# Big bang vs antimatter

ryanuser
Hi
My question is about absence of equal antimatter in our universe: If charge and energy is conserved, then this means there was matter before big bang that crunched and created more particles. The charge before and after big bang is conserved; just when two protons are accelerated toward each other and the result of their collision is three protons and an antiproton, one proton and an antiproton annihilate to conserve charge,mass, baryon no etc. so hasn't similar thing happened in Big Bang where antimatter was annihilated with some of the matter (again to conserve charge, energy, etc) and some more matter was left over?

Thanks

Mentor
Charge and energy are conserved within the evolution of our universe, this does not have to apply to the beginning (independent of how exactly that looked like).
Overall, the net electric charge in our universe is extremely close to zero as the number of protons and the number of electrons is very similar. You don't need to violate charge conservation to get a matter/antimatter asymmetry.
The mechanism that produced the observed asymmetry is unknown - the known differences between matter and antimatter are too small to explain the amount of matter we see.

Gold Member
The general topic is baryogenesis, the generic term for the hypothetical physical processes that produced an asymmetry (imbalance) between baryons and antibaryons produced in the very early universe. The baryonic matter that remains today, following the baryonic-antibaryonic matter annihilation, makes up the universe.

Baryogenesis conserves both charge and energy. Basically, there are an equal number of protons and electrons in our universe.

What isn't conserved during baryogenesis is baryon number, a number which is conserved* under the strong and weak nuclear forces.

* Well, it isn't conserved perfectly. But the tiny deviations aren't enough to explain the matter/anti-matter asymmetry we observe.

Gold Member
While discussing this problem it might be opportune to again refer to the Dirac-Milne universe model which solves the problem a different way; i.e. by proposing there is no asymmetry between matter and antimatter.

My own interest in the theory is yet again there is a claim that the "linear evolution of the scale factor with time which directly solves the age and horizon problems of a matter-dominated universe" produces a concordant model.

BTW Happy New Year Everyone!!

Garth

While discussing this problem it might be opportune to again refer to the Dirac-Milne universe model which solves the problem a different way; i.e. by proposing there is no asymmetry between matter and antimatter.

My own interest in the theory is yet again there is a claim that the "linear evolution of the scale factor with time which directly solves the age and horizon problems of a matter-dominated universe" produces a concordant model.

BTW Happy New Year Everyone!!

Garth
Doesn't fit the CMB, though.

Gold Member
Doesn't fit the CMB, though.
That is the question though, might it fit?

Several authors claim that the linearly expanding model does fit the CMB power- or possibly might be made to, as the standard model has been, fit by appropriate addition of DM and DE.
Cosmology of the Dirac-Milne Universe - Indico
First peak corresponds to acoustic scale given by sound horizon seen on last scattering surface. For Dirac-Milne, angular distance is 163 times larger than in ΛCDM one would expect a tiny angle! But, due to linear scale factor, sound horizon is much larger than in standard model Integrating from 40 MeV to ~7 eV (end of annihilation, cf BBN) yields acoustic scale around 1º
And http://[URL [Broken]] A Concordant “Freely Coasting” Cosmology[/URL]
The main point we make in this article is that in spite of a significantly different evolution, the recombination history of a linearly coasting cosmology can be expected to give the location of the primary acoustic peaks in the same range of angles as that given in Standard Cosmology.

It also provides an explanation for the low multipole anomaly as described in the 2013 Planck results.
Our overall conclusion is that the Planck data are remarkably consistent with the predictions of the base CDM cosmology. However, the mismatch with the temperature spectrum at low multipoles, evident in Figs. 1 and 39, and the existence of other “anomalies” at low multipoles, is possibly indicative that the model is incomplete.

Angular Correlation of the CMB in the R=ct Universe
We show that the Rh = ct Universe without inflation can account for the apparent absence in CMB angular correlation at angles θ >∼60◦ without invoking cosmic variance, providing additional motivation for pursuing this cosmology as a viable description of nature.
[/PLAIN] [Broken][/PLAIN] [Broken]

[/PLAIN] [Broken][/PLAIN] [Broken]Obviously a lot more work is required here before a definitive answer can be given, however, given the fact that the model does not need Inflation, can explain the low power anomaly and resolves a possible age problem in the early universe, perhaps the model ought to be re-examined.
[/PLAIN] [Broken][/PLAIN] [Broken]
Garth

Last edited by a moderator:
A model with equal amounts matter and anti-matter doesn't fit with the fact that the CMB exists.

