The Supermassive Podcast - 43: Most of the Universe is missing...

Episode Date: July 28, 2023

The Supermassive Team have spent the last three years attempting to talk about ALL the stuff in space. But that only makes up 5% of our universe. Where the heck is the rest?  This month, Izzie and Dr Becky dive into the topic of Dark Matter and Dark Energy... The stuff that makes up that missing 95% of the universe. With special thanks to Dr Alex Hall from the University of Edinburgh for covering the basics, and to Dr Stephen Wilkins from the University of Sussex for explaining how ESA's new space telescope Euclid will give us new insights to the field.  If you enjoy The Supermassive Podcast, please rate, review and follow us. Even better, why not vote for us at The British Podcast Awards?! https://www.britishpodcastawards.com/voting The Supermassive Podcast is a Boffin Media production by Izzie Clarke and Richard Hollingham. 

I always like to say that dark matter was one of the most begrudgingly agreed on theories in all of science. If you want to get direct evidence, you need to make some dark matter in the lab, I think. Dark energy, we know that it makes up roughly 70% of the universe. How do we know that? Hello and welcome to the super massive podcast from the Royal Astronomical Society with astrophysicist Dr Becky Smithers and me, science journalist Izzy Clark. Now for those keen beans who listen to the very end of every episode, we appreciate you by the way, you might have been expecting an episode on the sun, but there's been a little bit of a change of plan. but there's been a little bit of a change of plan. Yeah. So we have spent the last three years attempting to talk about all the stuff in space,

but in the grand scheme of things, all that stuff only makes up 5% of our universe. And quite frankly, that makes my head spin. So earlier in July, the European Space Agency launched a space telescope called Euclid, which is set off to give us important new insights into that remaining 95% of the universe that's missing, namely dark matter and dark energy. So that's given us a perfect opportunity to talk about all this stuff we've been just ignoring for the past three years. And so, you know, we just, we bumped the sun. Sorry, sun. Yeah, sorry. We'll get back to you at a later point but before we get on to that big old search for the rest of our universe can i just say thank you to who

say thank you i'm so excited can i just say thank you to everyone who has ever reviewed rated or followed this podcast recently it really helps us keep that shiny number one spot as the best astronomy podcast in the UK. And if you haven't done that yet, then please do because all of that good stuff really helps us keep making these episodes. Okay, that's my little rant done. Let's get on with it then. So obviously, Dr. Robert Massey, the Deputy Director of the Royal Astronomical Society, is here with us. So Robert, when did we realise that 95% of the universe is missing? It just sounds bonkers, doesn't it? Well, there were discussions about this early on. So even in the 19th century, Lord Kelvin, who was famous for things like temperature scale and a whole

bunch of stuff in physics, suggested dark stars, the idea that there might be some stars that weren't shining brightly enough to be seen. He had the idea of looking at the movement, the velocities of the stars that we could see and concluded there wasn't any evidence for this. But by the time you get to the 1930s, there are people starting to realise that something was going wrong, that there was actually matter that we couldn't see. A Swedish astronomer by the name of Knut Lundmark, who worked in Lund Observatory, published a paper in 1932 which suggested some dark matter. Fritz Zwicky made the key discovery. He used the Mount Wilson telescope in California. He looked at the Coma Cluster of galaxies. It's relatively nearby in the scheme of things, but still

millions and millions of light years away. And he estimated that there was dark matter, or what he described as dunkle materi, to be 400 times the visible mass of stars in that cluster. And I should also definitely mention Vera Rubin, who happened to get a gold medal from the Royal Astronomical Society back in 1994, only the second woman to do it, and that's a whole different story altogether. And she looked at the velocities of stars in galaxies themselves and found that explanation too, that the stars were moving too quickly and it could only be explained by dark matter. And that's why we get to the point where we now think it makes up a huge 26.5% of the universe. Then there's also dark matter, of course, and that was a dark energy, rather. That, of course, wasn't really a dark energy rather. That, of course,

wasn't really a thing for most of the 20th century. Einstein postulated this cosmological constant because he initially thought the universe was static and he needed something to balance the force of gravity. And then it went away when we discovered the expansion of the universe. But it re-emerged unexpectedly in 1998. Two teams looked at really distant explosions of stars, supernovae, and they were fainter than they expected. And so they concluded that the universe wasn't as everybody had thought until then, or most people had thought, slowing down in its expansion, but actually accelerating. And they rightly won the Nobel Prize for that discovery. And that's where we've been for the last 25 years. You know, we have this two thirds of the content of the universe that we don't really know what

it actually is. And my favorite thing about all this is that everyone always complains about the names. And I'm like, well, it's because we didn't understand what it was at the time, you know? So, I mean, you mentioned Zwicky's Dunkelmaterie. I'm going to pronounce it completely wrong. But I know that one of the phrases that was used was matière obscurée in the French as well, like obscured matter. And I feel like almost it's better than dark because dark, you know, people get confused as to what that necessarily means. And then obviously we just inherited the dark for dark energy. And dark energy doesn't really mean anything. It's just the name we give to the thing causing the accelerated expansion of the universe. But I love how you said, Robert, that like, you know, Vera Rubin, you know, observed the

velocities of stars and then the only thing it could be was dark matter. I feel like the only thing it could be, that part of your sentence probably took about 10 to 20 years for everyone to actually agree that that was the only thing it could be. That old thing. We tend to skim over the scientific process. Yeah. I always like to say that dark matter was one of the most begrudgingly agreed on theories in all of science. Cheers, Robert. Anyway, we'll catch up with you later in the show to take on our listener questions.

