Why the Universe is mostly made up of matter

From: Kevin Phyland	20/08/99 8:38:28
Subject: Matter in the Universe	post id: 31818
Hi every1, Can anyone tell me why the Universe is mostly made up of matter (as opposed to antimatter)? Is matter inherently more stable than antimatter? Were both created in the BB? Kevin Phyland. Wycheproof P-12 College.

From: Chris (Avatar)	20/08/99 10:50:45
Subject: re: Matter in the Universe	post id: 31840
Big question, Kevin! The question of why the universe appears to be matter dominated is one that has bothered cosmologists for a while. First lets have a quick look at why we think the universe is matter dominated. Firstly, when matter and anti-matter come into contact, they annihilate. The mass of both is completely converted to energy given by E = mc². We know that the earth is matter dominated, because we don't see the evidence of matter/antimatter annihilations about us everyday. Similarly we know the sun is matter dominated because it continually buffets us with the solar wind. In this way, we can see the solar system is matter dominated. In addition we know that the probes we have sent to all other planets haven't annihilated. We do see stellar (and other) collisions in our galaxy, and these collisions are consistent with matter-matter collisions rather than matter-antimatter annihilations. Also, in cosmic rays from the milky way galaxy protons outnumber antiprotons by 10⁴ - 1. The rest of the universe is a little more difficult to determine, although galaxy collisions do help us determine that both are either matter or antimatter (but not one of each). In addition, the so called "Annihilation Catastrophe" principle shows that it's well nigh impossible for large chunks of matter and antimatter to separate out of the early universe without annihilating. So we might infer that the visible universe is very probably matter dominated. Ok. How? According to quantum theory, matter-antimatter pairs are created out of the vacuum all the time. It takes an energy E=(2m)c² to create two particles of mass m, for example to create an electron-positron pair (m=9.1110^-31kg) it takes 1.610^-13J or about 1MeV. If this much energy is around (not a lot, really), then the particles are real and shoot off on their different ways. If the energy is not available they may still be created as virtual particles, only to disappear again very shortly after being created (for our electron positron pair, about 3.310^-22seconds!). In the early universe there was sufficient energy about to create real matter-antimatter pairs. The problem is that whenever matter and antimatter meet they annihilate, but then they're produced in even numbers again. It would appear that with an expanding universe, eventually there would be insufficient energy density to keep creating the pairs, leaving the existing ones to annihilate until nothing remains. So how come we see a matter dominated universe? Well, the universe is only "matter dominated" in comparison to the concentration of antimatter. In point of fact the universe is radiation* dominated. For every particle of matter that we see there are about 10⁸ photons of radiation. Now suppose there was a tiny asymmetry in matter/antimatter ratios in the early universe - we would expect most antimatter and matter to annihilate producing photons, and eventually leaving only the "left over" matter. It has been estimated that at the high temperature of the universe's first microsecond there were large numbers of thermal quark-antiquark pairs. Kolb and Turner (The Early Universe) estimate 30 million antiquarks for every 30 million and 1 quarks during this epoch. Over time most of the matter annihilates with antimatter leaving photons and a small amount of matter to dominate the universe. The best explanation for this asymmetry is in terms of baryogenesis occurring at a development period known as the Electroweak Phase Transition, the period when particles first acquire mass via the Higgs mechanism. I could go into more detailed explantion here if you like, but it's a bit technical so I'll wait to hear from you what level you're comfortable with. :o) In any case, that's the best story at the moment. If you have any questions please feel free to ask. Hope this helps! Chris

From: Kevin Phyland	20/08/99 13:59:51
Subject: re: Matter in the Universe	post id: 31883
Thanx again, Chris. I know a 'little' Electroweak theory and I've heard of higgs bosons, but can't claim guruship in either. Basically you're saying that a tiny excess of matter during the BB accounts for the apparent predominance of matter in our Universe (even if 1 part in 10,000 is antimatter)...right so far? Then how come there is ANY antimatter at all and does antimatter have an inverse analog to gravity, i.e. antigravity? Yours in cosmology Kevin.

From: Chris (Avatar)	20/08/99 14:53:12
Subject: re: Matter in the Universe	post id: 31902
There is antimatter because it can be created, or appear spontaneously. As mentioned above, for every particle created out of the vacuum there is an antiparticle created too. Also antimatter appears as a natural by-product of some reactions - eg proton-proton fusion produces a positron. The reason there isn't very much is because antimatter is necessarily short lived. As soon as it encounters normal matter they annihilate. The question of how antimatter reacts in gravitational fields has not yet been resolved conclusively. According to GR antimatter must "fall" at the same rate as matter - the two are gravitationally indistinguishable. However there are some models of gravity which are distinct from GR, and have not been ruled out yet, in which antimatter might fall slower or even the opposite way. This is due to different forces coupling with the mass in different ways. It is difficult to perform experiments to determine directly the gravitation of antimatter because (a)it is so much weaker than e/m and (b)only very small amounts of antimatter can be tested. However it is possible to rule out most models of antigravity using conservation of energy (the "Morrison argument"), the detectable effects of virtual antiparticles (the "Schiff argument"), and the absence of gravitational effect in kaon regeneration experiments (see Nieto and Goldman, "The Arguments Against 'Antigravity' and the Gravitational Acceleration of Antimatter," Physics Reports, v.205, No. 5, p.221). The other common candidate for "antigravity" - the cosmological constant originally introduced by Einstein - has nothing to do with antimatter at all. Getting back to baryogenesis, the important - and interesting - point is that it isn't good enough to just say that there was a wee bit more matter than antimatter at the BB and thats good enough. What we have to do is explain why there was a bit more matter than antimatter, given the symmetry with which it is generally created from the vacuum. The standard model of particle physics does a good job of explaining the asymmetry in terms of violations: Baryon number violation, C and CP violation, and thermodynamic non-equilibrium. Establishing that each of these is possible allows for a tiny asymmetry in the early universe and assures that it doesn't disappear. Hope this helps! Chris

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