B What caused the asymmetry between matter and antimatter in the early universe?

[calinvass]

What does quantum mechanics have to say about this aspect?
Matter usually differs by antimatter having opposite charge.
It is said that the Universe in the early stages should have created equal amounts of matter and antimatter.

The total charge is conserved no matter the interactions and if we go back in time indefinitely the universe has always been neutral. If in the early stages the universe was all EM radiation, the total charge had to be zero. This is also means a charge symmetry which feels a more natural course. But the can reason we don't have matter-antimatter symmetry have do with the randomness of nature. A classical universe would have evolves perfectly symmetrical. The whole Universe would've been a perfect sphere growing ( it doesn't matter how you model it, it remains classical). But in quantum terms, if we restarted the universe like 1000 times, would it go ~50% of times with more matter and ~50% times more antimatter? If this is the case it is extremely unlikely to be go equal parts. Can we apply this principle?

For example we flip a coin for like

10^{20}

times. We will get a ~50/50 ratio. However, in absolute values we can have something like

5.000001•10^{19}

tails and

4.999999•10^{19}

heads. But

0.000001 • 10^{19}

is very large and would be equivalent to the matter left in the Universe.

 

[phinds]

A classical universe would have evolves perfectly symmetrical. The whole Universe would've been a perfect sphere

That amounts to a personal theory. We don't know the shape of the universe and it might well have been (and still be) infinite in extent in which case it has no shape.

 

[calinvass]

That amounts to a personal theory. We don't know the shape of the universe and it might well have been (and still be) infinite in extent in which case it has no shape.

It is only an example in classical terms related to the Big Bang theory. Classical physics works accepts infinite precision in principle so if you start with a symmetrical system, the symmetry is preserved all the time. We know classical physics doesn't hold at fundamental levels.

 

[PeterDonis]

It is said that the Universe in the early stages should have created equal amounts of matter and antimatter.

Where is that said?

Bear in mind that we know about at least one kind of interaction--the weak interaction--that is not time reversal symmetric. We also know, of course, that our universe has no appreciable antimatter in it, at least not in any of the parts we can observe. So our actual universe does not appear to be matter-antimatter symmetric in this sense.

If in the early stages the universe was all EM radiation

It wasn't.

reason we don't have matter-antimatter symmetry have do with the randomness of nature

We're not sure, because we don't know exactly why our universe (or at least the observable part) has matter but no appreciable antimatter. This is an open area of research.

 

[calinvass]

Here is an example:

https://press.cern/backgrounders/matterantimatter-asymmetry

I've heard many theoreticians physicists saying the universe is dominated by matter because after the it is what it has left after some period of annihilation processes. I don't know if this is very well documented, but at least doesn't seem suspicious.

 

[PeterDonis]

I've heard many theoreticians physicists saying the universe is dominated by matter because after the it is what it has left after some period of annihilation processes.

That's true, but it doesn't explain why there was an excess of matter left over. A heuristic description of the basic idea is this: at the extremely high temperatures of the early universe, the temperature was much, much larger than the rest mass of every Standard Model particle, so all of them were basically in equilibrium, with particle-antiparticle pairs being continually created and destroyed but with the average amount of each not changing. But as the universe expanded and cooled, the temperature gradually dropped below the rest mass of various particles; and once the temperature is well below the rest mass of a particle, any existing particle-antiparticle pairs will be annihilated but no new ones will be created.

So if there are equal numbers of particles and antiparticles, on average, to start with, you would expect none of that particle to be left over once the temperature drops low enough--all that would be left is the radiation (primarily photons) produced by the annihilations. But that's not what we observe: we observe that there is still matter left over, now, when the temperature of the universe is only a few degrees Kelvin, way, way below the rest mass of every matter particle (electrons and quarks). So there must have been some initial asymmetry that favored matter, for any of it to be still left over now.

 

[DrChinese]

That's true, but it doesn't explain why there was an excess of matter left over. A heuristic description of the basic idea is this: at the extremely high temperatures of the early universe, the temperature was much, much larger than the rest mass of every Standard Model particle, so all of them were basically in equilibrium, with particle-antiparticle pairs being continually created and destroyed but with the average amount of each not changing. But as the universe expanded and cooled, the temperature gradually dropped below the rest mass of various particles; and once the temperature is well below the rest mass of a particle, any existing particle-antiparticle pairs will be annihilated but no new ones will be created.

So if there are equal numbers of particles and antiparticles, on average, to start with, you would expect none of that particle to be left over once the temperature drops low enough--all that would be left is the radiation (primarily photons) produced by the annihilations. But that's not what we observe: we observe that there is still matter left over, now, when the temperature of the universe is only a few degrees Kelvin, way, way below the rest mass of every matter particle (electrons and quarks). So there must have been some initial asymmetry that favored matter, for any of it to be still left over now.

Just to add to PeterDonis' excellent description: if there were no initial asymmetry of some kind, then you also might expect that as a worst case scenario that there are regions of the universe in which anti-matter dominated while in other regions (such as the Milky Way and everything near us) it is matter dominated. The idea being that the net amount of (matter - antimatter) is very nearly zero. Were that true, then there would ALSO be regions which were anti-matter dominated regions that were close to matter dominated regions. And if that were true, we would see all KINDS of radiation from the annihilation of the matter and anti-matter at such boundaries as those regions interact. We don't observe such signature by-products, so we conclude there are no such "boundary" regions at this time. Ergo, essentially everything in the observable universe is matter.