So it's quite convenient when almost all the astronomers and astrophysicists in the UK are in the same place at the same time. Earlier this month, the National Astronomy Meeting was held in Cardiff. So Becky went along to do some work, but also to corner some of those experts to tell us more about dark matter and dark energy. One of them was Dr. Alex Hall, a research fellow at the University of Edinburgh. And I loved this first question from you, Becky. at the University of Edinburgh. And I loved this first question from you, Becky. All right, Alex, let's start off with the big one because 95% of the universe is missing to us with this.

Does it bother you as much as it bothers me? It hugely bothers me. And it's one of the reasons that we're doing these amazing experiments to try to learn more because it's one of the most challenging problems in all of physics right yeah like the biggest problem in all the physics i'd say to be honest i mean it's almost embarrassing for us let's put it that way so let's let's break this down because we're talking about here about dark matter and dark energy so let's start with dark matter can you explain to people what dark matter is? So dark matter is this mysterious substance that seems to be filling the universe.

It's lurking in the darkness. It's this invisible stuff. We can't see it, but we can detect its gravitational effects. So that's how we know it's there. But we don't really know what it is. We just know that it's sort of lurking there. It provides the skeleton, the scaffolding on which galaxies like to like to reside and like to form. Yeah so I mean I mean how long have we known this

well how have we known that it's there but we can't see it and how how do you find evidence for something that you can't see? We think it's been there for, well, the idea of Darmada has been around for, I think, almost 100 years now. And it was originally thought up because we need to have some extra gravitational forces to explain the motions of galaxies. And since then, there's been a lot of other astronomical observations, Since then, there's been a lot of other astronomical observations, like very wide range of astronomical situations that we see that seem to require extra gravity than can be explained by what we see. So we infer its position indirectly. Yeah, and this is all based on the assumption that Einstein was right, right?

That general relativity is right, and there's more mass there than general relativity can explain. Yes, that's correct, because we're learning about it through its gravitational effects so we need to know about gravity and how the force of gravity works right. There are people who think that dark matter may be a manifestation of the wrong theory of gravity perhaps general, perhaps Einstein's theory is not correct. But most of the evidence suggests that actually gravity is working as expected, but there is this additional substance that is required to explain the observations. And I've got a question for you. Who do you think the smoking gun is going to come from?

Astrophysicists or particle physicists when it comes to dark matter and almost like proving that it is a real thing like you said we have indirect evidence for it right like who's who's the direct evidence going to come from so if you want to get direct evidence you need to really actually make some dark matter in the lab i think so i think particle physicists have that advantage. But what we can do as astronomers is we can look right back into the past of the universe, and we can see how dark matter has been changing with time,

how it's been clumping. We can learn a lot about the nature of the dark matter just through astronomy. But particle physicists, I'm hoping that particle physicists will make some in the lab one day, we can actually like see it on a screen. Yeah, I like how you said particle physicists have the advantage. I'd probably say it was a disadvantage because we're all sat here waiting on them like, come on, you've taken ages to figure it out. So that's, I guess that's dark matter done. So let's talk about dark energy now. Can you explain

what actually is dark energy? So dark energy is really the name that we give to whatever is causing the accelerating expansion of the universe. So the universe has been expanding since the Big Bang, but about seven or eight billion years ago, it started to accelerate in that expansion, right? So the foot's on the gas, it's starting to get faster and faster we don't understand why so dark energy is the substance that we've invented to explain that but we know even less about dark energy than we do about dark matter to be quite frank and you said substance there like is substance like the best description of it like what are what are some ideas to explain

what could be actually putting the foot on the gas as you said so it could be some new kind of field perhaps some new kind of matter that's never been seen before in the lab it would have to have very unusual properties but there are ideas out there about certain kinds of matter that could explain it going back to this idea of gravity being wrong that's actually a very active area of research this idea that yeah general relativity could be wrong on the largest scales so perhaps this acceleration is just a result of using this incorrect theory so could be could be that could be the word matter could be something else it's really not but well understood i feel like this is one really for the chalk users

as i call them, the theoretical physicists who are stuck at their blackboards. And you said that we know that the universe started to accelerate, expand seven, eight billion years or so ago. How do we know that? How do we actually observe that in the universe? Yeah, so we can look back to this period. We can look back into the history of the universe by looking at very distant galaxies, distant cosmic explosions, stars blowing up. And we can go back in time, essentially, because the light's taken that long to get to us. And so we can observe this phase in the universe's history, where that acceleration starts to take hold. So we're in a fairly kind of lucky position in that

