B Are Antiparticles the Key to Understanding the Universe?

[PeterDonis]

My understanding of Big bang theory is that the concentration of matter in the universe was due to quantum fluctuation in the early stages

That plus the gradual action of gravity over billions of years. The variation in matter distribution in the universe today is much greater than it was in the early universe, because the small fluctuations that were present then (due to quantum fluctuations, yes, but possibly not quite the way you are imagining--see below) have become greatly magnified by gravitational clumping.

would any fluctuation in the distribution of matter and antimatter in the early stages of the big bang lead to large areas dominated by matter and antimatter at this point?

The original fluctuations weren't in the distribution of matter and antimatter. They were in the inflaton field (the field that caused inflation). Those fluctuations got transferred to the fields we call "matter" and "radiation" (the ones that appear in the Standard Model of particle physics) when inflation ended (this process is called "reheating", which is a bit of a misnomer since there wasn't any previous "heating" or "cooling"). The process of reheating should, according to the Standard Model, have created matter and antimatter in equal quantities, which would have meant that, as the universe cooled, all of the matter and antimatter would have annihilated each other and left only radiation (photons). That would still be true even in the presence of fluctuations.

 

[alantheastronomer]

No, there shouldn't. The photons produced during annihilation have redshifted during the intervening time; their present temperature is the temperature of the CMB, because those photons are the CMB. See below.
No, we shouldn't. Recombination did not produce photons; it just drastically increased the mean free path of photons that already existed.

More precisely, before recombination, photons were constantly being created as electrons and nuclei combined into atoms, and then destroyed as they hit atoms just formed and split them apart again. The net effect was a constant photon number. Recombination simply established that constant photon number as free-traveling photons in a transparent universe, rather than an average of constant creation and destruction in an opaque universe.

While some hydrogen atoms were immediately reionized after recombining, the vast majority of electrons first fell to an excited state before cascading down to the first energy level emitting a lyman alpha photon.This IS what's called recombination. You're right, ordinarily there would be an equilibrium between absorptions and emissions, but as the universe expanded the photons became slightly redshifted enough and the mean free path became long enough that the matter became transparent and decoupling occurred. This is supposed to have happened when the matter was at a temperature of 3000 K. But a blackbody of 3000 K corresponds to an energy of 0.26 eV, while the energy of a lyman alpha photon is roughly 10 eV, so the signal from lyman alpha should dominate any blackbody radiation. But if, as you say, the CMB is due to gamma rays from the era of annihilation and the photon to baryon ratio is a billion to one, then any signal from the era of recombination would be redshifted beyond detection.
However, there's another puzzle involved besides there being an excess of matter over antimatter in the early universe, and that is; if protons and antiprotons, and electrons and positrons were created separately, how is it that the same imbalance occurred for both of them in order to maintain charge neutrality? Is it possible that the first matter to be produced was neutron-antineutron pairs that later decayed into protons, electrons and antineutrinos? Do you think that it's possible that the scant roughly fifteen minutes for a neutron to decay is enough time for the expansion of the universe to separate particles from antiparticles enough for a matter dominated universe?

 

[PeterDonis]

if protons and antiprotons, and electrons and positrons were created separately, how is it that the same imbalance occurred for both of them in order to maintain charge neutrality? Is it possible that the first matter to be produced was neutron-antineutron pairs that later decayed into protons, electrons and antineutrinos?

At the end of inflation, when reheating occurred and pumped a lot of energy into the Standard Model fields, the temperature was much, much too high for protons and neutrons to exist. It was just quarks and leptons. It was only when the temperature dropped low enough that the quarks became confined into nucleons. By that point, electroweak interactions would have equilibrated between quarks and leptons in order to achieve charge neutrality; at least, that's my understanding of the model.

Do you think that it's possible that the scant roughly fifteen minutes for a neutron to decay is enough time for the expansion of the universe to separate particles from antiparticles enough for a matter dominated universe?

