Why didn’t the big bang collapse in a singularity? What would a universe-sized black hole be like? Now that I think about it, do we live inside a giant black hole already? I discuss these questions and more in today’s Ask a Spaceman!

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Episode Transcript (Auto Generated)

Let's start by crushing you at first. Not much, just a little pressure here and there. But the pressure doesn't stop and we compress your body into a volume no bigger than a centimeter on a side, we don't stop the molecules of your body break down into their component atoms unable to remain against the pressure. We keep going, your body becoming incredibly unnaturally dense. Eventually electrons get squeezed out of atoms and then shoved back in by the extreme pressure. Almost all the particles of your body convert into neutrons, we keep going. The neutrons are able to resist the crushing pressure due to a strange quirk of quantum mechanics, but only for a while, they too succumb to the constant squeezing at some point. When the mass of your body is compressed into a volume, roughly the size of an atomic nucleus gravity takes over the gravitational pull at those densities is so high that no other known force can compete against it.

Once we reach that point, we don't have to keep the pressure up. Gravity takes over the work from here on out. It continues pulling on the material that once made up your body into a singularity. A point of infinite density surrounding that singularity is an event horizon. The mathematical edge of a nucleus size human mass black hole. If we were to repeat that process of never ending, squeezing on the mass of the earth, we would reach the tipping point. Once we compressed the earth's mass into a volume, roughly the size of a bean creating in the process, a bean sized black hole weighing nearly six times 10 to the 24 kg. That's a big bean. Giant stars are capable of creating black holes on their own once they die. And those black holes weighing a few times the mass of the sun are merely a few miles across smaller in diameter than a typical small town Sagittarius, a star, the black hole in the center of the milky way galaxy may have started out in a mall, but it's been feeding on material for billions of years with every gram of gas that it consumes.

It grows larger in both mass and radius. It currently weighs around 4 million solar masses giving its event horizon a radius of around 44 million kilometers. That's over 30 times the width of our sun. If we were to place Sagittarius, a star in our own solar system, it would stretch to the orbit of mercury completely consuming it. M 87 star. The black hole made famous by the image taken by the event horizon telescope weighs around 6 billion solar masses. Its event horizon is large enough to just barely fit our entire solar system inside of it. The largest known black hole with a precise mass estimate is known as ton 618 or ton 618 or ton 618. It's up to you how you want to pronounce it, which weighs somewhere around 40 billion solar masses. Its event horizon wouldn't just engulf our solar system, it would dwarf it. You could travel all the way out to the orbit of Pluto.

A journey that takes our best and fastest probes nearly a decade to complete and only be 1/80 of the way to to 61 eight's event horizon. Now, if you were to take all the matter in the observable universe, every floating dust grain, every atom of gas, every particle of dark matter and compress it into a single black hole. You would end up with a monster whose event horizon had a radius of roughly the size of the universe. Wait a minute, a black hole with the mass of the universe would have the size of the universe. Now, that is an interesting coincidence, don't you think in coincidences don't always demand explanations in science. Sometimes you know, a neutron star is just a neutron star after all, but they do pique our curiosity and they're worth a second look. So let's let's take a second look. If you were to compress all the material of our universe into a singularity. It would form a black hole the size of our universe.

Given that gigantic coincidence, what do we do with it? Well, let's compare and contrast black holes in the universe to see if we get anywhere. Let's see if anything interesting pops up. So the number one defining feature of a black hole is a singularity. It's a point of infinite density. You take all the material, all the mass that makes up a black hole and you crush it down to a literally infinitely tiny point, an infinitesimal a geometric singularity. Now you and me are we, we know the score, we know that singularities don't actually exist. Uh But we, but the singularities come about through our understanding of gravity, which comes about through general relativity. In general relativity says that singularities appear once you uh allow gravity to take over and overwhelm all of all other forces, we don't actually know what is happening inside of a singularity because we don't have a better theory of gravity. If we had a better theory of gravity, I would be able to tell you something else, but we don't have it.

So I can't. So right now, all I can tell you is that it is a singularity. That is what a black hole is. It is a point of infinite density. Now defining feature number two of a black hole is an event horizon. An event horizon comes about from the existence of a singularity. It is a consequence of the existence of a singularity in the event horizon. It's, it's not like the surface of a planet. It's not like uh even the upper layers of the fuzzy atmosphere of a gas giant. It, it, you can't see it, you can't touch it, you can't smell it. You can try to taste it, I suppose. I don't think you'll taste anything. An event horizon is a mathematical object. It, it just tells us if you get too close to a singularity, you can never escape. That's all it tells us. And this is the, the defining limit that if you get close but not too close to a singularity, then if you work hard enough, you can leave. But if you get too close, if you cross this invisible boundary, this mathematical line, then you are too close to the singularity and you will be trapped by it forever.