sense, because we can do that. We can look back and we can actually see this taking hold and track this expansion of the universe. That's how we know this is happening. So this is a big field, right? Dark matter, dark energy. It's a big problem. Lots of people working on it. So what's your individual little niche that you're working on right now? What's your baby that you care about? So I work a lot on a technique called weak lensing, weak gravitational lensing. This is a very powerful way of studying

where the dark matter is. It's a way of making maps of dark matter. It's a way of working out how the expansion of the universe is affecting dark matter structure. So it's a technique where you can indirectly see the dark matter through its So it's a technique where you can indirectly see the dark matter through its effect on galaxy images. When you look at galaxies, very distant galaxies, the light from those galaxies as it travels to your eye gets deflected by the gravitational

pull of all the intervening matter. And most of that matter is the dark matter. And what this does is it distorts the galaxies slightly. So it's like looking at the night sky through crinkled glass where certain parts of the sky are magnified, certain parts are demagnified, there's a smearing going on. And it tends to align galaxies, the images of galaxies. So it's a technique that is a fairly new technique. It's only been around 20 years or so. But it's a direct measurement of where the mass in the universe is. And so it's a really exciting and really great way of finding out where the dark matter is. So I work on this technique, in particular with upcoming missions to specifically probe

this yeah i was just about to say we're about to hear a lot more about the euclid space telescope this episode because this is something that it's aiming to do right it's to to take these images that allow you to you know take these shapes and work out what shape the crinkled glass is as you quite put it you must be so excited to see i mean euc you quite put it. You must be so excited to see, I mean, Euclid just launched, so you must be so excited to see the data start to come from it. Yeah, hugely excited. It's a really great time to be involved in this area. This satellite has been really built to do this measurement.

As a practitioner of weak lensing, I've got access to the best instrument you could possibly get to measure this effect. So it's really exciting and the images are going to be exquisite and it's going to be, you know, it's going to be a great few years. That was Alex Hall from the University of Edinburgh. Brilliant explanations in a rather echoey room at the National Astronomy Meeting in Cardiff. Yeah, sorry, the university buildings, they're not really built for podcast recordings. I do apologise. The amount of times I've gone in be like sorry can we shut that window can

we turn off the air conditioning oh that's controlled by someone else well can you find that someone else yeah uh no it was great so I can we go to this idea of making dark matter in the lab like Becky is that going to be? I would quite like to see this. Yeah. I mean, I think it depends on what dark matter turns out to be. So if dark matter is indeed a particle of some form that doesn't fit into our standard model of particle physics of, you know, here's what makes up protons and quarks and here's all the electrons and everything. So if it is indeed a particle, then I have no doubt that the particle physicists will figure out, you know, the maths of the underlying model and figure out a way to make it probably through collisions of other

particles if they're somehow related to the dark matter and matter in a way. And the only way that we'll know that we've actually made some is because the energy of all the particles that you could see before the collision is more than the energy of all the particles you could see after the collision because some energy has gone into making the dark matter that you could see before the collision is more than the energy of all the particles you could see after the collision because some energy has gone into making the dark matter that you could no longer see. But I mean, if it turns out to be astrophysical in nature, I don't think we could make it in a lab

because another idea for dark matter and one that's sort of still being worked on, you know, is that it could be primordial black holes. So primordial meaning has formed in the very early universe. And this would be from, you know, these tiny density fluctuations we talk about in the early universe of just gas like plumping together and then not plumping. If it clumps together enough so that you've got a density that's, you know, gives you a density where you have an object that light can't escape, you've got a black hole that's formed. So, you know, it could be anywhere from just, you know, a few grams of a black hole

to, you know, like sort of like earth mass fractions. So if that's the case, I don't think we're going to start making some primordial black holes in a lab because I don't think we have any way of making matter that dense. And also, would that be a good idea uh controlled experiment etc etc i don't think i personally would sign up for that one okay so tbc but if we look at dark energy we know that it makes up roughly i think robert said 67 earlier you know let's say roughly 70 of the of the universe how do we know that So that number doesn't come from that first evidence that we got for dark energy that Robert was talking about before, the supernova evidence. This actually comes from our cosmological models. So essentially what you do is you grab

every piece of evidence you can get about the universe. So every observation you can get your hands on, like the distribution of galaxies that we see around us today, the distribution of galaxies at greater distance, so earlier times in the universe. And even what the cosmic microwave background looked like, that first light in the universe that has these sort of imprints of where all the matter was in the universe back then. And so what you then do is you take a cosmological model that can fit to that and replicate the distribution of matter and how much the universe is expanded by. And out of that, you essentially get, okay, how much energy do you have to put as sort of like, if the universe has got X amount of energy, what fraction of that energy do you have to put into, you know, making

normal matter so that it clumps the way it does and making dark matter so that it gives us the sort of cosmic web structure that we see of that whole universe. And how much do you have to put into whatever's causing the expansion of the universe so that it can expand to its current size and there'd be that much space between the galaxies. And that's how you get at those fractions of, okay, well, the universe's energy budget means that 70% of all the energy has to go into dark energy and expanding the universe by accelerating it. And so that's how we know that, okay, if E equals MC squared, energy and mass are equivalent, then we can say, okay, 5% of the universe is made of matter and 25% is dark matter and the rest of it is all dark energy.

