Instant Genius - Our Universe could be trapped inside a black hole with no way out

Episode Date: December 8, 2025

It sounds like a theory plucked from the page of a science fiction novel, but according to Enrique Gaztañaga from the University of Portsmouth, our entire Universe could be trapped inside a black hole. It's a mind-blowing theory, but it could help us better understand the fundamental nature of our reality. But how is it even possible for us to be inside a black hole? Will we ever know for sure? And what could lie beyond its boundaries. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Hello and welcome to Instant Genius, a bite-sized masterclass in podcast form. Every Monday and Friday, you'll hear world-leading scientists and experts talking about the most fascinating ideas in science and technology today. I'm Ezi Pearson, commissioning editor at BBC Science Focus. In today's episode, I'm talking to cosmologist Dr. Enrique Gastanaga. Enrique is well used to exploring the mysteries of our universe and trying to answer one of humanity's most fundamental questions, how did all of this come to be? But his most recent study puts forward a rather unusual idea

that feels like it should be more at home in the pages of a science fiction novel. Apparently, it's possible that our universe and everything in it could actually be trapped inside of a black hole. Welcome to the show Enrique, Can you explain to us how you came up with this mind-blowing concept? Well, I guess the first thing to understand is what a black hole is, right? A black hole is an extremely simple object, but is a fascinating one. So it all relates to a concept that is what is the speed of propagation, you know, like when something happens, you know, at what speed, that happened,

propagates in space. For example, the light that we see from the sun, how long has it take to reach us? That will be an example of propagation. So according to the Newton's law, the propagation between, for example, the gravitational force between the sun and the Earth was an instant effect, instant propagation. And what this means is that the speed of propagation is infinite, that, you know, things propagate very fast. And one thing we have learned in physics that is very important is that infinites is a very nice mathematical concept that we use all the time to do all kinds of calculations, right? But it's an idealization. It's not, it doesn't correspond to anything in physics. In physics, nothing is infinite. I think this is a

very important theme to understand this concept of a black hole and also the black hole universe. So what happened is that James Bradley realized and show empirically that the speed of the propagation of light is finite. It's very, very fast, but finite. In particular, it takes like seven minutes for the light from the sun to reaches. But then when we see distance stars, it might take years, sometimes hundreds of years, and sometimes even hundreds of thousands and hundreds of millions of years, right? So when we look at the cosmos, it's really a time machine.

If the speed of propagation was infinite, then we will see everything about the universe at once. So the light coming from the distant past will come instantly to us, and it will be a total mess, right? You will see everything together at the same time. But this is not what we see. In astronomy, we see very distant objects, and we can see that they are really all. And this is because light has taken a long time to reach us, right?

So that's the most important thing to understand what a black hole is. A black hole comes from this idea that there is an escape velocity. Gravity attracts things, and it has an attraction. energy that bound things to Earth, right? But as you know, we can launch satellites into space. And this was achieved by passing a velocity above some critical value. This is called escape velocity. Now, the escape velocity depends on the mass of Earth. So given the mass of Earth, an object in the surface, a rocket in the surface, needs some velocity to be able to to fight back the gravitational attraction energy and to escape into space.

Okay. But now I mentioned that we have James Bradley and many other astronomers have figured out that there is a maximum speed of propagation, which is it coincides with the speed of light. It's this 300,000 kilometers per second. Now, the escape velocity will be the velocity you need to escape from the attraction of a planet. But now if you, what happens is that if the mass of that planet, for example, if you increase the mass of Earth by many thousands, and you make it very massive, then it turns out that the escape velocity will be larger than the speed of light, than the speed of propagation. right? So if the gravitational attraction is so large because you have a very big mass, there is a point where the escape velocity is larger than the maximum speed that you can have to escape, right, or the maximum speed that is allowed by the loss of the propagation.

So this is a fundamental thing we have found. It's called relativity, because this connection between the propagation speed, which is a maximum value, relates basically space and time. And this is why many times when we hear astronomers of physicists, they talk about space time. And the reason why space time exists is because they are linked by this maximum speed of propagation. And so a black hole is basically an object

such that the mass of that object is concentrated on a very small radius, so that the... escape velocity is larger than the maximum speed of propagation. And what this means, basically, is that you cannot escape. That's the summary, right? So basically, the other way we can make our earth a black hole is imagine we can squish earth into a smaller radius.

