The Invisible Universe

[00:00:01]All right. Very good morning everybody.

[00:00:03]How is everyone doing today?

[00:00:06]>> Oh, fantastic. Fantastic.

[00:00:09]Isn't this just the most incredible adventure that we are all on? I just think it's it's amazing, right? Every couple of days, like a whole new world to explore. I I just think it's fantastic. Um I hope you guys are having um as an amazing time as I am. Um who went to um Storway yesterday? Amazing place, right? Oh my good. What did you guys think of it?

[00:00:33]>> Did Did anybody notice? Everywhere I looked, it was like McCauley Road and like McCauley Farm. Did Did anybody see this?

[00:00:42]>> I thought I should have been like, "Guys, I I think I've I've come back, you know. It's um Yeah. So, um it felt um quite personal there." And it's just amazing every couple of days having a a whole new place to explore. It really is uh very exciting. So, I'm glad you guys are all having um a great time as well.

[00:01:00]Um can I just ask how many of you guys have been along to one of these talks before?

[00:01:05]Okay. Okay. Wonderful. So, um I've just loved um talking about some of the um things with you guys before. So, you know, we started out thinking a bit about um vacations in space and how we might soon be able to, you know, explore space for ourselves. And then I just love I'm a big movie geek. I love talking about some of the um space uh movies uh with you guys. Now, this morning is really exciting because we have the chance now to get into a bit more detail about some of the science out there in the cosmos. And we're going to be exploring why cosmologists think most of the universe is actually invisible. So, we're going to be thinking about what's called dark matter and dark energy. Thinking about why it's there and you know what is the um evidence for it because these are some of the biggest mysteries that we now have in cosmology.

[00:02:02]So, just um very quickly for those of you um who've coming on for the first time uh to introduce myself a little bit, my name is um Ed McCauley. So usually um most of the year I'm based in uh London where I'm a lecturer at Queen Mary University of London um where I lecture um physics and data science and it's absolutely a wonderful place. We have such great students there. So I'm actually kind of looking forward to uh the start of the new year there.

[00:02:33]Please do feel free to ask if you have any questions as you're going along or if you'd like to take any photos or anything like that. that is um absolutely fine. And of course, if you have any um questions afterwards, anything you wish you'd ask, please do feel free to um drop me an email. It's always great to get uh emails from you guys. Sometimes there's that question you wish you asked and it's just a few days later. Please do feel free to um drop me a quick email.

[00:03:01]Okay, so let's start by thinking about this kind of big picture for what are the ingredients that we currently think our universe is made out of. Now, if you go along to any cosmology conferences, you'll almost certainly see the uh cosmic pie chart. Okay, has anybody seen the cosmic pie chart before? It's really just a pie chart which breaks down the ingredients, the big pieces that we think our universe is made out of. And I'd like to use this as a bit of an outline, as a bit of an overview for what we're going to be thinking about this morning. Let's take a look at the pie chart, see what the big ingredients are. Okay. Now, the smallest part of the pie chart here, the um vanilla cheesecake, that's our regular massa. So our regular visible matter. So all the atoms, all the planets, all the stars, all the galaxies, all that stuff that we see when we look out at the cosmos, that only represents about 5% of what we think we can see in the universe.

[00:04:13]Now, the next biggest slice of the cosmic pie chart, the my personal favorite, the delicious pecan pie. It's very good. That represents the dark matter.

[00:04:26]We think that about a quarter of the total ingredients of the universe and most of the mass is made up of what we've kind of rather elusively called dark matter. Now I think a better name for this would have been invisible matter because it's not actually dark like you know things are dark like a lump of coal or something. It really is invisible. We think there's stuff there but we think we can't see it directly.

[00:04:56]So I would have preferred invisible matter but it's called dark matter. So that's what we have to work with. But just remember it's not really dark. It doesn't obscure light. It just seems to be completely invisible.

[00:05:09]We think we've got some ideas of, you know, how much of this stuff we need, what some of its properties could be, and maybe some idea of what it couldn't be. But when it comes to the biggest slice of the pie, we really don't have any so much of an idea at all. Okay, so this biggest slice here, the uh the dark chocolate cheesecake, this represents what we call dark energy. Okay.

[00:05:35]Now, it's called dark energy because it was discovered after dark matter and it's kind of borrowing that kind of dark terminology from the dark matter. Now, this part here, energy, okay, it's somewhat intentionally nebulous. Okay, it kind of conveys this idea that we think there's something there and maybe there's some energy associated with it, but we really don't have any clue whatsoever as to what this biggest slice of our cosmic pie chart um comprises.

[00:06:08]Okay, we thought until quite recently we might actually have some pretty good ideas as to what this is. But the very latest cosmological results, which have only really come out in the past year or two, they've really made this whole picture way more interesting. So, if anything, we now even more confused as to what this dark energy slice of the cosmic pie chart could be. We think we've got a reasonable idea of how much of it we need in the ingredients for our universe, but exactly what it is is quite a mystery.

[00:06:43]So, what we're going to be thinking about this morning is why do we think that most of our universe has to be invisible in order to explain the little visible slice of the universe that we can see directly. That's the plan for this morning.

[00:07:03]To give you guys a bit of context for what this picture looks a bit like if we're looking out in space, I'd actually like to start with something that might be a bit more familiar. I'd like to start with somewhat of a curious map of our own planets.

[00:07:22]What do we reckon is going on here?

