The Supermassive Podcast - 57: Untangling the Cosmic Web

Episode Date: October 2, 2024

Izzie and Dr Becky are untangling the cosmic web - the large scale structure of the universe - with help from Dr Chiara Mingarelli from Yale University. What the heck is it? What do we know about it? And can we use gravitational waves to "see" it? Plus, Dr Robert Massey is on hand to answer your questions. Got a question for the team? Contact us on [e-mail] or find us on Instagram, @SupermassivePod. The Supermassive Podcast is a Boffin Media Production. The producers are Izzie Clarke and Richard Hollingham. 

The cosmic web is kind of like the skeleton of the universe. The clusteriest of clusters and the voidiest of voids is the things that come to mind, right? If you get things that challenge that, well, then we just rethink the physics. Hello and welcome to the Supermassive podcast from the Royal Astronomical Society with me, science journalist Izzy Clark and astrophysicist Dr. Becky Smethurst. This month we're untangling the cosmic web, not the James Webb Space Telescope, but the large scale structure of the universe. What do we know about it and what can it tell us?

Yeah, I just thought it was about time we got into something really complicated because we're just taking a bit too easy with all this planetary science and our tour of the solar system. What, is your head not hurt very recently, Lizzie, when you've been recording? No, I'm due another headache. So here we go. As always, Dr. Robert Massey, the Deputy Director of the Royal Astronomical Society is here. So, Robert, how would you describe the cosmic web? Because it might not be something that everyone is familiar with. So let's begin this brain stretch right now. Exactly. Brain stretch, headache, questions I struggle to answer. You know, everything that makes a good Supermassive episode.

The good questions. so look i mean if you take a kind of casual look at the sky a very quick glance up you think okay randomly distributed stars but even in our own galaxy you look a bit longer you see the band of the milky way so you can quite easily deduce there's some structure in the way the stars are distributed and if you look on a much bigger scale if you map galaxies on a huge scale and you do that in 3d then it turns out they're grouped into these huge clusters and those clusters are on this filaments of this giant web and there are these filaments and voids and you do that in 3D, then it turns out they're grouped into these huge clusters. And those clusters are on these filaments of this giant web. And there are these filaments and voids. And that's the sort of bubbly structure of the universe on the bigger scale. And we didn't know about it until the 1980s, because that was when we got better telescopes. And we were able

to measure those distances more reliably and push them out further than before. And those galaxies, the galaxies like the one we live in, they're concentrated into these huge clusters. They tend to be concentrated along the nodes or the knots and strands of the web. And they're around voids where there are far fewer of them. And these are really big. These are billions of light years across. So they're certainly the biggest structures in the whole universe. So understandably, there's a lot of interest in understanding things on the biggest scales. Yeah, I like to joke that, you know, instead of like turtles all the way down, it's just filaments all the way exactly filaments bubbles so yeah whether you describe it as i don't know soap or spongy or something like that yeah i've always looked at it as like

a big sponge right and i like how you were saying you know we didn't discover this to the 80s because we almost couldn't zoom out far enough to see it right cheers robert we'll catch up with you later in the show for some more questions and this month's stargazing tips. So buckle up, everyone. The mind stretching continues because we're going to dive into the world of cosmology. So far, we've sort of talked about the structure of the cosmic web, but you're about to hear a little bit more on that. Why is it important and what is it made of and where the heck has it come from? These are all questions that I put to Dr. Chiara Mingarelli from Yale University. The cosmic web is the name that we give to this structure that

we can see on very large scales, on the largest scales. So let's just start where we are right now and then zoom out. Okay. So we're on the earth. We're in the Milky Way galaxy. Next door is the Andromeda galaxy. But we're part of a galaxy cluster. And then if you zoom out of our galaxy cluster, far away, there's another galaxy cluster. And there's like these filamentary structures that can connect these galaxies. And then if you zoom out again, you can see even more galaxy clusters with more of these filamentary structures that connect them. So they kind of looks like a brain with all of these bundles of neurons that are sort of talking to each other. Now, I'm not saying that we can talk to each other through the cosmic web. Yeah, okay.

But we are all connected. And so it begs the question, like, why is that there? And how did that form? Okay, so we've got this big connection of galaxies galaxy cluster I can just picture this like we start as a little spot and we move out and we've got this I don't want to say a tangle but it's kind of like a tangles some people might want to think of it as like a net kind of structure that's exactly right so to picture this is it a physical structure? What is it made of? Do we know? Right. So it's kind of like the skeleton of the universe, right? If you were to take this kind

of skeleton and then paint on hydrogen gas, then you would see it. And so it's mostly made up of gas, like the filamentary structures are really largely made up of gas. But then the big nodes, you know, that are the things that are being connected, there's a lot of hydrogen, but then you have other heavier elements that help to make up galaxies. And that's all formed by the early stars. So the earliest stars were only made up of hydrogen, because that's like a primordial element that was there in the beginning, hydrogen and helium. And then as stars burn, they can create these heavier elements. And so everything that we have was created in these early burning stars and then in supernova explosions.