Then because the same mass, you will be the radius, instead of being 6,000 kilometers, you should make the radius a few centimeters or millimeters, and then the gravitational energy will be so large that you will need escape velocity bigger than the speed of light, bigger than the bigger possible propagation speed. And what this means is that you generate a black hole, which means a trap surface.

We call this the event horizon. Event horizon means is the radius at which you can, can you need to put a mass so that you cannot escape out of this so this this concept might look a bit complicated but in terms of physics the diameter of the sun is 700,000 kilometers if you put all the mass of the sun which we call a solar mass into 2.9 kilometers so you have to squeeze it from 700,000 to 2.9 right so this is like a a huge squish. But if you do that, then the sand will become a black hole. What this means, basically, is that nothing will escape from the event horizon from these 2.9 kilometers.

Because basically, you will have to overcome this propagation speed, which is a maximum according to relativity. So basically, it's just a question of squeezing things. And what is a black hole then? Well, a black hole is just a something that has an event horizon, which means it's a trap surface. That's the concept of a black hole, right? So, you know, this idea was proposed long time ago, even long before Einstein, by several scientists. And recently, in the last, you know, in the last 10, 20 years, we have a lot of evidence of different kinds of black holes. We have found direct evidence of solar mass black holes, so black holes that have a few solar masses. But we have also found that in the center of most

massive galaxies, like ours, like the Milky Way, there are supermassive black holes which have millions, thousands of millions of solar masses. Each galaxy has such a supermassive black hole in the middle, right? So black holes are appearing everywhere. The example I put to understand black holes, which is the scale velocity, will be like, we call it a Newtonian analogy of a black hole.

In the theory of general relativity, the way we understand black holes is by understanding that mass and energy creates what it does, it causes a space time to curve. It creates.

it creates curvature. So you can, you probably have seen these images where you see like a sheet of some elastic material. So it could be like a sheet of clothes, right? And then you tense it out and then you put a heavy ball in. And then you see what happened is that what is a flat sheet, it curves. That's exactly how we can picture that. And then the planets, the reason why the orbit is not,

because there is a force, a gravitational force, but because the matter has created a curvature. And that curvature deforms space time. And then the planet just follow the curve space. So then black holes will be just the same as the sun. It can have the same mass, but it's just much smaller. is shrink. But from outside, it creates the same curvature.

It looks the same. It's only when you get very close to the vent horizon, very close to the surface, that then you see that the curvature is very high. And therefore, you will have very strong tidal effects that will make your life very difficult to be very close to the vent horizon

of a black hole. But it turns out that if the mass of the black hole is not like the solar mass, but it's much bigger. So I mentioned supermassive black holes, right? Like this is the center of a big galaxy, like the Milky Way or Andromeda Galaxy. These supermassive black holes has thousands of millions of solar masses. Then for those black holes, the event horizon corresponds to a much larger radiance. So basically, if the radiance,

of a solar mass black hole is 2.9 kilometers. If you say, well, what is the radii of the black hole in our in the Milky Way, in the center of the Milky Way? So that one has like 4 million solar masses. So then it will be 4 million times 3 kilometers. So now we're talking about 12 million kilometers. So now it's pretty big, right? And because this event horizon is bigger and big,

for larger and larger black holes of bigger and bigger masses, then the tidal forces and the curvature around the bend horizon are not so strong as for a solar mass. They are much milder. And in fact, if you extrapolate this idea and make it bigger and bigger and bigger, you can ask, okay, what is the biggest black hole we can imagine? Right?

And that will be a black hole with a mass that we observe in our universe. So let's take the whole universe and put all that mass into a black hole. Now the question is, what is that radius? Right? And this is easy to calculate. So if you have 100 billion stars, you have one galaxy. And then if you make a hundred billion galaxies, that's the universe.

So our universe have about 100 billion galaxies. Each galaxy has 100 billion stars. Then you ask, okay, what is the radius of a black hole that has the mass of the entire galaxy? And that radius is very large. It's about 15,000 million light years, right? So it's very large. So for that big radius, it turns out that if you calculate, okay, what is the largest distance we have ever seen in the universe? that's smaller than that radius.

