Why do we expect the Big Bang would have created little more matter than we observe? How do we know that almost all matter wasn't annihilated, and what's left really isn't just a rounding error after ~50% annihilated ~50%?
1. Because there isn't even close to the amount of radiation around we would see from such incredible amounts of anihlillation
2. Matter/antimatter annihilation is symmetric - it removes the same amount of matter and antimatter from the equation so however much stuff we started with we expect to still see the same amount of matter and antimatter and the antimatter doesn't seem to be around
Its worth emphasising that there is some matter/antimatter asymmetry in the standard model, they don't behave as exact "mirror opposites" particularly in the context of some kaon physics, but there isn't even close to enough asymmetry to explain the amount of matter and the lack of antimatter we observe.
2 particles, one of which is the anti-particle of the other form a system where the sum of all quantities that are conserved can be 0, so a transformation generating or destroying both particles is possible.
Nevertheless there are also other systems of particles where the sum of all conserved quantities can be 0 so there should exist transformations that generate all of them simultaneously.
Besides the systems of 2 particles, where generation and annihilation is well known, there are systems of 4 particles with zero sum of all conserved quantities (e.g. a quark, an anti-quark, an electron and an anti-neutrino) and also systems of 8 particles e.g. the 3 kinds of u quarks + the 3 kinds of d-quarks + electron + anti-neutrino.
The systems of 4 particles can actually be generated and destroyed in weak interactions, similarly like the the systems of 2 particles can be generated and destroyed in electromagnetic interactions.
For the systems of 8 particles, there is currently no theory about a mechanism of generation or annihilation, but my bet is that this is how matter was created during Big Bang (those 8 particles aggregate into 1 proton, 1 neutron, 1 electron and 1 neutrino).
For now we have a theory of Big Bang not from the starting point but from a little later when there already was normal matter as we know it.
There is no theory yet for the origin of Big Bang, but there is absolutely no reason to believe that at the beginning there was an electromagnetic interaction generating simultaneously equal numbers of particles and anti-particles. For that to be true, the matter and anti-matter should have been preceded by electromagnetic waves i.e. photons, of equivalent energy with the mass of the generated matter and anti-matter, which would then annihilate restoring the previous photons. Such a theory goes nowhere, so it does not match reality. Whatever started Big Bang, it was not an electromagnetic field generating pairs of particles and anti-particles.
There are many.
The Big Bang theory that is useful starts with matter at very high temperature and density, but otherwise not different from the matter that we know. That happens at some uncertain time interval after the beginning of Big Bang.
We may try to extrapolate towards time 0 and increasing temperatures and densities, but that reaches soon values of temperature and pressure high enough that we do not really know how matter behaves in those conditions and it does not matter anyway because it does not influence what happens later.
For the later evolution, it is enough to start the modeling of Big Bang from the moment when the temperature became low enough, e.g. of some tens of MeV, i.e. when matter was too hot for nuclei and atoms to exist, but cold enough so that it consisted of a plasma composed of protons, neutrons, electrons, positrons, photons and neutrinos, with only negligible quantities of heavier particles.
For the time 0 there is no theory that can predict anything quantitative.
I think the main point that is missing is that the standard model is very close to symmetric, so if you have some process that generates systems of 4 or 8 or however many particles from a neutral boson then exactly the same process should occur producing all the anti-particles of the above e.g. if you think you make
1 proton, 1 neutron, 1 electron and 1 neutrino
in such a way that all the numbers balance then you should also see a process that makes
1 anti-proton, 1 anti-neutron, 1 anti-electron and 1 anti-neutrino
Its also worth pointing out that pair production is not specifically an electromagnetic thing. For example the Z0 boson pair produces fermion+anti-fermion pairs in a very similar way to the photon and W bosons decay to lepton+anti-neutrino pairs in a similar way (although W bosons are charged so its a little different). The Higgs boson also decays into quark-anti-quark,lepton-anti-lepton or even boson-anti-boson pairs.
I did not claim that this is how matter was generated in the Big Bang, especially because for now there exists no theory for such a process.
My point is that this is a strange coincidence as it would provide matter in the right proportions. Unlike for electromagnetic interactions, there might be some asymmetry making the process generating the particles more probable than the process generating the antiparticles. Because the 2 processes are decoupled, they might have happened at different rates, which is not possible with electromagnetic generation and annihilation.
Of course, the correct explanation for Big Bang might be completely different, only the fact that there was no simultaneous generation of matter and anti-matter is certain.
The neutral Z0 boson behaves like the photons, so I did not mention it.
