The Supermassive Podcast - 23: The End of the Universe

Episode Date: November 26, 2021

What will the end of days actually look like? And how will it happen? This month, Izzie and Dr Becky explore the end of the Universe with cosmologist Dr Katie Mack - author of The End of Everything (Astrophysically Speaking). They also chat to astronomy journalist and author of The Unknown Universe, Dr Stuart Clark, who explains the role of dark energy. Plus, Dr Robert Massey takes on your questions and shares when it is best to see Jupiter, Saturn and Venus in December. Send your astronomy questions for January's Q&A to podcast[@]ras.ac.uk or tweet @RoyalAstroSoc. Book Club recommendations:  The Red Planet - Simon Morden  Apollo’s Muse - Mia Fineman and Beth Saunders  Fire and Ice - Natalie Starkey The Invisible Universe - Matt Bothwell The Mysteries of the Universe - Will Gater The Supermassive podcast is a Boffin Media production by Izzie Clarke and Richard Hollingham. 

Previously on the Supermassive Podcast. We're kicking off the new year with the beginning of everything, the Big Bang. That's right, and come December we'll explore how the universe might end, if it hasn't already happened by then. Hello, welcome to the Supermassive Podcast from the Royal Astronomical Society, with me, science journalist Izzy Clark and astrophysicist Dr Becky Smethurst. Yeah, this month it's all about the end of the universe. Can science tell us what the end of days would actually look like and how would that all happen? So just the fun stuff then.

Yeah, all of it. Very cheery, very cheery topic. So we'll be hearing from cosmologist Dr. Katie Mack, who's the author of The End of Everything, astrophysically speaking, to explore the options for our demise. Demise, Izzy. Such a stroke. Our demise. I'm leaning into it. I'm leaning into it. Plus, astronomy journalist and author Dr. Stuart Clarke explains the role that dark energy plays in all of this. And it's not the Supermassive podcast without Dr Robert Massey the deputy director of the Royal Astronomical

Society. So Robert so let's cut to the chase there's quite a few hypotheses behind the end of the universe aren't there? Well yeah and none of them particularly cheery if we're going to get down this thing I mean you know everything in cosmology is big at least at the moment and getting bigger so we've got two options for an open universe one is the sort of big freeze the heat death wherever it gets bigger and bigger and bigger and everything dies and all the stars run out of fuel and there's no more fuel for stars it's all a bit depressing really um and you get lots of black holes, I guess. So there's some excitement there. Yeah, exactly. So Becky's delighted. And then there's also the other kind of variation on that, the big rip, where the acceleration of the expansion of the universe increases until

everything is pulled to bits. So not a terribly cheery prospect. I console myself for the thought that 100 trillion years or so when it happens is a reasonably long time in the future. And then finally, you've got the closed option, which is the big crunch, which is a bit less favoured these days. And that's the idea that the expansion somehow reverses and the universe gets pulled back together by gravity until it collapses. So none of these are great options for the future, except, you know, not the sort of thing that keeps me awake at night, admittedly. Humans probably won't be around for that. I'm very grateful. Cheers, Robert.

We'll continue to contemplate our demise, as Izzy so beautifully put it, and we'll catch up with you later in the show. Now, I would like to clarify that you haven't stumbled into a philosophy podcast. This is still the podcast from the Royal Astronomical Society. But there's no doubt that the end of everything is something that philosophers and theologians have pondered for years. I spoke with cosmologist Dr. Katie Mack, assistant professor at North Carolina State University and author of The End of Everything,

about what our different end of the universe options are. Starting with, why do we think the universe will end in the first place? We can look at the way that the universe is evolving right now. And we can look at the way that the universe has evolved in the past. And we can say with pretty good certainty that the star formation, the process of forming new stars has reduced a lot over time and will continue to reduce over time because of the way the universe is expanding and the way the stars are using up gas. So here's something a little bit unsettling. We can look at the history of star formation and we can see that of all the stars that have ever

formed or ever will form, about 80 or 90% have already happened. Wow. 80 or 90 percent have already happened wow 80 to 90 percent i think it's actually i think it's actually 90 to 95 percent at this point um it's it's a lot all have already happened so so we're looking at the last five or ten percent of star formation that's that's how things are going we're basically almost done with the universe. Okay, so we've jumped straight in here. Enjoy listeners. Good luck with that one. Based on what we know about our universe and how it's behaving, and as you say, we've seen so many stars created