And in fact, this is how I bump into this idea that maybe we are inside the black hole. It's because you might have heard about this concept of dark energy. When we measure the galaxies around us, they are all expanding away from each other. So they are expanding away from us, but they are also expanding away from each other. And we interpret this as the fact that the whole unit,

is expanding, right? This is a property of gravity again. So gravity, because it's attractive, it doesn't like a static universe. It wants the universe to either collapse or expand. But the idea is that gravity forces the universe to expand or contract in a very specific way. We have actually measured that. We are measuring that the universe is expanding, but the way is expanding, which we say is expanding in an accelerated way. It's contrary to the loss of physics. The loss of physics will predict that the universe has to be, if it is expanding, it has to be decelerating.

Why? Because gravity is attractive. So the reason for the expansion is some big band, something that happened in the beginning. But the idea is that the universe is expanding because it has some initial energy. but that expansion has to be decelerated because gravity is attractive

but what we find is that the universe is not decelerating is accelerating and this is something we cannot explain with the loss of physics that we know that's why we put this name dark energy we put like a name of something mysterious that we don't know the only thing we know about dark energy is whatever causes the universe to accelerate

okay Well, if you look at this in more detail, it turns out that what this means is that we are inside an event horizon. It turns out that this accelerated expansion, what it means is that there is a maximum distance towards which you can expand. So this is something similar to this event horizon of a black hole because I mentioned before, that once you are inside the event horizon, the escape velocity is bigger than the speed of light, and therefore you cannot escape, right? So you are trapped.

That's exactly what we are measuring. When we measure cosmic expansion, we find direct evidence that we are trapped. And you know, the distance towards which we are trapped, you can convert it in units of millions of light years, and is exactly the same as the mass of a black hole, as the vent horizon corresponding to the mass that is inside that distance, right? So when we do the cosmic balance of how much mass there is in the universe,

and then by measuring the expansion you measure what is the maximum distance, the two numbers together tell us that we are inside the black hole, right? Because it's basically it corresponds to the vent horizon of that black hole. So that's why this idea, of the black hole universe came. I'm not the first one to have this idea. There has been several other scientists through time that have come with this idea.

The way in our papers we understand it is aligned with the current measurements, with the more recent measurements of this accelerated expansion. So now we have more data to formulate a formal model of this expansion. But that's how it came in to be. be. I mean, that's how the idea came into, right?

This is a really interesting idea that we could be inside, our entire universe could be inside of a black hole. But if we are, what's going on outside of it and other universes in the black holes that we see in our own universe? Yes, very good. So that's the next thing, right? So basically what I was saying before is that empirically, we seen evidence that we are inside a black hole because of, you know, how much mastery is, what are the distance, there is an event horizon, so all of these hints in this direction. Now, what does it mean? How is that possible? Why we are inside a black hole, right? Well, this is related to the question I asked before. Why are we expanding and not contracting? So the way to think about that is

related to the question you ask, what is outside, right? So the hint comes from looking around us. What we see around us is stars. And stars, we know because we have followed them in this time machine that is looking at the universe, how stars form. And basically they form from a cloud of gas. So you have this cloud of gas that is, we call it an overhaul. density, some region of space that has an over density, then it starts collapsing. And this becomes very, very nonlinear process very quickly. This collapse gives rise to stars. And many times they give rise to black holes.

Because, you know, a star is a very delicate balance between pressure that comes from, it depends on the material that is collapsing, how much. radiation there is, what are the nuclear processes going there. There is some pressure that oppose the collapse, but this pressure is sustained by some fuel, for example, nuclear reactions, right? Eventually, these nuclear reactions, the ones happening in the sun, will consume all the, now the sun is producing helium out of hydrogen. When it finishes all the helium, all the hydrogen, it will start consuming.

