the big difficulties with astrophysics, is you get a lot of snapshots of different things, but you can never look at one individual thing for a while to see how it evolves, evolves outside of maybe some star events like supernovas and stuff that do happen on a human time scale. Okay, so the question we're trying to ask is, okay, where is all the mass in this. If the truth of the universe is that gravity as we knew it was wrong, then what we would expect is that the location of the mass should be where we see all of the gas, as that's where we expect all the mass to be from the things we can see. But if the truth is that there's more mass, it's just all dark and non-interactive, then the location should be out where the stars are now. So those are two very different locations, and you can't modify gravity in any sane and simple way to explain that the mass is in a completely different spot.

0:31:00 Thomas: Yeah. Yeah. Hmm. Interesting.

0:31:53 Bryan: So this is where we apply by weak gravitational lensing in a particular mathematical form that we call mass mapping, to figure out where the mass actually is. But when we try to do this, you can think we run into the problem, okay, we'll look at the things in the back, ask how much it's been lensed. But the problem we run into is we don't know what the true unlensed images of everything in the background is, uh,

0:32:16 Thomas: Yeah. Well, wait, and is dark matter itself going to be lensing stuff?

0:32:22 Bryan: Yeah, dark matter is very massive, that's the entire point of why we created it, and that mass will cause gravity, and that will lens. So if dark matter does exist, we'll see its effect on lensing the background. If it doesn't, the lensing would be caused by the other mass we know is there, primarily the gas in the center.

0:32:25 Thomas: Yeah, yeah, yeah. Okay.

0:32:43 Thomas: Right. I guess the thing I'm wondering, and I know this is, I'm sure this is all for you science folks doing the sciencey science. I'm sure this is something that gets accounted for and all that. But just as a human, I'm thinking, well, if if dark matter is a thing that's potentially everywhere and we cannot see it, but it could lend stuff, isn't? Isn't there always a chance, like for anything you're seeing, like, oh, it could actually be different, but there's some dark matter we can't account for between here and there that's lensing it in a certain way.

0:33:10 Bryan: That basically always is a problem, particularly when you're trying to weigh something, that anything in projection, we call it, that is along the line of sight between us and it, and between it and the source of the light, will also lens that. And so this adds an element of uncertainty to any mass measurement. This is in addition to the uncertainty that's caused by not knowing what the original images look like. So the solution we've come up with for this is we've got a lot of low reliability data, but it turns out that as long as the data isn't intrinsically biased, if you throw in enough low reliability, uh, data and average, it's all, all together, you get a high reliable simple reliability result

0:33:32 Thomas: Right. Right. Mm-hmm. Yeah. Right? I was gonna say like, statistically speaking, it's like, okay, yeah, any one thing you could say like yeah, maybe there's a weird lensing happening that makes it, you know, maybe this is actually a totally normal galaxy, but we see it as a galaxy, it looks a little different. But if you do that times a gajillion all across the universe, I'm sure like statistically almost in the same way, it almost reminds me of like, what am I thinking of? It's not like cryptocurrency or something, but like where it's like, that's how you verify. It's not like you, know, exactly what's going on by one individual thing. It's that you have enough data that the odds that you're wrong about all of it get vanishingly small. Is it that kind of thing?

0:34:02 Bryan: Out of it yeah Yeah, there's a lot of mathematical theorems that goes into this, there's the central limit theorem is probably the most key, that when you take enough statistical information and average it together, it eventually, eventually the average result will approach the true effect that you're looking for. Now, in this case it gets a bit more complicated, if you think about two objects in different directions from the source of mass, they're going to be lensed differently, they're going to stretch, like one object directly above it will be stretched horizontally, one to the left will be stretched vertically. So you don't want to just average them in as they are, or everything will cancel out to zero. You want to kind of rotate things, align them, or when you're doing mass mapping, there are other techniques you use to make sure that you don't cancel it out, but the net result is that as long as your shape measurements are unbiased, you can in the end throw everything together, average it, and get out an unbiased of the mass that's in the center.

0:34:40 Thomas: Mm-hmm. Right. Right.