and most of the stars created. So where does this lead us with the end of everything? What are the possibilities, I guess, in how our universe could end? There are several different ways the universe might end. We don't know for certain at the moment what's going to happen. But all of them have something to do with the way the universe is currently expanding. The galaxy is getting farther apart, gas and SARS is being used up, all of that. For a long time, when astronomers looked at the expansion of the universe, there was this really big question of,

will it continue to expand forever? Or will the expansion someday stop, right? And that's kind of, there are sort of only a few logical possibilities if you're living in an expanding universe, based on what we understand of gravity and how it works on large scales in the cosmos. Basically, you can have the universe will continue expanding forever, or it will stop and turn around and everything will re-collapse, right? The possibility for re-collapse would be if the initial kickoff of the Big Bang was sort of not not strong enough and then all of the galaxies all the stuff in the universe has so much gravity it kind of slows down the expansion and stops it and then everything wants to fall together again so it flips essentially yeah once was previously expanding and now everything's coming back towards us so So the way that could happen is just that, you know,

everything in the universe that has mass has gravity, right? So galaxies and stars and dark matter even, which is this mysterious stuff that's most of the matter in the universe, all of that has gravity. So even though the universe is currently expanding, everything is kind of still pulling on everything else. And if the expansion is not powerful enough, then that could cause

everything to come together again. Sort of like if you throw a ball up into the air, you know, when you throw the ball up into the air, you give it an initial push and it's moving away from the earth, but the earth is still pulling on it. And so if you don't throw it hard enough, then it'll be slowing down and slowing down. It'll stop and then it'll come back and hit the earth again. If you threw it inhumanly fast, you know, 11.2 kilometers a second, the escape velocity of the Earth, then it would continue to be moving away from the Earth always, and it would go off into space. So it seemed like those were kind of the only possibilities when astronomers first started looking at this. And so the question was, was the big bang powerful enough or is gravity too strong right those are the kind of the options and and and this idea of it coming back in this is

what we call the big crunch right yes they've all got the right i'm gonna say they're fun names for quite a yeah interesting end of all time um so that was quite a popular theory. Is it still quite a popular theory? What are some of the other options? So in, in sort of the 1960s, that was the popular theory because we really thought that the gravity was going to win. But then in the late nineties, uh, astronomers measured the expansion of the universe more carefully and the history of the expansion and found that the expansion was actually not slowing down at all. It had been slowing down for billions of years, and then it's just started accelerating again and speeding up in this expansion. We didn't know why that was happening. And so astronomers use the term dark energy to talk about whatever it was that was making the universe expand faster.

And dark energy only started making the universe expand faster you know somewhere around five billion years ago if it's gonna keep expanding this way and always accelerating in this same manner then then what we end up with is what's usually called the heat death of the universe um it's also sometimes called the big freeze fun what happens there then it's it's a very cold ending so what happens there is that the universe expands and expands everything gets farther and farther apart the each galaxy gets more and more isolated the stars burn through their gas and then there's nothing to replenish it so everything kind of just fades away and dies in this cold empty lonely space it's a really It's a really depressing idea,

you know, black holes evaporate, and then matter decays, and everything is just left empty. And the reason it's called a heat death is that as everything is decaying, it's evolving toward disorder, toward entropy. What that leads to is a universe that is kind of the same temperature everywhere. It's a very cold temperature, but it's the same temperature everywhere. You have this uniformity of disorder, of waste heat. There's one thing that I've heard, which is called the big rip, which kind of sounds like the opposite of the big crunch. So is that what's going on? Talk me through that. So the big is is sort of a way for dark energy to go terribly wrong um so dark energy dark energy is is something that's making

the universe expand faster our guess about dark energy is that it's a cosmological constant which is just there's a certain amount of stretchiness in every bit of space that over time as the universe gets larger the cosmological constant is the thing that's really governing the expansion, but it's not like messing stuff up. It's just moving everything apart. It's stretching out empty space. You could imagine a different possibility for dark energy, one that isn't just constant throughout space, but something that builds up over time. Instead of just moving galaxies apart, it would build up within galaxies. It would build up within this room. You know, it starts to pull apart structures that already exist. So not only would it be pushing galaxies apart, it would also be building up in them and say.

