Instant Genius - Marcus Chown: Does the Big Bang really explain our Universe?

Episode Date: December 28, 2020

In the New Year issue of BBC Science Focus Magazine, we cover the biggest ideas that you need to understand in 2021. Over the next few episodes of the Science Focus Podcast, we’ll be talking to the experts who will explain these ideas in their own words. In this episode, we talk to science writer Marcus Chown, who tells us all about the major problems in our current understanding of cosmology. We discuss the Big Bang, dark matter, inflation, and what we still don't know about the formation of our Universe. Let us know what you think of the episode with a review or a comment wherever you listen to your podcasts. Subscribe to the Science Focus Podcast on these services: Acast, iTunes, Stitcher, RSS, Overcast Listen to more episodes of the Science Focus Podcast: Katie Mack: How will the Universe end? Dr Douglas Vakoch: Should we try to contact aliens? Dr Jacob Bleacher: Why do we need to go back to the Moon? Elisa Raffaella Ferrè: What happens to the brain in space? Dr Erin Macdonald: Is there science in Star Trek? Kathryn D. Sullivan: What is it really like to walk in space? Hosted on Acast. See acast.com/privacy for more information. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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available in print and in several digital formats throughout the world. Find out more at sciencefocus.com or look out for us in your app store. Hello and welcome to the Science Focus podcast. I'm Sarah Rigby, online assistant at BBC Science Focus magazine. In the new year issue, we cover the biggest ideas that you need to understand. stand in 2021. Over the next few episodes, we'll be talking to the experts who will explain these ideas in their own words. First up, we have science writer Marcus Chowne, who is going to tell us all about the major problems in our current understanding of cosmology. First of all, can you

please just explain to us what exactly is cosmology? Well, cosmology is really the ultimate science because it's the science which deals with the origin, the evolution and ultimate fate of the universe. So really it is the most daring of all sciences. So what is the standard model of cosmology? What do we currently understand about the way our universe was formed? Yeah, well, first of all, that's based on the Big Bang theory. So there is abundant evidence that's that our universe began in a hot, dense phase about, well, we think, 13.8,2 billion years ago, and has been expanding and cooling ever since. And that evidence is all around us.

When we look at the universe and the galaxies, which are the building blocks of the universe, we see that they're pretty much all flying away from us, like pieces of cosmic shrapnel in the aftermath of a Titanic explosion. And if you run that expansion backwards, like a move, in reverse, you do come to a time, as I say, about 13.8,000 billion years ago, when everything was compressed into a tiny volume, very, very small volume. And when you squeeze anything in a small volume, it gets hot. So if you squeeze the air in a bicycle pump, you know it gets hot. So the Big Bang was very, very hot. It was blisteringly hot. It was like the fireball of a nuclear

explosion. And that light, that heat had absolutely nowhere to go, because it was bottles. up in the universe, which by definition is all there is. So it's all around us today, greatly called by the expansion of the universe. And this afterglow of the Big Bang, we call the cosmic background radiation. And incredibly, it accounts for 99.9% of all the photons. That's all the particles of light in the universe are tied up in this afterglow. And we didn't spot it until 1965. We didn't spot it because it's mostly microwaves, which are a form of light that you know, your microwave oven uses or your mobile phone uses, but you can't see with a naked eye. But it was discovered in 1965, and this heat afterglow is really the main piece of evidence that the universe began in a big bang.

But, so we have this idea. So the Big Bang theory is, to recap, is simply that the universe began in this Titanic explosion. It was hot and dense. The universe expanded and cooled. And out of the cooling debris, they congealed the galaxies. about two trillion of them, among which is our own Milky Way galaxy. So that's the Big Bang. But the problem with the Big Bang theory is that it contradicts our observations of the universe in quite a few major ways. So because of that, we've had to bolt on new bits onto the basic Big Bang theory to make the standard model of cosmology.