0:35:43 Bryan: Yeah. So, getting into mass mapping is more complicated than we can really do here. But the end result is that people did this for the bullet cluster, and it came out to be pretty clearly that the, the mass was concentrated in the same location as where all the stars were, and this could only really be explained by there being a lot of dark matter there, which is much more massive than the gas in the center

0:36:03 Thomas: Oh. Oh, okay. So what I said earlier, the gas isn't the... Oh, apart from dark matter, the gas would be the most massive in total thing, but because of the dark matter for some reason tends to be around the stuff, the planets and the stars and the whatever.

0:36:12 Bryan: Yeah So the reason it's around them is they both react, um, to the collision the same way that the, um, stars pass through each other, They don't hit, and the dark matter, we're assuming, is non-interactive. There are some models of dark matter that have it being interactive, but it turns out that a non- interactive model of dark matter fits what we see here. So it passes through the center and ends up in the same place as all the stars and planets and stuff.

0:36:32 Thomas: Oh, gotcha. And when you say non-interactive, it still is affected by gravity. What's it non- interacting with, or what does that mean

0:36:50 Bryan: Yeah. So, it doesn't collide. Like, you don't have a collision of two dark matter particles. It doesn't have an electromagnetic charge, So you don't see it. So like when atoms, quote unquote, collide, it's really the electrons in the shell get close enough to repel each other. So in the model of dark matter, it doesn't have anything like that going on. And two particles would just pass by each other and only interact through gravity.

0:36:51 Thomas: Exactly? Okay. Okay, so whereas, are you saying the gas doesn't do that? Like the gas reacts a little differently?

0:37:21 Bryan: The gas does collide. It's atoms, much, some ions, which are basically atoms that have been stripped of their electrons and free electrons. And all of those will have electromagnetic interactions, which...

0:37:23 Thomas: Okay.

0:37:35 Thomas: I see. I thought the gas was just reacting gravitationally with each other. I didn't, that's where I was wrong. Okay, now it makes sense.

0:37:39 Bryan: Uh, No, Yeah, Yeah, Yeah, so the gas has all this electromagnetism going on, which is much, much more, uh, powerful than gravity, and so you get this collision-like effect in the metal. Yeah.

0:37:51 Thomas: Okay. Interesting.

0:37:52 Bryan: So putting this all together, the net result is that we, we can use gravitational lensing to look at this cluster, see. Okay, all of the them mass according to the lensing is around where the stars are, but if we calculate how much mass there is in the stars, that's not nearly enough. There should be more mass where the gas is, from the stuff we can see, and that means there must be invisible mass there where the stars are. We can't really explain this by modifying gravity, as the all the modifications to gravity, at least sane ones, really only modify the strength of of gravity.

0:38:28 Thomas: Mm-hmm.

0:38:28 Bryan: On these scales, they don't modify the direction of it. And if it did modify the direction, it would be very bizarre and a really big coincidence that it ends up looking like this. So gravitational lensing basically answered this question. And

0:38:36 Thomas: Yeah. Yeah. Oh, I see. So I feel like we were still waiting to get to the lensing, but, but so you're saying the lens, so, so you've been using lensing to look so are you looking at other objects around this bullet thingy and using how the bullet thingie is lensing that, those things, the light from those things to say, oh, Okay. I don't know why I hadn't made that connection where

0:38:44 Bryan: Yes. Yeah. Yeah. Sorry. Sorry, I didn't make that clear, but we're looking at things that are in the background of it. Like, we get accurate images of basically every galaxy we can see behind the bullet cluster, um, calculate the shape, uh, do some math with all of that to put it through to our calculations of gravitational lensing, and find what distribution of mass best explains this distribution of shapes of things in the back.

0:39:28 Thomas: The light has gone Yeah. How the light has, uh, bent around it. Okay, wow. Yeah.

0:39:29 Bryan: Yeah. Gravitational lens is basically why we're now very confident that dark matter exists.

0:39:40 Thomas: And isn't just like, uh, you know, some variable that we, some, some coefficient that, you know, Einstein didn't figure out or something? It's actually, it's actually a thing.

0:39:51 Bryan: Yeah. Now, obviously, this raises the new problem of what exactly it is, which to date we still unfortunately haven't solved. There have been a lot of attempts at it. Particle physicists have come up with a lot candidate particles, but in all of the big collider experiments, they've unfortunately been unable to prove any of them so far. It's still a mystery. Hopefully one will solve. But that's about where it stands with what dark matter is these days. But that doesn't mean that's all we can do with gravitational lensing.

