The Supermassive Podcast - BONUS - Are we Living in a Black Hole?

Episode Date: September 9, 2025

What would happen if the supermassive black hole at the centre of our galaxy disappeared? Dr Becky is back to tackle your toughest of black hole questions. Alongside science journalist Izzie Clarke and Dr Robert Massey, the team also discuss what the night sky would look like in an elliptical galaxy…and are we living inside a black hole? Join The Supermassive Club for ad-free listening and share your questions, images and more. Or email them to [e-mail] or on Instagram @SupermassivePod.The Supermassive Podcast is a Boffin Media production. The producers are Izzie Clarke and Richard Hollingham. Hosted on Acast. See acast.com/privacy for more information.

Hello and welcome to another bonus episode of the Supermassive podcast from the Royal Astronomical Society with me, science journalist Izzy Clark, the Society's deputy director, Dr. Robert Massey, and astrophysicist Dr. Becky Smethers, the Queen of Black Holes is back. We missed you. Oh, thank you. Thank you. I'm so sorry that I couldn't come with you to all these amazing podcast recordings you've been doing over the past couple months. You got to meet more astronauts without me. I was so, so jealous. Yes, I forgot the cardboard cut out of you. I am sorry. Yeah, I just wanted to apologize for the listeners that I did miss that. For those who didn't know, we're just going to get a little bit sad for a moment,

but everything is fine. But, yeah, the reason that I wasn't there was, unfortunately, I was diagnosed with early stage breast cancer a few months ago. So I've been going to treatment through surgery, ongoing. But I'm feeling good, no pain or anything like that. And, yeah, back to work. distract myself and everything like that so yeah it's just a lot of waiting around it turns out but i'm doing it okay i'm particularly grateful that i don't have to take any more black hole questions

yeah yeah i'm i've come back because robert just begged me they didn't want to take any of the black hole questions it's all of so Becky and i were messaging over you know while we were doing all these recordings and she's like how have they been i's like there are significantly less Taylor swift references hey and i missed them okay good yeah that's fine i'm not complaining I need more. I try to drop one in for you. Okay, so obviously we've just had the Q&A.

We had loads of black hole questions. So Becky, we've got more for you. Okay, great. So you haven't missed out too much. So let's start with this one from Sierra who says, Hi, Supermassive Pod. Thank you so much for causing monthly brain farts to my niece Maya of 13 and me.

I love brainfarts. Yeah, it's a great way. We were wondering, if a supermassive black hole at the center of a gallery, would just disappear. Puck it straight out. Yeah. What would happen to the rest of the galaxy?

Since they don't make up a lot of the mass of a galaxy, as Dr. Becky said in a previous episode, good. Someone's listening. Some one's doing their homework. Would the thousands of planets and stars keep spinning because of the momentum, orbiting nothing, or would everything fly away? Thank you so much for keeping up the monthly brain files. You are welcome, Sierra and Maya.

This is a great question. It's one I absolutely love because I think people think about, you know, galaxies with their supermassive black holes at the centre as being very similar to like the solar system with the planets and then the sun at the centre. And yeah, if you removed the sun from the solar system, right, the planets would just fly off in every direction because there'd be nothing there to keep them bound. But yeah, the sun is what, 99.99 something percent of the solar system in terms of its mass. So it really is the most dominant thing, whereas the supermassive black hole in a galaxy is. is less than 0.1%. 0.01% usually is where the correlation tends to lie. And that does change with time.

JWST is helping us find some galaxies where the black hole is like 10% of the galaxy's mass, which is crazy and we don't quite understand those yet. But generally in the nearby universe, black holes make up less than a percent of their galaxy's mass in stars. So it's a really interesting sort of thought experiment to say, okay, what would happen if you remove the black hole? because it's such a tiny, a very dense, massive part of the galaxy.

But overall, it doesn't have that, you know, big of an effect. And actually, it turns out if you removed it, it wouldn't have an effect at all. The stras, the gas, everything on stable orbit around the centre would carry on regardless. Which is a weird thing to wrap your head around. And the interesting thing is the reason that this happens is the reason that the galaxy is there or any galaxy is there at all. and we're here at all to answer this question is because of something called self-gravity. If self-gravity didn't exist, we would all just be one big accretion disk of gas around a black hole, one big like swirling disk of material of gas just trying to make its way down into the black hole.

