# Number of particles

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1. Jun 1, 2015

### Stephanus

Dear PF Forum,
I'm just curious about these things.
From what I read, the number of atoms in the universe is 1080 Hydrogen atoms.
And neutrino is a billion times more. 1.2 x 1089
But what it said is actually the number of protons + neutrons, right?
If yes, why?
Protons and neutrons, they both are created in Baryongenesis, what about electron? When were they created?
Leptogenesis they say, but it's hipothetical, right?

2. Jun 1, 2015

### Staff: Mentor

Basically, yes; it's the number of baryons, of which the only significant ones are protons and neutrons (the other baryons are all unstable and decay eventually into protons and neutrons anyway).

It probably is, yes, because the universe as a whole is expected to be electrically neutral, and the only significant charged particles are protons and electrons (all other charged particles in the Standard Model are either unstable or they are antimatter and therefore extremely rare).

At the end of inflation, when the energy stored in the inflaton field was transferred to energy contained in the various Standard Model particles. That would have included both matter and antimatter particles (so electrons and positrons, quarks and antiquarks, neutrinos and antineutrinos, etc.). Later on, as the universe cooled, the matter and antimatter particles annihilated each other, leaving behind a lot of photons and a small excess of matter particles.

3. Jun 1, 2015

### Chalnoth

At this level of accuracy, it doesn't matter. About 75% of the matter in our universe is Hydrogen, 25% Helium. The remainder that are heavier elements are so few that we can safely ignore them.

In this situation, the hydrogen atoms have a single proton, while the Helium atoms have two protons and two neutrons. If the total number of hydrogen atoms is exactly $10^{80}$, then the total number of helium atoms is about $3.3 \times 10^{79}$, which makes for about $1.3 \times 10^{80}$ nucleons (protons and neutrons) in all Helium atoms, for a total of $2.3 \times 10^{80}$ nucleons. That's such a tiny fraction of the neutrino number count that it's pretty much irrelevant when compared against neutrinos.

Electrons have been around since the end of inflation. So have the quarks that make up protons and neutrons. Baryogenesis didn't create the protons/neutrons, but it did create an imbalance between matter and anti-matter. Due to the conservation of electric charge, the imbalance in the number of protons had to be balanced by an equal number of negatively-charged particles. The only negatively-charged particles that are in the standard model and also stable are anti-protons and electrons. But we just said that we've got an imbalance between matter and anti-matter, so anti-protons can't fit the bill, leaving only electrons to retain conservation of charge for the protons.

4. Jun 1, 2015

### Stephanus

But electrons weren't created at the same time as protons or were they?
If they weren't created at the same time as protons, how the universe "knows" to create the same amount of electrons to at least match protons?

5. Jun 1, 2015

### Staff: Mentor

They were created at the same time as quarks. The temperature was too high at that point for quarks to be bound into protons.

6. Jun 1, 2015

### Chalnoth

Charge is conserved, so any process that created a positively-charged particle would necessarily create a negatively-charged particle. The total charge had to remain zero throughout, and electrons and protons are the only stable charged particles (aside from their anti-matter twins).

7. Jun 1, 2015

### Stephanus

Thank you, thank you for the answers.
Actually I want to know how the sun (and the stars) gets its energy.

If it's proton to proton reaction chain, then...

${^{1}_{1}H} + {^{1}_{1}H} = {^{1}_{2}D} + p^+$
${^{1}_{2}D} + {^{1}_{1}H} = {^{2}_{3}He}$
${^{2}_{3}He} + {^{2}_{3}He} = {^{2}_{4}He} + 2{^{1}_{1}H}$
This reaction seems involve 6 Hydrogens atom. And 6 electrons?
And the result only have 4 electrons?
This belongs to other thread I guess

And the result if neutrally charged only have 4 electrons?
But I'd like to know first before I learn nuclear reaction.
Is the universe (approximately) neutrally balance?
Thanks.

8. Jun 2, 2015

### Chalnoth

There's a reaction where a proton and electron can combine to form a neutron and a couple of neutrinos. In this reaction, the helium atom has two neutrons in addition to its two protons.

9. Jun 2, 2015

### Stephanus

Thanks Chalnoth for you invaluable answer. It's not the matter anti-matter balance that I want to know. It's the electrically balance that I want to kno.
Thanks.

10. Jun 2, 2015

### Stephanus

Ahh, of course. How foolish I am!

11. Jun 2, 2015

### ChrisVer

Also an addition to the chain you wrote down...
There are no electrons shown in those reactions, but nuclei... The circumstances where these reactions occur don't allow for atoms to exist, instead you are having a plasma state (ionized elements). Rephrasing: the electrons and nuclei are free.

12. Jun 2, 2015

### ChrisVer

And H+H-> D+p doesn't make much sense.

13. Jun 2, 2015

### Stephanus

Double foolish I am!. Forgot that also!
Okay..., nucleus' and electrons are free roaming. But the universe is somewhat electrically charge balance, right?

14. Jun 2, 2015

### Stephanus

But, that's what Wikipedia says

Last edited: Jun 2, 2015
15. Jun 2, 2015

### Chalnoth

16. Jun 2, 2015

### ChrisVer

Check post#2, stating that probably it is neutral... altough I'm not sure why would that be the case, and I don't know of anyway we'd measure the charge of the universe.

If you want to see the charge conservation (since it's conserved) then the electrons are free around and they meet a positron (which I mistook as a proton with that notation as Chalnoth said) coming from the 1st line and annihilate.

17. Jun 2, 2015

### Chalnoth

During inflation, the universe becomes almost completely empty, to the point that maybe one particle from the pre-inflationary regime made it into our observable universe.

All of the particles that we see around us were produced at the end of inflation, and the net charge of those particles had to be zero. There will be some tiny differences due to the fact that our universe is finite in extent, so that some of the matter/anti-matter particle pairs created in the very early universe may have become separated with only one member of the pair within our observable universe. But this number is going to be so small that it can be safely ignored.