Gold Member
A model with equal amounts matter and anti-matter doesn't fit with the fact that the CMB exists.
Why?
I thought the whole point of the model was that if antimatter is supposed to present a negative active gravitational mass then the universe will be composed of separated (mutually repulsive) domains of matter and antimatter. Each domain of hot plasma in the pre-recombination period contributes to the CMB whilst the combined effect of many domains does not gravitationally affect the evolution of the scale factor which therefore assumes the form of the Milne or empty universe.

I know the theory has problems, which doesn't? But in the present era of 'precision cosmology' where we know precisely what we don't know perhaps more attention ought to be paid to it.

In answer to the OP question about absence of equal antimatter in our universe my personal favourite borrows from Wheeler and Feyman's idea that a positron is an electron going backwards in time.

Thus the baryon asymmetry is coupled to the arrow of time; a matter universe expands forward in time from the BB whilst an anti-matter universe expands backwards in time. They might even be the same universe!!;)

Garth

Mentor
I thought the whole point of the model was that if antimatter is supposed to present a negative active gravitational mass
I hope AEgIS settles that question soon.
As 99% of the mass of nucleons comes from QCD and not the mass of valence quarks, those models were never well-motivated anyway.

Thus the baryon asymmetry is coupled to the arrow of time; a matter universe expands forward in time from the BB whilst an anti-matter universe expands backwards in time. They might even be the same universe!!;)
We know we had a rough antimatter/matter symmetry early on in the universe, otherwise we would not have so much energy in photons now.

Staff Emeritus
2021 Award
I hope AEgIS settles that question soon.

It's settled. AEgIS is an experiment in the category of "a direct measurement substantially weaker than indirect constraints". If antimatter fell up, a) you would be able to make a perpetual motion machine (let light fall down, convert it to an e+ e- pair, lift that up, annihilate it, and let the light fall down - lather, rinse, and repeat) and b) because nucleons contain antiquarks, you would see a composition-dependent gravitational interaction, strongly constrained by Eotvos-type experiments.

Gold Member
I hope AEgIS settles that question soon.
When do we expect the first results from AEgIS?
As 99% of the mass of nucleons comes from QCD and not the mass of valence quarks, those models were never well-motivated anyway.

We know we had a rough antimatter/matter symmetry early on in the universe, otherwise we would not have so much energy in photons now.
Good points, that's why I added a wink!

Garth

Gold Member
It's settled. AEgIS is an experiment in the category of "a direct measurement substantially weaker than indirect constraints". If antimatter fell up, a) you would be able to make a perpetual motion machine (let light fall down, convert it to an e+ e- pair, lift that up, annihilate it, and let the light fall down - lather, rinse, and repeat) and b) because nucleons contain antiquarks, you would see a composition-dependent gravitational interaction, strongly constrained by Eotvos-type experiments.
Confirmatory experiments are still welcome - perhaps we still have something to learn!

Garth

Mentor
nucleons contain antiquarks

Do they? Mesons are quark-antiquark pairs, but AFAIK nucleons (as opposed to antinucleons) are composed of three quarks; for example, a proton is two up quarks and a down quark. No antiquarks.

Why?
I thought the whole point of the model was that if antimatter is supposed to present a negative active gravitational mass then the universe will be composed of separated (mutually repulsive) domains of matter and antimatter. Each domain of hot plasma in the pre-recombination period contributes to the CMB whilst the combined effect of many domains does not gravitationally affect the evolution of the scale factor which therefore assumes the form of the Milne or empty universe.

I know the theory has problems, which doesn't? But in the present era of 'precision cosmology' where we know precisely what we don't know perhaps more attention ought to be paid to it.

In answer to the OP question about absence of equal antimatter in our universe my personal favourite borrows from Wheeler and Feyman's idea that a positron is an electron going backwards in time.

