The Universal Story
By / August 19, 2022

The Big Bang – Years Later: Fading

And after the chaos, from which it all started;
order descends, and we fade into darkness.

After the confusing chaos of the Big Bang, over a few years, everything becomes a bit more sensible. The energy of the Big Bang condenses into matter and the Universe generally cools down. The Universe is almost completely uniform, every corner is almost identical – the same temperature, the same density and has the same stuff in it. There are no ‘large scale structures’, no stars, no galaxies, none of the things we look into the night sky and see today. But slowly, it is starting to cool down and form something we might recognize. Let’s dive in, to the early Universe.

The Great Cooling: How “energy” turned into matter

An image showing the timeline of the Universe. It shows how, after the craziness of the early Universe, there was a bright line where suddenly, you could see light. Previously, because the Universe was full of particles you couldn’t really see anything, because light would have run into them and been scattered. Suddenly at 380,000 years, the Universe became transparent and this glow of light was created. We can still detect this light today.

One of the weird things about the early Universe, is that it doesn’t really have matter or light. Instead, it just has “energy”. What does that mean? How does “energy” turn into light and matter?

The physics behind this is complex, but it makes sense. Most people are familiar with the idea of “conservation” of energy in the Universe. We can’t really create or destroy energy, we kinda just turn it into different forms. When we burn stuff, we take matter, chemical bonds, and turn it into heat. And if we are burning things most thoroughly, like properly splitting an atom, we can get an enormous amount of energy from even a tiny bit of matter.

As the Universe was cooling, it was doing the opposite. All the energy that was created by the Big Bang was slowly condensed into matter. Now initially the Universe was very hot because huge amounts of energy was in energy form and all spread out. However a small amount of matter takes a lot of energy to create, so even creating small amounts of matter cooled the Universe a lot. (The Universe also expanded as well, see the below section).

Another thing worth noting is that, at a big scale, the Universe didn’t ‘clump’ as it cooled. This sounds really weird. Obviously, it did happen a bit, because on our scale it seems really clumpy, galaxies and stars formed, as did the Earth (see next blog posts). However, if you look at the Universe at a really big scale, at a galaxy level, for the most part, the Universe is evenly dense. There are no big central regions where there is a lot more matter or vast empty bits where there is a lot less. The Universe is very consistently a density of roughly 3 atoms per cubic centimeter, and while obviously planets and stars and galaxies get much denser, with trillions of atoms in a cubic centimeter, the vast vast majority of space is empty space in between these things, making the Universe very even.

How do we know all this? The Cosmic Microwave Background

This image is of the Cosmic Microwave Background. It was produced quite simply, some scientists pointed a telescope at the sky, trying to block out all the stars and see how much energy and heat there was in that bit of sky. They did this over and over again until they had a heat-map of the entire sky. What they got was amazing, it’s basically all the same temperature. The Universe, despite all the big things in it, galaxies, planets, stars, is basically dominated by this fading afterglow of the Big Bang. More details here.

The evidence of this process is surprisingly easy to find. In fact, almost all houses in the world have equipment that can detect it.

As the Universe got older and more and more of the energy created by the Big Bang turned into matter. However, not all of it has. There is still some leftover energy created by the Big Bang that is detectable today. This is called the ‘Cosmic Microwave Background’ or CMB. And you can detect this very easily, it is just static like you hear on a TV or radio station.

Most of the space on earth is now full of radiation. Whether it be from nearby technology, the Sun, or other particles passing through the earth, the Universe is pretty noisy. However, if you point a telescope at some empty pieces of sky, where you know there are no stars, you will find that even there, there is a low level of radiation. This radiation is not actually ‘coming’ from anywhere in terms of a source, like nearby stars. Instead, every part of the Universe has this leftover signature from the Big Bang. And this radiation is very very consistent wherever you look for it. In fact, it varies by less than 10-5 so, less than 0.001%.

This tells us some very fundamental things about the Universe. Firstly it tells you that the Big Bang happened ‘everywhere’. It didn’t just happen somewhere, like the center of the Universe, and spread out. If that happened, you’d expect the radiation to be uneven and have some “center”. Instead, it happened right at the center of every piece of space. Secondly, by looking at the spectrum of the radiation (basically what color it is) we can see that it is exactly the sort of radiation that is given off by a very hot object (it’s not X-rays or something weird like that).

We also know the exact moment this light was emitted and the process behind it. We can reproduce it in laboratories very easily because the Universe was not as crazy hot when this occurred (see the next blog post).

What about “expansion”? How did the Big Bang “expand” the Universe?