Okay. Okay. That makes sense. Does it? I'm so glad if it does. Does it? I'm so glad if it does. I think I'm starting to get deja vu. Last year, we went to the annual National Astronomy Meeting and chatted about the latest space telescope with Dr. Stephen Wilkins from the University of Sussex. And here we are again, a year later, talking about a space telescope at NAMM with Stephen Wilkins. Yeah, but it's a different space telescope. So last time it was the James Webb Space Telescope. This time I caught up with Steve just a few days after the European Space Agency's Euclid mission launched.

And, you know, Steve is so lucky that he gets to work with both of these missions. His science overlaps both of them. So he was the perfect person to talk to. And I wanted to know just how excited he was about it all. I mean, incredibly. I mean, I think we've had a really fortunate two years, you know, with JWST launching a year and a half ago, and now Euclid, which is really the next piece of our big puzzle to really understand how, you know, how galaxies work and how the universe works. So actually watching it launch on Saturday was, you know, fantastic. It's a shame we've got a while to wait for the first data there

because this is going to take a bit longer than it did with JWST. But yeah, really exciting couple of years ahead of us. Yeah, definitely. So this really is sort of astronomy's new toy. Can you explain, you know, how is it different either to JWST or even the Hubble Space Telescope as well? Because it looks quite similar to Hubble. That's right. So in terms of, I mean, it's actually a little bit smaller than Hubble and therefore quite a bit smaller than JWST.

But where it's really powerful is that it has a huge field of view. What I mean by this is the kind of area of the sky that it can observe at any one point. So the problem or the limitation of Hubble and JWST is that, you know, they're really good for looking at really tiny areas of sky, really, really sensitive. But, you know, with Hubble and JWST, you're never going to observe large patches of the sky. This is a problem if you want to make certain types of measurements. So, for example, if you want to learn about what the universe contains on the largest scales, so cosmology. contains on the largest scales, so cosmology, but also if you want to find rare things like supermassive black holes, even weirder phenomenon out there, you can't really do this particularly well with JWST or Hubble. Beyond that, I mean, it has kind of similar capabilities to both of them.

It doesn't go quite as fine to the infrared as JWST. It doesn't go quite as fine to the UV as Hubble does. It's kind of straddling between them. But like I said, the key difference really is this huge field of view. So, you know, over the six years of its mission, it's going to survey almost a third of the entire sky. Wow, that's really cool. So what is Euclid going to be doing over this six-year mission? What kind of data are we getting from it? I mean, with Hubble and JWST,

so it feels like anybody can just apply to use it for what they want. But you said sort of mission, like it's got a mission to do. That's right. So Euclid is much more focused in what it wants to do. So as part of the Euclid mission, it's really going to do these two very large surveys. One, we call the wide survey, which is around a third of the entire sky. And then the second, the deep survey, a much smaller area of sky, but still much larger than Hubble and JWST are going to observe during their lifetimes.

And so the idea is that with these two big surveys, we can answer a whole range of scientific questions. Now with these surveys, we're not just getting imaging, we're also going to get spectroscopy. So we're going to be able to get spectroscopic measurements, which gives us red shifts of very precise distances and other physical properties over millions of galaxies. So that's lots and lots of images and then obviously distances to that. So what's the eventual goal with Euclid, with all that data, you know, after the six years are finished? Well, the core mission is all about cosmology. So it's all about measuring the distribution of stuff in the universe. Because by doing that, we can actually learn about what the universe contains.

So we can answer questions about what dark matter is and dark energy. But with all of that data, it enables a huge amount of legacy science as well. So in particular, I'm really interested in finding the most distant, rare galaxies in the universe. So things like the very first supermassive black holes to form. That's what I really care about. But, you know, there's a whole range of other science that we can do, stretching all the way from, you know, finding new objects in our own solar system. I think we're expecting something like 100,000 new solar system objects, to finding tens of thousands of new exoplanets. And then like I said, all the way to the very most distant universe. So we can really tackle some of those questions. So it wasn't really designed to you know answer such

a broad amount of you know things in astronomy we're going to get it all for free from this kind of core cosmology survey nice i mean rare stuff i'm excited for as well because i study the really really rare things but nearby galaxies unlike you use the far away galaxies but i feel like the buzzwords around euclid are dark matter, dark energy. So especially in terms of dark energy, like how will just taking these images of the sky help us here? Yeah, so there's a couple of different probes that we can use to constrain both dark energy and dark matter.