0:39:58 Thomas: Oh, Yeah.

0:40:24 Bryan: When you boil it down, gravitational lens is a tool to measure the mass of things you might not be able to see, or even if you can see something, being able to measure of the mass is useful for, for instance, calibrating other estimates of the mass. Or some of the most interesting applications of it, measuring the mass of things that you don't even know are there at all. You can imagine a case that you do not see anything out in space, but when you look at a bunch of objects in that particular region, you see that they are all lined up, kind of streaked out in this circular pattern around nothing. That would imply that there's something there causing this lensing, and this would be one of the closest ways we could see a black hole, for instance, is that you couldn't see the hole itself, but you can see the effects of gravity on everything behind it. Well,

0:41:14 Thomas: Well, I was just gonna ask, I mean, this is again, this is just, just a kind of dumb, a layman's question, but what's would be the difference if dark matter was actually just, I don't know, a bunch of smaller black holes everywhere? Would that, how would that look different?

0:41:27 Bryan: It wouldn't necessarily look different. And in fact, black holes have been proposed as a candidate for dark matter. There are other reasons that we don't think dark matter is made out of black holes, which is really getting off onto a tangent. But basically because we believe black holes will evaporate over time and the smaller they are, the faster they evaporate. The most stable black holes are the largest one, so if dark matter were made of a lot of tiny black holes, they would very quickly evaporate and turn into something we could see. Yeah.

0:41:32 Thomas: Hmm. Yeah. Oh, okay. Interesting.

0:42:03 Bryan: But gravitationally, yes, black holes cause a gravity, um, so they could, if it weren't for the evaporation issue, have been, uh, dark matter. And, you know, in the hunt for dark matter, um, you might be familiar with the terms WIMPs and MACHOs, which were basically two different ways of classifying, uh, things that could be dark matter. And scientists, and particularly astrophysicists, are horrible with acronyms.

0:42:25 Thomas: Yeah. Was it like weakly interacting something particle or what?

0:42:27 Bryan: That yeah so wimp is weakly interacting interacting massive particle, which is basically what we think dark matter is, that it's some unknown type of particle which has a significant amount of mass and doesn't tend to interact with other instances of itself. So, we think dark matter is a WIMP. Now, Machinos, on the other hand, are another class of things which have fallen out of favor. I believe it stands for massively compact halo objects. It basically means things like planets which don't emit light on their own, but

0:42:33 Thomas: Massive particles. Okay. Massive Hmm.

0:43:05 Thomas: Hmm. Hmm.

0:43:06 Bryan: Are overall small enough, compact, that they don't tend to interact with other instances of themselves, and halo meaning they exist in the halo around galaxies. Other calculations of the way, um, galaxies form show that there just couldn't be enough mass in these. So MACHOs have fallen out of favor, but from a gravitational perspective, they would solve the problem if they were to exist. We just are pretty sure they don't.

0:43:32 Thomas: So this basically just could be a bunch of like small planets or something you're saying. So is that what the macho thingy is? Could have been.

0:43:37 Bryan: Could have been but there are other reasons that we don't believe there would have been that many small planets just out there wandering around. This is one case where gravitational lenses prove useful and demonstrating that yes, dark matter is a thing, but there are other ways it can be useful, and what I'm alluding to is picking up the masses of things that we don't know are there. So I gave the example of a black hole. But also happens is through the formation of the universe, there's a ton of dark matter around, and not all of it falls into the same gravitational wells that hold galaxies. So there's going to be loose dark matter, and a lot of it will clump together. You can imagine that there are going to be what we call halos, which is just, imagine a cloud of dark matter, which is attracted to itself by gravity, but it isn't deep enough to gather up enough gas that the gas can get together and ignite nuclear fusion.

0:43:43 Thomas: Okay.

0:44:36 Bryan: You might have a bit of gas there, but maybe not enough to even be visible, but but it would be to be picked up by gravitational lensing, it weren't for the fact that it were such a faint signal on any individual case.

0:44:49 Thomas: So it's almost like a galaxy of dark matter, kind of like just a big collection of dark matter, but it doesn't form stars or anything, so that could just be floating around anywhere really, right? Oh,

0:44:53 Bryan: Yeah. Yeah. Like, if you were to know where it was and go over to it, you would feel a gravitational force towards the center of it. You could go in an orbit around the center. You would just see nothing there.