So I'd just like to go back to something you mentioned just now, which is that you said that we know that the Big Bang happened because we can see that all the galaxies seem to be speeding away from us. Does that mean that then we are in the centre of the universe? Well, that's a very good question, because that would be the obvious conclusion, really. But it turns out that isn't the case. And the thing about the Big Bang expansion,

which is described by Einstein's theory of graph, which he came up within 1915. He applied his theory of gravity within about a year to the entire universe, the biggest gravitating mass that he could think of. And in that theory, what actually happens is that the space between the galaxies just gets greater and greater and greater. And this is the space between every galaxy and every other galaxy. So were we to sit on Andromeda, the nearest galaxy to us or some other galaxy, 10 billion light years away, we would see from that point of view, every other galaxy flying apart. I mean, you probably, you know, you can't get a direct analogy, but in astronomy books, very often they'll make the analogy with a rising cake with raisins in it. You know, as the cake rises and expands, every raisin recedes from every other raisin.

And if you could shrink yourself down to the size of a raisin, you would, well, obviously, it's difficult to see through the case. A cake mixture, but you see, every other raisin would be moving away from you. And not only that, the further way the raisin would be from you, the faster it would be moving. And that's exactly what we see. We call it the Hubble Law, because it was discovered by the American astronomer Edwin Hubble in 1929. So I'd just like to go for one of the other obvious questions about the Big Bang is that if the universe is expanding, what is it expanding into? do. Yeah, well, again, we are completely, we are really suffer from the fact that there is no everyday analogy between the universe and, well, there is no everyday analogy for the universe,

because basically the universe is a four-dimensional thing. And we, we only have experience of three dimensions. So every single analogy we use, whether it's a rising cake or whatever, there's always going to be, it's not going to be perfect. So, for instance, with the rising cake, people would say, well, what about the edge of the rising cake? Well, the universe may not have an edge. You know, it may be infinite. What Einstein's theory describes is a universe which is either contracting or expanding. It appears as is expanding. And where the space between every single galaxy is increasing, it says nothing about the edge. The universe could be infinite in extent and still expanding, or it could curve back on itself. So it would be like

the three-dimensional equivalent of the surface of a ball. And in fact, the current, according to the standard model of cosmology, which includes an ingredient called inflation, the universe could easily be infinite in an extent. So really, the problem we have is that the theory of gravity doesn't actually talk about any edge. It just describes a general expansion, and it could be of an infinite universe.

You just mentioned a period of inflation. What was that? Well, earlier on, I mentioned that there were several, in fact, three major contradictions between the basic Big Bang idea and what we observe. And for each of those, we have to tag on something, we have to bolt on something onto the theory. So it's a really, standard model of cosmology is a really rickety thing. So it's really, the article I'm writing about is, about a contradiction, yet another contradiction. So it's not really a surprise that we're going to find these contradictions because we know it's a rickety theory and there's got to be a better

one out there. But one of the things we bolt on is inflation. In fact, you've picked on the most complicated of all the three things to actually explain. Because one of the problems that we find when we look out of the universe, we see that it's pretty uniform in every direction. And more than that, the cosmic background radiation is afterglow of the Big Bang. It's very cool now. I should tell you, it's about minus 270 degrees Celsius, three degrees above the lowest temperature possible. But everywhere we look in the sky, it's perfectly even. It's pretty much exactly that temperature everywhere. So this creates a problem because if we run the movie of the universe backwards, you know, that run expansion backwards to the Big Bang,

we discover that parts of the universe, you know, for instance, which may be in opposite direction today, early on in the Big Bang, we're not in contact with each other. So there was no possibility of a signal going between them at the speed of light. So if there's nothing could travel between them, how could they know about each other's temperature? How could they have kept the same temperature?

You know? And so this, in order to explain this anomaly, we have to assume that the universe was a lot smaller earlier on than we naively think by running that movie backwards. Okay. So what we think is that the universe underwent, and if it was much smaller, then regions which today are very far apart and are at the same temperature were in contact and so could have equalized their temperature. So what we imagine is that there was this super fast expansion. The universe started off much smaller than we would naively expect. There was this super fast expansion which we call inflation. It was actually driven by the vacuum, what we call the quantum vacuum,

which incredibly had repulsive gravity. So this is a really weird idea. And the universe inflated tremendously fast in its first split second. And then when inflation ran out of steam, the kind of sedate expansion that we see today took over. So inflation has been likened to hydrogen bomb explosion compared to the stick of dynamite of the Hubble, or what we call the cosmic expansion we see today.