So for example, one of them is to try and map the distribution of matter in the universe. And one of the main tools that we use to do that map the distribution of matter in the universe. And one of the main tools that we use to do that is the phenomenon of gravitational lensing, or specifically weak gravitational lensing. And what this is, is basically whenever light travels by, you know, some collection of mass, be it in a galaxy or another structure, that light will get deflected by, you know, maybe a small amount, maybe a large amount. But we can actually measure those deflections and use that information to actually work out where the mass is in the universe. So the idea is that with Euclid, by using that technique and combining it with redshifts as well, we can actually make this

3D map, not just of galaxies, but where the mass is. So we know that galaxies, you know, they're just the, they're like the tip of the iceberg. They're the bits of the universe that we can see. the they're like the tip of the iceberg they're the bits of the universe that we can see what we really want to do is map the the kind of beneath the iceberg looking at all the dark matter structures as well and that's what we'll be able to do really beautifully with euclid so when you talk about red shifts as well that we get in from spectroscopy where you split the light through a prism and get this trace of how much light each wavelength you receive what do you mean by red shift here and how is that going to help us make this map of the universe? So redshift is really,

it's relating how fast the galaxy is moving away from us. And we know that in our current cosmological model where we have an expanding universe, how fast the galaxy is moving away from us actually tells us about the distance to that galaxy. So we can use those redshift measurements that are relatively easy to make to actually work out how far galaxies and other objects are.

And so with that information, we can then make this 3D map of where all the galaxies are in our universe. Nice. So it takes 2D to 3D. I like that. And then what about dark energy? We've talked about we've got a map of where all the stuff we can see is, and we've got a map of where all the stuff we can't see is. How does dark energy tie into all this? Well, dark energy is this really elusive thing

that we really don't understand. When we say dark energy, we just mean something which is causing this accelerated expansion of the universe. So, you know, if we had a universe that just contained matter, the universe's expansion should slow down over time because, you know, gravity exists. It's pulling things together. But what we actually observe and what we have observed

for almost 30 years now is that actually looks like today galaxies are moving faster and faster away from each other every single year. So this is this accelerated expansion. If we really want to probe this accelerated expansion, we essentially need to measure the distribution of galaxies in the universe and how fast they're moving away from each other, you know, at different times. So what we want to do here is really test some of the simple models of dark energy,

because we really have no idea. There is one kind of prevailing idea that dark energy is just some property of empty space, but by far that's not the only one. You know, it could be that Einstein's gravity is wrong for example it could be something even more exotic and so we really want to answer that question and we you know we've been trying to do this for decades and not really made you know the progress that we need to make and this is why we have something like Euclid. Great I mean so many huge areas of astrophysics seem to be resting on Euclid's shoulders now. Like I feel like it might be, does it trump JWST as astronomers' favorite new shiny toy? I have to be really careful politically

because I'm involved in both projects. I think my heart still lies with JWST because JWST is a little bit more valuable for my science areas of the very distant universe. But they're not in competition with each other, right? They're answering different questions and they're very complementary. So I want to use Euclid to find all of these rare things

like these distant supermassive black holes. And then I can study them in more detail with JWST. And it's great because we're going to have these two telescopes at exactly the same time allowing us to do this. Euclid's going to find all of these bright, the first supermassive black holes, and then I'll dissect them with JWST. That'll be tremendously exciting when we get to do that.

I think so. And I think, listeners, it'll tell you who's won because both me and Stephen are both currently wearing JWST t-shirts. So I think maybe JWST has taken Euclid there, but we are very excited about euclid and thank you so much steven for explaining that to us so well this is the super massive podcast from the royal astronomical society with me astrophysicist dr becky feathers and with

science journalist izzy clark this month we're exploring our missing universe. Where the heck is it? Safe to say when we said that we were going to do this topic, the questions came in. I'm hoping they're not too mind-bending, but let's see where we end up. So Becky Astronosurf asks, what do we know about black holes accreting dark matter? Surely this needs to be included in cosmological simulations so i love this ashrana sir i actually had this exact thought when i was writing up my phd thesis

so imagine me slightly sleep deprived um at my wits end from reading too much like two months in being like you really i feel like you had like chalkboards all around you like it was just paper all over the desk of me trying to keep up references. It's gone wild. Yeah, anyway. So I was sort of doing this study of talking about what black hole mass

in the center of galaxies, these supermassive black holes, is correlated with. Was it like the mass in stars in a galaxy? Or was it the mass of dark matter in a galaxy? And there was a lot of evidence leading to this. And so this is when I had this sort of eureka moment. I'm like, are they correlated?