And then recently, we also know that some of the heaviest elements like gold and platinum were created by merging dead cores of stars called neutron stars. That's also really exciting. Where has this come from? Where does this begin? How do we even begin to unpack this? Yeah, that's a great question. So it all started about 380,000 years after the Big Bang.

There was this cosmic soup and the universe was so hot that light and particles that make up you and I, like protons and neutrons and, you know, things that we call baryonic material, which is just stuff that you can touch with your hands, right? Your desk, your watch, your headphones. All of this was in this primordial goop, right? Which included light. It was so hot that even light couldn't escape from this really hot soup. And inside of this soup, there were quantum fluctuations from the Big Bang that had been amplified by inflation in the early universe. Right after the Big Bang, about 10 seconds, the universe inflated to be an

enormous size compared to what it was. And so the small fluctuations that had happened right at the quantum level after the Big Bang then became huge, right? Because everything expanded in all directions. And so those tiny little fluctuations are now really big. And so some of them are troughs and dark matter can go into those troughs and it forms these places that then regular matter wants to go later. So when the universe can cool down and then light can finally escape from this cosmic soup, we get the cosmic microwave background that maybe some of the listeners are familiar with. It's at 2.7 degrees Kelvin.

But where I was going with this is that those fluctuations created these little nests for regular matter to fall into and to cool off. That's how we formed galaxies and galaxy clusters. These little cosmic potholes that were really tiny at the beginning of the universe and then grew to enormous sizes now are the homes of lots of regular baryonic matter, including and also dark matter that surrounds them. So it's like the dark matter is their house. Yeah. Right. And then all of the baryonic matter goes inside and then it forms all of these structures as it cools down. But it has to cool down because when it's hot, stuff is moving everywhere. But as it cools down, then it can like go to its neighbor's house, see what's going on, find out what's happening. It can get together in

little groups and then you can form galaxies and then galaxies can eventually merge and they get bigger. And that's how we think the universe works. In a nutshell. Yeah. Okay. Thank you. Just blow my mind. Thanks very much. So I think this is a really interesting idea. So we're saying we've got these pockets of matter, essentially, and that's where we have our galaxies forming and clustering together. And so are we saying, like, matter can travel between these webs from, say, one little cluster of galaxies, say, over here in the left to, say, another one over here on the right?

You know, obviously we're talking about astronomically massive scales, but they are connected. They can, what, matter flows through them? Is it correct to say that? Well, it's hard to say if it's flowing or not, because in order to see any kind of flow, we would have to observe it moving. Okay. And these distances are so vast, that it's kind of like trying to watch a turtle go 100 kilometers. And you're wondering like, can the turtle actually travel that far? And then if you just watch the turtle for a second, you're like, this turtle doesn't move, but if you give it enough time, it's going to move and it can go a hundred kilometers, right? I'm Canadian. And so I tend to use the metric system. Hey, we're all fine with that. That's fine.

Okay, good. So it's really a timescale issue. We certainly know that we can see the filaments, which means that there's at least hydrogen gas in those filaments. But you know, which way it's flowing and if it's flowing or if it's just there, it's kind of hard to say. Our universe is expanding. So what does that mean for the cosmic web? And how do we know how that interaction plays out? Or how does that interaction play out if you've got galaxies merging as well? Right.

That's a great question. So each one of these nodes has galaxies in it, and they're all gravitationally bound, these galactic superclusters. And so the force of gravity inside of those is stronger than the acceleration of the universe. But what that means is that eventually we're going to get really far away from the other superclusters. And so like these filaments are going to get thinner and thinner and thinner as we move further away. Right. So imagine like a piece of toffee getting, you know, thinner and thinner and thinner as we move further away. So imagine like a piece of toffee getting thinner and thinner and thinner as it goes. And eventually there won't be anything left. We can already see some of

these super clusters accelerating away from us. And so in the future, if we as a species can survive another 100 million billion years, what we see today is going to be completely different from what our descendants will see, right? We might be very much alone in a little island in our little super cluster because the other ones will have accelerated away. So does this mean that as our universe expands, the formations of galaxies, does that slow down? You know, they're not going to have that, I want to say like swap of hydrogen to help continue to build them. Is that what we're talking about here?