So this is a really weird thing. So we've tagged on this period of inflation to explain what we call the horizon problem, which is why the temperature of the universe is the same everywhere. We've tagged on inflation, but I should tell you that we don't understand any of the microscopic physics of inflation, we think it was driven by some as-yet-un-discovered field called the inflaton,

and 40 years after inflation was proposed, we still do not understand the basic physics of that. So, but, you know, this is a tremendous thing we're doing. You know, we came down from the trees, God knows how many million years ago, onto the African plane. We have a brain, you know, which is three pounds of jelly and water. and yet we've actually speculated on, you know, we have a theory of the universe

and we can speculate on its origin. So, I mean, I think we ought to be pat ourselves a bit on the back and perhaps not worry too much that our theory has a lot of holes in it because it turns out that we believe that the universe began in an unusual state of the vacuum. So we have a, it's an odd idea. We think of the vacuum as empty.

But ever since the 1920s, when there arose of what we call quantum theory, which is our theory of the microscopic world of atoms, we've discovered that empty space is not empty. It's seizing with microscopic particles popping into existence and popping out again. And there's a possibility that the vacuum has a higher energy or higher energy states, just like an atom has higher and higher energy states. So we believe that the universe began in this. what we call inflationary vacuum, which was of high energy,

with an enormous amount of energy concentrated in it. And this vacuum had a very unusual property. It had repulsive gravity. And its repulsive gravity caused this inflationary vacuum to expand. And as you've got more and more of this stuff, you had more repulsion. So it expanded faster and faster. Now this vacuum contained no matter at that time.

It just contained energy, and it expanded faster and faster and faster. But it was what we call a quantum thing. And quantum things are inherently unpredictable. And all over this ever-expanding quantum vacuum, chunks started decaying into normal vacuum. So you can imagine, like, I don't know, imagine a coats being eaten by moths. So all over this high-energy vacuum was being eaten away. And in the places where it was eaten away, it decayed into normal vacuum.

So that's the everyday vacuum that we see around us. And that, the energy of this inflationary vacuum had to go somewhere. And it went into creating matter and heating it to a blisteringly high temperature. So it went into creating big bangs. So in this picture, we have this ever inflating, ever faster inflating inflationary vacuum. And all over it, tiny little bubbles of normal vacuum reforming. in which there are Big Bang Universes. And we live in one of those big bang bubbles

in the ever-inflating inflationary vacuum. Now, it turns out you only need about a kilogram of an inflationary vacuum with about a kilogram of mass energy to start this whole process. And our laws of physics, quantum theory, do allow matter to appear out of nothing. So there is the possibility

that our universe simply a chunk of inflationary vacuum popped out of nothing and began expanding and created everything that we see around us. Because the net energy of our universe is zero. The net energy of our universe is zero. Incredibly. When you look at the mass, the kinetic energy in all the galaxies flying apart, and then you look at the gravitational energy, binding it all together, it turns out that our universe has a net energy of zero. So it didn't actually take a lot to create it. So that's a weird thing. But the next question is, of course, if inflation preceded the Big Bang,

what started inflation or could inflate? It turns out that inflation could not have started in the eternal past. So it cannot have started. It has to have started at some point. So in a way, we're pushing that origin question backwards. I think I gave you too much of an explanation. I tied myself in knots. So I'd just like to pick up on one thing that you said there.

So in your analogy of the motholes in a coat, so our entire universe could just be one of many of these motholes, one of many universes. Exactly, exactly. But not only that, all those other motholes are forever out of contact with us because the inflationary vacuum is just expanding far from far from far from furs. and no light could ever reach us. So they would be forever out of contact with us.