[00:07:24]Maybe there's some features we could recognize. We can see Florida and the States over here and some parts of Europe.

[00:07:33]But it's clearly quite an unusual map of the world. Does anybody have any ideas as to what's could be going on here with my slightly unusual map of the world?

[00:07:41]>> Yeah.

[00:07:42]>> Like Yeah.

[00:07:46]>> Yeah. Exactly right. Okay. So, this is a map of the world, but showing where the lights are. Okay. So imagine if you had to make uh an atlas of the planet earth, but the only information you had was where the city lights are. You'd end up with a quite distorted view of the surface of planet Earth. You would miss out all of these deserts and all this kind of stuff and you know, Antarctica and the Sahara Desert. They would all be completely missing from your map.

[00:08:18]Of course, we know that if we look at planet Earth in the daytime, we see all these beautiful land masses and oceans and things like that and u you get a much more complete picture. But if you were only looking at the Earth at night and trying to build your map on where you see the city lights, you'd end up with a very incomplete picture of where the land mass on planet Earth is.

[00:08:42]And in a lot of ways, what we see in cosmology when we look out in space, it's somewhat similar. So all these beautiful stars and galaxies we can see, in some ways they're just the kind of tip of the iceberg of what we think has to be out there.

[00:08:59]So I'd like to share with you guys a bit of an equivalent picture. So we've got some beautiful clusters of galaxies here. So each one of these points of light is a galaxy. We've got a nice kind of cluster over here. And we've got some other clusters of galaxies over here.

[00:09:16]Now, this is not a photograph from a telescope. This is actually the results from a simulation. So, building simulated galaxies on a computer, putting those ingredients in, letting the science work, and seeing what we get.

[00:09:33]And we can only get these visible galaxies in a kind of pattern that matches what we actually see if we include in our simulations an underlying amount of this dark matter. And because it's a simulation, we can just turn it on and see where it is. So in our simulation, we can take a look and we can see that we have to have this underlying framework of dark matter and it is very beautiful here. So we know that this needs to be going on kind of underneath the surface in our simulation in order to give us these galaxies in the correct place. Those visible galaxies that we know uh of course are going to be there.

[00:10:15]Simulations are actually one of the ways that we know we that dark matter has to be there. Because if you try and simulate what we see in the universe only with that visible matter, only with those regular atoms, you can never get the distribution of galaxies, the kind of galaxies correct. You can only make it work if we have all this beautiful dark matter in there as well.

[00:10:40]But the story of dark matter actually starts decades before these kinds of cosmological simulations. And it actually starts by observing um galaxies and how they rotate.

[00:10:55]In this animation, this shows what we would expect to see with galaxy rotation. So a galaxy got a lot of mass in the center there. Maybe you've got actually a black hole in the center of a galaxy. And you would expect the stars near the center to be rotating faster than near the edge. But what we actually see is something like this. We see that the g stars near the edge of the galaxy are rotating just as fast as the stars in the center.

[00:11:24]And this was really bit of a puzzle for kind of many decades when it was discovered because you know in our solar system the inner planets they rotate a lot faster than the outer one. So we think what we might get this what we call a rotation curve like this. So our distance from the center and how fast it's going. So, we'd think that the velocity is going to get slower, but what we actually see is something like this. As we move further and further out from the galaxy, we get what we call a flat rotation curve, which is just a way of saying that the stars near the edge of the galaxy are rotating at about the same speed as the ones in the center.

[00:12:02]So, this is what we would expect. And of course, this is what we're observing.

[00:12:09]When this was first discovered, there were quite a few kind of competing theories which might explain this. One explanation that could have explained this kind of rotation curve business is that maybe gravity works differently at those kind of very large scales um that we see galaxies because you know we can test gravity very well on earth and in our solar system and you know if you learned about Isaac Newton and gravity that works fantastically well in the solar system but maybe on scales as big as galaxies maybe gravity works a little bit differently. So that was one theory.

[00:12:45]The other theory that could explain this uh rotation curve is that maybe there's just more matter in the galaxy that's invisible. And if you put more stuff in the galaxy and you spread that matter more evenly out, then you can get those kind of flatter rotation curves. So if we go back to this picture of a beautiful spiral galaxy here, now this is kind of very much like what our own Milky Way galaxy might look like. This is the visible matter. If we put a kind of big old nebulous cloud of this invisible dark matter floating around the galaxy, all this extra matter, it's going to change the dynamics of the galaxy and it's going to make it easier for the stars near the edge to be rotating more slowly than the stars near the center.

[00:13:38]So for many years there was a bit of a kind of um we had these two theories kind of does gravity work differently in galaxies or maybe do we have dark matter. So this was kind of the first piece of the puzzle one of the first pieces of evidence that maybe there is this strange dark matter in the universe.

[00:13:57]Since then, we've had a couple of other very key observations which really steer us to this conclusion that it's not that gravity is working differently. It's that there is this invisible matter which is surrounding the galaxies.

[00:14:12]To give you guys a bit of context for how this works, I'd actually like to return to our very beautiful map of our own planet Earth. Okay? And I'd like to share with you guys how some kind of dynamics work on planet Earth because that actually applies in space and that actually helps to give us an idea for why we think there's this observational evidence for dark matter.

[00:14:36]Now I'd like to start by taking you guys on a bit of an imaginary journey and I'd like to start in um LA. Is anybody here from California?

[00:14:46]>> Oh okay. So you guys, you must have flown over. Did you fly over here to Heathrow airport? Something like that.