Exactly. Pulling at them. And that's not just galaxies. That's like objects in space. It applies to everything. Okay. Yes. Yes. Everything. And that leads to a big rip, which is where basically what happens is first the dark energy builds up and it starts to move galaxies within clusters apart from each other, you know, sort of, then it would be pulling stars off the edges of galaxies and kind of unraveling galaxies. And then it would start to pull apart solar systems, you know, take the planets away from the stars, and then it would build up within stars and planets and explode stars and planets and keep going until it's pulling apart atoms and nuclei and just ripping apart the entire universe itself. Yeah. And, you know, your professional opinion, you've written a book about all of this, what do you think is the most likely outcome i

mean i i really think that the heat death is the most likely basically because it's it's the most depressing option it is the most depressing option yes that's true but it also gives us the most time so you know okay okay you can weigh those things out um it it's it's the simplest option in the sense that it's it's just a straight extrapolation from what's happening now. But that seems to be the straightforward one. And just to put listeners at ease, how far do we think we're talking here? So if we're talking about the heat death, we're talking about something so far into

the future that we really don't have words to describe that timescale. I mean, you know, trillions, trillions, trillions, trillions, you could do that all day. So very, very far into the distant future. With something like the Big Rip, we can actually put pretty good constraints. We can put measurements of, you know, the earliest that could possibly happen based on our current observations. And we get something like 200 billion years, which is a pretty long time. I mean, by 100 billion years from now, all the other galaxies will be so far apart, we will no longer be able to see them. Most of the stars in our galaxy will be dead. You know, 100 billion years is a long time. It is not something that I think is an

immediate concern. That was Dr. Katie mack assistant professor at north carolina state university and author of the end of everything astrophysically speaking so not to freak everyone out becky but is there a state where the universe could end now like really very suddenly um well mathematically speaking yes not to not to freak everyone out you know we're sticking with the cheery topics today aren't we but yes mathematically speaking it could be due to something called vacuum decay right and it's all to do with the higgs field do you remember the higgs boson that the large hadron collider discovered way back in CERN a few years ago, right? It's the stuff that gives particles mass, right? And so this field permeates the entire universe.

And so what we like to measure about it is its potential, how much energy it has in that field, right? And if it's in the lowest possible energy state that it could be in, like a teenager, I guess, right? That's what we call a true vacuum. Okay, so we talk about like vacuum energy and vacuum states. That's what we're talking about. We're talking about the lowest energy possible. And that's a very stable place it can be. And you can think of it as sort of like a ball that's rolled to the bottom of a hill, right? It's not

going to move anywhere. It's very stable. But imagine if there was like a little divot in the side of the hill that you set the ball rolling down and the ball stopped in this little divot. It would think it was in the lowest possible energy state, but actually it wouldn't be. It's what we call a false vacuum. So that if you gave that ball a little boost of energy, it could pop out of the divot and roll back down somewhere else. So a false vacuum is not stable. It's what we call met stable a brief push could could just you know send everything tumbling and so the question is whether the higgs field is stable

or not or if there are pockets of the universe that are not stable where it has some different energy in that region right and if some massive big energy producing event happened in that region of the universe that wasn't stable could that push it over the edge and send it tumbling down the hill now right this is where we have to be like what would happen if that did happen right it would essentially create this bubble in the universe that would expand at the speed of light obliterating everything in its path as it went and this is why it's like could the universe end very suddenly without any knowledge of it because we wouldn't have any warning that that was coming yes technically but it all depends on this idea of the higgs field and everything like that so you know measurements of this but well idea of the Higgs field and everything like that. So, you know, measurements of this, well, very newly discovered Higgs field, let's put it like that, you know, are not exactly encouraging because, you know, what are we actually measuring in that sense as well. And those quantum fluctuations, it's very, very unlikely,

right? When we think about them too. So not to panic anybody, mathematically speaking, this isn't impossible, but it's also not very likely. And when I say not very likely, I mean like vanishingly small. Okay. So we can still sleep at night. Fine. Yeah. Like I'm not worried about about it like it doesn't keep me up so it shouldn't keep you up yeah okay noted and looking on twitter there's a lot of questions on something called the big bounce this is the idea that we could bounce back essentially if we ended in a big crunch then there would be another big bang and everything starts all over again is that really likely what what does the general field think about the big bounce?