But within our bubble, there could be an infinity of universes like ours. Because we don't see the whole of our bubble. We only see out to a horizon around our bubble. So the universe, because the universe was born, so because the universe was born, let's say 14 billion years ago, we can only see the light from the galaxies that has taken less than 14 billion years to get to us. Because the universe expanded faster than light in the beginning,

the edge of the universe is about 42 billion light years away. So it began 14 billion years ago, but the edge is about 42 billion light years away. But over that, so around the edge of our universe, like a membrane around a soap above, is this horizon. Think of it again as like horizon at the sea. We know there's more of the ocean over the horizon at sea. We know there's more of the universe over our horizon. I mean, every year, more of the universe comes over the horizon

because the horizon expands. So according to inflation, there could be an infinite amount of the universe beyond our horizon. So if you imagine all of our, everything we see, our two trillion galaxies in our little bubble, it could be that there are an infinite number of other bubbles beyond us, all within this giant mothole. So you do.

It does seem quite incredible, doesn't it, really, the picture that we created. But I suppose the thing to take home is that we don't believe that the Big Bang was a one-off. You know, we believe that if inflation is correct, these kind of Big Bang universities were spawned all over this inflationary vacuum. So that's a slightly different picture because people say, Well, what happened before the Big Bang? And there were other big bangs going off right across this vacuum. If you could even talk about time,

because you may not be able to talk about time in this, you know, before the Big Bang. Because we think that time itself was born along with space and matter and energy in the Big Bang. So what does that mean in a physical sense? Time was created. How can time be created? Well, who knows? I mean, you know, now you're going from deep questions to even deeper questions. I mean, according to the Big Bang Solutions of Einstein's theory of gravity, you know, time does begin at that point.

But again, if you want to ask what is time, that's something that I've tried to answer in the next article I've written for science focus. But we're really at sea. We're pretty certain that time as we... perceive it does not exist. So for instance, we're all, every, everything that we commonly think about time is probably not correct. I mean, we probably think there's a flow of time. But when you think about it, something only flows with respect to something else. So, you know, a river only flows with respect to a river bank. So if time flows, it must be, it must flow with respect to something else, maybe a second type of time. So the minute you start thinking about things like that, you begin to realize how ridiculous

it is. And we think that, again, if you imagine the expansion of the universe running backwards, the universe would not shrink down in a very, in a uniform way, because matter is not spread uniformly around the universe. So as you got very close to the big crunch, which is kind of a mirror image of the big bang, the variations in density of the universe would get greater and greater and greater. So the variations in gravity would get greater and greater in a thing. effect, space and time would be torn apart. So we're pretty certain that space and time did not actually exist at the very beginning. And they somehow emerged from something more fundamental. And really, that is the big problem in physics today to find out what was the more fundamental thing

that space and time emerged from. But of course, we don't know. Okay, thank you. So inflation, you said, was one of the three problems. with our Big Bang theory that we need to bolt extra bits on to make it make sense. So what are the other two? Fortunately, the other two are a lot easier to explain. One of them is that, well, look at the contradictions between the standard model or the Big Bang and what we observe. And a major, major contradiction is that the theory predicts that we shouldn't be having

this conversation. So it predicts that we shouldn't exist, which is a pretty critical. problem with the Big Bang theory. And the reason for that is that we believe that the universe started off pretty much pretty uniform. We can see from the cosmic background radiation, the afterglow of the Big Bang. It's pretty uniform around the sky. That was coming from matter very soon after the universe began.

So we know that it was pretty smoothly spread matter. We know it wasn't perfectly smoothly spread. and a Nobel Prize has gone for finding the slight variations in this background radiation. And we believe that those variations were caused by quantum processes during the inflation era, so a lot earlier than that. But these variations were really tiny, like about a few parts in 100,000, something like that. So basically, there was a slight lumpiness, a slight tiny clumpiness in the material that emerged from the Big Bang.

And the clumps which had slightly more matter, they had slightly more gravity, so they dragged in more material. And it's a kind of a process akin to the rich getting ever richer. And this is how galaxies were formed, so the galaxies like her own Milky Way. But when you do the calculations, you find that you need much, much longer than the 13.82 billion years the universe has been in existence to make a galaxy like the Milky Way. You need something like 10 times as long. So something has to have speeded up galaxy formation. And the best bet is that there's other matter in the universe, okay? So currently we think that there is this stuff called dark matter.