[00:14:51]Okay, great. So if you were flying from LA to Heathrow airport, you might say, "Okay, we're on a schedule. We don't want to miss the boat. Let's just go as directly as we can." You think, "Okay, that's got to be our path from LA to Heathrow, taking the most direct path."

[00:15:10]If you look at the in-flight map as you're flying along, your plane's actually going to go something like this. You're going to go up through Canada and kind of Greenland, and then you're going to come in like this. And you might say to the pilot, "What is going on like this? We got to get on the cruise and don't want to miss the boat."

[00:15:25]Well, we don't have time to take a detour over Greenland. Okay.

[00:15:31]Of course, we all know what the uh solution to this is. This map is just a flat projection of the spherical planet that we're living on. And it makes a lot more sense actually if you look at it in a round Earth like this. Okay. So if we're looking on the Earth as a surface like this, that's our great circle path.

[00:15:51]So the one that looks like it's going further. And if you look at it on a sphere, it's obvious that that's the shortest path. Whereas if you plot that path, which looks like the most direct path on the map, you can see that you're actually taking a bit more of a detour there.

[00:16:09]The story here is that the shortest path between two points isn't always totally straightforward.

[00:16:17]And the reason this is important in astronomy is because light always wants to take the shortest path between two points. Now, usually that's pretty straightforward. If you think about, you know, I've got like a little laser beam here. The laser beam is always taking the shortest path from the laser point onto the screen because the space around us is completely flat.

[00:16:38]But you might have heard that space isn't always completely flat as well.

[00:16:42]Space gets curved by the matter which is in there. Very much in the same kind of way as you know, our planet Earth. It's not a flat surface, it's a spherical surface. mass out in space curves the space and that causes the path that light takes to be curved kind of in the same way as this. And this actually gives us another insight into the distribution of matter out there in the cosmos. So I've got another um animation to kind of show you guys what's going on. Suppose we're observing a galaxy like this over here and we're there in planet Earth and maybe there's a galaxy cluster in between us and that galaxy cluster is filled with this invisible dark matter.

[00:17:25]We can't see the invisible dark matter in the galaxy cluster directly, but that matter is causing spaceime around that area to be curved.

[00:17:37]So instead of just taking a direct path to us from the galaxy, the light looks like it's taking this curved path.

[00:17:45]And this is something that we call gravitational lensing because it really is acting exactly like a lens. You know, if you wear glasses, you're looking through a magnifying glass that really is bending light in exactly the same way. And we have these amazing things happen in space where we have big clusters of galaxy and we have this matter and that's causing us to get distorted views of the galaxies that we're looking at.

[00:18:12]This is just an animation but we really can observe this amazing kind of stuff if we look out at the distribution of galaxies here. So this is an absolutely beautiful sensational image from JWSC from just a few years ago observing some distant galaxies and you guys can probably see here. Can you see this kind of distortion over here that if we look over on this side we've got some galaxies being distorted and stretched here and it really looks almost like someone's put a big glass magnifying lens in the middle of this uh image here. So, we think what must be going on is that there's got to be a big cluster of dark matter here causing this lensing, causing the light to be stretched.

[00:18:58]And what it's actually causing, it's giving multiple images of the same galaxies. So, a lot of these um kind of red curved out galaxies, we're getting several different images of the same thing. And by looking very carefully at that we can actually infer how much mass we have to have between us and the thing we're observing.

[00:19:20]So this is what is called gravitational lensing and it's a really very important technique in astronomy because it allows us to see the distribution of matter even if it's invisible. Um and this has been one of the developments in the past maybe kind of few decades of astron a astronomy and cosmology which has allowed us to really map out um where the dark matter has to be.

[00:19:47]It was actually combining gravitational lensing with other forms of astronomy that really gave us the key insight that okay most of the matter in the universe has to be dark matter and it really can't be modified gravity.

[00:20:03]It was actually this really very famous result called um the bullet cluster. So there's actually a few things going on in this image. It's actually a a composite image. So all of these galaxies are essentially the kind of visible galaxies what you'd see in a in a photograph of this cluster.

[00:20:22]But these kind of pink and blue patches over here, these are actually um superimposed images of different things going on in the cluster. So these blue patches over here and over here on this side of the cluster, they're where the mass has been measured by this technique of gravitational lensing. So by very carefully looking at these background galaxies and seeing where they're getting how they're getting distorted, um the researchers on this project mapped out these blue patches of where there's dark matter.

[00:21:00]These pink patches here actually tell us where the hot gas in the cluster is. And they're measured from X-rays because when this gas gets really hot, it emits X-rays. So that's kind of mapping us where the hot gas is. And that was measured with a an X-ray telescope. So a telescope that instead of using visible photons, it measures X-rays.

[00:21:22]And this single data set of the bullet cluster really gave very conclusive evidence that there has to be dark matter in the cosmos and it can't be some form of modified gravity. And the explanation is what what you get when you look at the dynamics of the cluster.

[00:21:43]I have another animation here which really explains what is going on with this cluster and why it really must be dark matter and it can't be some modifications to gravity. So what we actually had in the bullet cluster is we had two different clusters and they're coming together and they merged.

[00:22:02]Now each cluster has hot gas in the middle surrounded by some dark matter.

[00:22:09]But let's think what happens when these two clusters merge. when they come together, the hot gas interacts with each other and kind of sticks together and it gets left in the middle.