Well, philosophically speaking, I certainly like that idea. Like it's very, it feels very cyclical, right? You know, it's that sort of like endless cycle, right? And it all comes back to what Katie was talking about, this sort of balance between matter and dark energy in the universe, whether we'll have a big crunch or not, right? It's obviously the first thing you need is if there's going to be a big bounce, you need the big crunch first. Then to know whether there'll be a big bounce or not, or even if our universe started

in a big bounce is a bit harder, right? Because we're relying on the maths to tell us maybe what happened here and something that we weren't there for and what we won't be there for in the future, right? So there are a couple of hypotheses that say that this would be possible mathematically so something called loop quantum cosmology fun which is a fun one to say right it uses the math to show that basically if a previous universe did collapse you know we get to that point where if we try and explain the physics of what's happening in the universe, and we try and track it all the way back to the very beginning, we get to like 10 to the minus 36 seconds, right before the beginning before all our physics breaks down, right, where none of the forces make sense anymore, we need quantum gravity describe everything that's going on. So in this,

in this hypothesis, they say that basically, when you get to that point, this is when quantum gravity kicks in all these random fluctuations, and it starts to dominate and it's repulsive. Repulsive with, you know, everything crushed into this incredibly tiny point before it quite reaches singularity, but still incredibly, incredibly dense repulsive. And so this is what in this hypothesis, they say causes the big bounce is this repulsive quantum gravity right there's lots of cosmological hypotheses that you know come up with this and say this is also possible as well but observing something you know is how we prove that this happened right so observing something in the universe and i don't know how realistic that would be because you know how we have a record of this

even happening there are some astronomers that claim that the really massive super massive black holes that you have right in the early days of the universe, like we don't know how they got so big so quickly. They claim that perhaps that's because they formed in an earlier universe and it was, you know, their remnants of this big bounce, perhaps. I personally am like, it's probably more likely we just don't know something about black holes. Add it to the list. But there are some that claim it could be evidence yeah so we have quite a few options when it comes to the end of everything lucky host and from what katie was saying dark energy has a big role to play in this

but how do we find out more about this mysterious phenomenon? Joining us now is astronomy journalist and author of The Unknown Universe, Dr. Stuart Clark. Stuart, it is literally your job to translate what us experts say so the public can understand. So please explain to all the listeners, what do we know about dark energy? Yeah, this is the fascinating thing, Becky. I mean, on one hand, we seem to know an awful lot. But when you boil it all down, we don't know very much at all. So we know that there appears to be something accelerating the expansion of the universe. And there's no natural candidate for that, it seems, in most of or all of physics, really. So we have one possible idea, which is called the

cosmological constant. But that's difficult to understand exactly what the nature of that is. And so we continue to try to measure the expansion of the universe and the rate at which galaxies cluster together in order to try to understand the properties of whatever this mysterious component, this dark energy is, and see if it's consistent with this cosmological constant we talk about. Yeah, classic physicists, right? Giving it a name, but not knowing what exactly it is. Exactly. As they say, naming is not explaining, but at least it's a start. At least, yeah. So I guess people get a bit confused when we talk about expansion of the universe and then we link dark energy to the death of the universe as well.

So can you explain what role it plays in perhaps the end of the universe? Yes. So the dark energy accelerates the expansion of the universe. So there would appear to be no way in which it could ever collapse into a big crunch again, way in which it could ever collapse into a big crunch again that we're constantly being driven further and further apart from our sort of extra galactic neighbors and so eventually the universe is just you know going to become so large and the matter spread out so much that everything will disappear over our observable horizon so So we won't even see other galaxies. We know it's a bit of a mystery.

There's lots of work going into it. So how can we actually study dark energy? What are those missions and telescopes looking for? How does that work? Yes, without knowing exactly what the dark energy is. And so being able to construct a sort of detector that interacts with it, you have to look at its effects in the universe. And this is something that astronomers are very used to doing, because this is how we study gravity. We just study the movement of celestial

objects in the universe. And we relate that to the underlying force of gravity. Well, with dark energy, we're looking at something that modifies the effects of gravity. And so again, you study the motions in the universe. And one thing that dark energy will definitely do is as it resists the collapse of matter on regions into big clusters of galaxies. So if we measure the way that galaxies are distributed across the universe, we can check that with our predictions from general relativity and see if there's a mismatch. The mismatch should be telling us about the dark energy, the strength of it, the variation of it through time and through space. And if we see those kinds of variations, then it's not an energy as such. It's not a cosmological constant anymore, but it could be, say, a modification to gravity of a behavior that we hadn't anticipated

in the gravitational force or it could be a new force altogether an unanticipated fifth fundamental force and that's sometimes called the the quintessence and both of those options allow it to vary through time and space whereas the cosmological constant is uniform everywhere. It's weird, isn't it, when we think about this, about the different options it could be, because I feel like as an astronomer, an astrophysicist, I'm more interested in what it does and figuring that out from the missions.