Okay, it doesn't give out any light, or it certainly doesn't give enough light for us to currently detect. But it has a gravitational effect. And this stuff, with its own gravity, would have speeded up galaxy formation. basically the dark matter would clump, and it would then pull in the visible stars or material that made stars afterwards. So that's a universe really in which basically is mostly dark matter, kind of like thinking of them as mountains, a mountain range, and the visible stuff is like the frosting of snow on the mountains, that's all it is. So we've come to this idea that most of the universe is invisible.

And then the third, so we have to bolt on dark matter. We bolt it on inflation. We have to bolt on dark matter onto the basic model. And the third thing is dark energy. In 1998, contrary to all expectations, we discovered that the expansion of the universe is speeding up. Now, you would expect it would be slowing down because the only force we knew that was operating on the large scale in the universe was gravity.

And gravity is like an invisible world. web of elastic, you know, joining all the galaxies and kind of breaking the expansion. So we expected that when we look back in time, we would see that the acceleration, that, sorry, the universe was expanded faster in the past and it would be slowing down its expansion. But we found that it's in fact speeding up. So we've had to postulate the existence of this stuff called dark energy, which fills all of space, it's invisible, and it's got repulsive gravity. And its repulsive gravity is speeding up the expansion of the universe. Now immediately, people say, okay, didn't inflation have a vacuum with repulsive gravity?

Yes, it does. So that's really weird. The universe started out with a vacuum state with repulsive gravity that triggered cosmic expansion. That vanished. And then it turns out something like about 5 billion years ago, this other stuff with repulsive gravity took over the expansion. It gained control of the universe. So a big question would be, could there possibly be a connection between these two?

They both have exactly the same property. So the dark energy accounts for two-thirds of the mass energy of the universe. So unbelievably, until basically 30 years ago, you know, 32 years ago, we had missed the major mass component of the universe. So adding up all these, so basically the standard model is the big bang. The universe began in a hot-dense phase, has been expanding, calling ever since the galaxy is congealing out of that debris. Then we bolt on inflation, super fast expansion very early on, and then we bolt on dark matter. We need six times as much of that as we

have an ordinary matter that makes up atoms and you and me in the stars. And then we bolt on dark energy, which accounts for two-thirds of the mass of the universe. So when you add all these other components, they account for more than 95% of the mass of the universe. So basically, the stars and galaxies that we see are only about less than 5% of the mass of the universe. And incredibly, we're only seen half of those. Because a lot of the normal matter is in the form of very hot or very cold gas between the galaxies that we can't see with our telescopes. So we really have only seen 2.5% of the universe. So you can see that although we've made an awful lot of progress, it's a pretty shocking position to be in. I mean, a matter.

Imagine if Darwin had, was trying to come up with a theory of biology and he only knew about elephants and, I don't know, oak trees. You know, he didn't know about insects and he didn't know about fish and you didn't know about all these other things. Well, cosmologists are kind of in that position where they built this fantastic edifice, their standard model of cosmology, based on the 2.5% that they can actually see. and the other components are really just an admission of their ignorance. So we know that dark energy accounts for, I can tell you exactly, actually, it accounts for 68.3% of the mass of the universe. And we know that dark matter accounts for 26.8% of the universe. And we know the ordinary matter that we're made of is only 4.9% of the universe.

Wow. So, but we don't actually know what the dark match really. is and we don't actually know what the dark energy is. So there's obviously something wrong. I'll just tell you one last thing about dark energy, and that is that we think it's the property of the vacuum, the empty space. So it's kind of springy.

Empty space is kind of springy. We don't know why. But when we use the best theory to predict what the energy of empty space should be, which is quantum theory, we get a number, which is one, followed by 120 zeros, bigger than what we observe. So that's the biggest discrepancy between a prediction and observation

in the history of science. So they're probably telling us that there's something wrong with our picture. So we've got our Big Bang model and we've added on dark matter and dark energy and inflation and it still doesn't explain everything. Not at all, no, because basically dark matter and dark energy are just names, labels for things we don't understand. We know something basic about dark energy. We know that it has this repulsive gravity. We know something basic about dark matter. You know, we know how much of it there is.

