marce said:
- I never quite understood , while people generally believe they must search for the reason of matter, anti-matter unbalance. I mean the big-bang was a violation of the energy conservation law. As far as I understand in the first moment there was only energy which later condensed into matter and anti-matter. But the energy was there before, so why would anti-matter and matter be created in perfect balance ? Meaning we have energy , matter and anti-matter, but there's no anti-energy , right ?
M.
All processes in the Standard Model balance out all new matter particles with an equal number of antimatter particles. So, if you started from pure energy and that energy condensed to matter using only Standard Model processes, then there ought to be the same amount of matter and antimatter.
For example, if I (hypothetically) go into my laboratory and produce a pair of photons (which is about as close to pure energy as you can get) with a combined energy equivalent of the mass of a soccer ball, half of the matter produced when they collide and condense into matter will be ordinary matter and exactly half of the matter produced will be antimatter.
Three of the four fundamental forces (electromagnetism, the strong force and gravity) treat matter and antimatter in exactly the same way. But, one portion of one of those fundamental forces (the W boson interactions of the weak force which transforms particles into different kinds of particles that causes the beta decay of radioactive atoms like uranium and plutonium) behaves differently when matter and antimatter particles, respectively are involved, which is called CP violation.
But, while there are processes in the Standard Model that interact differently when matter is involved than they do when antimatter is involved, in some very subtle ways, the differences are not big enough to create a universe that is as rich in matter and as poor in antimatter as we do, if it started from pure energy (let alone to do so in just half an hour or less which is what our current observations and laws of nature imply it must have if the universe did start from pure energy).
There are basically two possible resolutions to this situation.
One is that the universe began at the Big Bang, not with pure energy, but with some amount of matter than was greater than the amount of antimatter that was present at the start. Specifically, the number of matter particles in the universe minus the number of antimatter particles in the universe (by particle, not by mass or energy), must have been equal to the number of matter particles that we observe today. This possibility is completely consistent with all known laws of physics. But, this possibility forces us to abandon what some people see as the extremely beautiful idea that the universe arose from pure energy.
The other possibility is that there are physical laws in addition to those found in the Standard Model that only have easily observable effects in circumstances similar to those in close vicinity (in time and space) to the Big Bang. More precisely, this needs to have happened within about half an hour or less from the Big Bang, (i.e. by the end of the process that created the first atoms called "Big Bang Nucleosynthesis which is a well proven theory about what happened not long after the Big Bang) and maybe even within as little as ten seconds from the Big Bang (i.e. before atoms started to form).
There are myriad proposals for ways that the laws of physics could be different to allow this to happen. But, many of these proposals are either impossible to falsify (e.g. the missing antimatter is out there in a place where it can't be observed), or are contradicted by observation because they imply other things that we should be able to see and don't see (e.g. they require proton decay to occur with a mean lifetime that is not many times longer than the age of the universe, which we know doesn't happen).
In 1967, Professor A. D. Sakharov, a Russian physicist, figured out that any proposal that worked to explain why we have so much matter relative to antimatter if the universe started from pure energy had to do three things (1) violate the law stating that new matter particles must be exactly matched by the same number and type of antimatter particles, (2) have equations in a form that include a preferred direction in time, and (3) must take effect when the universe is either rapidly heating up or rapidly cooling. But, those limitations still leave all sort of room for new physics which are beyond the Standard Model that could cause a matter dominated universe to arise from a starting point of pure energy.
No one has found an acceptable solution to the new physics that could do that, but lots of physicists aren't willing to give up the pure hypothesis that the Big Bang involved pure energy rather than a mix of pure energy, matter, and at least somewhat less antimatter (or no antimatter).
So, this is called an "unsolved problem."
But, as you note, there is no real profound reason that the initial conditions of the universe should involve zero matter, when we already are perfectly comfortable with the idea that the initial conditions of the universe did not involve a zero energy.
To be fair, this concern isn't entirely aesthetic. Any amount of energy can fit into a point-like space. But, the kind of matter particles that need to be present for the first option to be true, can't fit in a single point in space. Each one needs to have its own distinct space coordinate, and we need something like 10^90 (i.e. a one with ninety zeros after it, which is a very, very big number) different points for them to be in, even if there is no antimatter at all to start with. This means that in addition to having a starting point with non-zero energy and non-zero matter that we must also have a starting point for the universe that has non-zero volume in space, even though those points can be jammed extremely close to each other initially.
This detracts even further from the image of the Big Bang as pure energy at a single point, although there is still no profound reason that the universe has to start from pure energy at a single point, aside from the fact that this just seems way more elegant.
For example, a non-zero energy, non-zero matter, non-zero volume Big Bang initial conditions scenario works just fine if you have a cyclic cosmology in which the Big Bang comes at the tail end of the "Big Crunch" of another universe that preceded it. This kind of cyclic cosmology also solves other problems that physicists trying to imagine a universe starting from a single point of pure energy have to invent concepts like "cosmological inflation" to explain. And, cyclic cosmology also eliminates the problematic question of how something could have come into existence out of nothing that you described as non-conservation of energy in your initial post.
Of course, almost by definition, any kind of theories about what came before the Big Bang can't really be proven or disproven, even if we can speculate that they make sense and fit all of the available evidence more straightforwardly than alternative possibilities. In contrast, at least in principle, if the matter-antimatter asymmetry of the universe is due to new physics, there is every reason to think that it might be possible to prove that those new physics are correct in some kind of experiment or observation, because they might have some very subtle effects that also apply in non-Big Bang situations.
At any rate, until we discover any new laws of physics that only apply in extremely high energy, rapidly cooling environments, we're going to have to come to terms with the fact that in the "almost initial" conditions of the universe, at least (in standard versions of cosmology, certainly by the time that half an hour had elapsed since the Big Bang), the universe had lots of matter and not much antimatter, for no particular reason known to us.