But I think physicists will just want to know what it is, right? Yes. And put a pin in it. Yes, it's the perfect marriage, I think, really, of astronomy and physics, and that you have to combine forces in this way. And when we think of the work that physicists do, it's always about sort of the smallest scale of things, the fundamental interactions, the fundamental particles, the building blocks of the very universe that we have around us. And when we think of what astronomers do, it's always about the very

largest things, and what their behavior, how they come together. And what absolutely fascinates me about all of this is just how interrelated those two things are, you know, the very biggest things are the results of the very smallest interactions that take place in the universe. And I find that awe-inspiring. So who are the big players in all of this? Which telescopes, which missions do we have to look forward to that are actually going to actually try and do all of this? Yeah, the first big player to go for this is ESA. Yeah, the first big player to go for this is ESA.

And next year, they launch the Euclid Space Telescope, which is going to study a billion galaxies. That's its aim across the sky. It's looking out to about 10 billion light years. And hopefully out of that, we can do, well, we will, we'll be doing some really high class statistics. The mission itself, and I don't pretend to understand the full technical details of these cameras and the telescope, but I've talked to quite a number of the engineers and scientists involved, and it is mind-blowing the precision at which they are working. I mean, Euclid is a thing of extreme beauty,

just from the engineering point of view. And I think the science that it's going to allow us to do is every bit as great. And then following on from Euclid, of course, we've got things like the Roman Space Telescope from NASA, and big ground-based service things coming from the rubin observatory the hobby ebony um did i pronounce that right telescope um and these are all going to help fill in the gap so euclid sort of goes wide and i mean substantially deep but other telescopes will go even deeper in sort of pinhole or more pinhole-like views of the universe. So it's that classic way that we have sort of developed for studying cosmology, big wide surveys and then small spotlights on very deep parts of the universe

so that we can get the best of both worlds. It's a very exciting sort of five to ten year period coming up for dark energy. That's what I wanted to ask. As someone who has watched this play out over however many years, do you think in maybe the next decade you can say, yeah, we know what dark energy is, or are we looking at 15 years, 20 years, 30? What do you think i sincerely hope i can explain it in 10 years because um i've you know i'm what do i how do i put this it's not exactly that i'm fed up

but um i'd like a different headline than we don't understand what dark energy is great thank you for joining us. That was brilliant. This is the Supermassive podcast from the Royal Astronomical Society with me, astrophysicist Dr. Becky Featherst and science journalist Izzy Clark. This month, it's all about the end of the universe. And Robert and Becky will be taking on your questions in just a moment. But as promised, I mean, albeit a few months ago, sorry about that, we've got our book club. So over to you two, let's start things off, you know, what good space reads have you got for our listeners? So I have The Red Planet by Simon Maud and I actually have a real book as well. You know

how many times I don't read a real book, I just read things on Kindle. It was so nice to read it properly. I'm literally looking at my copy of this right now. So Simon Warden, he's a planetary geologist, right? Rather than necessarily an astronomer necessarily. But this book is so great. I've really enjoyed it. It's about rocks, right? Who doesn't love rocks? But it's rocks on Mars as well, right? It tells the history of Mars through, you know, what we see on its surface. And it talks about all the latest research that's pieced together to sort of tell the story of Mars, essentially. And it's just a fascinating read. You know, it's obviously about astronomy in the sense that it's about Mars, but it's really nice to sort of read it in a geology sense.

It's something very different from what I do every day. You know, it's my job. So it's nice to read something a bit different. Nice. about you what's on your bookshelf yeah uh I've been looking at a couple of things um I should say I was actually vegging out with COVID over the last couple of weeks so wasn't reading anything erudite at all but watching endless box sets which I do recommend if you get it so um however back back to you know exercising my brain again um back in October it was observe the moonlight so I started to check out a book I'd been given a while ago Apollo's However, back to exercising my brain again, back in October, it was the Reserve of the Moonlight. So I started to check out a book I'd been given a while ago, Apollo's Muse, by two curators at the Metropolitan Museum in New York, Mia Finneman and Beth Saunders.