A weird property of our Universe is that it has a horizon. You can’t see more than 13.8 billion light-years away from yourself, because the light being emitted hasn’t got to us yet, since it started its journey at the beginning of the Universe. This is true everywhere in our Universe. This picture shows how two galaxies that are really far apart at the beginning of the Universe, will not be able to see each other until there has been enough time for the light to travel.

Another weird element of our Universe is that there is a hard limit on how far we can see. Because light takes time to travel from stars to us, we can only see stars that are 13.8 billion light-years away from us. In a few billion years, we will be able to see stars a few billion light-years further away. And the same thing holds true for observers in other galaxies. There is no way for them to see us until a certain amount of time passes and the light being emitted by all these stars has had long enough to get to us. The entire Universe is slowly opening up and being able to see more and more of itself.

Now the speed of light is the universal speed limit of the Universe. It’s one of the few rules we really just have not been able to break, in any context. Things cannot move faster than the speed of light.

However, as we talked about before, the Universe is very consistent in its structure. Initially, when we built our first telescopes, we kept thinking there would not be any galaxies much further away from us. However, then we built better telescopes and saw more and more galaxies that were further and further away. We thought we’d hit the limit again. But then, with better telescopes, we just kept seeing more galaxies. This pattern has continued until now we have telescopes that can just see to the limit of 13.8 billion light-years away. And we keep finding more and more galaxies. As we talked about before, the Universe really does seem to be consistent in structure. So it’s basically certain that there are a whole bunch of galaxies out there that we can’t see yet, because they are too far away, that we will eventually be able to see.

Now, these two things cause a bit of a problem for the Big Bang. How did all these galaxies get so far away from each other in the first place if we can’t break the light speed barrier?

This is where we get the evidence for the Big Bang expanding the Universe. Clearly very early on, to give these galaxies all enough time to form and turn into stars, very early in the history of the Universe, all the stuff in the Universe must have gotten really far away from each other really quickly. And it must have gotten away from each other, somehow at faster than the speed of light.

The way this happened was a very rapid expansion of the Universe right after the Big Bang. And on a completely different scale to the expansion of the Universe we see now. To put some numbers on it, going back to the previous post, in the first 10−36 seconds after the distance from the closest things in the Universe to the furthest away things in the Universe probably expanded by 1028.

Now, this smashes the light speed barrier? How is that okay?

This is where we’ve been a bit flippant when talking about ‘expansion’. The expansion of the Universe does look like things just getting further away from each other. The problem is, it actually isn’t. It’s actually the space in the Universe itself expanding. These two things sound very similar but they are slightly different. If you were in one of the galaxies, there is no real sense in which you would be moving. The wind wouldn’t be ‘blowing through your hair’. Instead it’s just that all the space is getting bigger between you. This sort of expansion isn’t two dots on a canvas moving further apart, towards the edge of the canvas. Its the whole canvas being stretched, getting bigger, without the dots feeling like anything is changing, they just see everything getting further away.

The Big Bang: Thanks for sticking with us, we really tried to have it make sense

This is our favorite image of the Big Bang at The Universal Story. It’s not of the Big Bang. It’s a strange attractor – a diagram of a weird piece of mathematics. It’s just pretty.

So that’s the Big Bang. Frankly, that Indigenous Australian story about the serpent spewing forth all the animals has a lot going for it in our view. The Big Bang is so weird. Its catchy name makes you think that you get it, but it is a lot more complicated than you would think.

The Big Bang wasn’t a massive bomb that magically appeared into the blank canvas of an empty Universe that exploded fully formed galaxies and stars everywhere. Instead, it was the moment that the canvas, space, time, and an enormous amount of energy came into existence. We have no answers about what happened before it, we just don’t know.

The Big Bang happened simultaneously everywhere across our Universe, and we can still feel its warm afterglow today, at every single point in the Universe. Billionths of a second after, the Universe was so dense and hot that we just don’t really understand what was going on. Then the Universe rapidly expanded throwing the energy out all across, faster than the speed of light, so that even 13.8 billion years later, there are still large hunks of the Universe we can’t see.

As it got less dense, and less hot, more and more normal particles were able to form. Eventually an almost perfectly even mist of super hot cooling of gas condensed from the energy over thousands of years. In the next few posts, we’ll see this gas cool and the tiny variations in it clump into galaxies and stars.

But we are still billions of years away from that. First, we need to get through, the cosmological dark ages.

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Related posts:

The Big Bang: Seconds Later The Cosmological Dark Ages: Abyss The Big Bang: What happened?