That is the half-life for free neutron decay, but as I understand the theory of Big Bang nucleosynthesis, the vast majority of the neutrons were not free; they were bound into nuclei (deuterium, helium, and lithium). Nucleosynthesis was completed within about the first three minutes (hence the title of Weinberg's popular book from the late 1970s).

As for separating particles and antiparticles during that time, I don't know off the top of my head by what factor the universe is believed to have expanded in the first three minutes, but I don't think that's the key point anyway. I think the key point is that the expansion was decelerating (because the universe was radiation dominated), and I think a decelerating expansion won't separate particles and antiparticles the way you are thinking. I think you would need an inflationary expansion to pull particles and antiparticles apart faster than they could annihilate each other, much less to pull them apart to the extent that we now have our entire observable universe filled with matter, as far as we can tell.

 

[hmmm27]

I don't mean to disrupt a serious conversation, but why is the prevailing opinion that matter/anti-matter creation should be even at the beginning ? Not that I don't understand how to flip a coin, but there's an already existing polarity bias, of sorts - behind (towards the big bang point) and ahead (the other way). So maybe something as simple as a particle creation happening while being pushed out of a gravity well makes matter, pushed into makes anti-matter: there's still anti-matter creation, but the bias is clearly towards matter.

 

[PeterDonis]

why is the prevailing opinion that matter/anti-matter creation should be even at the beginning ?

To the extent that this is a prevailing opinion (I'm not sure to what extent that actually is), I think it's because we have no reason, based on our best current physical theories, to expect that the "reheating" process at the end of inflation, that transferred the energy density of the "false vacuum" state of the inflaton field to the Standard Model fields, would have had any bias towards matter; we would expect it to excite matter and antimatter Standard Model fields equally.

there's an already existing polarity bias, of sorts - behind (towards the big bang point) and ahead (the other way)

This is just another way of saying that the big bang is to the past. How is that a "polarity bias"?

something as simple as a particle creation happening while being pushed out of a gravity well makes matter, pushed into makes anti-matter

The big bang and the universe's evolution since then was not a process of anything being "pushed out of a gravity well".

Also, please review the PF rules on personal speculation.

 

[hmmm27]

The big bang and the universe's evolution since then was not a process of anything being "pushed out of a gravity well"

.

Ah, right. I think I'll pass on trying to argue that antimatter is created backwards in time.

 

[Justin Hunt]

Do Cosmologists consider that there could have been exotic particles during the extreme conditions of the big bang? Super symmetry predicts larger particles, but we have been unable to create any of them in our particle accelerators so far. If these particles due exist at the higher energy levels, wouldn't the early universe been dominated by them or has the LHC ruled out this possibility (not sure if the energy level at the LHC is comparable to the energy level of the BB when particles were being created)?

 

[diogenesNY]

I recall a description in the book _Cauldrons in the Cosmos_
http://www.worldcat.org/title/cauld...physics/oclc/4435817210&referer=brief_results

that there was a speculation of highly massive exotic particles that would form and then rapidly decay during a period of unimaginably high energy levels in the _very_ early universe - way higher than we could ever conceivably get with our current (or foreseeablely future) apparatus. This was a 1988 book, however, and this content may well be superseded by current research. Of note, there is a 2005 (revised?) edition. Not sure what the differences or possible revisions are.

diogenesNY

 

[sadovnik]

In my opinion ''Antiparticles'' are Dirac's negative
virtual particles ( -E=Mc^2) and ''Cosmology''
is first of all Dirac's ''vacuum sea'' with parameter: T=0K
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[PeterDonis]

In my opinion ''Antiparticles'' are Dirac's negative
virtual particles ( -E=Mc^2) and ''Cosmology''
is first of all Dirac's ''vacuum sea'' with parameter: T=0K

Your opinion has been known to be wrong for decades now. Dirac's speculation was an interesting early one but the model was fairly quickly found not to work; one key reason why is that it could not explain why bosons have antiparticles.