And it covers some of the themes that I did in my book with Alexandra Luska. But it's got fantastic photography and a really rich collection of images from the mid-19th century onwards. So I thoroughly recommend it if you like visual books. It's not just a coffee table work. It really covers the images in great depth. I was going to say it looks stunning. The image of the moon on the front cover is absolutely gorgeous. It's absolutely beautiful.

It looks like 3D to me. I think that's the sort of plastic covering on it giving it that impression. But it's really, really nice. It's a fantastic thing. And there's lots of, you know, really early images from the 1860s and before and so on, just when photography was getting going. You know, astronomers try these things out. I love seeing that kind of stuff. So do take a look at that.

And there's also, I have started to read properly Natalie Starkey's book, Fire and Ice. And I knew the premise of this talking about the moons and the volcanoes and cryovolcanism we've talked about in the solar system. But what I hadn't appreciated was just how they make the analogy with terrestrial volcanoes. So that technique of geology and geophysics gets applied across the whole solar system. It's this really nice symmetry, even though you're talking about ice rather than lava in some cases you know the techniques still work so again a really actually it's a really great overview of how this stuff is being done and you know talking about the different planets and it supplements the episode we had a few a couple of months ago as well oh yeah lovely um i've been dipping into the

invisible universe by matt bodwell oh nice yeah yeah it's good so it's it's for anyone that looks up at the night sky with just pure wonder so i think that is probably most of us yeah um and it's just a journey through things that we can't see with the naked eye um and matt himself is the public astronomer at the university of cambridge so it's just really well explained there's some history thrown in there too. And if there are some more complex topics, it just brings you along for the whole journey. It's really great. So that came out earlier this month. And then something which I would say is intended for children,

but I think it absolutely works for adults as well, is Will Gator's The Mystery of the Universe. Again, it's just taking you through page by page like these incredible illustrations of different parts of the solar system gorgeous galaxies all of that and it's just a lovely book I think for children that are starting to get a bit more interested in our universe but also has stunning illustrations to back it up as well so you mean i don't have any kids but i literally just keep it on the side i'm like oh lovely little coffee break look through that um so yeah i'm thinking christmas presents for my

daughter actually so thanks yeah yes let's hope she doesn't listen to this now come on that sounds so much like a book i had as a kid though as always you know what you mean you do like it seems silly to say our kids need the visuals but like it does capture your imagination and it was definitely one of the things that like inspired me to to become an astronomer and astrophysicist today so it's so important to have that kind of a book exist so that's great exactly i think it's really important and obviously we'll put all of our book recommendations in the show notes for anyone that wants to take a look. So questions about the end of the universe.

There were a few. They're pretty big ones. Let's see what we can do. Becky, can you take on this one from Domizaki? He asks, is there an upper limit to the mass of a super massive black hole i feel like some really weird things would start to happen if multiple super massive black holes merged could that cause the end of the universe great question because it's about black holes um my favorite thing it's amazing for me to think that we're actually living or starting to

enter the epoch where black holes might be reaching their most massive that they can get to wow i don't think a lot of people realize um but there's a black hole called ton 618 it's an incredibly bright quasar and we think it's around about 64 billion times the mass of the sun. So it's big. And people have tried to work out before what would be the upper limit to a supermassive black hole and come up with somewhere in the region

of 60 to 90 billion times the mass of the sun. So it's getting into that region. And the reason that there is this limit to how big supermassive black holes can grow, at least by what something we call accretion, not by, I mean,

uh, Domizaki mentioned sort of the merging together, but we'll get to that in a bit, but like by accretion where black holes are taking matter in under gravity, there's actually something that limits them. And it's the fact that you have something called the innermost stable circular orbit. The last point around a black hole that you can actually get something orbiting the innermost stable circular orbit, the last point

around a black hole that you can actually get something orbiting the black hole happily, right? Right. Yeah. And it's about three times further out than what we call the event horizon, right? Where you'd be traveling at the speed of light to escape. It's about three times further out. Now, as the black hole gets bigger, that gets pushed further and further out. It gets pushed beyond what we call the self-gravitational radius, where say if you put a star in orbit around the black hole or some gas that is usually the case, its gravity holding it together would be stronger than the gravity trying to pull it into the black hole terran part right so you get to the point where the the innermost stable circular orbit is beyond the self-gravitational radius so something will

always stay together essentially if you've got gas around there it will happily orbit the black hole forever it will never actually fall in fall into the black hole. Yeah. That's so weird, isn't it? I find that really hard. Really, like really weird. Okay. It's really strange, isn't it? And it comes back to this idea that like, yes, okay, black holes, you know, have very strong gravity, but they don't suck stuff in. We don't, you know, they're not these like hoovers or whirlpools that pull stuff in. And so we are reaching the limit where supermassive black holes are possibly the most massive they could get. But you could still merge two of those together or more of them, which I think is what Domizaki is saying. Now, supermassive black holes are found in the center of galaxies.