The charged W bosons cannot generate particle/anti-particle pairs because that would violate the conservation laws. They generate simultaneously 4 elementary particles: quark, anti-quark, lepton and anti-lepton.
The quark and the anti-quark remain bound as one meson.
The reverse event is also possible, but highly improbable, because it is unlikely for a meson, a charged lepton and a neutrino to collide simultaneously, in order to annihilate into W bosons. What happens frequently is the equivalent event when 2 of the 3 collide and transform into a W boson together with the antiparticle of the third.
These type of events involving W bosons are what I was referring to as the generation/annihilation of systems of 4 elementary particles through weak interactions.
W^+ -> positron + electron neutrino
W^- -> muon + muon antineutrino
are allowed as are processes like
W^+ -> up quark + anti-down quark
Unlike the stable photon, the W bosons have a negligible lifetime.
So in fact those processes listed by you that result in 2 particles are 4-particle processes.
Two particles collide, either a quark and an anti-quark giving other 2 particles, a lepton + an anti-lepton (in your exemples positron + electron neutrino or muon + muon antineutrino) or a lepton collides with an anti-lepton giving a quark and an anti-quark (in your exemple up quark + anti-down quark).
So there are always 4 particles, either quark, anti-quark, lepton and anti-lepton, or there may be both at input and at output a lepton and an anti-lepton, which includes the case of scattering through weak interactions, or there maybe both at input and at output a quark and an anti-quark, which includes the case when a meson decays into another meson through a weak interaction.
All the interactions involving W bosons are 4-particle interactions, where the W boson is an intermediate particle that exists only during a negligible time.
The 4 particles may be all at the input, all at the output, 3 at the input and 1 at the output, 1 at the input and 3 at the output or the most frequent case, 2 at the input and 2 at the output.
Like all the interactions between elementary particles all these cases are equivalent and all the variants are interchangeable by moving a particle from the input to its anti-particle at the output or vice-versa.
In most cases you need to pay attention only to the 4 particles participating in the weak interaction and not to the intermediate W boson.
The existence of the W boson matters only when you need to compute various things because this intermediate particle transforms diagram nodes with 4 edges that cannot be computed into diagram nodes with 3 edges that can be computed with the methods of quantum electrodynamics.
To close approximation the kinetic energy of something is 1/2 * mv^2 and the mass energy is m c^2 so to have a meaningful contribution to the total energy thespeed has to be close to the speed of light. We generally don't observe big things (stars, black holes, galaxies) moving anywhere near to that fast and their energy is basically entirely due to their mass rather than their speed.
If you have a sparse gas with a random distribution of velocities and you let it sit for a while, you will find that the outliers on the upper end have have moved further away from cluster center. They have boiled off.
Now focus on single random object and narrow down your field of view so you won't see the edge. Look at other objects. They will seem to be moving away directly from the point you focused on with velocities proportional to distance from it.
I made such simulation and calculated that the relative velocities of other points face directly away from the point I'm observing. And they really do.
There's definitely a center of this system, yet from the point of view of any object far enough from the edge to not be able to see it, it seems like there's no center and other objects just move away from us, and from each other with speed proportional to the distance.
Same way that you can't figure out where the center is, you can't figure any special direction or plane.
Try sumulating this model yourself. Everything looks exactly as if you were in the center of Big Bang, even when you are not.
And you wouldn't be able to tell what's your speed when you look at how other stuff moves. Everythin would look like moving away from you with speed proportional to the distance.
What if kinetic energy is the only thing responsible for galaxies moving away from each other? Or at least the bulk of it? Maybe the part that we interpret as a period of super fast inflation right after the big bang? Maybe this rapid inflation is just a way of interpreting matter having huge amount of kinetic energy relative to each other right from the beginning?
This is exactly what we would expect if the universe is expanding isotropically but under the "galaxies actually moving away from each other" hypothesis this is only possible if Earth is exactly at the center of a sort of "explosion of galaxies".
A second issue with this theory is that we see objects with redshifts that would mean that if their apparent motion is actually real motion then they would be moving away from us faster than the speed of light. As far as we know this is completely impossible for actual motion but is exactly what we would expect from expansion.
Start with bunch of objects at coordinates 0,0,0. Give them random velocities from 0 to c. Then just let them move according to Newtons law.
Now focus on single random object and narrow down your field of view so you won't see the edge. Look at other objects. They will seem to be moving away directly from the point you focused on with velocities proportional to distance from it.
I made such simulation and made calculations to ensure that the velocities of other points face directly away from the point I'm observing. And they really do.