Thus the baryon asymmetry is coupled to the arrow of time; a matter universe expands forward in time from the BB whilst an anti-matter universe expands backwards in time. They might even be the same universe!!;)

Garth
At the time the CMB was emitted, our universe was uniform to one part in 100,000. A universe with separate domains of non-relativistic matter and anti-matter couldn't ever get anywhere nearly that uniform in density.

Torbjorn_L
Staff Emeritus
2021 Award
Do they?

Yes, they do. They do not contain valence antiquarks, but they contain antiquarks. Valence quarks are simply the difference between the quarks and antiquarks. It is difficult to impose a difference in gravitation between matter and antimatter without it showing up as a difference in attraction between nuclei and electrons, and that in turn manifests as a composition dependent reaction to gravity.

As far as the CMB, I am not 100% sure that one couldn't get to this level of uniformity even with separate matter and antimatter domains. I remember talking to de Rujula about this maybe 15 years ago. You certainly cannot get this way if there are large gaps between matter domains and antimatter domains (large = larger than the gaps within the matter domain) but it wasn't clear (at least at the time) that you couldn't make this work. Have there been calculations/simulations since then?

One can always solve this another way - the visible universe is matter, and "over there somewhere" past the horizon is a region of antimatter. I don't think this is excluded, but it is certainly not satisfying.

Torbjorn_L
Mentor
They do not contain valence antiquarks, but they contain antiquarks.

Ah, ok. Yes, I was only talking about the valence quarks.

Gold Member
At the time the CMB was emitted, our universe was uniform to one part in 100,000. A universe with separate domains of non-relativistic matter and anti-matter couldn't ever get anywhere nearly that uniform in density.
In agreement with what Vanadium 50 said, from the Introducing the Dirac-Milne universe paper
the global size of domains of matter and antimattermust be approximatively that of the observable universe.

I personally do not much like the Dirac-Milne universe except as an answer to the OP question.

Garth

Mentor
It's settled. AEgIS is an experiment in the category of "a direct measurement substantially weaker than indirect constraints".
Sure. Still good to get a direction measurement that is free of all (real or imagined) loopholes.
When do we expect the first results from AEgIS?
I tried to find some plans, but all I saw is "we will take some data in 2015".

Yes, they do. They do not contain valence antiquarks, but they contain antiquarks. Valence quarks are simply the difference between the quarks and antiquarks. It is difficult to impose a difference in gravitation between matter and antimatter without it showing up as a difference in attraction between nuclei and electrons, and that in turn manifests as a composition dependent reaction to gravity.

As far as the CMB, I am not 100% sure that one couldn't get to this level of uniformity even with separate matter and antimatter domains. I remember talking to de Rujula about this maybe 15 years ago. You certainly cannot get this way if there are large gaps between matter domains and antimatter domains (large = larger than the gaps within the matter domain) but it wasn't clear (at least at the time) that you couldn't make this work. Have there been calculations/simulations since then?
Pretty sure you'd need large gaps to avoid annihilation as long as gravity was the only thing keeping the matter and anti-matter regions separated. And if there was any significant amount of annihilation going on, it would be flagrantly obvious in the CMB.

One can always solve this another way - the visible universe is matter, and "over there somewhere" past the horizon is a region of antimatter. I don't think this is excluded, but it is certainly not satisfying.
This certainly cannot occur with inflation, and probably can't occur with any of the proposed alternatives to inflation either. The problem is that the matter particles are producing during reheating, and locations on the CMB that are separated by more than about a degree were never in causal contact from reheating to the time the CMB was emitted. The matter/anti-matter disparity would have had to occur after reheating.

Gold Member
In agreement with what Vanadium 50 said, from the Introducing the Dirac-Milne universe paper

Abstract: "Faced with this unsatisfactory situation, we study an unconventional cosmology, the Dirac-Milne universe, a matter-antimatter symmetric cosmology, in which antimatter is supposed to present a negative active gravitational mass."
If this holds true I would expect that e.g. as long as an electron and a positron are moving towards each other the attractive and repelling gravity of the system should be balanced. However no more after annihilation due to photon production. This seems weird and I am not sure if this this reasoning is correct. What do you think?