Well, so what we're talking about is, you know, competing forces. So we know, for example, like the universe is expanding right now, but we're humans and we're here because the forces that keeps our cells together is stronger than the force of gravity that's expanding everywhere. And so similarly, in our super cluster right now, the force of gravity and all of the dark matter that's holding us together is stronger than the expansion of the universe. It's making everything else fly out. But eventually, what will happen is something called the heat death of the universe.

And that means that eventually, the galaxies will all merge. They're supermassive. Black holes will merge. All of the stars will eventually burn out and we'll have just a black hole universe, right, where everything is in some sort of black hole. And then the black holes start to evaporate. And that takes a really long time. They emit something called

Hawking radiation. But all that means is that they eventually lose all of their mass to radiation. And the universe just becomes this like cold, dead place. Okay. I mean, it sounds lovely, doesn't it? Yes. Yes. Exactly. How much can we actually know about the cosmic web? Because obviously, a big part of this is dark matter. So can we study it? Or does that depend on being able to detect dark matter, which in itself is a whole other podcast episode? Yeah, we can study it in different ways. We can study the cosmic web by looking at large surveys of galaxies. We have really big telescopes that look at the sky, and then we can map the sky and we can see these filamentary structures emerging. So we can actually see the cosmic web. We can also make large computer simulations like Illustrious. And if you look up Illustrious

on the internet, you can find lots of beautiful animations that show you where the dark matter has to be, where the regular matter is. And then what's really amazing is that you can match the two, right? That these simulations have to match what we actually see with our telescopes in order to be credible. So one of the fun things you can do with the simulations is that you can turn off dark matter. And then what you see is that like you can't make a universe, right? There's no scaffolding. There's no skeleton for the matter to like clump on and cool down on. There's no like little house that all the baryons can go into and chill out right like they're just kind of floating everywhere and it's really difficult to create any kind of large-scale structure as we call it which is this this cosmic web oh i really enjoyed speaking with kiara yeah she's

great yeah yeah it's so good we've got part two of an interview with her coming up in just a moment because there was literally too much to cover turns out covering the biggest thing in the entire universe can't do it in 10 minutes it's quite difficult yeah so becky some follow-up questions to that how important is dark matter for this web light structure hugely i mean it literally wouldn't exist without it so dark matter i think it's often referred to as like the scaffold of the cosmic web, right? It holds galaxies together and it holds together clusters of galaxies as well. And therefore, if you keep going on that, it holds together the whole structure of the web. So without it, like in just a few billion years, those structures would completely disperse, right? And it just wouldn't be held together anymore. And

that's because gravity just wouldn't be strong enough to hold it together against all the random motions that these galaxies have in different directions right like we talk about sort of like red shift when we look out into the universe right and all the galaxies appear to be moving away from us but that's like an overall global thing that's going on or universal thing that's going on but like think about how andromeda is actually coming towards us towards the milky way because in our little local group everything's got random motions with respect to each other yeah and so it's the dark matter that sort of pervades this entire structure and sort of connects galaxies along these filaments that holds everything together and it's just without it

would be like a whole house of cards that falls apart this is one of the actual like big pieces of evidence we have for dark matter is that we can't get the universe to exist to look like it does without it yeah absolutely and so is there a repeatable pattern within the web itself like can we see that or is it just random we don't think it repeats no so there are recurring like similar structures like filaments walls voids that robert was talking about before they show up everywhere and in every direction we look but we don't see like the same patterns of those structures repeating no it's not like some sort of weird fractal or anything like that so when we go out to large enough scales beyond around 300 million

light years across the universe starts to look what we call homogeneous so it looks the same in all directions on a large scale but like if you zoom into those smaller scales it's still very different in terms of like a pattern yeah okay and are there any standout features within the cosmic web is that something that cosmologists would study? Yeah, definitely. The clusteriest of clusters and the voidiest of voids is the things that come to mind, right? The biggest structures, so like the Booty's Void is like known as a super void. It's so big. It's 60 megaparsecs across or something so 330 million light years across that's about the size when we start to you know things start to become homogeneous and yet it's that size like it's just incredible and then you've got huge superclusters as well like the laniakea supercluster for example that's 500 million light years across so that's you know

this huge thing and that actually includes like our local group of galaxies like the milky way as well we're technically part of this laniakea super cluster you've got the virgo super cluster as well as a famous one there's these huge huge structures in the universe and what's really interesting about them is when we find these structures like there was recently i think we even talked about in a previous podcast right like the big ring and the big arc and huge mega structures that people find is how do they get that big in the first place? But when we find these incredible big structures, we want to try and explain them. And sometimes they're so big that actually our cosmology and our model of cosmology can't actually do that yet.