0:45:11 Thomas: But wait a minute, could you have a singularity of dark mater? Or would we just not know because we don't really

0:45:14 Bryan: A singularity, um, of, but uh, so yeah I'm just thinking about uh the specific definition of a singularity uh this

0:45:20 Thomas: Oh was that well, whatever essentially a black hole formed out of dark matter's what I'm getting at, you know, like, like, where there's enough of it together that the gravity itself gets so strong that it does the black hole type stuff.

0:45:23 Bryan: Yeah yeah I I think Yeah I don't think you can actually get a black hole out of it. Um, the problem is that that it has too low of density, that when you go to to a smaller, smaller region, will contain less dark matter. And as you keep going to a smaller and smaller region, you'll never have a region where the amount contained within is dense enough that on the edge of it, the escape velocity reaches the speed of light. Dark matter tends to be moving too quickly for a strong cusp like that to form. It's actually pretty hard to get that amount of of matter in space.

The way that traditional black holes form is from stars exploding and, in part, exploding inward, pushing the cores inside. And that's the most common mechanism for black holes forming, is that you have the supernova explosion pushing. It both pushes the outsides off, but the explosion is throughout it, and it also pushes the insides in, and that's able to get things dense enough. But with just dark matter floating around, you never achieve the critical density, and even if you do slightly achieve it, you then run into the evaporation problem, that it'll just evaporate away and then you just have dark matter again.

0:45:55 Thomas: Hmm. Really? Oh, interesting. Okay. I don't know why I thought my conception of how, I don'T know, space worked was that eventually, like, everything, gravity will just pull everything together and make a bunch of black holes out of everything. Is that not true?

0:46:57 Bryan: On some scale, some of that will happen. Um, so once a black hole is formed, it, yeah, there is the evaporation issue, but in most cases, the surrounding media is dense enough that things will slowly fall into it and that black hole will grow. But there are also going to be a lot of regions of space where this will never happen. So space is expanding. So there are going to be a lot pockets of space that are never going to practically interact with other places where a black hole does form, and so if they can never achieve the critical density within it to form a black hole, um, nothing will ever form there, and it'll just be a loose halo.

0:47:35 Thomas: Okay. Well, Oh, I've never thought about that before. All right. Well, where are we on our we our black? Gravitational lensing lecture here?

0:47:37 Bryan: Yeah so we have our loose dark matter halo. And practically speaking, it's more complicated than just loose dark matter. We call it a cosmic web, that it's more what you would think of as a big three-dimensional spider's web of dark matter. Some places it's collapsed together and is very dense. Some places where it's a bit looser. So you'll have these long filaments connecting clusters to each other, and then big halos around the cluster. And then if you look at a few filaments together, you'll get what we call walls, that kind of exist between the filaments. So it's very complex, complex, interesting three-dimensional structure. All of this could, in theory, be analyzed through gravitational lensing. You just have to kind of sort the universe in the right way. So, for instance, if you want to look at filaments, you can identify two clusters that seem to be relatively close together in space, then look at the space between them and look at is there their lensing on either side of that empty space, and use the amount of lensing there to calculate, okay, how massive is the invisible filament between them.

And then you can get an even larger scale, and rather than just looking at individual filaments or walls or halos, let's just look at everything. Let's look at the full distribution of mass in the universe and see if there are interesting patterns in it. And this gets into what we call cosmic shear. This is basically, okay, we know there's mass there, but let's not even think about it from the perspective of any individual pieces of mass.

0:49:17 Bryan: Let's just look at all the potential lenses in the universe and see what relationships they have to each other.

0:49:25 Thomas: Make a kaleidoscope out of it.

0:49:26 Bryan: It's basically, ignore the mass in between. Look at all the lenses we have and ask, okay, how correlated are the, not looking at the lenses, looking at the sources in the universe. So, how correlated are the shapes of these? So you can imagine if there were no lensing, then there would be little correlation beyond the very short scales where things evolved from the same events. But outside of that, you wouldn't see much correlation. If there is lensing, then you'll see correlation, and the amount of that will depend on the particular distribution of mass. So if you have a very clumpy universe, you'll see kind of spikes in the correlation function, where there are certain distances where things tend to be very well correlated with each other. And if the universe, like, imagine we had a perfectly spoof the universe, this is the same as there being no mass to lens at all. We won't see any correlation. So, bigger the difference we see in this correlation function, the more spikes we see and at what scales we see this tell you about basically how clumpy is the universe

0:49:38 Thomas: Yeah, I like my universe really clumpy. I think clumps are better. I'm pro-clump.

