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Brian Cox debunked the Big Bang! Wait what?
🕑 Added 2024-01-29 16:30:02 +0000 UTC[This is a transcript of the video.]
What happened before the Big Bang? I got this question on twitter and thought, well, the answer is that we don’t know. Now, I’m quite fond of short videos, but that’s a little too short even for my taste. Then I saw that Brian Cox has supposedly “debunked” the Big Bang with a creation story and thought, He did what? Then I watched the clip that’s narrated by Brian Cox, and I think I know what happened there. Let me explain.
Observations tell us that the universe expands. If we use those observations and take the equations that Einstein gave us, we can calculate what happened in the distant past. Since the universe expands, in the past matter must have been more concentrated. Indeed, if we trace the equations back then the density of matter just increases and increases, until it’s infinitely large. This infinite density happens at a finite time of about 13 point 8 billion years, and is what we usually refer to as the “Big Bang”.
Mathematically, this Big Bang is a singularity in several measurable quantities, like the energy density of matter, but also the curvature of space-time. The first relevant thing to note is that this doesn’t mean there wasn’t anything before the Big Bang. It’s just that if there was something before the big bang, it isn’t in any way connected to what happened after, so we’d never know of it.
To see what I mean, take a super simple example of a singularity, that in the function one over x squared, when x goes to zero. It’s not like there’s no curve for x smaller than zero. It’s just that there’s no connection between the two parts. The same could be the case for the Big Bang singularity. There was something before it, but it’s not in any way connected to us.
That said, most physicists don’t think that the singularity is even there to begin with. That’s because in all other cases where we have a singularity in a physically measurable quantity, it just means that the theory breaks down and we need to use a better one.
A good example is hydrodynamics that describes how liquids behaves. Surfaces of drops can have points with infinite curvature, like when a drop of water pinches off a tap. That pinch off point is a singularity. But of course, if you look at the drop on the scale of single atoms, there is nothing singular about it. It’s just that if we approximate all those atoms with a smooth fluid that these singularities appear. They’re a consequence of this approximation.
That’s why physicists think, the Big Bang singularity just tells us that we need a better theory. We don’t have that theory so the smart thing to do would be to just leave it at that. But of course, that’s no fun, so physicists have come up with a lot of speculations for what might have happened instead of a big bang singularity.
And there is very little data in the way of their speculations. The young galaxies that the Webb telescope sees were probably created a few hundred million years after the Big Bang. The Cosmic Microwave Background was created approximately three hundred thousand years after the Big Bang. And we have no observations from earlier on.
Physicists have used particle colliders to study what highly compressed matter does, and we can use this data to extrapolate back to somewhat before the creation of the cosmic microwave background, to around the time when the Higgs field condensed. But this still doesn’t get us anywhere near the big bang.
This is why in theoretical physics you find many alternative ideas for the Beginning of the universe, depending on how you change the equations, because there’s no data to tell those ideas apart. And this brings me to Brian Cox’s supposed debunking of the Big Bang.
In this clip from the BBC, he’s clearly talking about the currently most popular theory for what happened before the big bang. It’s called eternal inflation. In eternal inflation, the creation of our universe is not singular, so strictly speaking there isn’t any bang. According to this theory, our universe is created by a quantum fluctuation in a field called the “inflaton”.
The inflaton creates this fluctuation, and that creates a bubble which then rapidly inflates, hence the name. At some point the inflation stops, and all the energy from the inflaton field is converted into matter. All the matter around us is created only at that point when the inflaton field dumps its energy. In the literature, this event is usually referred to as “reheating”.
What happened with Brian Cox’s explanation is that he made it sound as if the reheating is the big bang. I mean, listen to it yourself: “As inflation ended, the ocean of energy was converted into matter. Big Bang.”
So basically he just used the word “Big Bang” for something completely different. It’s somewhat of a disease among science communicators that I’ve complained about before because it causes a lot of confusions, but let me not go there again. Maybe more importantly, even leaving aside that we don’t know whether eternal inflation is even correct, it doesn’t remove the Big Bang per se, it just removes the big bang in our universe.
You see, in eternal inflation, there are infinitely many of those quantum fluctuations and each gives rise to a new universe. This entire multiverse of eternal inflation however must also have had a beginning at finite time, an then you can ask, well, what happened before that. So eternal inflation just moves the problem elsewhere.
And of course there are other ideas for the beginning of our universe besides inflation. For example, there’s the idea of a big bounce, in which there was an earlier phase of the universe which contracted. Matter became very dense, but not infinitely so, and now it’s expanding again. If those bounces repeat, it’s called a cyclic universe. You can also just push the big bang all the way back to minus infinity and say that for half of infinity very little happened, and then the universe sudden began expanding. It’s not a very popular idea probably because the entire point is it being infinitely lame. Or there is the Hartle-hawking no-boundary proposal that says before the big bang there was no space-time, there was just a 4-dimensional space, and then one direction of that space turned to time.
These ideas are all more or less compatible with observations because you can always make sure that the deviations from Einstein’s equations are unmeasurably small. This doesn’t mean these ideas are wrong, it’s that we can’t tell whether they’re right or wrong. So what happened before the big bang? Physicists have a lot of speculations, but the short answer is that we don’t know, and since there is so little information left over from whatever happened, we might never know.
Comments
"Just moves the problem elsewhere" (~6:07/8) IMO, that is the problem with other "theories", such as string theory, god, etc. The problem is just shifted.
What you mean is a theory that goes back to Lee Smolin. If every BH would be the birth of a new universe, a kind of selection would lead to universes with the most possible Bhs. Don´t think, that it´s compatible with infaltion, we do know nothing about what happens inside a BH and this theory doesn´t need eternal inflation to describe a multiverse.
Walter Lerchner
I have a question in regard of the eternal inflation theory: Some physicists consider black holes singularities that possibly giving rise to "baby universes" - I assume these "baby universes" would be subject to the same inflaton field as our universe. My question: is eternal inflation compatible, or maybe it would even require baby universes to arise within the singularity of black holes? Or am I just confusing things?
The topic covered today by Sabine in a simple but comprehensive way, is really a fascinating one. I would also say that the topic classified here as "Cosmology", as marked a the end of the written version, is really and example of "Philosophy", because it is all about non-verified but interesting speculation and, in the case of eternal inflation (and probably in all cases covered here), probably also unknowable. First I was confused about the word "debunked" in the title, but then, after reading the whole article and looking at the title again, I saw it was said as a if it were a dubious thing. Which it is. Can't debunk an unknowable with another. Or stop people with inquisitive minds to come up with more of these.