And so we have to think, okay, are we missing something in our best model of the universe? Is there some law of physics that needs tweaking? Or is it just that these are sort of like the far, far reaches of like the sizes that you're going to get in terms of galaxies? You're right on the tail of the distribution. So how can we study the cosmic web? Well, first up, we can study it and create maps of all the galaxies that we see thanks to the telescopes and observatories that we have. We can also observe it at different wavelengths and specifically look for the hydrogen gas that connects all of these galaxies together. There's also computer simulations too, but more recently, we can study the cosmic web with gravitational waves. Now, this is an

early application of using gravitational waves to actually observe the universe, but it's something that Dr. Chiara Mangarelli from Yale University specializes in. The field of gravitational waves is simultaneously very new and very old. Einstein thought of gravitational waves when he was writing down his theory of general relativity, which relates the curvature of space-time to matter, and that gravity is actually this curvature of space-time. Then the idea of waves of space-time came out, and then he thought, well, these are so small, we'll never be able to detect them. And in fact, it took 100 years from Einstein's prediction of gravitational waves to the first detection of gravitational waves by LIGO. And so that was announced in 2016.

We now have engineering technologies that can make these measurements, but these measurements were predicted a while ago. And so what I study are gravitational waves from supermassive black holes. And there's now evidence for these gravitational waves as of last year, right? So this is now a brand new field. And the experiment that I work with is not LIGO, but it's called Nanograv. Okay, so what's that about? What's Nanograv about? So Nanograv is about studying gravitational waves that are light years long. So LIGO found gravitational waves from black holes that were maybe 30 times the mass of the sun. And now it's found about a hundred of those signals that have been published.

And those signals last a fraction of a second. Now with gravitational waves that are light years long, that come from supermassive black holes that are a billion times the mass of the sun, you can't use anything on earth. It's not big enough. And so we turned the whole galaxy into a gravitational wave detector to look for these signals. So the reason that it's really important to study these very low frequency gravitational waves or these very long wavelengths of gravitational waves is that it gives us a completely different way of observing the universe. So far, we are really good at observing the universe in light and gravitational waves are not light. It's ripples in the fabric of space-time itself. And so things like merging supermassive

black holes create very clean signals. If you were looking at these galaxies with telescopes, then you can only get so far before you hit gas and dust and things that obscure your view, or millions or billions of stars, and you just can't see through that. But with gravitational waves, they don't care about any of that. They are the space-time of fabric itself that's moving. They just plow through everything. And those ripples give us a really clean signal. We can basically hear the universe as well as see it, right? It's like we're unmuting the universe and we're hearing all sorts of really interesting stuff. How does that work?

That's a great question. And I love talking about it. So thank you for indulging me. There's an astronomer called Jocelyn Bell Burnell, who when she was a PhD student at Cambridge discovered pulsars. We know her well, we've had her on the show. So yeah. And we're so grateful to her because she revealed pulsars. Now, some pulsars are really stable clocks. And these are called the millisecond pulsars. And they were found about 20 years after Jocelyn found pulsars in general.

So there's a specific subclass that are almost perfect clocks. They can keep time to better than 100 nanoseconds over a decade. So gravitational waves change the distances between objects. And so if you and I were sitting across the room from each other and a gravitational wave came down from the ceiling and into the floor, we would get closer together and then further away from each other without us feeling like we're moving. We're not moving, but the space-time between us is changing. And so if you think about that happening in outer space and in our galaxy,

these pulsars with their perfect clocks come closer to us, and then their signals arrive early, and then further away from us, and then their signals arrive late. And so they're doing this bobbing, like they're on the surface of the ocean. If you imagine a combination of a buoy that's also a lighthouse, like a buoy with a spinning signal on it, right? That's bobbing up and down. And you can measure that. Then you can figure out what the waves are doing underneath it, right? So we use those to look for the gravitational waves that are transiting through our galaxy that come from supermassive black holes. And we need something like that that's stable over really long timescales, because the gravitational waves that we're looking for have light years long wavelengths. So we need a clock that'll be good for light years that's in outer space. And millisecond

pulsars are these perfect clocks. That's amazing. And so how do these millisecond clocks and measuring that with gravitational waves then loop back into understanding the cosmic web? The cosmic web has all of these galaxies in it. And when galaxies merge, their supermassive black holes should merge. And when supermassive black holes merge, they create gravitational waves. Now, gravitational waves have a lot of properties that are similar to light waves. They can interfere with each other. And so what we found last year as Nanograv, and also the European Pulsar Timing Array, and an Australian and Indian colleagues as well, we found evidence for a gravitational