helium and then it will start consuming whatever it's left and at the end there will be nothing else to produce pressure. Then gravity will win and it will make either a black hole or a big explosion we call it a nova or

a supernova. So that's what we see. Every day we look at the sky and we see new stars forming or black holes forming or supernova explosions. Now if this is what we see in our universe, you may wonder if this is also how our universe was created. Imagine now a larger background, much larger than our universe,

where one very large cloud collapse. And maybe that cloud is our universe that collapsed through gravity, and then it makes a black hole. So basically when a cloud collapse very quickly, especially if it is very massive, it makes a black hole very quickly because the bigger the mass, the bigger the radius, the event horizon is also called gravitational radius. So the gravitational radius associated to the whole mass of our universe, as I said before, is enormous. So it will make a black hole very quickly, right? And then it continues collapsing inside the black hole. But the problem is that once you collapse inside a black hole, there is no way out.

And so the idea is that there has to be a mechanism that after the collapse makes the universe bounce. And that bounce is what we call the Big Bang. Now the universe is expanding, but it now is inside the black hole, which means that it will never, there is an event horizon and it will never pass this event horizon. And that's what we call dark energy. So we measure this dark energy, we don't understand.

But it's very simple. Mathematically, you can show that this dark energy can be interpreted as this event horizon where we collapse. Wishing you could be there live for the big game, soaking up the atmosphere in the crowd. But too often, life gets busy. or the price hold you back.

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it really resembles the big bang that we see. Yeah. If black holes are, like, famously, nothing can escape a black hole. It's a singularity, everything's trapped within it. So does that basically mean that that's what the limit of our universe would be if we were stuck inside this black hole? It's just the event horizon. Yeah, I mean, one of the reasons why this theory of either cyclic universe

or bouncing universe are not very popular is that famously Penrose, Roger Penrose, who got the Nobel Prize in physics in 2020, precisely for this reason, right, put out some mathematical theorems saying that, you know, once you collapse into a black hole, then a singularity is unavoidable. Now, what is a singularity? Well, a singularity is an infinite.

Now remember at the beginning of my presentation, I was saying in physics there are no singularities, right? But in mathematics, there are. So Roger Penrose showed that under very generic conditions, once you collapse and make a black hole, then you will make a singularity. And that makes black holes a bit weird because immediately we know that this doesn't make sense. This is not physical, right? But we see black holes. Then what is going on inside? We know there cannot be a singularity

because singularities don't exist. There are no infinites in physics. But because nothing can come out, we look at the black hole and we don't know what is inside, right? So these are called sometimes they say, well, but nature does not have naked singularities. It's not naked because the event horizon protects the singularity. So protect us from seeing a singularity. A lot of physicists, and I'm talking like 50 years ago or 60 years ago,

think that the inside of a black hole hides the answer to a new theory of physics, one that combines gravity and quantum mechanics. And this new theory sometimes is called quantum gravity. And The idea of this theory is that, well, when you get close to a singularity, you are putting so much energy into a small piece of space, right? Into a singularity will mean that you will put all the mass of the black hole in a single point without extension, right? So the volume will be zero. The density will be the mass over the volume. So the mass is fixed, but the volume is zero.

It goes to zero and this creates, we call it a direct function, a direct delta function, which basically is a singularity. So that's what the general relativity, the theory that Einstein used to understand gravity, seem to be telling us, right? And so then physicists like Hawking and many others immediately so, well, wait a minute, when you put things into these very small scales, you have to take into account quantum mechanics, right? Because quantum mechanics is another theory that appear at the same time as relativity.

And basically, quantum mechanics, again, is this idea that infinites don't exist. So what is saying is that, yes, you can divide space or you can divide matter to very small pieces, but these pieces cannot be zero. You cannot divide as much as you want. And this is called sometimes the uncertainty principle, which is saying that you cannot consider infinitely small pieces of action. That's what the uncertainty principle tells us. And this applies to everything.