The universe is expanding and galaxies are getting further away from each other. So mergers of galaxies are getting rarer and rarer as the universe expands more. It's only those very close together where the gravity is strong enough, like Andromeda and the Milky Way to eventually pull them together. But most supermassive black holes are going to get really far apart. So it's not likely that we'd ever have some period where you have a huge sort of supermassive black hole mergers that would perhaps set off huge gravitational waves that perhaps could disrupt the universe in some way, like I think Domizaki's asking. So that situation is not likely at all. i think where people correlate supermassive black holes and and sort of the beginning or the end of the universe is because of this idea of a singularity right inside the event horizon we

mathematically describe black holes as infinitely dense in an infinitesimally small point which is how we describe the beginning of the universe too but they're very different singularities completely right so like a black hole singularity the the only direction you know for something if you think about like the future of of something around a black hole its only future is to end up in the singularity whereas you know the big bang theory you have the singularity at the beginning of the universe but the future was always moving away from that, right? Because everything was expanding. So it's two very different singularities. Right. Okay.

Well, Domizaki, I hope that answers your question. Robert, Brian Ross wants to know, is there an actual end of the universe or will it be some kind of phase transition that can bring about another universe? So stare into that crystal ball, please. Yeah, absolutely. And all I can say is I'm very glad we've got people like katie talking about this on the on the podcast as well um yeah so this is the idea that uh the universe somehow could you

know you could have another universe emerging in a stretched out our universe if you get the conditions where uh the sufficient energy dark energy. It's possible that that increases over time. Ask a decent theoretician about how this works. There might be circumstances in which a new Big Bang, a new universe could emerge in the far, far, far, far future. So it's an interesting philosophical question as well, I guess, as to whether or not there is an end to the universe. You know, if you just run the clock forward, I suppose you would say that when nothing is really changing, that sort of implies that's the end of it.

But it would still presumably run on into the future. So a very open question. And I think it's really hard to test as well. There is the WFIRST, the Nancy Grace Roman Space Telescope is supposed to be trying to understand how dark energy might have changed over time. So if we saw that changing, that suggests that this kind of thing could be at least possible, but it's pretty speculative, I guess. Yeah, okay. And Becky Alboran asks,

if neither the centre nor the edge of the universe is observable, then how can we possibly speculate on the lifespan? So first and foremost, there is no center of the universe. The Big Bang happened everywhere. Expansion is happening everywhere. I know as we look out, we see everything moving away from us, but that's our perspective. It's nothing to do with the universe itself.

Hop yourself to another galaxy, you'd see the same thing, right? We are the center of the observable universe because we can see the same distance in all directions around us. So you are the center of the observable universe because we can see the same distance in all directions around us. So you are the center of the observable universe. I love that. It's what I tell myself every morning. And we don't need to see the edge of the universe either because we don't need to measure the size to work out how long it's been around for or know what's going to happen in the future, right? We're not measuring the speed of expansion, right? Which would be, you know, like the speed of your

car. We're measuring the rate of expansion and the rate of acceleration, right? So it's a very different thing that you're measuring. The unit that we measure it in is basically a distance divided by a time divided by a distance. So the two distances cancel each other out and you get something which is just like one divided by time, which if you invert, you get time, you get how long it's been around for, right? Because you don't need to know the size, you only need to know the rate at which it's expanding and you can work out, well, okay, well, how long ago was that at zero? And that's how you know how long the universe has been around for. And as we've discussed all the way through the end of the universe, we just need to know this balance between matter and dark energy

to work out what will happen. We don't necessarily have a timeframe for that. There are some estimates depending on, you know, if we were to have a big crunch, if we were to have a big crunch if we were to have a big rip how long would that take but it's not like we've put a date in the calendar for tuesday the 29th of november in you know the 100 trillion years time or something right there's there's not a specific day that that would necessarily happen on okay well i hope that answers everyone's questions there's a lot in there. But if you want to send in questions for a future episode, then email podcast at ras.ac.uk or tweet at Royal Astro Sock. And these actually can be about anything. Our January episode for 2022 is going to be a Q&A special. So if you just want to send in an email, then just do it. Now's the chance. special so if you just want to send in an email then just do it now's the chance so robert what can we see in the night sky this month hopefully it's not the rest of space moving towards us in a big crunch or a vacuum decay we wouldn't know because it would be at the speed of light so yeah it's if you if the universe is going to end i guess not knowing about it at all