wave background. And so a background is kind of like the cosmic microwave background. We have the superposition of all of these signals, and that signal had an amplitude, and it changed as a function of frequency or wavelength that we had already predicted. So what we found was this cosmic signature of the cosmic merger history of all of the supermassive black holes. And this was a huge discovery because before then, we didn't even know that supermassive black holes. And this was a huge discovery because before then we didn't even know that supermassive black holes merged at all. But now we do know that they merge, at least we strongly suspect that they do. And so this gravitational wave background,

you know, is similar to the CMB or the cosmic microwave background radiation in a lot of ways, that the first thing that we did is find the overall signal, right? Now we can measure the structure that's in the cosmic microwave background to one part in 100,000, right? We also think that there should be some sort of fluctuation in the gravitational wave background, and that there could be structure within it. And that structure has two different flavors. And so this is a new result that my research group is working on. So one of those fluctuations is well known, something that I led a team to discover back in 2013, that nearby supermassive black holes are going to create

areas that have more gravitational wave power than others. And so start to make these potholes and pockmarks in the map of the gravitational wave background because you have these screaming loud signals. So that's one form of something called anisotropy, which basically means one side of the map looks different from the other side of the map. So what are the differences? Okay. So that's one way that you can get it. Another way, and this is coming back to the large-scale structure, is to look at very low frequencies. And if we look at very low frequencies where there's millions of merging supermassive black holes, well, we know from electromagnetic observations, from observations of regular light, that there is large-scale structure. There's this cosmic web.

And so shouldn't the supermassive black holes also follow this cosmic web structure? And the answer is yes, but it's going to be really hard to find because these nearby supermassive black holes are going to be screaming loud. And so we'll need to find a way to get rid of all of these pesky, nearby screaming supermassive black holes so that we can find this very low frequency hum, like an om at the end of yoga class, coming from this cosmic web structure of supermassive black holes. And so how important is that hum? What can it tell us? So that hum should follow the large scale structure. It should have a signal that maps on to these filamentary structures and to the nodes. So we should be able to predict where to find louder

hums and weaker hums. And in fact, we can use what we know about these maps of the universe to try to predict where there should be more humming and less humming to try to find that signal in the data. And so does that correspond with like these higher points of, you know, you've got more clusters that if there's a loud hum, that's where your galaxy clusters are going to be. And then where it's quieter is not much. Exactly. That's perfect. You've got, you totally got it. You nailed it. Amazing.

Gold star. Black hole. Whew. More mind stretching stuff. You hole. Whew, more mind-stretching stuff. You wanted the headache, Izzy. I'm going to go and get the paracetamol now. Thank you so much to Dr. Chiara Mingarelli from Yale University. This is the Supermassive podcast

from the Royal Astronomical Society with me, astrophysicist Dr Becky Smethurst, and science journalist Izzy Clark. This month we're attempting to untangle the science of the cosmic web. Are you two ready for the questions? Just about. Ready as ever. I could say I was born ready, but that would be exactly what it's going to be.

Okay, Becky, so David Walton says, Firstly, thank you so much for your excellent podcast they always brighten up my sky when they appear you are all stars in my eyes oh that's nice my question is given the exponential expansion of the universe does that mean that the filaments of the cosmic web will gradually thin out and disappear so this is something Kiara's already covered and I think we can all agree that the answer to that part is yes so the second half of david's question is if that is the case is it possible to calculate how long the process will take very best wishes to you all and keep up the excellent podcast hi david first of all great great question now as we know and as kiara said yes okay this is going to happen the the cosmic

web will gradually thin out and disappear due to the expansion of the universe now we know, and as Chiara said, yes, okay, this is going to happen. The cosmic web will gradually thin out and disappear due to the expansion of the universe. Now we know the rate of expansion of the universe. We know the rate it is accelerating at, but extrapolating forward is a little bit difficult because, you know, there are different models for what the expansion is going to do. So that does affect things slightly. However, the timescales involved involved are really really quite long so i think in the grand scheme of things i can give you like an an earmark figure for what would happen we think in around about 100 billion years because of the accelerated expansion and bear in mind you

know the universe currently has 13.7 billion years this is far in the future in terms of its sort of history that any galaxy that's not bound to our local group so Milky Way and Andromeda they'll have moved so far away that we actually won't be able to see them beyond the observable universe so the structure will still exist but it'll be so thin that we won't even be able to see it it's only in a trillion years time that structures like within clusters and the web itself will actually begin to be affected right so it will be stretched out and then over trillions to tens of trillions hundreds of trillions of years we think is when eventually that gravitationally bound sort of nature of

clusters will actually be overtaken by the universe's expansion and i think that just puts it into perspective like okay yes the universe's expansion. And I think that just puts it into perspective, like, okay, yes, the universe is expanding at an incredible rate, but, you know, gravity does its job really well in holding things together, thanks to, you know, dark matter, as we talked about before.