For example, there are not infinitely small pieces of energy, but energy comes in quantums, very small quantums, and that's why it's called quantum mechanics, right? So then a lot of physicists, once they understood what the black hole is and the singularity theorems, and even before that, they were trying to figure out

how to combine gravity with quantum mechanics. And they think the key is inside the black hole. The idea we propose is, that long before you reach these very scales where quantum gravity are relevant, which are, these are, they are called plank scales, because plank was a physicist that invented this quantum concept with a plank scale. So before you need to quantize gravity, like considering quantum of space time, which is what gravity is, you reach a state which is, which is a state which is called nuclear

saturation. This is very well known in astrophysics. So basically, there are objects like a neutron star which is like a super atom. This is a star that is as big as the sun in mass,

but is much, much smaller. So a typical neutron star has a radius of 10 kilometers. So just a little bit more than a black hole. And this is like a super, atom is basically it has the same density as the nuclei of an atom, but it has the size of a star. So it turns out that the density of a neutron star is the same as the density of a nuclei,

which I think is pretty remarkable. You know, quantum mechanics doesn't like you to squish things beyond some limit. Right? So you can put things together. but there is a maximum you can squeeze things together. And this is called the Pauli exclusion principle. Basically, what it means is that

if you take two neutrons and you try to squeeze them together, right? There is a point where they don't like to be on top of each other. They exclude each other. You cannot put them on the same position in space. And you can see that

this is very related to this singularity. So quantum mechanics, of the stars are already telling us that there is a limit towards which you can squish matter and when this happens what happened is that you have a collapse you are collapsing things so the collapse makes the density higher and higher and higher but it saturates it gets to a point where the density starts to not increasing anymore and so you get to a brief instant of time where the The density becomes constant and the radius becomes constant.

When this happens, you have like an infinite pressure going against you. It's a quantum pressure. We call it degenerate pressure. It's a pressure that doesn't like degenerate state. So quantum mechanics doesn't want to exist and to be in the same state. So the singularity is not necessarily avoided because of the quantization of space time by quantum gravity, but it's just a natural consequence of quantum mechanics doesn't allow you to squeeze matter all the way. And then what happens is that we see this

process in a stars. When you do that to a star, what happened is what? Supernova. The stars explode, right? And that's the big bang. So you are squeezing all these matter together. You get to quantum degeneracy state, and then you get a supernova, you get an explosion that doesn't like it to be a squish anymore, and that explosion, we believe, is the big ban. Now, what you have is an expanding universe, but you are now inside the black hole. Now, answering to your question, what is outside, right? I have to explain this a little bit to understand that, well, if we really form like a cloud in a bigger background,

you can imagine that outside you will find something similar to what we find inside. You will find other stars, other black holes of different masses. So you will find something similar to what we see inside. They will also

be outside. But we cannot see it because it's kind of hard to see something has to come inside the black hole to be able to see it, but even if it comes in, because we are expanding inside, it's going to be difficult for us to see something coming in. This is part of what we need to study more in detail. Like, what will be the consequences of our black hole being in the middle of a bigger background,

like another universe, like a parent universe, where there are other black holes like ours. So you can imagine that each of these black holes, if they are big enough, they will be like universes, right? So there will be universes. And then inside one universe, you have another one because there are black holes inside our universe. And inside those black holes, there could be other black holes. Right. So there is a whole hierarchy of potential black holes. And each of these black holes has a characteristic expansion.

and collapse in time, depending on the mass. If the black hole has a very large mass, it has a very long time to collapse and expand. Whereas if the black hole is small, it will collapse and expand very quickly. So that also explains why we need a very large black hole to allow stars and galaxies to form, right? You need a very back...

If the black hole was much smaller, like there are black holes in our unit, that we don't think there are universes inside there. And the reason is because the time for collapse and expansion is so fast that nothing happens in that time. I mean, to give you an example, the supermassive black hole in our galaxy, which has a million solar masses, will collapse and expand in one second. So in one second, you cannot form a galaxy or star, right? And so there is this connection between why the universe,

is so big, the universe we live on. And the answer is, well, it has to be very big to have time to make stars and planets and galaxies. So this is a great theory that you've come up with. And as you said, there's definitely certain things that look like it could be possible. But is there any way to actually test whether this is the universe we're living in and whether our universe actually is inside of a black hole? Very good question. So Well, the first evident is how we find it out, right? There is this dark energy we don't understand. And so we measure dark energy.

We don't understand what it is. So an interpretation of what it is is that we are inside a black hole. You cannot say that this is a prediction of the theory, but this is how we build the theory. But it agrees very well. So it explains at least one fact very well. Now, the other fact is I didn't explain very carefully this collapse. Well, one of the conditions in Penrose theorem is that there is no curvature.

