Why does our Universe have more electrons than positrons?

[Leo Liu]

In the known universe, the number of electrons seems to be greater than that of positrons since electrons are within every atom around us. However, when a gamma ray approaches a nucleus, a pair consisting of an electron and a positron, can be created from pure energy. If all matters are created from energy, why don't we see an equal quantity of positrons and electrons?

 

[Drakkith]

No one knows. It's one of the unsolved problems in physics and is known as baryon asymmetry: https://en.wikipedia.org/wiki/Baryon_asymmetry

 

[phyzguy]

Note that an average cubic meter of the universe today contains approimately 1 hydrogen atom and 1 billion CMB photons. So at one point there were nearly equal amounts of matter and anti-matter, but approximately 1 part per billion more matter than antimatter. The matter and antimatter annhilated, creating photons, and the fact that matter that remains today is due to the slight (1 part per billion) excess of matter over antimatter. As @Drakkith said, no one really knows why there was slightly more matter than antimatter.

 

[ohwilleke]

No one knows. It's one of the unsolved problems in physics and is known as baryon asymmetry: https://en.wikipedia.org/wiki/Baryon_asymmetry

The excess of electrons over positrons is a corollary of (1) baryon asymmetry and (2) global electromagnetic charge neutrality in the Universe, but isn't strictly the same thing since electrons and positrons are leptons.

The number of baryons in the universe is about 4*10^79, and the number of neutrinos in the universe is about 1.2*10^89. We know that the ratio of baryon antimatter to baryon matter (and the ratio of charged leptons to charged antileptons) is on the order of 10^-11. And, we know that to considerable precision there are 2 neutrons for every 14 protons in the universe (this is a confirmed prediction of Big Bang Nucleosynthesis), and that the number of charged leptons is almost identical to the number of protons in the universe.

In the Standard Model of Particle Physics, baryon number, B (quarks divided by three minus antiquarks divided by three) and lepton number, L (leptons minus antileptons) are conserved separately at all but the highest energies, while sphaleron interactions violate the separate conservation of baryon number and lepton number at very high energies, but still conserve baryon number minus lepton number (B-L). The energies at which sphaleron interactions should take place are about 10 TeV (i.e. 10^4 GeV) and up. See , e.g., Koichi Funakub, "Status of the Electroweak Baryogenesis" ("[W]e find that the sphaleron process is in chemical equilibrium at T between 100 GeV and 10^12 GeV.") Even at the LHC we haven't actually observed a single sphaleron interaction, although it isn't impossible that this could happen in small numbers at some future collider (despite the nominal 13 TeV power of the LHC, it still doesn't get enough energy in a small enough place to produce sphaleron interactions in frequencies large enough to be detected over background events, for reasons beyond the scope of this post and at the fringe of my ability to explain in technical detail).

Baryon asymmetry succinctly stated is the fact that global baryon number in the universe is greater than zero (the aggregate baryon number of the Universe is roughly 4*10^79).

It is widely accepted, and has been proven, that if the Standard Model is correct regarding baryon number, lepton number and B-L conservation, and given the measured values of charge parity conservation (CP) violating processes in the Standard Model, that that the baryon number of the initial conditions is positive and non-zero in the absence of BSM physics immediately after the Big Bang. The factors to determine the initial conditions of the baryon number and lepton number of the universe are known as Sakharov’s conditions (Yoshimura is also sometimes given credit for them).

Of course, BSM physics at near Big Bang temperatures that are many orders of magnitude above what we can test in the lab, or via "natural experiments" that we can observe via astronomy as low z values, would be a lot less shocking that BSM physics within the Standard Model of Particle Physics' well explored domain of applicability.

But, we don't know if there is a lepton asymmetry corresponding to baryon asymmetry because we haven't measured to sufficient precision the ratio of neutrinos to antineutrinos in the universe. I've seen estimates that the ratio of neutrinos to antineutrinos is within 3% of each other, but only in a highly model dependent estimate, because this is difficult to measure. There are hints that the number of antineutrinos in the universe exceed the number of neutrinos in the Universe (making the aggregate value of L in the universe negative and the aggregate value of B-L in the universe positive), but those hints are not statistically significant.

So, the magnitude and sign of the aggregate lepton number of the Universe, and of B-L in the universe is very sensitive to the ratio of neutrinos to anti-neutrinos in the Universe, since there are roughly 10^9 times more neutrinos in the universe than there are baryons and charged leptons combined. To know if the aggregate L or B-L in the Universe is zero, we'd need to measure that ratio to a precision of parts per billion. Of course, if the neutrino-antineutrino ratio is sufficiently far from one (e.g. 1.03), far less precision is necessary to rule out the possibility that L or B-L are zero.

Even if dark matter carries baryon number or lepton number, the critical number for the overall values for the universe of aggregate L and B-L is still almost entirely a function of the ratio of neutrinos than antineutrinos, because neutrinos probably have masses on the order of 10^-3 eV/c^2 (meV), while dark matter candidates (other than axion-like particles) generally have masses on the order of 10^3 eV/c^2 (keV) or more. So, even though there is roughly 10 times as much dark matter (by aggregate mass) as there is ordinary matter (by aggregate mass) in the lambda cold dark matter paradigm, the number of dark matter particles is 100,000 times or more smaller than the number of neutrinos in the Universe.

 

[Buzz Bloom]

The number of baryons in the universe is about 4*10^79, and the number of neutrinos in the universe is about 1.2*10^89.

Hi ohwilleke:

I assume by in the universe you meant in the observable universe.

Regards,
Buzz

 

[phyzguy]

Hi ohwilleke:

I assume by in the universe you meant in the observable universe.

Regards,
Buzz

Since the part of the universe outside the observable universe is, by definition, unobservable, how could we possibly know what is in it?

 

[weirdoguy]

how could we possibly know what is in it?

We can't, but nonetheless I think it's really important to add adjective "observable" when we mean observable universe, because not doing so leads to a lot of misconceptions among laymen.

 

[ohwilleke]

FWIW, I meant the observable universe. It is customary to omit the qualifier as wordy when the distinction isn't at issue, but arguably in the baryon asymmetry problem it is at issue (because one way to explain it is to say that the missing anti-matter exists but only outside the observable universe, e.g. before the Big Bang along the lines of https://arxiv.org/abs/2002.07550 ), so that is fair.