Abstract: "Faced with this unsatisfactory situation, we study an unconventional cosmology, the Dirac-Milne universe, a matter-antimatter symmetric cosmology, in which antimatter is supposed to present a negative active gravitational mass."
If this holds true I would expect that e.g. as long as an electron and a positron are moving towards each other the attractive and repelling gravity of the system should be balanced. However no more after annihilation due to photon production. This seems weird and I am not sure if this this reasoning is correct. What do you think?
It's not correct. For electrons, the electric force is more than $10^{40}$ times stronger than the gravitational force. So for individual protons or electrons, the gravitational force is so weak that it might as well not exist. It's not until you have large collections of protons/electrons that the gravitational force starts to be noticeable.

Gold Member
It's not correct. For electrons, the electric force is more than $10^{40}$ times stronger than the gravitational force. So for individual protons or electrons, the gravitational force is so weak that it might as well not exist. It's not until you have large collections of protons/electrons that the gravitational force starts to be noticeable.
Thanks, I am aware of this huge factor, but my question is about the principle. Would you agree that before annihilation attractive and repelling gravity is exactly balanced?

To my understanding in the context "antimatter is supposed to present a negative active gravitational mass" the description 'negative gravitational mass' means negative mass, so, referring to the electron-positron pair, things like momentum and gravitational force should cancel each other out. If 'negative gravitational mass' is compatible with positive mass and is positive mass, then only the gravitational force should be cancelled out. In any case there should be the states: no gravity (exactly zero) - annihilation - gravity, if my reasoning is correct.

And further, what would result in case the early universe contains exactly the same amounts of positive and negative matter?

I don't want to insist and appreciate any comment.

Gold Member
The ALPHA experiment at CERN has already constrained the gravitational mass of antihydrogen to less than 110 times its inertial mass in the 'down' direction and less than 65 times its inertial mass in the 'up' direction. This,of course, is inconclusive, but, suggests the odds are roughly 110 to 65 antihydrogen does not exhibit anti-gravity properties; re: http://alpha.web.cern.ch/. As noted, a more precise measure is expected later this year after more antihydrogen is created.

Mentor
The ALPHA experiment at CERN has already constrained the gravitational mass of antihydrogen to less than 110 times its inertial mass in the 'down' direction and less than 65 times its inertial mass in the 'up' direction. This,of course, is inconclusive, but, suggests the odds are roughly 110 to 65 antihydrogen does not exhibit anti-gravity properties;
No it does not. If we assume the true value is within the range of -1 to 1 times the matter value, then it tells us nothing. The positive values are slightly favored (maybe 2-3%), but the study is not sensitive to that difference yet.

Note that you can never give odds based on experimental results, unless you use Bayesian statistics and invent some prior. Measurements just give likelihood ratios.

In any case there should be the states: no gravity (exactly zero) - annihilation - gravity, if my reasoning is correct.
Probably, which makes that idea quite odd.

Gold Member
mfb said:
...Note that you can never give odds based on experimental results, unless you use Bayesian statistics and invent some prior. Measurements just give likelihood ratios.
I believe this is a case you can apply the binomial distribution to estimate the odds antihydrogen is gravitationally attractive or repulsive wrt 'ordinary' matter. According to my calculator, the 50% confidence interval is about 63% in favor of attractive vs 36% repulsive.

Last edited:
Mentor
I believe this is a case you can apply the binomial distribution to estimate the odds antihydrogen is gravitationally attractive or repulsive wrt 'ordinary' matter.
You cannot.
Imagine a different experiment: two observers are one meter apart, a bullet gets shots in such a way both can see when the bullet crosses their position along the path. The observers manually press a button, the time difference gets recorded. You repeat this experiment a few times and get a measured flight time of (-0.02 +- 0.02) seconds. What is the probability the bullet actually has a negative flight time, which means it is going backwards?

Very close to zero, and your experimental result does not change that - you are just not sensitive to the actual, small, positive flight time of the order of a millisecond.

Thanks, I am aware of this huge factor, but my question is about the principle. Would you agree that before annihilation attractive and repelling gravity is exactly balanced?