0:50:37 Bryan: Yeah. Yeah, the clumps make it interesting. So this question, basically how clumpy the universe is, what cosmic share measures and is basically what the Euclid Space Telescope is trying to look at it.

0:50:40 Thomas: Yeah.

0:50:50 Thomas: Now, how pulpy is our universe? Is a lot of pulp in our universe? I like clumps. I don't really like pulp, though. Can we get a reduced pulp universe?

0:50:57 Bryan: Well, in my mind, pulp is all of the galaxy. So from that perspective, I like pulp.

0:51:00 Thomas: Oh, I thought that was the clumps.

0:51:02 Bryan: Now, the clones of the dark matter, the pulp is the galaxies. Okay.

0:51:04 Thomas: Oh, okay. All right, all right. You've talked me into pulp. I'll accept pulp as well. Well, so this is what your Euclidean telescope, this is what Euclid dreamed up with his telescope, huh?

0:51:07 Bryan: Okay. Yeah. Yeah, back in 300 BC, he, he had all of this, uh, figured out, uh, wrote it down, and we're finally going back to his notes and making a telescope, based off of them.

0:51:21 Thomas: Yeah, like, like Da VinCI's flying machine, yeah, it's like that, right,

0:51:25 Bryan: So of the main observables of the Euclid telescope is cosmic shear, that, and and it's a measure of both how clumpy the universe is and how clunky it was at various points back in time. So one way to think about this is, if you know how far away the various sources are, you can look at only specific sources. So if we do lensing with only the sources close to us, we'll get, okay, how cl bumpy is the universe very recently? If we look at all of the lenses far away from us, we'll see the effect of mass from present day out to very far back in the past.

0:52:02 Thomas: And assuming you left the universe out and didn't put the lid on it, it is probably going to get pretty, like clumpier over time, I would assume, I

0:52:03 Bryan: And yeah, yeah, yeah, so this technique called tomography will give you a formula for the evolution of clumpiness in the universe, and then once you look into all this, this actually, goes and clumpiness, and of the universe turns out to be a very, very important measurement as it relates back to basically a lot of the fundamental physical constants that make up the universe. This, along with other factors like what percentage of the universe is mass, can give you very strong constraints of what the universe is made out of, and even indirectly what dark energy is, which

0:52:26 Thomas: Love it, hmm, oh, Dark energy now, now we are on a, a whole different, okay, yeah.

0:52:49 Bryan: Yeah. Yeah. Dark energy is something even more mysterious than dark matter. We might not know what dark matter is, but we know roughly speaking what it it does. And dark energy, um, we know what it does at very large scales, but we haven't constrained it well enough that we have a firm grasp on exactly how it does. It's hard to describe it without getting into math, uh, But it's basically what, when Einstein was going through general relativity, he found that there was this one extra, um, variable at the end which he could set to whatever he wanted that just fell out of things. And he called that a cosmological constant. And he eventually decided, okay, that's junk, I'm going to get rid of it. It does weird things, it causes the universe not just to expand but to accelerate an expansion, and that's just silly, I'm Gonna forget about that, it was a mistake to even think about it, it doesn't exist.

0:53:12 Thomas: Yeah. Yeah. And when You say that, it's like he was needing to make his equations work or something, right? And it's not, well, in order to make them work, I Got to do this and

0:53:50 Bryan: Well. No. He didn't even Uh, need it to make it work. He was just doing the math and it turned out that it allowed this one extra parameter, the value of which could be set however you wanted, but if there was any of it at all, it would cause the of the universe to accelerate. And he said, okay, well, we're just going to set that to zero.

0:54:09 Thomas: He just Said that can't be true first, and it turned out to be true, right? So

0:54:11 Bryan: Yeah, it Turned out once we got into some very accurate measurements of the universe that the expansion is in fact accelerating. And so we had to come back to this and say, well, maybe that cosmological concept was actually right after all.

0:54:28 Thomas: Fascinating.