Because the dark matter ends up in the black hole. And so I raced downstairs in the physics department, probably fell over and turned to my sort of simulator friend, you know, who does these cosmological simulations and was like, is this a thing? Do you simulate it? Have you got numbers on it? And they basically just sighed

at me. Oh no. Yeah. And you know, then you're like, oh, so first of all, I haven't had a eureka moment. People have thought about this before. And also it's obviously really difficult. And it turns out, yeah, it is very difficult. And this process of black holes growing with dark matter doesn't tend to be included in cosmological simulations. And one of the reasons for that is because one of the pieces of evidence for what we have for what dark matter is like is that it's collisionless. So I think we've talked about this on the podcast before, but there's a really famous image of what's known as the bullet cluster, which is two clusters of galaxies that have collided together. And the galaxies of stars have just passed straight through,

gone past each other and ended up on other sides. They've not collided at all. In the same way that if you had two galaxies merged together, no two stars would collide either. Yeah. The distance between the two stars is too great and the probability is incredibly, incredibly small. Whereas all of the gas that you find between the galaxies in the cluster,

the intracluster medium, has collided and sort of been left in the middle. In that collision, it's heated up and it starts glowing x-rays. So when you look at the bullet cluster, you can be like, okay, here's where all the x-ray emission is coming from, right in the middle. There's actually nothing there because it's the only way you can see the gases through x-rays. The stars in the galaxies are all on the outskirts. And then you look where all the dark matter is and you find it traced the galaxies of stars. So the dark matter didn't collide in the middle like the gas did. So it's collisionless. Now that's a really big deal when you think about how black holes grow. Because if you have material spiraling around a black hole, we think of black holes as hoovers, but actually most stuff's just trapped in orbits around black holes.

And to get it out of an orbit, you have to have a collision between two particles. And where one particle will lose energy and so will fall into the black hole and one particle will gain a bit of energy. And so if you've got dark matter in that, what we call the accretion disk, the sort of disk of swirling material that will eventually be accreted, eaten, eaten, eaten by the black hole. Eaten, it's a new word. If you've got dark matter there, it's not going to collide with anything. So it won't make it into the black hole. And so this is why it tends to be left out of cosmological simulations. Now, there are some people who've done individual galaxy simulations to be like, OK, if you do have some dark matter around over the full lifetime of a galaxy,

probably some of it's going to be like direct trajectory on the bullseye into the supermassive black hole at the center will actually make it in. And sort of the upper limit that people put on this is about 10%. That's the amount of matter in a black hole that is dark matter is around about 10%. So it's not very high. That's a real upper limit. You have to have the perfect conditions for that. So this is why it doesn't tend to be included in our simulations either. So my eureka moment was, I came very, very far down very quickly. It did not help my mood while writing my thesis let's put it that way it lasted 30 seconds but it was a great 30 seconds yeah the walk back up the stairs to my

office was a lot sadder and slower than the run down the stairs i mean i just think this is incredible so i just want to throw in an extra thing here do we say that it then behaves quite predictably the more that you study it the more that you can try and predict how it might interact say with galaxies and other you know objects interacting and things like that yeah i mean it's interesting like this is sort of all the information we have on dark matter comes from astrophysics and so you're looking at from huge distances and you have to be clever with what you, you managed to figure out about it. So collisionless, yeah. Doesn't absorb or emit or reflect light. Big questions are, okay. So that's one of the main fundamental forces of

physics, electromagnetism. All right. So it doesn't interact with light, but what about like the strong force that binds atoms together or the weak force that's to do with radioactivity you know we know gravity it definitely does because we see it bending space but there's no visible matter that we can see there that's causing the bending of space so it's it's sort of like what can we actually figure out from our astrophysical observations like that and then we feed that to the particle physicists to be like have fun with that okay okay we're still waiting by the way we have this joke between astrophysicists and particle physicists that we're like keep looking at our watches like you still not figured that one out yet yeah we've done the legwork, come on. Okay, so Robert Mariana on Instagram wants to know, can humans or planet Earth feel its presence?

I'm still reflecting on Becca's terrible sad moment during that, but I think she's recovered well. Well, this is a really good question, isn't it? I mean, the answer is not really very much at all. And if it's so weakly interactive, it's not really surprising. It's very generally thinly distributed, even though there's a lot of it. So there's a very, very weak gravitational force because gravity extends over infinity. It's just that it's a tiny, tiny force for us. So feel not really, it's just not something we'd be aware of. I found an article on this a few years ago suggesting that about a milligram of dark matter might pass through us over an 80 year lifetime so yeah pretty small really i mean what's that that's about uh just trying to work out what

that is as a fraction of a teaspoon teeny teeny speck of stuff before it's a proton's worth of like matter in terms of weight mass per teaspoon per teaspoon there you go so teeny teeny stuff and you know your whole lifetime adding up to that that tiny sat thousandth of the gram so no not really very much um and i i guess if it was something we could feel you know we'd have we'd know a lot more about it already it'd be much easier to detect on a local level rather than just seeing its influence into some galaxies in the way that stars move through space. This is why those big dark matter experiments like the Bulby Mine, they're buried really far underground in the hope that a random dark matter particle will come in and

hit something and transfer some energy. And they've been searching for like, what, 30 years? It's science. Exactly, yeah. It's the science where you do the biggest possible lump of stuff to try and see things happening. Yeah, we need a really, really big mine or we need, you know, at the South Pole,

we need a really, really big lump of ice to try and see if anything's happening. And so far, nothing. Maybe they just have to wait for 80 years, you know, for that one milligram. Exactly. There's not been going long enough. That's a depressing prospect if you're a PhD student there.