To my understanding in the context "antimatter is supposed to present a negative active gravitational mass" the description 'negative gravitational mass' means negative mass, so, referring to the electron-positron pair, things like momentum and gravitational force should cancel each other out. If 'negative gravitational mass' is compatible with positive mass and is positive mass, then only the gravitational force should be cancelled out. In any case there should be the states: no gravity (exactly zero) - annihilation - gravity, if my reasoning is correct.

And further, what would result in case the early universe contains exactly the same amounts of positive and negative matter?

I don't want to insist and appreciate any comment.
I think you're making up a different situation than is being discussed. Yes, you have to be careful about how to assign the negative mass for anti-matter. Basically this means that anti-matter picks up a minus sign for its mass when it occurs in the gravitational force, but still has a positive value for the mass when considering acceleration. This would mean that an anti-matter universe would behave, gravitationally, exactly like a matter universe. But if you had two clumps of mass, one made of anti-matter and the other made of matter, then they would gravitationally repel one another.

Honestly I think that this view fails before it even gets started for the simple reason that we know of common particles that are their own anti-particles: photons. The only way to fit photons into this pictures is to make it so that they do not generate any gravitational field at all. And making it so that photons generate no gravitational field would rather drastically alter the expansion of our universe at very early times, almost certainly resulting in dramatically different BBN predictions.

Quds Akbar

The law of conservation of matter and energy does not apply to the creation of the Universe. What you are doing by assuming that there was time before the Big Bang is like saying that there is a more south point than the most South point on a circle without flipping or turning its orientation. It doesn't work that way, that's what makes the "what banged" in the Big Bang so challenging. You're idea however might work if there was a Multiverse and matter could exist between Universes. However, it is a good way to go beyond that like you did, that is what I do not like about the Big Bang.

Gold Member
Honestly I think that this view fails before it even gets started for the simple reason that we know of common particles that are their own anti-particles: photons. The only way to fit photons into this pictures is to make it so that they do not generate any gravitational field at all. And making it so that photons generate no gravitational field would rather drastically alter the expansion of our universe at very early times, almost certainly resulting in dramatically different BBN predictions.
Ok, this is a very convincing point, so my scenario seems purely fictional. Even so, I have a question.
Basically this means that anti-matter picks up a minus sign for its mass when it occurs in the gravitational force, but still has a positive value for the mass when considering acceleration.
Why "but still has a positive value for the mass when considering acceleration"? Isn't ##F=-ma## then?
This would mean that an anti-matter universe would behave, gravitationally, exactly like a matter universe. But if you had two clumps of mass, one made of anti-matter and the other made of matter, then they would gravitationally repel one another.
Understand, thanks. So, a universe filled with with anti-matter (minus sign for the matter) wouldn't imply that the matter density is negative, right?

Last edited:
Quds Akbar
Ok, this is a very convincing point, so my scenario seems purely fictional. Even so, I have a question.

Why "but still has a positive value for the mass when considering acceleration"? Isn't ##F=-ma## then?

Understand, thanks.
I did not understand the question properly, could you please paraphrase it. I also did think of it that way, until I saw that diagram in a book. However I do stress that this idea may be considered and would be considered since the fact that a multiverse exists could be a possibility, which is what your theory is based on. But that is a very good perspective to look at it.
Thanks

Torbjorn_L
The law of conservation of matter and energy does not apply to the creation of the Universe.

Similarly, 'creation' or emergence wouldn't apply in such a case. The universe would just be. (E.g. no-boundary topologies work.)

But that depends on what you mean by "the Universe". The Hot Big Bang, where matter emerged and baryogenesis happened, emerges out of inflation. If inflation is eternal and consists of a multiverse, does the term "universe" apply? And how does it work? And what would it say on matter/antimatter asymmetry?

The OP would be answered with the idea of baryogenesis and the observation that it isn't obviously wrong (and very nearly good enough as of lately).

Well, what you'd do is have two masses for an object: $m_i$ and $m_g$. For normal matter, $m_i = m_g$. For anti-matter, $m_g = -m_i$. The inertial mass, $m_i$ is always positive. The term $m_i$ appears in the acceleration equation, while $m_g$ appears in the gravitational force.