[00:22:20]But the dark matter, it doesn't interact with anything else and it just kind of passes through. So we end up with these two clumps of dark matter either side with the hot gas in the middle.

[00:22:32]So what we end up with is the hot gas in the middle and then the two clumps of dark matter either side. And this is really very difficult, almost impossible to explain with any kind of modifications to gravity or anything like that. So it really is kind of very compelling evidence that most of the matter in our universe, it's got to be this very strange invisible dark matter.

[00:22:57]So this is a really beautiful simulation um of some dark matter here. So each one of these points is a big kind of blob of dark matter and where we get these um kind of central clusters. That's where we get galaxies forming. And we know what's going on here because it's a simulation. So we can just see for ourselves what's going on and where everything is. And this is really very advanced cosmological simulation because what we see going on here is actually the hot gas in the simulation. So, as the dark matter clusters are kind of getting bigger and bigger, they're drawing in all of this hot gas from the surrounding areas and then they're kind of feeding back those elements into the cosmic web and that's actually triggering all of the the star formation and all the amazing stuff, all the amazing life that we now have know in our solar system.

[00:23:53]So it really is very beautiful and this is an extraordinarily complicated uh convoluted um simulation took so much computing power but it really actually gives us a very good picture of the universe as we actually observe it.

[00:24:10]And we can only simulate the universe as we observe it if we include this underlying dark matter. And then what we end up with is we get these points of kind of hot gas and things and they actually you can kind of start to see some of them are forming these little spiral galaxies here. So each one of these points is a whole other galaxy very much like our own and we've got the kind of hot gas and the disusion and then some of them are starting to form spirals there.

[00:24:37]So we're now up to about 10 billion years uh since the big bang. So, we've just watched a lot of cosmic history just fly by in a couple of minutes. And then what we end up with towards the end of the simulation, it's showing us, you know, where the visible matter in the universe is. And we end up with these clusters of galaxies and we end up with these little smaller what they're called dwarf galaxies. They're almost like kind of suburbs of the main galaxies. And we get this really very accurate picture.

[00:25:08]And we could only generate that if we include um much more dark matter in our simulations than the uh visible matter.

[00:25:19]So we end up with this kind of beautiful picture. This is another simulated image from that same illustrous simulation and it looks very much like the kind of images that we get if we look into very deep space you know with the James Web Space Telescope with the Hubble Space Telescope and we observe clusters of galaxies.

[00:25:39]But the amazing thing is that everything here has just come out of a simulation.

[00:25:43]There are no real galaxies in here at all, but they match very well with what we actually see if we include the correct amounts of dark matter.

[00:25:55]So, we think we know that we have to have dark matter there. We have some idea of how much dark matter we need.

[00:26:02]And we know that it's got to be what we say non-interacting. So, it just kind of floats by each other. It only really seems to interact with gravity.

[00:26:11]But we really don't have any idea as to what the dark matter actually is.

[00:26:16]There's plenty of theories, plenty of ideas. My colleagues in particle physics are working very hard to see if they can detect some dark matter particles. But currently the nature of what dark matter actually is. It's one of the biggest open questions in cosmology because you know it represents such a large fraction of the mass in our own universe and we really don't have any idea as to what it actually is.

[00:26:42]If you remember the cosmic pie chart from the beginning, you'll remember that that dark matter though, that's not even the biggest slice of the cosmic pie chart. And that most of our universe, not even the matter in our universe, most of the what we loosely call energy in our universe. We think it has to be what we call dark energy. And we actually know even less about dark energy than we do about the dark matter.

[00:27:06]So with the dark matter, we know it's invisible. we pretty confident that it's some form of matter. We just kind of don't know exactly what kind of matter.

[00:27:17]With dark energy, we're really even less sure. So, we know there's kind of something there, but to call it even energy, that's probably a bit more specific than um you know, maybe perhaps we should be to give you guys a bit of context for why we think dark energy is there. Um I'd like to share with you a slightly um a kind of more earthbound example, a bit of an analogy by thinking about what happens if we're looking at a tree rings. So maybe a bit more of a a natural example, bit more of a granded example because looking at tree rings, they actually allow us to look back into the past into what's happened into the life of the tree. And when we're trying to map out the universe to look for dark energy, we actually do a kind of similar kind of thing but just on a cosmic scale. So I thought it might be a bit of useful context to see how this works with like a tree ring and then we can see how we can do it for the entire universe. That's the plan basically. So do you guys remember doing this? Maybe you do this in school. You know if you have a a tree ring a slice of the trees each year the tree adds a new ring as it grows.

[00:28:29]And then from the width of each ring you can see how much the tree grew in that particular year. So just by taking a slice through the tree you in this one slice you actually get the whole history the whole growth history of the tree. So you can just count the number of rings.

[00:28:48]So you can count the number of rings how far back in time you want to look and then you can say okay maybe 15 years ago the tree grew this much. So for every year into the past of the tree, you can see how much growth there is. And what that allows you to do is you can make a graph like this. So it starts over on this side. This is basically kind of where we are over here. So this is kind of present day and they're looking at lots of different tree rings, not just one. And then, you know, you can count how many years back. You can just count the tree rings and then you can see how wide each tree ring is in a given year.

[00:29:25]So what you can say is maybe we can look back so let's say 1976 how much were the trees growing in that given year and that allows you to kind of work out um how these trees are growing on our own planet.