Yeah, can you imagine the sales pitch? Like, I can't promise you that we'll find anything i can't promise you that it might be on your retirement party this is worse than me looking at future space missions and thinking 50 50 when i see that okay i'm becky alec and kunta on twitter asks how are prominent end of the universe scenarios like the Big Freeze and the Big Rip impacted by the influence of dark matter and dark energy? What would be their exact influences on these scenarios? So Becky, can you just give us a very brief recap on the Big Freeze and the Big Rip? Well, actually, let's start with the dark matter and dark energy because then it makes more sense what they are. Okay, fine. So it turns out that they are hugely, hugely influential, right?

Because what happens at the end of the universe, there is going to be an end, all depends on the balance between how much stuff you have creating gravity pulling inwards versus how much dark energy you have pushing outwards. And if that's perfectly balanced, great. Eventually we just reach this happy medium and the universe reaches a size and we don't do anything else anymore. Of course, when it's not balanced is when you start to get these big rip scenarios that Alicante was talking about. So the big rip is essentially dark energy wins over matter as it

turns out there's more of it and it accelerates the universe to the point where everything is just completely torn apart there's so much space between galaxies that they're probably over each other's like horizons of what you can see like completely okay huge the big freeze is an interesting one because that's actually when you don't have dark energy and you just have expansion and it just keeps expanding, but it doesn't accelerate expand. So you just end up with everything really fast paced, but everything's just really cold because nothing's barely moving anymore without the influence of gravity. But then of course you have the opposite as well, where if there's more matter, like dark matter, normal matter, then there is dark energy and that starts to win.

Eventually you'll expand to a point and then you will start to reel everything back in again in what's known as a big crunch yeah and so all of these scenarios for what is the end of the universe depend on what is that perfect sort of balance in our universe and people have been trying to measure it yeah being like how does it compare because we have this thing called the critical density and so you measure these fractions of how much stuff there is based on this critical density. And so you're like, are they equal? Are they not equal? The number that we're getting is basically like, it looks like they're pretty equal, but the uncertainty is still pretty big.

Well, it's getting smaller and smaller, but it's still like erring on, it could be on the side of like the big rip, basically. So it's really interesting. And if you want to know more about this session i can highly recommend katie mack's book the end of everything astrophysically speaking where she talks about all these different scenarios yeah great question lovely okay and robert adrian nowick from south america has been in touch and he says i love your podcast and i enjoy every episode my question is will the Euclid mission data be publicly available like NASA does with all new missions by putting them on the Mikulski Archive for space telescopes? Or will the data only be available for a few people?

Well, I checked with actually Steve Wilkins to ask about this because I thought, I don't know, who knows? Generally, you'd approve of the principle. And he said that, yes, the answer is yes, at least eventually. Now, that doesn't mean that when the data's out, if it's made publicly available, that you can analyze it yourself and publish a paper on the back of it, because sometimes it's the result of proposals that have come from groups. And so they get first dibs on analyzing or at least publishing the results. But in principle, yes, it should be possible, at least least eventually to see this and to do your own work on it. I mean, certainly there

are people looking at the reams and reams of Hubble data over what the last three decades and still publishing papers on the back of that. So yeah, the good thing is I think that we're in an era now, I guess, driven by large-scale fast internet access that this is possible and you can see this stuff for yourself. Yeah. It's becoming less of the norm. The default was usually like six months proprietary period access to the data, just you before it's made public, maybe a year. But now it's becoming more of the norm that stuff is released. So I know with Euclid, they have a timeline for data releases. So like data release one, data release two, like two and three and four

years, et cetera, into the six year mission, which they will do a lot of the sort of reduction of the data. So cleaning it up and, you know, making it into a, you know, a nice accessible form before it's released. And when they do that, they'll probably get the sort of low hanging fruit science results for themselves. And then, you know, it would be such an overwhelming amount of data though, that, you know, for people to dive in and do any analysis faster than the team who is an expert on looking at Euclid data is unlikely, but it's still public, you know,

for people to access if they want. Yeah. Yeah, it makes sense. Lovely. And if you're listening to this and you're thinking, I have a question for the team, then get in touch. You can email podcast at ras.ac.uk, tweet at Royal Astro Sock, or we're on Instagram at SupermassivePod. So send us your questions and we'll try and get to it in a future episode.