And we know that it has to be there because we can't explain the existence of our own galaxy, the Milky Way, without it. But beyond that, we don't really know what they are. I mean, dark matter. I mean, we've been hunting for dark matter for a long time. And we've experiments on the earth because it doesn't interact via, it doesn't give out light. That means it doesn't interact by what we call the electromagnetic force.

So everything, you know, everything on earth interacts by the electromagnetic force. I mean, when you try and touch something, I don't know, you touch a table, you don't actually touch the table. The electromagnetic force from the electrons in your thumb, reaches out and it's repelled by the electromagnetic force from the table. So if you don't have that interaction, there's no way to detect it, really, except to if it just runs into you. So a lot of experiments have been set up in mines all around the globe, hoping that a piece of dark matter will just bump into the experiment and give it a kick.

then of course we've hoped also that we would well I should say there are many possible candidates for the dark matter and mostly the candidates are subatomic particles that haven't been discovered or that are predicted by certain theories

and cynically as someone with an astronomy background I'd say that's because they're more particle physicists than they're astronomers because it's still possible that the dark matter could be in the form of primordial black holes, something like black holes that were formed from the Big Bang, left over from the Big Bang, not produced by stars which explode and their cores collapsed, but left over from the Big Bang. It's still possible there are some mass windows that are still open.

But most physicists believe that the dark matter is some kind of subatomic particle. and you may have heard of the term WIMP, which is weakly interacting massive particle. Basically, this is a massive particle up to maybe 100 times the mass of the proton that doesn't interact via light. And the reason that these particles are so popular is that there is a speculative theory of particle physics called supersymmetry, which does predict the existence of these. Basically, every particle has another particle with a different kind of spin.

So the electron has a partner called a selectron, and the neutrino has a particle called a s neutrino. And these are the, you know, because of this, this is a very attractive theory of particle physics. These have been the prime candidates for the dark matter. The problem is that they were expected to appear at the large Hadron Collider by now. So that's the giant atom smasher, which straddles the border between France and Switzerland, where the Higgs particle, you may have heard, was found in 2012, but there's been no sign of

supersymmetry. And physicists haven't quite given up, but they're getting quite demoralized. So other candidates are coming forward. There's a thing called an axon. There could be another type of neutrino called a sterile neutrino. Or we might be completely wrong. I mean, it could be. that there is no dark matter, but there is something wrong with our theory of gravity. So when we look at another well-known place where we see evidence of dark matter is in the rotation of spiral galaxies.

So our galaxy, when we look at the spiral galaxies like our own, we see that the stars in the outer regions are circling far, far too fast. You know, like children on a speedy-up merry-go-round, and they should spin off into space. They don't. So in most barro galaxies, you need at least 10 times more invisible stuff

whose gravity is holding onto that, those stars, than visible stuff. So there's evidence for dark matter there. But an equally viable alternative would be that the gravity, gravity on the very large scales or in the outer regions of galaxies is stronger than we would expect from Einstein's theory. And there's been great resistance to that idea

because the Einstein's theory is so elegant and people don't want to mutilate it, but that's equally possible. Okay, so this whole standard model of cosmology, we call it Lambda CDM, don't we? We do, yes. So this Lambda CDM model, as you mentioned in your article, still has some big problems with it.

Could you take us through what those problems are, please? Okay, well, the problem with Lambda CDM is, first and foremost, but before I even start, I should tell you that the problems are we actually don't know what dark matter is and we don't know what dark energy is and we don't know the details of inflation. So even at the beginning, we don't know what 95% of the universe is. So that's actually a major problem and that tells you that the theory is good, but it's clearly not the theory. You know, we need a seamless theory in which dark energy and dark matter and all these things are all seamlessly described, and we haven't got that. We just have this botched theory in which we just have the Big Bang, we bolted on all these things. So it's not a surprise that we found yet more contradictions. And one of them in particular is really exercising cosmologists, and it's called the Hubble Tension.

