I love The Straight Dope. For 35 years, people have written in and asked some of the most difficult-to-answer questions on any topic you can think of; the staffers, writing under the pseudonym Cecil Adams, do their best to get to the bottom of their questions. Well, they also have a message board, and I saw one of the most difficult questions I’ve ever seen there:
Where does all the matter in the universe come from?
I’m no[t an] astrophysicist but I understand a little about the Big Bang Theory and also that there’s lots of stuff we don’t know or probably ever will know about it.
But the universe is awfully big and must have an awful lot of matter in the form of asteroids, stars, asteroids and suchlike. Did all matter in the universe originally exist at the centre of the Big Bang or is new matter being constantly created? If so, how?
None of the responses up there even begin to do this question justice, so let’s take a look ourselves. First off, there are two possible interpretations of this question, and I need to choose which one to answer. Is the question asking:
- Why is the Universe full of stuff? That is, why is there anything with any energy at all instead of nothing? Or…
- Why is the Universe full of matter? Energy could take any shape or form, but why matter, and how did it get there?
I’m assuming the second one (although if someone wants to ask the first, I’ll give it a shot); Why is the Universe full of normal matter? This isn’t something we expect, mind you. Here’s what we know as normal matter:
protons, neutrons, and electrons make up all the planets, stars, gas, and dust that we know and observe in the Universe. But the problem is, whenever we go into a laboratory and try to make some of this normal matter, for every particle of normal matter we make, we also make one of antimatter. But the Universe as we know it is made up almost exclusively of matter, with almost no antimatter. Every galaxy we see is matter, not antimatter. Every cloud of gas and dust we see is matter, not antimatter.
Why? If we take a look at wikipedia’s Unsolved Problems in Physics article, this is the second one on the list. But there is a whole bunch we do know about it, even if we don’t know the entire story.
First off, and this is the first rule of any scientific inquiry, is no cop-outs. That means, we would never just say, “Oh, it had to be there from the start of the big bang.” No; we want to figure it out, so we want to start with equal amounts of matter and anti-matter, and see if we can make more of one type than the other, naturally. We call this process Baryogenesis (other articles here and here).
So what do we need to make it happen? We need three things, known as the Sakharov conditions:
- You need an interaction that violates Baryon number conservation, which means you need to be able to make more protons than antiprotons or something akin to that.
- You need to violate CP-symmetry, which means (in English) that you need particles and antiparticles to decay into their various products at different rates.
- You need to be out of thermal equilibrium.
So, how can we do that? Let’s give you my favorite example; let’s assume that at some high enough energy, there are superheavy particles called X. The X has a charge +4/3, and there’s also the anti-X, with charge -4/3. When the Universe is very hot, it can be stable, and you make equal numbers of X and anti-X particles. Now, these X and anti-X particles aren’t stable, and they decay. Maybe the X-particles decay like this:
And make a positron and an anti-down quark (1/3 of an anti-baryon). Or maybe they decay into two up quarks instead (2/3 of a baryon). But let’s say the anti-X-particles also decay (because decays happen when the Universe cools and becomes unstable — that’s condition 3): they can decay into electrons and a down quark (1/3 of a baryon), or two anti-up quarks (2/3 of an anti-baryon). But what if the X decays into 49% positrons and anti-downs and into 51% two ups, and the anti-X decays into 51% electrons and downs and 49% two anti-ups? Well, that’s what can happen if you violate CP-symmetry (that’s condition two). Let’s put it all together and see, at the end of the day, what are you left with? A bunch of particles and antiparticles that will find each other and annihilate (down/antidown, up/antiup, and electron/positron), but then you’ll have that 2% left over! And what is that 2% made up of? Electrons, down quarks, and up quarks — just the stuff you need to make normal matter! And so you meet all three of these conditions, and just like that, you can make more matter than antimatter, starting with equal amounts of both!
Now, we aren’t sure that this is how it happens, nor are we sure that any of these related methods is how it happens either. But we can pretty definitively say that the Universe is made up of matter and not antimatter, and that there are a number of physical ways to make more matter than antimatter in the Universe. It isn’t being created now, nor is it fair to just assume it was created at the Big Bang, but it looks like we can make it rather shortly after the Big Bang, and pretty much all in one go. And that tiny little asymmetry, the little extra bit of matter that was created as opposed to antimatter, makes up and gives rise to everything (except the Microwave Background) that we see today. Pretty neat!