Saturday, August 27, 2022

We don't know how the universe began, and we will never know

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]

Did the universe come out of a black hole? Will the big bang repeat? Was the universe created from strings? Physicists have a lot of ideas about how the universe began, and I am constantly asked to comment on them. In this video I want to explain why you should not take these ideas seriously. Why not? That’s what we’ll talk about today.

The first evidence that the universe expands was discovered by Edwin Hubble who saw that nearby galaxies all move away from us. How this could happen was explained by none other than Albert Einstein. Yes, that guy again. His theory of general relativity says that space responds to the matter and energy in it by expanding.

And so, as time passes, matter and energy in the universe become more thinly diluted on average. I say “on average” because inside of galaxies, matter doesn’t dilute but actually clumps and space doesn’t expand. But in this video we’ll only look at the average over the entire universe.

So we know that the universe expands and on average matter in it dilutes. But if the universe expands today, this means if we look back in time the matter must have been squeezed together, so the density was higher. And a higher density of matter means a higher temperature. This tells us that in the early universe, matter was dense and hot. Really hot. At some point, matter must have been so hot that atoms couldn’t keep electrons around them. And even earlier, there wouldn’t even have been individual atomic nuclei, just a plasma of elementary particles like quarks and gluons and photons and so on. It’s like the alphabet soup of physics.

And before that? We don’t know. We don’t know because we have never tested what matter does at energy densities higher than those which the Large Hadron Collider can produce.

However, we can just ignore this difficulty, and continue using Einstein’s equations further back in time, assuming that nothing changes. What we find then is that the energy density of matter must once have been infinitely large. This is a singularity and it’s where our extrapolation into the past breaks down. The moment at which this happens is approximately thirteen point seven billion years in the past and it’s called the Big Bang.

The Big Bang didn’t happen at any particular place in space, it happened everywhere. I explained this in more detail in this earlier video.

Now, most physicists, me included, think that the Big Bang singularity is a mathematical artifact and not what really happened. It probably just means that Einstein’s theory stops working and we should be using a better one. We think that’s what’s going on, because when singularities occur in other cases in physics, that’s the reason. For example, when a drop of water pinches off a tap, then the surface curvature of the water has a singular point. But this happens only if we describe the water as a smooth fluid. If we would take into account that it’s actually made of atoms, then the singularity would go away.

Something like that is probably also why we get the Big Bang singularity. We should be using a better theory, one that includes the quantum properties of space. Unfortunately, we don’t have the theory for this calculation. And so, all that we can reliably say is: If we extrapolate Einstein’s equations back in time, we get the Big Bang singularity. We think that this isn’t physically correct. So we don’t know how the universe began. And that’s it.

Then how come that you constantly read about all those other ideas for how the universe began? Because you were sitting around at your dentist and had nothing else to do. Ok, but why do physicists put forward such ideas when the answer is we just don’t know. Like, you may have recently seen some videos about how our universe was allegedly born from a black hole.

The issue is that physicists can’t accept the scientifically honest answer: We don’t know, and leave it at that. Instead, they change the extrapolation back in time by using a different set of equations. And then you can do all kind of other things, really pretty much anything you want.

But wait, this is science, right? You don’t just get to make up equations. Unless possible you’re decorating a black board for a TV crew. Though, actually, I did this once and later they asked me what those equations were and I had to tell them they don’t mean anything which was really embarrassing. So even in this case my advice would be, you shouldn’t make up equations. But in cosmology they do it anyway. Here’s why.

Suppose you’re throwing a stone and you calculate where it falls using Newton’s laws. If I give you the initial position and velocity, you can calculate where the stone lands. We call the initial position and velocity the “initial state”, and the equation by which you calculate what happens the “evolution law”.

You can also use this equation the other way round: if you know the final state, that is, the position and velocity at the moment the stone landed, you can calculate where it’s been at any time in between, and where it came from. It’s kind of like when my kids have chocolate all over their fingers, I can deduce where that came from.

Okay, but you didn’t come here to hear me talk about stones, this video was supposedly about the universe. Well, in physics all theories we currently have work this way, even that for the entire universe. The equations are more complicated, alright, but we still have an initial state and an evolution law. We put in some initial state, calculate how it would look like today, and compare that with our observations to see if it’s correct.

But wait. In this case we can only tell that the initial state and the equations *together give the correct prediction for the final state. How can we tell that the equations alone are correct?

Let’s look at the stone example again. You could throw many stones from different places with different initial velocities and check that they always land where the equations say. You could also, say, take a video of the flight of the stone and check that the position at any moment agrees with the equations. I don't think that video would kill it on TikTok, but you never know, people watch the weirdest shit.

But in cosmology we can’t do that. We have only one universe, so we can’t test the equations by changing the initial conditions. And we can’t take any snapshots in between because we’d have to wait 13 billion years. In cosmology we only have observations of the final state, that is, where the stone lands.

That’s a problem. Because then you can take whatever equation you want and use it to calculate what happened earlier. And for each possible equation there will be *some earlier state that, if you use the equation in the other direction, will agree with the final position and velocity that you observed. So it seems like in cosmology we can only test a combination of initial state and equation but not find out what either is separately. And then we can’t say anything about how the universe began.

That sounds bad. But the situation isn’t quite as bad for two reasons.

First: the equations we use for the entire universe have been confirmed by *other observations in which we *can use the standard scientific methods. There are many experiments which show that Einstein’s equations of General Relativity are correct, for example redshift in the gravitational field, or the perihelion precession of Mercury, and so on. We then take these well-confirmed equations and apply them to the entire universe.