So the universe is expanding, and we describe how fast it's expanding by what we call the Hubble constant. And that tells you basically how much faster a galaxy is moving than another galaxy if it's what we call a megaparsec further away. It's 3 million light years further away. This gives us an idea of how fast it's expanding. So we can measure that today. When we look at objects flying away from us, we can measure what that speed is, that Hubble constant is. But we also have a second way of measuring because the cosmic background radiation

is basically a treasure trove of cosmological information. Because when it was formed, the universe didn't have any galaxies or anything. It was just this fluid of this hot fluid of subatomic particles, electrons and protons and helium nuclei and photons and photons, particles of light. And you can imagine it. It's just like a fluid, like a sloshing about, like water sloshing about in the bath.

And if you think, imagine water sloshing around in the bath. You know, you can get one hummock. You can get two hummocks. You can get a whole range of different, what we call sloshing modes. And we see these across the cosmic background radiation as variations in the temperature,

what we call cosmic ripples. And from them we can deduce major cosmological parameters. So, you know, now, now we very often say that the universe is, well, I just told you that the universe was 68.3% dark energy, 26.8% dark matter and 4.9% ordinary matter. Well, that is precision cosmology. All of those numbers come from examining these sloshing modes, these temperature variations of the afterglow of the Big Bang. And we can also deduce what the expansion rate was when that radiation was broke free of matter, which was about 380,000 years after the Big Bang.

That's where the cosmic background radiation comes from. And we can then extrapolate that expansion speed to the present day, because we know how much matter there was in the universe. Its gravity would have been breaking the expansion of the universe. We then know that dark energy took over and began speeding up the universe, expansion about five billion years ago, just before the Earth was born. And then we get this discrepancy. We find that the universe is expanding about 10% faster,

and it should have been, according to what the cosmic background radiation tells us. So there's an internal inconsistency in the theory. Now, before we think this is really significant, I should tell you that measuring the expansion rate today is very, very hard. measuring it from the cosmic background radiation is really, really easy. But to measure it in today's universe, you literally have to know the distance to objects. Because, and that is very, very hard. So we have to find what we call standard candles.

So we have to find things like pulsating stars, supernovae, these are exploding stars, which we believe have a standard brightness. And so when we look at them across the universe, they would be like a, you know, a string of 100 watt light bulbs. So if we see a 100-watt light bulb that's half as bright as another one, we know it's twice as far away, that kind of thing. And so there's a lot of uncertainty. However, multiple research teams who have measured the size of the universe and the expansion rate of the universe have all come to pretty much the same conclusion and that the expansion is 10% bigger than it should be. So that is a real problem.

If this holds up, then it's a real problem. But we would expect some kind of paradox like this because we know that the theory is not perfect. We know that it's a botched job. And in fact, paradoxes like this are fantastically important. They are what physicists dream of. In 1900, all of what we call classical physics was perfect. And great physicists of the day, like Lord Kelvin,

said that we only needed to effectively, crossed the T's and dot the eyes, we basically had come to the end of physics. But there was one observation, and it was the observation of the light, the way the light varied with temperature in a furnace. And what we call that black body radiation. And this one anomaly led to a revolution in physics, quantum theory, and us recognizing that everything that we'd learned before was actually incorrect. That quantum theory was correct. We live in a quantum universe and the classical physics that came before was only an approximation. It wasn't correct. So we look for these tiny little anomalies. Now, why could the universe be speeding up faster? Well, there's possibilities. We don't know anything

about the dark matter. It's a kind of a weird assumption of physicists that it's going to be made of one type of particle. But when we look at the visible universe, we see that it's very complex. You There's 92 different elements. You know, those are made of several different types of quark and electron. You know, there's a lot of stuff, a lot of different stuff. Why could the dark universe not be made of lots of stuff that may be interact by dark forces, you know? So we have dark atoms and dark planets and dark stars and maybe dark life. Who knows?

So that's all, that's a possibility. Another possibility is that the dark matter consists. of particles, some of which have decayed since the Big Bang. So they've decayed into light, into photons. And if that happens, then there'll be less matter around. And the matter is ultimately, you know, gravity was what was breaking the universe's expansion. So if less of that around, there'll be less breaking of expansion and the universe could expand a bit faster. But it seems that it doesn't matter what you do. You can't resolve this tension. Because the trouble is, this standard picture fits pretty much everything we see.