[00:29:40]The reason um people do this kind of study for looking at trees. Well for one you know you can learn about trees but also you can learn about the environment that the trees are growing in. So you know trees they need sunlight to grow and they need carbon dioxide in the atmosphere. And by looking at how these trees are growing that's one way that you can trace how these kind of things are changing in the environment on earth that the trees are growing in. So by looking at slices of tree wings you can say okay what were the conditions like for growing trees you know half a century ago.

[00:30:12]So for one you learn about the tree but you also learn about the environment that the tree is growing in.

[00:30:19]A lot of what we do in cosmology is actually doing a similar kind of thing as this essentially on a cosmic scale.

[00:30:26]So we map out how big did the universe used to be at given points in its history and then we use that same kind of growth history to map out what are the ingredients that we need in order to explain that growth. And there's two pieces of the puzzle. You need to know how big did the universe used to be and how long ago did the universe used to be that size. Those are the two pieces of the puzzle. And what we're going to focus on now is how do we go about getting both of those pieces of the puzzle.

[00:31:01]Now the first piece of the puzzle is looking back to h the past of the universe. What did the universe look like in the past? And in a sense, this is kind of the easy part because whenever you look up at anything in space, you're always looking into the past because light, even at the speed of light, still takes a long time to reach us.

[00:31:26]So, I've got this scale diagram here.

[00:31:28]Can you guys see planet Earth over here?

[00:31:30]Okay. Now, to scale, and this is actually just from one image all the way over here, we've got the moon. Okay. So this is the size of the moon to scale and also the distance.

[00:31:43]So usually when you see the Earth and the Moon to scale, you don't seem quite so far apart because most people don't have such an exceedingly huge screen to show them on. Okay.

[00:31:57]Does anybody know how long lights takes to reach us from the moon?

[00:32:02]>> How long? Not that long. Not that long.

[00:32:06]>> So I think you're thinking about the sun. You're exactly right. That's how long light takes to recharge from the sun. Very good. Very good. So, how long?

[00:32:13]>> Two and a half seconds.

[00:32:14]>> Two. Yeah. So, that's the round trip time. Round trip time. So, if I'm over here and I send a signal to the moon and then I want to wait for it to come back, it's 2 and 1/2 seconds for a round trip.

[00:32:24]So, 1 and a half seconds one way. And you're exactly right. So, 8 and a half minutes for light to reach us from the sun.

[00:32:32]So when you look up at the moon, next time you look up at the moon, remember you're looking 1 and a half seconds back in time.

[00:32:41]Now I always have to do the mandatory safety disclaimer. Do not ever look directly at the sun, but just remember that light from the sun is coming 8 and a half minutes uh from the past.

[00:32:55]And this applies to everything that we're observing out in space.

[00:33:01]So what about if we bring into the same scale the planet Jupiter? Now this is the size of planet Jupiter to the same size as Earth and the moon and that distance. Okay. Now there is no way I could include the distance of planet Jupiter on the same scale. Does anybody know roughly how long to takes to reach us from Jupiter?

[00:33:24]Does anybody want to have a guess?

[00:33:27]>> 20. Yeah, very very good guess. So depending on exactly where Jupiter is in its orbit, it takes somewhere between half an hour to an hour to reach us.

[00:33:36]Has anybody ever seen Jupiter through a telescope? Yeah, you got a question.

[00:33:40]>> I I have, but that's light reflected from the sun off of Jupiter, is it not?

[00:33:44]>> You're exactly. Yeah, that you're exactly right. So So the So Jupiter doesn't produce light of itself. So the light we're seeing from Jupiter has gone on an even longer round trip. So it's taken a long time, you know, kind of hour or two to reach Jupiter and then taken half an hour, an hour to come and uh reach us. So yeah, exactly right. Um who has seen Jupiter through a telescope? Okay, cool. Could you guys see much of the features like the red spot, this kind of stuff? I always think it's amazing seeing this through the Earth through a telescope. So just remember if you're looking at Jupiter and you don't even need a telescope, you know, you can just see Jupiter in the sky. You're looking somewhere between half an hour to an hour back in time.

[00:34:22]Okay? So, you know, if it's, you know, half past 10 at the moment, if there was a big old clock in Jupiter, it's going to be saying something like half past 9, 9:00, something like that. You're looking um half an hour, an hour back in time.

[00:34:37]How many of you guys came on our stargazing night a few nights ago?

[00:34:42]Do you guys remember roughly how long light takes to reach us from the stars?

[00:34:49]>> So, do you Some of them are millions of years.

[00:34:53]>> Millions. So mo so most of the visible stars we can see they're not quite as far as as millions of years. Yeah. So the closest star to earth um after the sun of course light takes about five years to reach us. But most of the constellations that you see when you look out on the starry night sky that light is going to be taking hundreds of years maybe a couple of thousands of years to reach us.

[00:35:21]Now, what I kind of hope you might be able to kind of take away is if you think this distance from the Earth to the moon, okay, this distance over here, that represents a light travel time of 1.3 seconds.

[00:35:36]Think how much longer 500 years is than 1.3 seconds.

[00:35:43]That's how much further away even our nearest stars are than just the moon.

[00:35:49]Okay. So, even those stars that you can just see on a good clear night just with your own eyes, you're looking hundreds of years into the past, almost all of the stars you're going to be seeing with your own eyes, that light left the star before you were born. And while that light was traveling towards you, you know, you were born and you grew up and you were able to observe it. So, just by looking at the stars, we're looking thousands of years into the past.