This, however, doesn’t entirely solve the problem. That’s because in cosmology we use further assumptions besides Einstein’s equations. For example, we use the cosmological principle about which I talked in an earlier video, or we assume that the universe contains dark matter and dark energy and so on. So, saying that we trust the equations because they work in other cases doesn’t justify the current cosmological model.

But we have a second reason which *does justify it. It’s that Einstein’s equations together with their initial values in the early universe provide a simple *explanation for the observations we make today. When I say simple I mean simple in a quantitative way: you need few numbers to specify it. If you used a different equation, then the initial state would be more difficult. You’d need to put in more numbers. And the theory wouldn’t explain as much.

Just think of the equations as a kind of machine. You put in some assumptions about how the universe began, do the maths, and you get out a prediction for how it looks like today. This is a good explanation if the prediction agrees with observations *and the initial state was simple. The simpler the better. And for this you only need the observations from today, you don’t need to wait some billion years. Unless of course you would like to. You know what? Let's just wait together.

Okay. How about you wait, and we talk again in 10 billion years.

While you wait, the cosmologists who aren’t patient enough justify using one particular equation and one particular initial state by showing that this *combination is a simple explanation in the sense that we can calculate a lot of data from it. The simplest explanation that we have found is the standard model for cosmology, which is also called LamdaCMD, and it’s based on Einstein’s equations.

This model explains for example how our observations of the cosmic microwave background fits together with our observations of galactic filaments. They came out of the same initial distribution of matter, the alphabet soup of the early universe. If we used a different equation, there’d still be some initial state, but it wouldn’t be simple any more.

The requirement that an explanation is simple is super important. And it’s not just because otherwise people fall asleep before your done explaining. It’s because without it we can’t do science at all. Take the idea that the Earth was created 6000 years ago with all dinosaur bones in place because god made it so. This isn’t wrong. But it isn’t simple, so it’s not a scientific explanation. Evolution and geology in contrast are simple explanations for how those dinosaur bones ended up where they are. I explained this in more detail in my new book Existential Physics which has just appeared.

That said, let us then look at what physicists do when they talk about different ideas for how the universe began. For this, they change the equations as we go back in time. Typically, the equations are very similar to Einstein’s equations at the present time, but they differ early in the universe. And then they also need a different initial state, so you might no longer find a Big Bang. As I said earlier, you can always do this, because for any evolution law there will be some initial state that will give you the right prediction for today.

The problem is that this makes a simple explanation more complicated, so these theories are not scientifically justifiable. They don’t improve the explanatory power of the standard cosmological model. Another way to put it is that all those complicated ideas for how the universe began are unnecessary to explain what we observe.

It’s actually worse. Because you might think we just have to wait for better observations and then maybe we’ll see that the current cosmological model is no longer the simplest explanation. But if there was an earlier phase of the universe that was indeed more complicated than the simple initial state that we use today, we couldn’t use the scientific method to decide whether it’s correct or not. The scientific method as we know it just doesn’t cover this case. Science fails!

Sure, making better observations can help us improve the current models a little more. But eventually we’ll run into this problem that more complicated explanations are always possible, and never scientifically justified.

So what’s with all those ideas about the early universe. Here’s one that’s been kind of popular recently, an idea that was put forward by Nikodem Poplawski. For this, you change general relativity by adding new terms to the equations called torsion. This removes the big bang singularity, and replaces it with a bounce. Our universe then came out of a bottleneck that’s quite similar to a black hole, just without the singularity. Can you do this? You can certainly do this in the sense that there’s maths for it. But on that count you can do many other things. Like broccoli. There’s maths for broccoli. So why not make the universe out of broccoli?

I know this sounds crazy, but there are a lot of examples for this, like Penrose’s cyclic cosmology that we talked about some months ago. Or the ekpyrotic universe which starts with a collision of higher dimensional membranes. Or the idea that we came out of a 5-dimensional black hole which made headlines a few years ago. Or the idea that the universe began with a gas of strings which seems to never have been particularly popular. Or the no-boundary proposal which has it that the universe began with only space and no time, an idea put forward by Jim Hartle and Stephen Hawking. Or geometrogenesis, which is the idea that the universe began as a highly connected network that then lost most of its connections and condensed into something that is indistinguishable from the space we inhabit. And so on. 

Have you ever wondered how come there are so many different ideas for the early universe? It’s because by the method that physicists currently use, there are infinitely many stories you can invent for the early universe.

The physicists who work on this always come up with some predictions for observables. But since these hypotheses are already unnecessarily complicated anyway, you can make them fit to any possible observation. And even if you’d rule out some of them, there are infinitely many others you could make up.

This doesn’t mean that these ideas are wrong. It just means that we can’t tell if they’re right or wrong. My friend Tim Palmer suggested to call them ascientific. When it comes to the question how the universe began, we are facing the limits of science itself. It’s a question I think we’ll never be able to answer. Just like we'll never be able to answer the question of why women pluck off their eyebrows and then paint them back on. Some questions defy answers.

So if you read yet another headline about some physicist who thinks our universe could have begun this way or that way, you should really read this as a creation myth written in the language of mathematics. It’s not wrong, but it isn’t scientific either. The Big Bang is the simplest explanation we know, and that is probably wrong, and that’s it. That’s all that science can tell us.

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