And we're not that, stars, planets, everything have formed instead. And it's because, yes, okay, the pull of the black hole's gravity is very strong, but at a certain distance away from it, it's weakened enough. I might still feel the pull, but it's weakened enough that the gravity between, say, two gas particles next to each other

is stronger than the pull from the back hole, which means that you can collapse a gas cow down into a star and planets can form around it rather than that star being shredded apart and being pulled in towards the back hole. So if you didn't have self-gravity, i.e. gravity being stronger in an object's self so it can hold together as a star

then we would just all be one big swirling like disk of material around the black hole instead of an actual galaxy with planets and people living on this planet to ask this question in the first place. Ooh, okay, great, thank you. And Robert Paul Whitmarsh asks, Dear Supermassive,

how would the night sky appear to inhabitants of a planet with an elliptical galaxy like M87, where the star system is at the same relative distance from the core that the sun is from the centre of the Milky Way. Would the absence of A-class and earlier main-sequent stars cause the sky to be barren of stars, or would the higher density of cooler stars still give those inhabitants enough stars to make their constellations?

Love the podcast, and wishing Dr. Becky a speedy recovery. Thank you, Paul. Paul, Wittmarsh, Loo's Astronomy Society. Yeah, hi Paul, and as listeners might guess, we know each other quite well from Lewis in East Sussex where we both live, and I hope his broken arm. Your broken arm is getting better too.

No, everyone's in the wars. I know, and clearly he's got time to write questions like this. Presumably one-armed on keyboard. It is a really good sort of head-scratching question. An interesting one. I mean, so elliptical galaxies. I mean, first of all, they have a lot less dust than spiral galaxies, like the one we live in, and far fewer younger stars.

And if you want to think about when we'll end up in that situation ourselves, as well, after the Milky Way has collided with Andromeda in about four and a half billion years, you know, strip out a lot of the gas material might cause a burst of star formation, but the aftermath might, oh, Becky's looking at me now, do you think? Well, you know, the, models debate.

If I was going to put my real, okay, this is my research, the hat on it. You've got to do this. The, andromeda is much bigger than the Milky Way. The mass ratio between Andromeda and the Milky Way, andromeda is much bigger than the Milky Way. So you could argue that this might not be what's called a major merger is more of a,

Even if it was a major merger, anyway, there's a lot of reports now that says if you merge together two spiral galaxies, yes, it will sort of at first destroy the spiral galaxy nature and become a big elliptical blob, but the gas will actually stick around and reform a disc afterwards and then start forming more stars. So it might not be the end of star formation in the moon. of star formation in the Milky Way, but still, carry on because it's a very good point and now I know. I was like, if we were writing the discussion section of this paper, then I feel like we should have. We'd have it, we'd have it. Yeah, this is very cool. Anyway, so now I have to think about it. Completely derailed.

That's all right. No, no, it's good. It's good to know. And so elliptical galaxies anyway, regardless, they have many more stars that are old and red. And they tend to be fainter because the brighter stars, the ones that, you know, if you go look at And the winter sky constellation like Orion has got quite a few bright stars that with relatively short lives and faint stars live a lot longer. So if you run time forward and you run out of material, then you end up with a galaxy that's got more and more red dwarfs as a proportion of its stellar content. So there wouldn't, if you were in that hypothetical position, you might have a higher density of stars.

It depends exactly where you are and what kind of galaxy it is. But you wouldn't have that kind of defined Milky Way in the sky because the shape of the galaxy is. different. You know, you're in an ellipsoid rather than in a flat disk one. And you might be looking towards a better kind of defined or a better brighter galactic centre because you haven't got as much dust there obscuring the view. So that's interesting in its own right. And a dim of view in other directions. And I think given how faint red dwarf stars are at some point when you run the clock forward, you're going to get to the point where these objects, you know, if you've got a galaxy that's made up, and mostly red dwarf stars, we can't even see the nearest one to the sun, which is also

the nearest star to the sun, proxona centauri, it's in the southern hemisphere. It's a hundred times fainter than the eye can see, so you need a telescope to see it. So I think, yeah, constellations pretty much disappear. You know, if the density is a bit high, you might have a smattering of stars in the sky, but it wouldn't be anything like as nice of view as we enjoy from the earth. So terrible, really. So, yeah, not a great view. Fascinating astrophysically, You know, you can see towards the center of these galaxies, see the black hole in the center, all that stuff much more easily. But, yeah, it wouldn't be the kind of exciting view that we enjoy on Earth. Ah, great question, Paul.