But if we start from this cloud, that cloud, because it has an over-density, it will have a curvature. A small curvature, because it's very large cloud, but that will be something we can measure. So we can actually measure the curvature. So the prediction will be if our universe started from a cloud that collapsed then we would expect it to have a curvature and a spatial curvature.

And we can measure a spatial curvature by looking at triangles in the sky and seeing if the triangles if all the angles of the triangles add to 180 degree. So if you do a triangle in a flat, space in a piece of paper, and no matter what triangle you draw, the angles of the triangle,

the three angles of a triangle, they always have to 180 degrees. This is an amazing property, which was discovered by Euclid, a few hundred years before our time. But if you do a triangle on Earth, so Earth has a curvature, it has a radius of curvature, which is the radius of Earth, if you now do a triangle. triangle between three cities on the surface of Earth and measure the angles formed by that triangle they don't add to 180. So that's a way to measure the radius of Earth, which in fact is similar to the way it was measured by the Greeks. It's related to that

way. So basically the curvature of Earth can be measured by doing trigonometry on the surface of Earth. In the same way in the universe, we can measure the curvature of our space time around us by doing trigonometry with the position of galaxies in our universe. It's a bit more elaborate, but I don't want to spend a lot of time explaining that, but that's one of the things we do, right? And basically, so far, we have measured that the curvature is very small. In percentage, we say that the curvature is less than 2%, which means that basically you can convert the energy in the universe and the mass in the universe to a scale, and then the radius of curvature will also be a scale. And this is saying

that the radius of curvature is so big that it will only contribute to 2% to the total radius that we measure. Now, in the very near future, with missions like the Euclid satellite, the ESA-Euclid satellite, and we are expecting to measure the curvature with a very high precision. And the prediction of the black hole universe is that if really the big band started as a collapse, we would expect that there is a curvature. We are currently estimating what this curvature could be. And there is some indirect ways of predicting what it will be. And so that if we in the near future measure curvature, positive curvature for the universe,

I will say that this will be a very good indication that we started from a black hole. There are other indications. One of them is that when we look at the perturbations inside our universe, the fact that we started from a collapse, from a collapsing cloud, also set up a cutoff on the largest perturbations that we do. can measure. And this cut-off have already been measured in the Cosmicuioid background. Again, this is called an anomaly that we don't understand. One possible explanation is that this corresponds to a finite curvature that I just mentioned. So there is a way to interpret this cut-off, this anomaly in the

cosmic Macquay background, as evidence for curvature. More evidence will be that, you know, if the universe really collapsed before the pigman, we know that black holes could have formed during the collapse phase. And not only black holes, you can also form neutron stars. These neutron stars and these black holes, we believe they will survive the bounce, the bouncing phase. And they will appear here nowadays as relics. So it's like doing archaeology. By looking at Newton stars and looking at black holes, maybe these are relics of the collapsing phase. So remember I told you that we live next to a supermassive black hole

in our galaxy, and that all galaxies have these supermassive black holes. Well, it turns out that now with a James Webb telescope, the infrared space telescope, we are finding that very old galaxies, very early galaxies, at the very beginning of the universe, they also seem to have these very supermassive black holes already. So this means that it's very hard to make these supermassive black holes from gravitational collapse alone,

and it's much simpler to understand them as relics from the collapsed phase. So there are many things like this. There is also gravitational waves from the bounds and from the collapse. There are also what we call e-modes and b-modes that are, you know, asymmetries that can be imprinted into a structure that we see in the universe. And this will appear if you have a finite universe collapsing. So we are still working out all the details of the theory, but my impression is that we haven't even figured out all the possible consequences of this. pre-BigBan epoch. So basically, you know, the model can reproduce a lot of the successes of the Big Ban,

like the Cosmicure background, nucleosynthesis, you know, galaxy evolution. But there are these small differences that we should figure out. And my impression is that there could be evidence for this collapsing phase that we still haven't figured out. We are working on it, basically. It's like a very new theory. Thank you for listening to this episode of Instant Genius, brought to you by the team behind BBC Science Focus. That was Enrique Gaztenaga.

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