[00:36:15]So the furthest stars we can see in our own galaxy. They're kind of thousands of years.

[00:36:22]Now the next nearest galaxy outside of the Milky Way is the beautiful Andromeda galaxy. So it's another spiral galaxy somewhat like our own Milky Way. Does anybody know how long light takes to reach us from Andromeda? Yeah.

[00:36:37]>> Yeah. Exactly. Right. Just two and a half million years. Yeah. Has anybody seen Andromeda?

[00:36:42]Yeah. So, um, it's it's it's actually quite a large galaxy. It's just it's quite diffuse. Um, I've pointed my eyes in the direction of Andromeda and kind of thought maybe I can see a bit of a kind of hazy patch there. But yeah, if you're looking at Andromeda, you're looking 2 and a half million years into the past just in our nearest galaxy. So, as we're looking further and further out into space, we're looking further and further into the past.

[00:37:10]Now this is a amazingly beautiful image from the Hubble Space Telescope of the Hubble Deep Field. Has anybody seen this before? The Hubble Deep Fields.

[00:37:21]Absolutely beautiful image. So we you've got a few stars here. So that that's a star, but most of the points of light here are whole other galaxies. So every single one of these little tiny patches of light here is a whole other galaxy containing maybe a 100 billion stars kind of the same size as our own Milky Way. So if we just kind of have a look in some more detail here, each one of these little points here is a whole other galaxy.

[00:37:49]Light from most of these galaxies has been traveling towards us for billions of years. So even some of the closest light has been traveling for several billion years, the light from most of these galaxies has taken longer than the lifetime of planet Earth to reach us. So planet Earth is about 4 and a half billion years old. The light from most of these galaxies has taken longer than that to reach us. So when the light left these little galaxies over here, planet Earth did not even exist. And just while that light was traveling towards us, planet Earth formed and you know we kind of life evolved and we built our telescopes and we're able to observe this. Some of the more distant galaxies in this image, so the kind of really redder ones, the faint ones, light's been traveling to us for something about like 10 billion years. So almost the entire age of the universe. So we can kind of look back almost all the way through our cosmic history.

[00:38:49]So you might be wondering, well, how do we actually know how far away these galaxies are? Okay, and a lot of what we do in cosmology, it's actually about measuring distances, measuring how far away galaxies are. And the reason is because if we know how far away they are, we know how far back in time we're looking. So people say, you know, telescopes, they're kind of like time machines. And this really is true because they really do directly allow us to look back in time. And the key part of that is we need to know how far back in time we're looking. So we need to know what are the distances to these galaxies.

[00:39:30]And one of the most precise ways we have to measure distances to galaxies is actually by looking at exploding stars called supernova.

[00:39:41]I have an animation here of what goes on in one particular kind of supernova called a a type 1A. What we have is a white dwarf over here and it's getting more mass. It's suck hoovering up more mass from this other companion star. And when it reaches this mass here, a bit more than one and a half bit less than one and a half times the mass of our own sun. The whole thing explodes because it's upset the the balance of pressure and um gravity there in the star. and the whole thing explodes and maybe it leaves behind very beautiful uh remnant supernova remnant there nebula.

[00:40:15]The reason these particular kind of supernova are so useful to study is because they generally always explode when they reach about that same kind of mass about 1.4 times the mass of our own sun.

[00:40:30]And what that means is they kind of explode with what we call like a standardizable brightness.

[00:40:36]So we can go and see a supernova exploding and we know okay we know how bright it must have been. We can use that to figure out the distance. Now that's a kind of animation. It looks a bit less dramatic in kind of real life if you're observing it through a telescope. So what it might look like a bit more like in a telescope if you're looking at a galaxy here. We've got a supernova going off here. And over a period of about a month or so what we see is the brightness in that supernova increases.

[00:41:03]And so it takes maybe a week or two to increase. It reaches a peak brightness and then it maybe takes a week or two to decrease.

[00:41:12]And this kind of gives us a very characteristic shape. And this peak brightness and how long this whole supernova takes. That really gives us a lot of information in terms of how bright the supernova is and how far away it is. So we can measure that peak brightness and we know how much light must have been um kind of idiated in the original explosion and then with a bit of geometry we can work out exactly how far away our supernova is. So this is one of the most precise tools we have to measure how far away very very distant galaxies are. And it allows us to build the first piece of that kind of cosmic um timeline puzzle that we were thinking about with the trees there.

[00:41:58]So if we think about a cosmic timeline here, this is a bit like that same diagram I showed you before with um the um those tree rings here. So here we are sitting over here and then as we go this way we're looking billions and billions of years ago. Okay? So we're looking one billion year all the way to about 7 billion years. So this time over here this is looking back to about half the age of the universe.

[00:42:26]And every time we look at a supernova we can work out how far away it is. We can work out how far in the past it is.

[00:42:34]So the first piece of this puzzle is if you look at a hundreds of supernova, you can start populating this timeline here and it looks something like this.

[00:42:44]So this is a collection of supernova and each one of these points is a supernova explosion in a distant galaxy. And because we know how bright the explosion is and you know we've got some idea of what's going on with the explosion we can say okay if this supernova is exploding with this brightness it must be 4 something billion years ago.

[00:43:06]So that's the first piece of this kind of cosmic puzzle our cosmic timeline here.

[00:43:12]But the second piece we need to know is okay let's suppose we're looking at a galaxy 6 billion years ago.