And Becky, Tommy says, hello, I'm a long-time listener and first-time questioner from Vermont, USA. I have a question that's been on my mind about living inside a black hole. If our universe is inside a black hole, how are galaxies moving apart if everything that goes inside a black hole? if everything that goes inside a black hole gets pulled towards its singularity even if it's theoretical that we are inside a black hole wouldn't the expanding universe

immediately disprove being inside a black hole or could that possibly be the end result of the big crunch and we will eventually meet our end inside this black hole cheers I feel like I've said black hole so many times I'm just like it doesn't make any sense anymore welcome to my life yeah good question Tommy

and I think you really hit the nail on the head they're like, wouldn't the expanding universe immediately disprove it? Pretty much, yeah. I think what's interesting with this is that people gravitate toward this idea of, you know, the universe being inside a black hole. I don't just mean people who's in the general public. I mean, as I should have this as well. We've seen a few papers come out that sort of explore this idea because there's so many

similarities to, if we think about what a black hole is made up of, we've got what we think is, you know, called the singularity where all the matter is condensed into, you know, a tiny, tiny, infinitescally small, infinitely dense point in the very center of this sort of sphere of unknown space where we don't get any light from, the edge of which is this horizon. So there's this horizon that we can't see over into the black hole. And then we think about the universe, right? And we think about looking out into the infinite space and there's this cosmic horizon where we don't get any light from it because the universe hasn't been around for long enough for

like to reach us yet at the speed that it's traveling at. And if we think about the expanding universe and we rewind time, we end up with, you know, this idea of the Big Bang theory where all of time, space, matter, everything was condensed into an infinitely small, infinitely dense point that we know as a singularity. So people are like, well, that seems very similar to what we know of about a black call, which is where people get this idea from. But despite that, there is a whole load of evidence to the contrary, right, including the fact that the universe is expanding, as you say. And there are some people that go, well, maybe white holes exist, which are like the opposite of black holes. And maybe that could explain how the universe is expanding. But it's

one of those things where it's like, you know, it's a straw man argument, right? Where it's just the house of cards, just all waiting to tumble and fall because there's so many like assumptions that you have to get to from there. And this idea did crop up fairly recently. And I'm wondering, tell me why, whether that's why you've asked this question, but there was a paper that got quite a bit of coverage sort of in science media in the past few months, which was a paper that came out looking at the spin directions of galaxies seen with the James Webb Space Telescope. So it's about 250 of them that basically someone said, oh, two thirds of them all rotate, I think it was clockwise and a third of them anti-clockwise or the other way around, whichever way around it was.

It doesn't matter if we flip ourselves to the side of the universe. It would have been the other way around anyway. So saying, oh, well, if there's this, like, you know, sort of preference for the galaxies to spin one way or another, that suggests there's some overarching force that wants them to do that. So perhaps it could be the fact that we're in a spinning black hole. And that's why, you know, they're spinning one way or the other. That was like 250 galaxies in the other universe.

And when that, you know, when that went sort of, like, quite viral on sort of like, you know, sort of science media, I was like, well, hang on a minute. like a load of people have done this with galaxies in the local universe with like a million of them and found no difference like that it was 50-50 so I think I trust the study that had a million galaxies rather than 250 necessarily obviously that's the local universe not the early universe and telescopes like you could for example are probably also going to probe this and that's going to be a you know a full sky survey so we'll be able to test this again and you know in the earlier universe not just the local universe so you know never say never but I

I think there's a lot of evidence to the contrary that we're inside a black hole in the universe. And yeah, one of them is definitely the expansion of the universe as well, Tommy, as you said. Amazing. Okay, thank you both. And keep the questions coming. It's at SupermassivePod on Instagram or email your questions to podcast at rass.ac.com. And we'll try and cover them in a future episode. We'll be back in a few weeks time in an episode about major space weather events.

You haven't heard of the Carrington event. Get ready. because this is the one we're all scared of. Until then, everybody, happy stargazing.