And in fact, you will eventually reach the singularity and be crushed to oblivion. The event horizon marks the boundary between what happens in the outside universe out here. And then what happens inside of a black hole? We can't see inside of a black hole because nothing can escape. That's why they look black. But you can cross the event horizon and be alive in principle, especially if it's a big enough black hole, you can be inside of that event horizon. And there's the singularity over there, there's the outside universe which you can still see because light is still pouring in from the outside, you can never go back, but you can see outside eventually you'll die, eventually you'll hit the singularity. But you know, you got some time the event horizon separates the universe of the black hole from the universe of you know, the universe. OK. So that's defining features. Number one and number two of a black hole. So let's ask, does the universe have a singularity? 00 yeah, it does. We call it the Big Bang. It's a point of infinite density where all the matter and all the stuff, all the radiation, every speck of dust was crunched down into a point of infinite density.

Yes, I know. Once again, it's really probably something else but we don't know what that something else is. So for, for right now we're gonna go with singularity. OK. What about defining feature? Number two? Does the universe have an event horizon? Uh Yeah, it does. Our universe is expanding and, and the cool thing about expansion is that very quickly, very easily and very naturally you can get expansion that goes faster than light. Remember the speed of light limit is a speed limit for how things can travel through space. It's not a limit to how much space itself can behave. You will never ever see a rocket ship blasting in front of your face going faster than the speed of light. That's what that speed limit is all about. But uh like the current expansion rate of the universe, we know very precisely and we're arguing over the finer details, but that's a different episode. It's around 70 kilometers per second per mega parsec. That means if I look at a galaxy one mega parsec away, 1 million parsec which is around three I three and change million light years.

If I look at a galaxy one mega part SEK away, it is receding away from me at 70 kilometers per second. That's the speed. If I look at a galaxy twice as far away, two mega parsec away, then that galaxy is receding away from me at twice that at 140 kilometers per second. If I look at a galaxy that is 10 mega parsec away, then it is receding away from me at you guessed it 700 kilometers per second. This is just because the more distant galaxy has more space between us and it to do the expansion. And so it picks up a greater speed. You can do this at home. You can buy a, a giant rubber band, I don't know from giant rubber bands dot com. Put you at one end, put your friend at the other and then start to have someone else pull it, stretch it at a constant rate. And if you uh you know, get a, a radar gun and start measuring the speed at which they are receiving away from you the farther away they are, the faster they will be receiving because there's more rubber band to do the stretching for you.

That speed just goes up and up and up and up and up. And there is a point a distance about 13 billion light years away where the speed crosses the speed of light. What that means is, yeah, there are Galaxies on the other side of that invisible line. It's a big universe out there. And we can see those Galaxies because they emitted their light billions of years ago. And the light is just now reaching it and it was oh cool. There's, there's a galaxy farther away than 13 billion light years. But it means that that galaxy is receding away from us now faster than the speed of light, which means if we got in a rocket ship, we can never reach it. And eventually the light from that galaxy will fade away to nothing. Folks. That's an event horizon. There's a part of the universe inside that invisible line that I can access. And there's a part of the universe outside that invisible line that I can't just like being inside of a black hole. If I cross the event horizon of a black hole, I can access things inside the event horizon. If, if you come in one end and I come in the other, we can meet up, we can shake hands, you know, we'll, we'll die together.

Maybe we should hold hands while we're dying. Isn't that romantic? And we can see the light coming from the outside universe that's filtering into the black hole, but we can never reach it. Same as the universe. So, so wait a minute, black holes have a singularity. We black holes have an event horizon. We have a singularity. We have an event horizon. This led some researchers in the 19 seventies to wonder aloud if the universe was indeed a giant black hole and were living inside of it. That's fascinating, isn't it? They share these two essential ingredients in common except I don't mean to be a wet blanket in my episodes. But here we are, there's one tiny little, almost insignificant difference that is going to completely and totally forever ruin the fun, sorry, not sorry. And that's the fact that the singularity of a black hole is different than the singularity of the Big Bang. And that has to do with time. We need to take a quick break folks and I need to tell you that this episode is brought to you by better help.