[00:43:20]What was the size of the universe relative to now? How much has the size of the universe changed since we've been looking at it? That's the second piece of the cosmic puzzle. So, the last thing we're going to be thinking about this morning is how do we go about getting this last piece of the cosmic puzzle and putting this kind of uh cosmic history together.

[00:43:44]So in order to figure out how much the size of our universe has changed, we actually need to look very closely at the lights that we're observing from galaxies. So in this animation here, we've got an example of a galaxy. And here we are over on Earth here, and the light's coming from the galaxy. But if the universe is changing size, if the distance from that galaxy changes while the light's traveling towards us, what that does is it actually stretches out the wavelength of light that is coming towards us. So if that light maybe was originally blue when it left, if the distance between Earth and that galaxy increases as the light was traveling towards us, that light actually gets shifted to a redder color. And we call that a red shift. If the distance is increasing, that causes the light to stretch out.

[00:44:37]So if we look very carefully at exactly the essentially the color of light in these distant galaxies, we can work out this red shift and that tells us how much the size of the universe has changed while that light's traveling towards us.

[00:44:55]Now you might be wondering, okay, let's suppose we see some red light over here.

[00:45:00]How do we know that the light wasn't just red when it left the galaxy? and maybe the universe is the same size.

[00:45:09]This is the last piece of the puzzle that we need to put together this kind of cosmic growth history. Okay, and it involves actually looking at the spectrum of light. So, um, have you guys looked at, you know, prisms or even, you know, rainbows, that kind of thing? We got to look very carefully at the spectrum of light that we're observing.

[00:45:28]So, I've got this animation here of a prism. So it's just taking some white light in here and it's just splitting the light into all the different colors.

[00:45:38]And essentially what we do on our telescopes is we put a kind of similar kind of device like this on our telescope. So it's a device which splits the light into all the different colors.

[00:45:48]So instead of just being like a camera where you get a few different colors, we get a very exact pattern of what the colors what the pattern of light is that we're observing.

[00:45:58]Very often when we observe distant galaxies and we look at the spectrum of light, we don't get a whole rainbow of light, what we actually get is a pattern of lines like this. So we only get light in very distinct wavelengths in very distinct colors.

[00:46:17]And the reason is that the light is actually emitted from the atoms in the galaxy. And in atoms, atoms can only release light at very specific wavelengths. So that comes from the structure of the atom. So atoms have very distinct structures. Light is only allowed to have certain transitions and that gives us these very specific patterns of light that we observe.

[00:46:45]Now we can study atoms very carefully in the lab and we can see what colors of light um they emit. And in a lab on Earth, we never see atoms emit patterns of light like this. It maybe might look something like this. Okay? So, it's the same kind of idea. You know, we've got two lines here and then a bit of a gap and then a bit of a long line and then this other red line like this. So, on Earth, if we look at very carefully at the pattern of light we get from atoms, we get something like this. And then if we look at light from a distant galaxy, we might get a pattern like this. Now we know that it's not physically possible for atoms to emit light in this kind of pattern. It must have been emitted like this and then stretched out by this red shift effects.

[00:47:32]So we can actually be quite conclusive and quite specific about exactly how much that light has got stretched out if we can look at the spectra of light from these galaxies.

[00:47:44]And this is the last piece of the puzzle that we need to build our cosmic growth history. So if we return to this timeline here, so remember we've got how long ago these galaxies uh are looking, when did these supernova explode? That's from the brightness of the supernova.

[00:48:04]We can now put on this next piece of the puzzle. So how much our size of our universe has changed by looking at the red shift of light. So, how much that light has got stretched out, that tells us how much the size of our universe has changed. And this is what it looks like when we combine those two pieces of information.

[00:48:25]So, we're looking over here.

[00:48:27]This is essentially where we're sitting out here and we're looking back at the whole history of our universe here. So, if we look up top there, we've got size of the universe relative to now. So, a size of one is the current size of our universe. So this isn't actually telling us in absolute terms how big our universe is. It's telling us how the size of our universe has changed.

[00:48:50]And then if we look back in time to something about what is this like 5 billion years something like that our universe was 34 of its current size.

[00:49:02]So at every point every single one of these supernovo gives us one of these amazing points. It tells us if you look back in time 5 billion years, how big was our universe relative to now? So we call this the expansion history of our universe. And it's something that you can map out with supernoving.

[00:49:25]What's really interesting is if I put a little straight line on here starting over here, this straight line corresponds to a universe steadily expanding. Okay, at a steady speed.

[00:49:40]It's just going along at a nice steady speed. So if you look even you know 7 billion years ago about half the age of the universe for a good few billion years the universe is just very happily expanding at a nice kind of steady rate getting bigger and bigger but can you guys see somewhere about three billion years ago something really interesting happens.

[00:50:01]Can you guys see the universe has started expanding faster and faster than just that nice steady straight line there? And it's actually getting faster and faster as we get to the present.

[00:50:16]>> Yeah, it was a big surprise. Big surprise. So before this discovery, um quite a lot of the best theories thought that maybe the universe was going to slow down expanding and that maybe it was going to start getting smaller and maybe the universe was going to end with what's called a a big crunch. So this is a really kind of big surprise that the universe is not just continuing to expand, but it's expanding faster and faster. Now, this might look a bit kind of esoteric, but this really is as strange as imagine if you had like a like a football or tennis ball or something like that. Imagine if you threw the tennis ball up. Okay, we know what's going to happen to the tennis ball. It's going to slow down. It's going to stop and it's going to come back down.