You know, and this is why, you know, when people ask, why isn't the space between stars expanding in the Milky Way? It's because, well, gravity is stronger on those smaller scales to hold everything together. So like, you know, the space between stars isn't getting bigger in the Milky Way, it's because, well, gravity is stronger on those smaller scales to hold everything together. So like, you know, the space between stars isn't getting bigger in the Milky Way because everything's bound by gravity. And so over a much larger scale is when sort of the acceleration of the universe starts to take hold, but still gravity locally is the strongest thing until trillions to tens of trillions of years time when eventually that is overcome.

At least we think based on our current models of what's happening in terms of the expansion until trillions to tens of trillions of years time when eventually that is overcome. At least, we think, based on our current models of what's happening in terms of the expansion rate of the universe. Okay, thanks, Becky. I hope that answers your question, David. And Robert, Adrian111 asks, can Euclid help with mapping the cosmic web? Will it make a dark matter and dark energy map? So Euclid is this telescope

that we've all been talking about recently so excited yeah it's great release next year so okay good stuff but back to adrian's question yeah it's good it's good stuff adrian one more one it's a good question to ask the answer is yes because euclid is designed to map galaxies out 10 billion light years away across a third of the sky so looking back a long way into the past in the universe as well so it'll definitely help us make a map of the web because the whole objective is to make a 3d map of a chunk of the universe so not the whole universe that we can see by any means but quite a big bit of it and when you look at things like phenomena like gravitational lensing and the bending of light by gravity

that's a way of mapping dark matter and understanding exactly where it is, because you see this lensing and you know, if you don't associate it with visible matter, or even implied visible matter, normal matter, as it's called, then you know there's dark matter there. Now, dark energy is sort of more pervasive and uniform, and we have really, it's fair to say, not a good idea of what it is still, even compared with dark matter, where there are at least more candidates. So mapping it is a bit of a challenge but by getting the spectra of the galaxy so looking what you when i talk about a spectrum you think about a rainbow think about the light being dispersed across colors and then think about that happening in radio and x-ray and as well but for this this in this case optical infrared and infrared so i'm sorry infrared

we can use that to get infrared please yeah no, I do need to be corrected on these things. Mostly infrared, the UK, yeah. But we can work out how fast galaxies are moving through redshift, basically by seeing how the lines in the spectrum are shifted, and then understand the expansion of the universe, and then we can deduce how much of an effect dark energy is having. I mean, we have a good idea overall, but just verifying that, thinking about all that stuff.

So Euclid is making a big contribution to that too so the answer to your question is yeah we'll get better maps as a result including dark matter yeah the dark energy one's so exciting as well the idea that we can like trace the expansion rate of the universe by looking back and further where the distance is is it's just so cool i'm so excited and someday we'll know what it is right well fingers crossed you know come on yeah oh, cosmologists. Come on, yeah. Oh, come on, Becky. You need to change fields. You need a Nobel Prize, surely, you know.

Yeah. No pressure. Yeah, no pressure. Okay, and Becky, there's not a name on this one, but they've asked, how sure are we about the structure of the cosmic web? How precise can we be, especially in far areas? Yeah, that's a pretty good question.

So, I mean mean we're pretty sure about the structure because many surveys have looked at this and all seen the same thing and obviously with newer telescopes new observatories we're pushing to higher red shifts or greater distances away from us all the time like with euclid as we just talked about but as we do that obviously we're only seeing the brightest of galaxies we're not seeing the faintest of galaxies when we go to those huge distances you know even with something like james webb right like you're still not going to see the faintest of things even though it's got this incredible sensitivity and light collecting power we know there's some we're still missing however the brightest galaxies still

do trace that overall structure right and as i as I said before, 300 million light years is when things start to look homogeneous and the same in all directions and have that overarching structure of the filaments and the clusters and the voids and the knots. So we know that it's sort of the same everywhere else we look. Now, to put context on that number