You know, life can get mildly complicated occasionally and sometimes you are faced with very tough decisions where the path for just isn't clear. It's hard to weigh the benefits or like both options are terrible. I remember navigating so many choices in my career of what major to go. When I remember in college, uh faced with this decision, I was originally a computer science major and I was faced with this decision of potentially going into physics and that was a tough, like unclear decision and I ended up making it and never looking back and never regretting it. But like, life can be tough and it'd be nice to know the future. Um, it's kind of fun that you, we don't, but like, sometimes not knowing the future is a little bit difficult. Therapy can help just help you organize your thoughts and clear your thinking so that you can take control of your own choices. If you're thinking of starting therapy, give better help a try. It's all online. It's designed to be convenient, flexible and suited to your schedule.

You just fill out a brief questionnaire and off you go let therapy be your map in life with better help visit, better help dot com slash spaceman today to get 10% off your first month. That's better help he LP dot com slash spaceman. If you cross the event horizon of a black hole, boom, you are now inside of a black hole. The singularity, the black hole singularity is in your future, you will hit it. You are guaranteed by the iron laws dictated by Albert himself to reach the singularity in a finite amount of time. According to a watch on your wrist, your local time, you will strike the singularity. The amount of time depends on the mass of a black hole for something like Sagitta say star, you've got like seven seconds cross the event horizon, seven seconds later, you will hit the singularity. You can't escape it. It's kind of creepy no matter where you turn, the singularity is always in front of you and you cannot stay still inside of a black hole and you just keep moving towards the singularity, no matter how hard you try.

It's really weird and creepy but it's there and the singularity lies in your future. But the cosmological singularity, the Big Bang singularity that's in our past. Yeah, it was a singularity and everybody in the universe can point to it, but you can't go there, you can't reach it. You can't travel into the past of our own universe and travel to the cosmological singularity. They're different. That was the 19 seventies when this idea was floated around in the 19 nineties, we discovered dark energy which just rains on everybody's Cosmological parade. The expansion of the universe is accelerating and that very, very much does not look like a black hole. The inside of the event horizons of black holes is not expanding. Yes, if you accumulate matter like the black hole can get bigger, but the inside of space is always rushing inwards towards the singularity, not outwards towards the event horizon. Now there are extensions, modifications to general relativity that you know, to say that maybe kind of sort of the universe is a white hole, you know, white holes are the mere images of black holes.

You still have a singularity, you still have an event horizon. Uh But the difference of a white hole is, is constantly spewing stuff out if you are inside of a white hole and you will be ejected from it faster than the speed of light. And if you are outside of a white hole, you can never get inside of it. And that maybe that's like the universe because like here we are being accelerated away from a singularity question mark. Now, in order to make this work, it doesn't work in vanilla general relativity, you have to add extensions and modifications, you have to work to what you hope to be, you know, a a quantum theory of gravity or something or just something different than Einstein. Ah but extensions to general relativity have been largely ruled out. Thanks to Patreon. That's Patreon dot com slash PM. Sutter is your contributions that are killing any extensions and modifications to general relativity. So keep it up and keep Einstein's dream alive in this show. Thank you so much for your support. That's Patreon dot com slash PM Sutter. No, it's not that it's not Patreon. I was just kidding. It's killing novas, you know, these, these observations we made a few years ago where neutron stars merged together and we saw both the gravitational wave from that event and the flash of light from that event that allowed us to test a lot of theories of extensions to general relativity.

A lot of modifications to general relativity, almost all modifications to general relativity say that the speed of gravity is different than the speed of light. It's slower. Turns out they're not, they're like the exact same to some ridiculously insane precision. And so we can just rule those models out. And it's those models that say that you need in order to get the maths work to say that our universe is a white hole. So it doesn't look like our, we're living inside of a white hole either. OK. So the modern day universe doesn't appear to be a black hole. But what about the early universe? I mean, there was literally a singularity in our past. So what stopped all the matter in the early days of the cosmos from collapsing back into a giant black hole? Why didn't it collapse? Like how did we get a universe if we know that a long time ago, our universe was much smaller and much denser. Why didn't it just form a giant black hole? Why are we here? One word density? No, wait, I need two words, density difference. We have to ask, what is a black hole?