So, Robert, what can we see in the night sky over the next few weeks? Well, the nice thing at this time of year, I think it's almost one of the best times of year for looking at the sky. It's still warm, the nights start to draw in a bit. The latter isn't necessarily something everybody looks forward to, but if you like looking at the sky, it's good. At least by the end of August, it's changing very rapidly as we head towards the equinox and moving into the autumn and so on. So, it's a really nice time still to look up at the Milky Way, the Summer Triangle, Dene, Vega and Alto, really, really prominent as you get even slightly later into the evening. So

get your binoculars out if you've got them and have a look at that beautiful Milky Way, particularly on a moonless night. And I was thinking of different targets you can see just with a pair of binoculars. And some that came to mind are there's the Wild Duck Cluster down in Scutum, so lower down in the sky. There's the fabulous Coathanger Asterism, which is a genuine star cluster that looks like a coat hanger. And I promise you, it's absolutely unmistakable if you check this out. It's technically all Brocky's cluster, but everybody else I know thinks so. That's the coat hanger. That's in Vulpecula, just over to the side of the Milky Way. And there's also the bottom of Cygnus, if you're in the Northern Hemisphere, at the top, if you're in the Southern Hemisphere, there's the beautiful

double star Albireo, which has a nice blue and yellow contrast. And that's quite easy to see in binoculars as well. Now, in terms of the solar system, Venus is now moving between the Earth and the Sun, so it's not really visible in August. But it's time to look at other planets. And one that's coming up, which hopefully will please Dr. Becky, is that Saturn will be at opposition at the end of August, which means that it's at its best for the year. It's been gone for so long. It was in the morning sky. I love Saturn, but I wasn't going to get up for it in the morning sky. But now I'm like, it's fucking midnight. I should ask whether it's Taylor Swift, Saturn, but it's pretty high above the southern

horizon. It's better than it was last year, but, but it's pretty high above the southern horizon. It's better than it was last year, but local midnight, which means if you're on summertime, about one o'clock in the morning is when it's at its highest. And after that, it'll be pretty visible for the next few months over the autumn. So a good time to start looking for it without having to get up at a horrendous hour. And the highlight, I guess, of the month or for many people will be the Perseids meteor shower. Now, this is a shower of meteors, incoming tiny bits of natural space debris burning up in the Earth's atmosphere. And when there's a shower, it means that we're passing through the tail of a comet, in this case,

one by the name of Swiss Tuttle. It was last near the Earth about 40 years ago, but it's left a trail of debris through the solar system. The Earth goes through that and those bits burn up in the atmosphere. And that's when you see meteors or shooting stars. And this year, the peak is around the 12th to the 13th of August. And the moon that night will be a really thin crescent. Now, that's a good thing because what it means is there isn't any moonlight. So the sky, if you're in a good place away from the lights of towns and cities and the sky is clear, it'll be nice and dark and you'll see more meteors. You might see as many as, say, a few tens an hour, which is really a nice sight. You can sit there, it's generally still quite warm, get a drink out, enjoy the view. Some people do things like lie down on sun lounges in the middle

of the night and look up at it. Generally best as well to, again, look in the small hours of the morning because that's when you're on the side of the earth that's moving into the debris, so you get more meteors. But if you look earlier on, sometimes you get what are called Earth grazers where they're just glancing the Earth's atmosphere, so you get longer trails as a result. So I think if you're out and about, I don't know whether you're at, say, some festival or you're in the Mediterranean or in a dark place, it's a holiday season after all, then do take a look.

Happy days! And it's a weekend. I imagine that even if people are on holiday. I hadn't even caught that. That's even better, isn't it? It's exactly the 12, then do take a look. Happy days. And it's a weekend. I imagine that, you know, even if people are on holiday. I hadn't even caught that. That's even better, isn't it? It's exactly the 12th, Sunday the 13th. So I'm like, right, even if you're not going to holiday, plan a camping weekend or something.

Just get somewhere dark. Oh, definitely. No, I remember, you know, I remember seeing them many years ago, a folk festival and just sitting out with people out on their tents and looking up and thinking, this is great. You know, it was about midnight. Admittedly, we'd probably all had a bit to drink, so I wouldn't entirely rely on our counts of meteors,

not that we were really trying, but it was a great view. It's a really publicly accessible show. And it's so much more fun with a group, right, as well, because you can all sort of keep an eye on a different patch of the sky. And, you know, when a couple of you all see the same one, everyone gets that excited. I saw it. It was actually nice.

Exactly. And there's some people, I guess, they'll try and take pictures of them as well. That's sometimes more of a challenge because you need to leave your camera in for a long time. If you see a meteor, it's too late to have photographed it. So you need to leave your – the people I've set up for kind of looking over the whole sky over a period of time. But if you do have any pictures and tweet us, we'd love to see them. Yeah, absolutely.

And, well, I think that is it for this month. We'll be back in a few weeks' time with another bonus episode. And then we bring back the sun. Yeah, absolutely. And well, I think that is it for this month. We'll be back in a few weeks time with another bonus episode and then we bring back the sun. It's time. We bumped it. We're very, very apologetic to it. And obviously, as Robert said,

tweet us if you try some astronomy at home. It's at Royal Astros on Twitter or you can email any questions you have to podcast.ras.ac.uk or DM Instagram

at supermassivepod. are we on threads as well now is he for those on threads or we are actually my instagram had a weird little glitch and it wouldn't let me do it for a bit but we are also on threads now so there you go send us some questions on threads at supermassivepod as well and we'll always try and cover them in a future episode whether it's a main episode or a bonus episode we're just giving you all the episodes but until next time everybody happy stargazing