of 300 million light years, Andromeda is 2.5 million light years away. The edge of the observable universe is 13.7 billion light years andromeda is 2.5 million light years away the edge of the observable universe is 13.7 billion light years away so 300 million light years is pretty close you know in the grand scheme of things right so that's why we're pretty sure that the cosmic web is a structure that pervades at least the entire observable universe anyway okay yeah well i was about to go into the unobservable universe but that no we don't we don't have time for that okay robert gashept wants to know how do you test the models of the cosmic web against reality when reality provides a relative snapshot yeah gashept

a good and deep question there i think think it's fair to say. So thank you for asking that. I mean, look, to a certain extent, the answer is it's sort of the best we can do. We have the universe we can see, and that includes the universe going back into the past because of the fact that things are so far away that the signal takes so long to reach us. And so we can look at those distant objects and test those models, and we can try and trace the evolution of the universe over time. And that includes the filaments and voids of the cosmic web. And you're quite right that this is limited, that we have to make assumptions in this. But it seems to be okay.

We think we understand, even though we can't actually detect it very easily, we understand the physics of even the first few seconds of the universe after the Big Bang until the present day and how things will end up in the future but we need to use as many different techniques as possible to test those ideas you know and obviously the more that come back in a consistent way with what you understand already that's great and on the other hand if you get things that challenge that well then we just rethink the physics and think about what's going on so yeah i mean it's it is a snapshot that's all we can do we don't have any means of traveling forward in time for real or traveling backward in time for real but we do at

least we're at least we're able to see the history of the universe unfold and that really helps as well happens in the future well that's why we need to understand things like dark energy and how that might or might not change over time i always think that's why people who do simulations like my colleagues are always slightly smug when they press play. You know, it's like, oh, we could press play. We could see some evolving. And also, if it's running into the future, who's going to be around to check, right?

Exactly. I think, Robert, you literally just said my favorite throwaway comment. Oh, we'll just rethink the physics. Yeah, okay, fine. That's, you know, that's pretty much it, right? No biggie. We'll get on it exactly okay well if you're listening to this and you want to send in any questions then please

do you can email podcast at ras.ac.uk or find us on instagram at supermassivepod okay so shall we finish up with some stargazing? Robert, what can we see in the night sky this month? So we've edged past the equinox on the 22nd of September. So we are looking now, you know, we're in the autumn. It's chilly. The nights are drawing in a bit. Officially. Officially, exactly.

This is the hill I will die on. Officially. The autumn doesn't start until the equinox. All those people who are like, autumn starts on the 1st of September. I'm like, sit down. It's still summer until the 22nd of September. We've got to hold on. In the UK particularly, we need to hold on to, in the UK particularly,

we need to hold on to the idea of summer. But yeah, if you're in the Southern Hemisphere, obviously looking forward to warmer days. But I mean, where we are, at least it means you don't have to stay out quite so late to see the stars, which is a big plus. And so in the sky, you've still got, you know, the wonderful Milky Way in the summer

and basically autumn triangle of Vega, Denim and Altair. And that's the time when you can see now constellations like Pegasus, the winged horse, the big square and Andromeda with the Andromeda galaxy in it, which you can even see with your eye if you're in a dark enough sight. And if you pick up a pair of binoculars or a small telescope, you see this elongated haze. It doesn't look like the pictures on the whole. It's really difficult to see, but it is nice to know you're seeing something two and a half million light years away. Yeah. I always think my eyes are going when i i'm in a dark enough sky and i'm like there's something there's something weird yeah wow that's the focus on and what's

wrong with my eye yeah and later on you've got you know they're groups like perseus and that wonderful double cluster but one thing we are waiting for um at the time of recording is comet sushin chan atlas which was discovered last year, January, both by Purple Mountain Observatory and the Atlas Robotic Telescope. And it might be visible with the unaided eye with the naked eye in early October. It really depends on whether it's done well

as it goes around the sun, as it makes its closest passage, perihelion, or whether it broke apart, because a lot of comets do that. But if it has survived, it might be a good naked eye object, and certainly something that would be easy to pick out in binoculars after the sun sets. There is the hypothetical possibility that if it's really bright, and I'm not holding my breath for this, but if that happened, you do sometimes see as

well that you get tails stretching up from the horizon, even if the comet's nucleus, the coma of the comet, the head of the comet is below it. So that's another intriguing possibility. But my suspicion is it won't be quite that good. But it might be something people pick out in astrophotos. Those telltale two tails of a comet. Oh, yeah. I mean, wouldn't it be amazing? You know, and well, sometimes some comets, I mean, there are records of ones with even weirdly six tails and all that kind of thing. I think that's a bit, you know, it's probably not going to happen like that, but it is something to look out for. I think that's a bit, you know, it's probably not going to happen like that, but it is something to look out for. And all I can suggest is that when we get into October, keep an eye, problematic though it might be, on social media channels.