I know I I listed its two defining features singularity and event horizon. But what is it? It's a thing that exists in space time. Planets are things that exist in spacetime stars are things that exist in space time. You define a planet. By contrast, you take a bunch of rock and water and squish them together in a little ball and then outside of that ball, there's the vacuum of space. Boom planet. You take a bunch of hydrogen and helium gas and squeeze it down to such high densities that it triggers nuclear reactions in the core. And then you have to surround that ball with nothing with empty space time. Boom, that's how you define a star. You define it through contrast, black holes are objects that exist in space time. In order to build a black hole, you have to take a bunch of stuff. It doesn't matter what take and have it exist in space and time and then squeeze that down into a very small volume so that you get your point, you get your singularity, you get your event horizon, you get all that good stuff and then surrounding the black hole, you need empty space.

Another way to put it, you need to start with empty space and then you cram a bunch of stuff in the middle of it. Boom, you make a black hole, you needed a density difference in order to trigger the gravitational collapse of a black hole. If I have a bunch of atoms and they're all spread out uniformly and they're moving around, there's no density difference. You can't form a black hole. You need a contrast, you need some more material over here than over there. So you can get a density difference and you can collapse into a black hole. You need stuff in empty space. The early universe was different. There was no outside. It was just the universe because if there's an outside, the the universe is literally all the stuff. And so if there's an outside, that's a side, that's a thing and that should count as part of the universe. The universe has no edge has no boundary, has no inside. It has no outside. It is just the universe. Yes, our early universe had incredibly high density but it had high density everywhere. Everything was equal.

There were no differences and so black holes couldn't form. So black holes couldn't form in the early universe because there were no density differences. You couldn't get any, any little hook for gravity to do its thing. Because as soon as there's a little difference then that that little pocket of extra high density has a little bit more gravitational pull. And so it can start the process of building bigger and bigger stuff and collapse into a black hole. But with no density differences that can happen. But what about the whole thing? What prevented all the material from collapsing? Well, we have to account for the fact that the early universe was dynamic, it was expanding, it wasn't static, it wasn't just sitting there. That's, that's another thing about black hole formation. You need static featureless space time as your background as your stage that you use to start building your black hole. But the universe you now today is expanding. And in the past, it was also expanding, it was moving, space itself was expanding.

This makes everything different because space itself is expanding. You need an enormous amount of material to rec collapse. We don't know how the universe started. That's a different episode. Feel free to ask and we can talk about the options and the ideas out there. But what we do know is that our universe started in an expanding state, that's what it was. And so there was all this material and yes, you can fill up a universe with stuff and if you have enough stuff, like if you imagine the present-day universe and it's expanding and it's full of matter, you imagine all that gravitational attraction of all that matter, all working together to slow down that expansion to overwhelm it and then reverse it and turn the Big Bang into a big crunch. And it's very worthwhile to ask why that didn't happen in the early universe. Well, if honestly, if it can't happen today, it was never gonna happen. The universe never had enough stuff to recap.

And just to be clear, that wouldn't lead to the existence of a black hole. If all the material in our early universe collapsed, it would not create a black hole. Why? Because a black hole is a thing that exists within space And if you're recaps the entire universe, you are collapsing space itself. Remember space right now is expanding. If you were to reverse that, then space would be contracting and all the universe, not just the stuff, but space itself would collapse into a singularity. You would have a breakdown of the entire space time framework itself in that singularity that's different than a black hole where you take a bunch of stuff and you create a puncture, you create a point within spacetime spacetime still exists around a black hole. There is no such thing as around the universe. So if you recaps the universe, you rec collapse all of it. It's still a singularity for sure, but it's still different than a black hole. And honestly, we never had enough stuff, we never had enough stuff to do it.

Why? Well, we don't know that's also a different episode. I love it. How every episode about black holes leads to 10 more potential topics. Feel free to ask. And also thank you to Michael P on email, Don Dion. Email, Nicholas be on Facebook, David Kay on email and John and Amelia F on email for the questions that led to today's episode. I hope you enjoyed it. Please consider going to patreon dot com slash pm, Sutter to contribute two more of these episodes. I'd like to thank my top Patreon contributors this month. We've got Justin G, Chris L. Barbara K Duncan M Corey D, Justin Z Naia Scott M Rob H, Justin Lewis M John W Alexis Gilbert M Joshua John S Thomas D Simon G Aaron J, Jessica Kay and Valerie H that's patreon dot com slash PM. Sorry, keep those questions coming and I will see you next time for more complete knowledge of time and space.

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