Even in completely flat Newtonian universe there could have been a sort of Big Bang with epicenter and it could be as simple as "give matter random speeds" and we living on a one speck of matter would have no way to figure out that there is a center or where is it.
When I asked about this on physics stack exchane I got a shurg that, yeah cosomology is basically that but with Einstein not Newton.
All that talk about spacetime inflating is just a result of matter 'dragging' the spacetime along as it moves.
The faster than light galaxies far away are not a problem because the speed of their movement that we measure is sum of their kinetic movement (which could be almost at light speed) and expansion of the space time between us and them as the spacetime is 'dragged' by them with GR. But you can equally well interpret the math and data as galaxies roughly at rest and all the speed coming from spacetime expansion and for I have no idea what reasons people actually prefer to do that.
US 708 is going fast and it's still only 0.4 percent the speed of light.
if you're talking about relative speeds between astronomical objects being very high due to cosmic expansion, i'm not well enough versed in the physics to know why or explain how, but the energy locked up in that movement doesn't seem as practical to talk about with regards to creating work.
we know how to harness kinetic and thermal energy, as far as I know we're not yet able to surf the cosmic expansion, except inadvertently.
Their sum might easily be faster than light and they still might have insane kinetic energy.
Th obvious suggestion is that if there was equal amounts of antimatter near us, we would likely be annihilated already. But that doesn't explain why so little can be found remotely either.
The potential for matter and antimatter to alternate is an interesting proposal that could explain our little island of stability in the universe.
Or it could be something else entirely. I'm a software engineer, not a physicist.
Because while CP symmetry is violated CPT symmetry always holds as far as we know?
Traveling in opposite directions and back in time?
(Serious question; I’m not a professional physicist)
When particle and photon meet the result is particle and photon just moving differently. You probably might think of this as old particle and photon disappearing and new paricle and new photon appearing.
We define "matter" to be what there is in greater abundance in the local neighborhood. Thus we define protons and neutrons (or up and down quarks) as matter, along with electrons.
But what if that's wrong? What if electrons are anti-particles? That would go some distance toward "matter and antimatter in equal amounts" (though not, I think, all the way there).
Is there any physical reason that, if we identify the up quark, say, as matter, then the electron also has to be matter rather than antimatter? (I think there is reason that if the up quark is matter, the down quark has to also be matter, but I could be wrong on that one, too.) Can anyone with better knowledge than mine shed some light here?
Electrons can't, in any sense, be "antimatter". That's because there antimatter as a concept only applies to a particle's relationship to its antiparticle -- it's not a feature of a single type of particle, only of two types of particle. Which must be variant excitations of the same quantum field.
That's almost irrelevant, however. The big bang should have created roughly equal amounts of matter and antimatter for each quantum field; an excess of protons cannot be balanced by deficit of positrons, as I believe you were suggesting.
Protons and positrons do not annihilate, nor would protons and electrons for that matter. Only protons and anti-protons, or electrons and positrons, have that reaction.
(Though, aside: Protons are composite particles. None of the composites are electrons, though, so this still applies.)
If the Big Bang didn’t create an equal quantity of matter and antimatter, and given that antimatter is still being produced by various cosmological sources and at a lower rate for proton-antiproton pairs than for electron-positron pairs, would that imply that the universe has a non-zero net electric charge which is still changing over time?
And if so, what would a universe with a significant non-zero electric charge look like?
And if not, because Noether, could the combination of e.g. antiprotons and positrons into anti-neutrons turn out to be stable and one possible candidate for dark matter?
(Full blogpost: https://kitsunesoftware.wordpress.com/2021/02/09/baryon-asym...)
All the events that happened after that, e.g. the decay of a part of the neutrons into protons and electrons, the annihilation of positrons with electrons and various generations and annihilations of particle/anti-particle pairs, have not changed the total electric charge, so it has remained zero.
The electric forces are extremely strong and they ensure that matter is on average electrically neutral, so the only long-distance forces are magnetic and gravitational.
Any region with electric charge would generate strong electric fields with various obvious effects.
To rephrase my two questions about that observation:
1) What if the observed baryon asymmetry is an illusion because anti-neutrons are stable, unlike their mater counterparts? Would that explain anything?
2) If baryon asymmetry is real and not an illusion, what else should we expect to see?
> The electric forces are extremely strong and they ensure that matter is on average electrically neutral, so the only long-distance forces are magnetic and gravitational.
Which is why my question is specifically with regard to the observed baryon asymmetry as that might (in the case of question 2) mean the charges can’t be balanced.
> Any region with electric charge would generate strong electric fields with various obvious effects.
If the charge was distributed isotropically rather than in a limited region?