See what people are saying to get that kind of alert of when it's visible. I'll certainly comment on it if I see it. Yeah, just point your phone towards the eastern sky before sunrise and just see what your phone picks up. And then maybe you'll be able to spot it more easily and just see if it's there and yeah you can see it from where you are that's what i usually do and from what i can tell that i mean there's there's a chance singular sunrise but i think the best prospect is after sunset in the october's go but we'll see you know who knows i mean just keep an eye on what people say but you're right becky i mean your eye is some your camera rather on your phone is can be so much better at picking these things out and for those of us that you know went out and saw comet neo wise a few years back how are we hoping that this might compare because you could

see neowise neowise was easy wasn't it yeah but you but it was straightforward and you could go out and say there's a comet honestly don't know and it's it's really hard to say i personally don't think it'll be quite like that because i think it's close to the sun in the sky and so you're going to see it against a bright sky it's going to be more challenging but who knows you know the perennial comment is comets and becky will like this are a bit like cats in that they have tails and they do what they want so you know yeah i think it is predicted to be brighter than neo-wise but as you say because it's next to the bright sky yeah it won't look as like i don't think it'll look as cool because it won't be next the dark sky like neowise was yeah fine yeah neowise was just the highlight of 2020 take me back well actually

no don't take me back but it so depends on what happens as it goes around the sun you know who knows but we'll see keep an eye out and the other thing to mention of course in october saturn is still good it's pretty much at its best for the year it's a bit thin ringed so slightly disappointing slightly disappointing. And that's if you'd like rings anyway, then it's going to, you know, as we've said before, those will disappear next year. And there's also a sort of nice low level background of meteors. There's modest meteor showers like the Reconids and the Southern Taurids. They're not huge numbers, but maybe they add up to a few more meteors than normal. So I'm not suggesting, you know, going out night after night looking for them. But if you you happen to be out you might stand a bit more chance of seeing them and it's just something to enjoy i mean who doesn't like seeing a shooting star right this might sound like a

really stupid question but no such thing well i'm gonna i've got a question about saturn why is it that it's we're going to stop seeing its rings like this is something i can't quite wrap my head around in terms of how we picture it and how it's going to be over the next few months. Can you explain that a bit more? It's the angle of the planet. So what you have to imagine is that the orbits are tilted with respect to each other.

Essentially, you imagine the Earth's orbit and Saturn's orbit are inclined with respect to each other. So there will be periods of time when Saturn goes around the sun, when it's lined up edge on with us. imagine sometimes it's like north sometimes south and at those points when it's further north and south the rings are wide open but it does go through this point when it's lined up precisely with us and that's when the rings are edge on so yeah it's just like being able to see a bit above and a bit below it. Yeah, you know, those spinning tops that sort of widen in the middle. You know, you see them sort of like kids toys and stuff like and you see them wobbling around.

Right. Essentially, what we're seeing is Saturn wobble as it orbits the sun. And that means that sometimes the rings wobble away from us. So we see their underside and wobble towards us so that we see their top side. And then sometimes they're just like perfectly edge on. Yeah, it's kind of like, you how like earth's axis right is tilted and so for you know half the year the north pole just doesn't see the sun yeah we're getting into that part of the air because earth will wobble away from it so if you think of it like that like the earth's equator like extends out from itself with some rings you can imagine sort of like again the same thing as this is wobbling around of the rings and it only

happens twice every 30 years so every 15 years sat Saturn takes about 30 years to go around the sun. Then we see that kind of view. So after next year, it'll open up nicely. And it'll also be much better for the northern hemisphere as well. It's been quite low in the sky for us for many years now. So as we go into the second half of the decade, it'll be really, really good for us. So that is something to look forward to, wide open rings and high in the sky as well that's that's the follow-up to lord of the rings isn't it well thank you for clearing that up because it's something i've been thinking about a lot

since our last recording i was like but it's hard to picture right it's one of those things where you're like you're trying to picture what's going on your head and it requires a lot of 3d spatial okay and with that i think that's it for this episode. We'll be back soon with another bonus episode. And then after that, we're exploring strange stars. And I'm taking that to be quite loosely, whatever bizarre, weird stars we want it to be. Brill, there's a lot of those in the universes

that you will not be able to find. We've got quite the selection. Yes. Of course, contact us if you try some astronomy at home. It's at supermassivepod on Instagram, or you can email your questions to podcast.ris.ac.uk, and we'll try and cover them in a future episode. But until then, everybody, happy stargazing.