0:54:29 Bryan: Now the thing is, the cosmological constant isn't the only way to have accelerating expansion. It's one way and it will cause the expansion according to one very particular formula, but you could also have some sort of exotic substance, substance there that doesn't behave like any substance, uh, we have, but um, it behaves in just the right way that it would cause this expansion. And it can have, it could follow any number of different formulas. We know it's accelerating, um, and we know that the amount it's accelerating is pretty close to what we would expect if it were a cosmological constant, but it, it could still be some other substance. So one of the big questions astrophysicists have is, well, it would be very interesting if we were to measure it exactly and prove that it's actually slightly off from the formula for a cosmological constant. That would prove definitively that dark energy is some substance and give us one more thing that goes into the mix of the universe. Now, it could be that it is a substance that has exactly the same formula as a cosmology constant.

0:54:30 Thomas: Mmm.

0:54:44 Thomas: Lambda.

0:55:33 Bryan: And if it is a cosmological constant, we can never rule out between the two. But we have to do the experiment to see, okay, how close is it to this value? And if by chance it's a bit off, then we'll find that this value, which we call lambda, this is just the variable we've chosen into a sign for this, the value of a cosmological constant will be negative one. So if we measure it very carefully and we say, okay, lambda is very close to minus 1.02, there's a chance it could be minus one, but let's look a bit harder. And then we look harder, do more experiences, like no, no, no, it's definitely not minus one, it's close to it but not minus 1, so we have some new substance here, and that would prove definitively that dark energy is a thing and not just a cosmological constant, and that would be very interesting and open up even new avenues of trying to figure out what that thing is.

0:56:32 Thomas: Wow. But, but that thing isn't, this isn't the same as dark matter or whatever, like but

0:56:37 Bryan: It would. It's very different, despite, despite the name being deceptively similar.

0:56:43 Thomas: I guess, what kind of thing are you, could it, even, what does that mean to be a thing instead of constant, a, that's a hard thing to fathom.

0:56:50 Bryan: Well, a thing, it could be like some particle or not even necessarily a particle. This is getting a bit outside my field of expertise. It's deep into theoretical physics and how do you exactly explain this? Dark matter is well explained by being some sort of exotic particle, but the problem is almost any particle that can think of would probably cause the expansion of the universe to decelerate and slow down.

0:57:00 Thomas: Right. Oh.

0:57:19 Bryan: You would have to have something really exotic to cause it to accelerate. Because if think about anything that has mass, the mass is going to attract itself. And so the mass attracting itself, it might not be enough to cause the extension to stop, but it would at least slow it down, but

0:57:20 Thomas: Right. Yeah. Wait.

0:57:36 Thomas: But I thought space itself was like expanding, right? It

0:57:37 Bryan: Yeah. And uh, the attraction of mass affects that, it affects how much space itself expands, yeah.

0:57:48 Thomas: Does, oh, okay, I, I thought it, it was going to affect like, so if space itself is expanding but stuff wants to stay close together wouldn't it still be that the universe is expanding, it's just the stuff is staying close together? Or do those have to be the same thing?

0:57:56 Bryan: Well, This gets kind of into relativity and that, if you imagine you have an infinite universe and all the stuff wants to stay together, the effect of the stuff staying together is basically space doesn't expand, or expands less.

0:58:05 Thomas: Ah. Mm-hmm. Oh. Okay. Because it's sort of like inextricably tied to the stuff, there's not, okay, yeah, that's hard. To All right, we'll put a pin in that.

0:58:24 Bryan: So putting it all together, the plan of the Euclid telescope is to gather data on a very large number of source objects that we imagine are being lensed by all of the mass between us and them, looking at maybe a third of the sky overall. All. Then we put all of this into a ton of different mathematical formulae, look at it from a bunch of different ways, slice things up by distance. And then we throw in other science we know as well to try to figure out which solution to the nature of the universe makes things fit the data we have the best. And hopefully that'll give us a very good constraints on this parameter lambda, and tell us is dark energy plausibly a cosmological constant or not. And also give us data on a lot of other parameters that I haven't even, uh, discussed here.

0:59:18 Thomas: Yeah. You haven't, you mean you haven't told us all of astrophysics and communicated that to us in about an hour?

0:59:21 Bryan: Oh, 90 of it, but I left a few things by the road.