So it tells us how galaxies form. And this is kind of like, it's got lots of interlocking pieces. And if you change the picture in one way, you might solve, you might think you've solved this, what we call Hubble tension, between the, you know,

the university's expanding 10% faster than it should. But then you find that you've unfixed something else. So now you can't make galaxies. So it seems that this tension is so, great, that if it's real, we won't easily be able to resolve it without some revolution in our thinking. So some completely new model of cosmology, something is radically different as quantum theory was from the physics that came before it. So do you think the Big Bang theory is something that we're going to keep, or do you think eventually we're just going to

ditch it and say, well, that's not working? So it clearly, you know, we'll find something better than it explains it better. Well, again, you have to be ever so careful here because in science, words change. So once upon a time, the universe meant the sun and the six visible planets, you know. And then later, beginning of the 20th century, it began to mean the band of stars we could see across the sky, which we now know is the Milky Way. And then later in the 20s, we realized that that's only one galaxy among two trillion others. So the word universe has kept changing. And we have to remember that the word Big Bang keeps changing as well.

So the Big Bang theory is really that in its most simple form is that the universe began in this hot-dense phase, you know, this explosion, expanded and galaxies formed out the cooling debris. But all these things have been tagged on. And we also call, so now we call the Big Bang theory the Big Bang plus inflation. so that you know when you say and we call the Big Bang Theory inflation plus all these other things inflation plus dark matter whatever so you have to you have to say

what do you mean what do you think is going to be overthrown you think this if we're talking about the standard model of cosmology yes it will be because it is incomplete in exactly the same way that we have a standard model of particle physics

which explains that everything's made of effectively two quarks and one, what we call a lepton, electron. There are some other there are some other heavier quarks and heavier leptons as well. We can describe all of them, and we can describe how they interact via three forces. But the theory does not tell us why the three forces have the strength they have and why the particles have the masses they have. So that tells us that there's something massively missing from our theory and that there is a better theory out there that explains those things. So similarly, we have this standard model of cosmology, which has got lots of holes in it. So we have a name, we have a thing, dark matter,

which is basically a word for our ignorance, and we have dark matter, which is another word for our ignorance, but we're hoping that we'll get a better theory, which will tell us what dark matter is, dark energy is, and how they fit together. Are you confident that this is a problem that we're ever going to solve? I'm not confident because, you know, now we're really, really asking philosophical questions. Is it possible for the universe to understand itself? Because basically we are part of the universe, we are made of the stuff that the universe is made of. Is it possible for us to understand it?

I mean, this is another question like, can the brain understand the brain? Well, the brain can understand the brain because it isn't just one brain understanding the brain. You know, there are multiple thousands of researchers all over the world trying to understand the brain. So that problem could probably be solved, although it's probably going to be hundreds of years in the future. Can the universe be understood completely by its occupants? I don't really know. I mean, ultimately, difficult philosophical question, because when we do science, tend to be able to do experiments and we do other experiments to confirm the results of those

experiments and that's or refute them and that's how science advances. How do you do that when you only have one example? You know, this is a real problem. And the theory as well has spawned something even worse, which is the multiverse. So when I talked to you earlier about the ever-inflating inflationary vacuum and these little bubbles of ordinary vacuum forming in it, Big Bang universes, as is just one of those. And those others are completely out of touch with us. So we can never, you know, we can never know what's happening for sure in these other bubbles. So that means that there are domains of physics beyond what we can actually observe. And that is very philosophically, very difficult for science. How do we actually, how do we, you know, most of what we, what we,

Most of what's out there we can't ever observe, can we actually have a coherent theory about it? Thank you for listening to this episode of the Science Focus podcast. That was Marcus Chowne, talking about the cracks in our understanding of cosmology. The New Year issue of BBC Science Focus magazine is on sale on the 29th of December. Pick up a copy to find out about even more of the ideas you need to know in 2021, including virtual reality therapy, what rewilding the podcast, planet will do for the climate and how viruses can save lives. Thank you for listening to the Science Focus podcast from the BBC Science Focus magazine team.

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