it's not so terrible but anyway being the way I'd want to go. Being more positive, being more positive. Yeah, it's a nice time of year to be looking up at the sky. You've still got Jupiter, Saturn and Venus low in the sky now heading towards sunset. Absolutely beautiful if you're out. Now it gets dark at sensible time when you're coming home from work. Have a look along the southern horizon, this beautiful, bright Jupiter, somewhat faint to Saturn and Venus, very low down. But if you have a good horizon and a pair of binoculars, you can see it as a lovely crescent. So do have a look at that.

That's a really special sight. With Jupiter, Saturn and Venus nicely along the southern horizon, if you watch on the 8th and the 9th of December, that's when the crescent moon will be moving past them. So that's always another great photo op as well. Who doesn't like a crescent moon or as Becky refers to it as something else? Don't set her off. We won't say it. It freaks some people out. I just love how they're all lined up as well. Cause it's just a reminder of like the ecliptic and the plane of the solar system, isn't it? When you see them all in a line like that. It's, it's really fantastic view. And I've seen some really fantastic photos. And please tweet those

as well if you're doing that. Even a smartphone would probably pick them up pretty well. But the crescent Venus is always a really, really nice sight. So there is that. And then we're moving into winter. So you've got the fabulous constellation of Orion is getting easy to see now. The hallmark of the winter sky. the, you know, it has a red star, Betelgeuse at the top left corner in the northern hemisphere, Rigel at the bottom left, and this blue-white star, and this beautiful sort of box shape with a belt and a sword, and hanging down in the middle of the sword is the nebula. So, you know, pick up a pair of binoculars, it's very easy to spot. And also, more of a challenge, but actually not too hard to find. I thought about,

you know, Orion, we talk about that a lot. It's an obvious constellation. Above Betelgeuse in the top left corner is a star cluster, which is sometimes called the 37 star cluster, because it looks like the number 37. Now I'm not making this up. There's not some kind of, you know, divine intervention. It really does look like that.

And if you look at it upside down, it looks like LE. So do try and take a look at that. It's just above Betelgeuse. You should be able to see it with a pair of binoculars. And the final thing I have to mention is that we have a comet that's bright enough to pick up, at least with binoculars, called Comet Leonard. It goes by the name of C2021A1. It was the first comet discovered this year. It's currently underneath the plough at the end of November, early December. It'll be moving down fairly quickly uh and the first week of december should be a good time to spot it so the time the best thing to do i'm afraid is to get up quite early before dawn although that's that's less hard in the middle of winter and look due east underneath the plow get a

finder chap to find it because you know it's not going to be like near wise last year it's not going to be obvious obvious with your eye but it might be bright enough to see if you're in a dark sky place with your eye you know so so do uh do try that too but it's a nice christmas treat or at least early christmas treat because by the uh the middle of the month it's getting really hard to spot but that first week of december is a good time to check it out i bet a long exposure on a phone might pick it out if you probably like if you get up in the morning and crack the curtains and you sort of prop the phone against the window looking east and then just be like, I'm just going to leave it for 10 seconds,

I bet it might pop a bit at your picket door. I think it will, and I think I've seen loads of people taking pictures of it, and that's the great thing about even smartphones, the cameras are so sensitive that I think, yeah, there's a very good chance. And if you do try that, and you'll probably get an image with a bit of noise on it and so on, just compare it with one of the Finder charts, or if you really know what you're doing, you can get the data into programs like Stellarium,

which are completely free, and then you can see exactly where it is and then check it out. Yeah, I tried. I have tried, but I've struggled so far. But I live in quite a light, polluted place. I'm hoping that when the moon's out of the way, when it's a bit brighter in early December, it'll be a good time to check it out.

Well, I think that is it for this month. We'll be back in December with a chat about eclipses and expeditions. And we'll be starting 2022 with that Q&A. So get your questions in. Yes, send them all in. You can email podcast at ras.ac.uk or you can tweet them in to at Royal Astrosoc. Until then, happy stargazing, everyone.