0:59:25 Thomas: Yeah. Is so I remember the lawrence like Krauss uh back when he was kind in better standing. I remember watching a lot of those lectures and he was talking about something. I don't know. I'm wondering if this Lambda thing is the same thing. I feel like he was talking about whether or not the universe was, I don't know, closed, flat or open or something like that. To my memory, is that, you know what I'm talking about? Is that related to what you're talking about here?

0:59:47 Bryan: So that's related, that's, uh, the curvature of the universe is another one of these, uh, physical parameters um that we want to study. And it turns out that as far as we, we can tell, the universe is very close to being flat, curved and open being the other two directions it can go in. And this is another thing that would be interesting to see. As far as we've been able to measure, it's consistent with being flat. And flat is like, basically the curvature parameter being exactly zero. But either side of that would give either a positive, positive curvature or negative curvature, those being closed or open.

1:00:28 Thomas: And is that a curvature in a, in a like a fourth dimensional sense? Because it's like the, okay yeah, hard to

1:00:32 Bryan: Yeah, kind of. It's Like difference between a globe or sphere has positive curvature, and that if you go out in two perpendicular directions, you'll eventually get a bit closer to those, those two positions will start to get closer to each other the farther out you go, until eventually you go out far enough, you get together. Now, on a universal scale, you might not ever actually get back together, it might just be something more complicated with how it works. And that negative curvature is kind of the opposite, you can imagine what they call a saddle point, just because the shape of a horse's saddle is negatively curved. In one direction it curves up, and the other it curves down. And so if you go in two perpendicular directions, one curves up, the other curves down, those points get farther away from each other.

1:01:24 Bryan: And then flat is where they get farther way from each, but exactly according to Pythagorean's theorem.

1:01:32 Thomas: Gotcha.

1:01:33 Bryan: So as far as we can tell by looking at the shape of the universe, it's very close to flat and could plausibly be flat, but we don't know for sure. And if we find that it's not flat, uh, that would be very interesting and uh, give us insight into both the early, um, evolution of the universe and uh, possible late evolution of it. Oh, so

1:01:55 Thomas: So that's a different thing, And so if the lambda thing isn't, isn't this curvature, what, what is it, again, just so I'm on the same page, right?

1:02:01 Bryan: It's basically the free parameter in the, uh, what we call the dark energy equation of state, which is, it's a relationship between the pressure of dark energy and how much it's expanded. So think about, uh, about you have a gas and you have it in an enclosed container, and on the top of the container, you've got one loose wall and you can push the wall up or down. If you push that wall down, the volume in it gets smaller, then the gas gets denser and exerts more pressure outwards. And if you pull the wall outwards, you If there's gas gets less dense and you have less pressure.

So the parameter we're looking for, for the equation of state basically describes what's the relationship between the volume and the pressure. And what this value is determines the substance could be. And different substances react to this differently. So for instance, if you have a solid object in there, where it's going to bounce back differently from a gas, and a gas bounces back differently from a liquid, and I guess also bounces back different from light, light has its own equation of state. Figuring out the equation of state of dark energy will give insight into what it could be.

1:02:34 Thomas: Okay. Okay. So we know there's a dark energy you're saying like that. So there's something in this Tupperware we're thinking about. That's for sure. And so you're saying if we can find the, the Lambda, which is essentially the, amount, the, I don't know, density or whatever it is would change given the amount you're pushing this lid up or down. That gives you an indication of what the thing is. We know there is something in there. It's like Pulp Fiction. Like there's, you know, you open the briefcase, there's light. There's big shiny thing. You know, there is something in there. We just don't know what. And if we knew kind of essentially how it reacted in this way, that would help tell us what it is.

1:03:26 Bryan: Yeah. Yeah. Yeah. Yeah.

1:03:57 Bryan: So The Weird thing about dark energy, which is, uh, why it can't be explained by any of the things we know exist, is that it seems to react the opposite way to changes in volume, so We talk about like a gas, if you decrease the volume the pressure increases, but Dark energy is the opposite, where if you Increase The volume the pressure increases, so this means that as the universe gets bigger and the dark energy can fill up more space, the pressure it exerts increases. So you can kind of intuit from this why this causes acceleration of the universe, because the more space it has to fill, the more pressure it has, and the more it pushes out, that causes it to expand even more, you get even more expansion, then it fills even more space, it pushes even more, and

1:03:58 Thomas: Oh, okay. Hmm. Hmm. Oh, I see.