[00:50:55]Imagine how strange it would be if you threw the tennis ball up and it goes up and up and it just gets faster. And then your your tennis ball is gone. you got to get a new tennis ball and it's just flying up into space getting faster and faster the faster it's going away from you. You say, "Well, that was a crazy tennis ball. I'm kind of glad that's gone, but you know, I kind of need to get a new tennis ball." That's what's happening to every galaxy in the universe. Every galaxy in the universe is getting further and further away, and it's going away faster and faster. And it really is very, very strange.

[00:51:27]And we can only explain this if a big fraction of the universe, so something like 70%. Is full of this very mysterious stuff that we call dark energy. But we really don't understand at all what it might be. Okay. Um, and until maybe a year or two ago, it was looking like dark energy might be just the simplest, most standard explanation for what we think dark energy might be.

[00:51:53]But actually some very very exciting super recent hot off the press cosmology results looking very carefully at this growth history seem to suggest that maybe dark energy is accelerating the universe in an even stranger way than we thought. So there might be even more kind of interesting ingredients to that cosmic pie chart than we first thought.

[00:52:16]So this really is a very very uh big mystery. It's one of the biggest open questions in cosmology.

[00:52:25]what all of these pieces of the cosmic pie chart tell us. So the visible matter, the dark matter, and the dark energy, they've allowed us to build what's become known as somewhat of the kind of standard model of cosmology of what our universe looks like. And looks kind of something like this. So this is a bit of a diagram where we've got time going along over here. So there's our present day and this is looking all the way back to the big bang. Now, of course, we have three dimensions of space in this diagram. They're just showing you it with two. Okay? So, you can think of this as almost like slices through the universe. Starting at the big bang and then you can see the universe is expanding kind of nice and steadily and then about three billion years ago that expansion starts getting faster and faster. This is a kind of overview of most everything we think we know about our universe.

[00:53:18]So we think it started nearly 14 billion years ago and we now think you know there's this dark energy accelerating expansion and that most of the matter there is just this u invisible dark matter. This is a kind of current picture of the universe.

[00:53:38]Now, there's been some little refinements to the numbers there, but those big those big pieces of the pie chart, they really remain a bit of a mystery.

[00:53:50]So, we're kind of coming up for the hour and um just before we wrap things up, I'd just like to mention you guys um you know, I've always been fascinated with, you know, trying to understand space and time and this kind of thing. And this summer, I was so excited to publish my very first book on the subject. It's called First Steps in Spacetime. And I know some of you guys, you've already bought a copy, guys. Thank you so much for buying a copy. Honestly, it really does mean uh so much to see the kind of book kind of uh coming to life and people reading it. So, it is available in the um ship essential shop on uh deck 6. Still couple of copies left. Okay.

[00:54:30]So, if you're kind of really curious about this and maybe you'd like to find out a bit more about space and time, um I really hope you might um enjoy this book. Just to give you guys a bit of a heads up, this book is kind of somewhere between the idea is it kind of goes into sort of more in depth than like a popular science book, but going to be more accessible than like a university textbook. Um so, it's definitely not what people call like a popular science book. Now, I hope that doesn't mean that it's like an unpopular science book, okay?

[00:55:04]I just mean that it actually goes into more depth about the actual theory than you might find in a popular science book. So, there are actually, you know, the actual equations, we actually think about the actual theories it involves.

[00:55:14]But, you know, if you really are curious about what's called the theory of special relativity, uh this was kind of really one of the first theories to think about space and time as intrinsic things uh themselves, then I really hope you might enjoy And of course, we'll be more than happy to answer any questions you might have about the book um if you're uh reading it uh on the cruise.

[00:55:37]Just before we wrap things up um for this morning, just a quick announcement about the next talk coming up. Okay, so we're going to be back here on Thursday at uh 10:00 a.m. and we're going to be discussing a really big uh really fascinating topic. We're going to be focusing on the search for life in space.

[00:55:57]Now for a long time any talk about life in space and extraterrestrials it really was very much the preserve of kind of you know the slightly more kind of fringe kind kind of field of research.

[00:56:11]It certainly wasn't even the kind of thing that you'd have in, you know, university research departments.

[00:56:16]But in the past few years, the search for life has become really very serious and there's so many very exciting places in the cosmos where we think there might potentially be promising uh science where we might be able to fight life. So it really is a very exciting very authentic field of research and I am very excited to share some of the kind of latest results in this field uh with you guys. So I hope um you know if you're able to that you know you might be able to join us for that. That's actually going to be our last talk uh for this series. Okay. We're going to be thinking about uh life in space. Um now we're kind of coming up for the hour. So I think if people do have any questions I think maybe we'll let some let people go first and then you know we can come down and chat if you've got any questions at the front there. But for the timing I just want to say it's just such a joy to see you guys all coming here to you know share what we know about the uh the universe and I just love chatting with you guys you know on ship. Your questions just absolutely fantastic. I love, you know, thinking about all this kind of stuff and just going on this absolutely amazing adventure with you guys. I mean, it's just like it really is absolutely fantastic. So, I hope you guys have an absolutely wonderful and amazing time for the rest of your time on this uh expedition. I hope you enjoy all of the other amazing destinations that we have.

[00:57:41]If you have already bought the book, I really hope you enjoyed and thank you so much. So happy to talk about this. But for the time being, I just want to say guys, thank you all so much for coming.

[00:57:50]So, thank you.