2
$\begingroup$

My question is about the creation of the Universe. The current theory I know is that matter is the debris left after matter/antimatter annihilation at the dawn of the Universe. The question is, wouldn't such an event produce approximately equal amounts of matter and antimatter in which case there should be far less matter in the cosmos, even given the immense amounts of background radiation? Why would such an event favour the production of matter over antimatter?

My conception of matter/antimatter relations is that they are sort of like stereo isomers of one another; when space is split, it produces equal amounts of both while absorbing cosmic energy. But in biological science, only left stereoisomers exist in living organisms. These spontaneously revert to equal amounts of both left and right stereoisomeric forms at the death of the organism. Could something similar be happening at the quantum level? Or else, where is the store of all that antimatter and what are the possible mechanisms keeping us apart from it?

$\endgroup$
  • 1
    $\begingroup$ Welcome on physics SE :) What research did you do on the topic yet? $\endgroup$ – Sanya Nov 27 '16 at 20:05
  • $\begingroup$ This is an unsolved problem in physics. We suspect there must be processes that do not conserve baryon and/or lepton number, and that there are more sources of CP violation than currently observed. $\endgroup$ – dukwon Nov 27 '16 at 21:59
1
$\begingroup$

Early in the universe there were almost the same numbers of electrons and positrons, since the production of those in the radiation dominated universe frompair production was in equilibrium with electron-positron annihilation until the temperature dropped at below about 1 Mev's, the mass of a pair. At that point annihilation proceeded to eliminate the excess of positrons, and leave behind the electrons.

As @dukwon said in his comment there is some slight asymmetry that should have been the cause of more electrons than positrons, that remains an unsolved problem in physics.

The excess elimination occurred leaving behind approximately 1 in a billion electrons. Since the universe is electrically neutral (overall, approximately) the number of leftover electrons is about the same as the number of protons. See also Ratio of electrons and protons in Universe

1 in a billion imbalance of electrons and positrons comes from the fact that the baryon to photon ratio is about 1 per billion, and before annihilation there would have been roughly the same numbers of photons as leptons in equilibrium (none of this is exact, there were other leptons, and other charged particles, but those were the dominant ones)

See the thermal history of the universe in calculations at, for instance, http://www.helsinki.fi/~hkurkisu/cosmology/Cosmo6.pdf

Roughly there is 1 baryon per about $10^9 photons. That can be deduced from thermodynamic equilibrium in cosmological evolution, and the abundance of light elements from the Big Bang nucleosynthesis (BBN). It's an important number that also helps determine the light nucleon percentages formed early after the Big Bang. See http://www.astronomy.ohio-state.edu/~dhw/A5682/notes7.pdf. See also Wikipedia at https://en.m.wikipedia.org/wiki/Big_Bang_nucleosynthesis.

$\endgroup$
  • $\begingroup$ in the last paragraph "per 10^9" what? or should it be antibaryon there? in general you have to qualify your numbers more to become intelligible.( 1 positron in a million electrons) $\endgroup$ – anna v Nov 28 '16 at 7:21
  • $\begingroup$ Thank you, an oversight. It's per that many photons. I'll edit $\endgroup$ – Bob Bee Nov 28 '16 at 18:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.