1:04:48 Bryan: You basically get a feedback loop that the universe is going to expand faster and faster over time because of the very weird way dark energy works.

1:04:58 Thomas: So, okay, so the thing we, the reason we know there is a thing in the Tupperware is because of the, not only the, not the expansion, but the rate of increase of expansion over time. Okay. Okay, it and the same kind of thing where we're talking about dark matter, how much of this could just be instead of a thing in a box, it's just, you know, one of our theories about something is different. Has that possibility been kind of eliminated?

1:05:07 Bryan: Yeah.

1:05:20 Bryan: Well, no, in the case of dark energy, and that's kind of what we're looking at. So, lambda lambda Lambda actually refers specifically to the cosmological constant, are a lot of other things that it could be. And for the equation of state, the cosm logical constant will have a particular value of minus one. I forget offhand what variable is actually used, but let's just call it L. It'll have L of minus 1. So if we find that the value of L is very close to minus one, then it's plausible that dark energy is a cosmological constant, which actually is just part of general relativity. It isn't actually a substance. It's just a way the universe works. It is just kind of...

1:05:25 Thomas: Okay. Okay, it just happens to be some default value you into put the formula, and you don't know why. We can't really say why. It's just what our universe is.

1:06:10 Bryan: Yeah, The Universe was made such and such a value for the density, of it was programmed in and it follows this, but it could also be some substance of a given density which which follows the equation of state with a value close to minus one. We know it's pretty close, we don't know if it's exactly minus one. It could be a substance that has it at minus one, and we would have a very hard time distinguishing it from from it being basically just the cosmological constant. So there is the open question of, okay, is this just an extension to general relativity as a cosmological content, or is it some new exotic substance? And if we find out equation that of state of the value is very definitely not minus one, then we can say with a high degree of certainty, it is a new substance.

1:07:10 Bryan: If it is minus one then, we don't really know which of those, and we'll have to find some other angle to look at the problem to try to figure out which it is.

1:07:21 Thomas: And gravitational lensing is key to that process. Did

1:07:24 Bryan: Because Gravitational lensing, by looking back through time, we can look at how the universe has accelerated through time, and depending on exactly what the equation of state is will determine how much it accelerates over time. So we get basically a history of the universe.

1:07:39 Thomas: Well, that's super cool. And that's something you're kind of directly working on. All

1:07:46 Bryan: So we're working on uh getting the Euclid Space Telescope going so that it can analyze data on many many sources across the sky to try to get as much data as possible to try and figure this out. Yep.

1:08:00 Thomas: Right. Well, I think we have topic for another day. We have a little bit more about how you actually do that, how the Euclid telescope and how other things actually give you that data, right? All right. Well, I'm looking forward to that. Well, thanks as always. Of course, Dr. Bryan Gillis, you don't have anything to plug. Do you have anybody else's stuff to plug like you did the last time?

1:08:11 Bryan: Oh Yes. I do, actually. Uh So uh one thing that occurred to me that people might be interested in, if you're finding this interesting, is that at my workplace, we do public talks regularly. And during the pandemic, we did all of these remotely. And as part of that, we recorded all of those and uploaded them online. So there's an archive of talks we gave during the epidemic, a couple by me, many others by other people working there, which you can find online. So the site for this is on Vimeo.com/RoyalOBS. That's V-I-M-E-O dot C-O-M slash R-O-Y-A-L O-B-S. That will take you to the Royal Observatory's Vimeos site where you can watch any of the 60 or so talks that we recorded and put up online on various subjects.

1:08:21 Thomas: All Right.

1:09:13 Bryan: Subjects, uh, so for instance, for the ones um I did, um, I did, uh, one on how the study of astrophysics coped, uh, throughout the pandemic, another on how people, um, who are outside of science can contribute to science if they're interested in it, um, there are a lot, um, on

1:09:29 Thomas: I almost think we should just talking about some of those topics here.

1:09:30 Bryan: Yeah, we, can do that if you want. It won't take much prep for me to turn them into episodes, and uh, since I already have all the slides for those, so that's certainly one idea we can talk about.

1:09:41 Thomas: Absolutely. Cool plug. I love it. Thanks as always and see you again next time.

1:09:46 Bryan: All right. Thank you very much, Thomas.