Matter v Antimatter I: The Baryon Asymmetry

By Mark Trodden | July 24, 2008 3:05 pm

I’m in the middle of a couple of posts about the matter-antimatter asymmetry of the universe and have found that I keep referring to things I posted back on my old blog a long time ago. This became so frequent that I’ve decided to post a slightly edited version of these here, and in my next post, as preludes to some newer material that I’m getting to.

Antimatter is just like ordinary matter in every way, except that every quantity you can think of (apart from mass and spin), is reversed. As an example, the electron is a particle with a specific mass and carrying a specific amount of negative electric charge. The antiparticle of the electron is a positron, which has the identical mass to an electron, but precisely the opposite charge. The thing about particles and their antiparticles is that, if one puts them together, the net value of any quantity (called a quantum number by physicists) carried by the pair of them is zero. Therefore, a particle and an antiparticle together are merely mass which, thanks to Einstein’s E=mc2, can be converted entirely into energy. As a result of this, when matter and antimatter come together, they annihilate, producing energy in the form of light (photons).

We know so much about antimatter for two reasons. The first is that it is a natural part of quantum field theories, which we use to describe matter, and which are among the best-tested theories in all of science. The second is that we can make and investigate antimatter in large amounts. For example, the purpose of the Fermi National Accelerator Laboratory near Chicago is to make vast numbers of antiprotons to study how they annihilate with protons.

Antimatter is important in cosmology because of the extreme temperatures and densities of the early universe. One consequence of such an extreme environment is that there is so much energy around that any kind of matter (including antimatter) can be created. Therefore, in the early universe, one expects there to have been equal amounts of both matter and antimatter and then, as the universe cooled, for these particles to find each other, annihilate, and leave our present universe with very little matter around (and an equally small amount of antimatter).

This is clearly at odds with what we observe in the universe, where we have relatively large amounts of matter and essentially no evidence of primordial antimatter. In fact, this asymmetry between matter and antimatter can be made quantitative (for baryons such as protons and neutrons) through observations of the abundances of light elements in the universe (Big Bang Nucleosynthesis – BBN) and also from the pattern of anisotropies in the cosmic microwave background radiation (CMB). Thus, there is clear quantitative evidence that the universe is composed of matter, with negligible antimatter.

This all constitutes a puzzle for cosmologists. How did the universe evolve from early times, in which there were equal numbers of baryons and antibaryons, to the present universe, in which there is a precisely measured baryon asymmetry of the universe (BAU)?

Potential solutions to this puzzle provide a wonderful example of the interplay between particle physics and cosmology. A beautiful feature of many theories beyond the standard model of particle physics is that, when considered in the context of the expanding universe, they automatically contain such a dynamical mechanism that can, in principle, explain the origin of the BAU. The generation of the BAU through one of these mechanisms is what is known as baryogenesis. This isn’t enough of course; we don’t yet know which, if any, of these theories might be the right one. However, upcoming experiments, such as those at the Large Hadron Collider (LHC), provide the exciting possibility of either ruling out some of them or providing significant evidence for one of them.

Over the course of my next few posts I’ll try to explain how some of these mechanisms work, and how they illustrate the particle-cosmology connection.

  • DaveC

    Mark, is there experimental evidence that antimatter obeys the equivalence principle? I was trying to think of some but couldn’t. Can we be sure it couples to gravity in the same way as matter does?

  • Matt

    Small typo… You write: “Antimatter is just like ordinary matter in every way, except that every quantity you can think of (apart from mass and spin), is reversed.” But spin is also reversed, at least in the sense that for a massless particle with spin, the antiparticle has opposite helicity. Like the story for left-handed neutrinos and right-handed antineutrinos in the minimal Standard Model….

  • ST

    If antimatter coupled differently to gravity, I would guess that you might be able to cook up scenarios were energy is not conserved. Because we know that light and ordinary matter both couple to gravity through the equivalence principle, and matter plus antimmater annihilate to give light.

  • ST

    Also, there are many particles whose anti-particles are the particles themselves (eg: photon). So then the gravitational coupling would have to be species-dependent, which sounds implauisible.

    Still, it is an interesting question. Direct experimental test are probably very difficult.

  • Ellipsis

    DaveC: there is indeed some. See e.g.
    John Baez maintains (although written by others) a good web page on this

  • Tyler

    very interesting topic of course, looking forward to more

  • Xerxes

    This phrasing always makes me grumble: “[Annihilated particle-antiparticle pairs are] converted entirely into energy … in the form of light (photons).” Photons are not a form of energy any more than electrons are. Photons have energy. They also have spin and momentum. Electrons have energy. They also have spin and charge and momentum and mass. Everything has energy!

    It makes no more sense to say that a positron-electron pair that annihilates into two gamma rays has “converted entirely into energy” than to say they are “converted entirely into momentum” or “converted entirely into spin”, which are other properties of the resulting two-photon state. I would say something like:

    “Matter-antimatter pairs can annihilate into ANY other state that has zero quantum numbers and conserves energy and momentum. Since photons have zero mass and all-zero quantum numbers, any annihilation can result in two photons. For pairs with higher mass or momentum, more exotic things are possible. For example, in a high-energy electron-positron collider, you could hope to produce top quark-antiquark pairs.”

  • Mark

    Grumble away. It is close enough and one tries not to get bogged down in too many technicalities when trying to write a semi-popular account.

  • Mark

    Dave C – I think ellipsis got to your point, but it is a good question and to the best of my knowledge it is extremely difficult to test this experimentally because of the difficulty of canceling out other effects.

    Matt – fair enough, I was really just meaning that spin 1/2 remains spin 1/2, for example.

    ST – thanks for taking this on. It is actually pretty easy to get the coupling to different species to be different. One way to do it is to have a scalar field determine Newton’s constant (as in Brans-Dicke theories) and then to have differing functions of this field enter in the coupling between gravity and the different species. Getting different couplings to matter and antimatter of the same species isn’t something obvious to be immediately though.

  • Yoo

    Are the typical theories for baryogenesis based on fundamental differences between matter and antimatter, or are they usually based on random asymmetries that arise between the two? (I guess the latter sort of theories would require baryogenesis to occur before inflation …)

  • Mark

    Baryogenesis has to happen after inflation Yoo, since inflation would dilute away all matter (asymmetrically populated or not). All baryogenesis mechanisms I know about are based on the violations of the three so-called Sakharov conditions. See my next post (probably Monday) for what these are and an example of how they can be violated.

  • Kurt

    Why are some particles like neutrons ( i think) their own anti-particles?
    really what i am asking is how do we know the different between a particle and an anti particle if it is the same?
    i hope that makes sense.
    I busted out my old copy of griffiths – intro to particle physics and i can’t really find the answer in there.

  • jinxyte

    Slightly off-topic and nothing to add, physics-wise, but have you guys seen this Obamaism from Slate? Since it trades in metaphors of matter v. antimatter and baryogenesis, I thought Cosmic Variance readers might appreciate it.

    Barackyogenesis (buh-RAK-ee-oh-JEN-uh-sis) n. An attempt to explain why there were more Barackyons than anti-Barackyons in the Democratic Party, a condition necessary for the formation of heavier election matters.

    Example: There must be some initial perturbation to explain the Barackyogenesis problem. Many political scientists have suggested that an imbalanced density of subatomic particles known as “charm quarks” in the candidates led to the dominance of Barackyons over anti-Barackyons.

  • Jason Dick


    Neutrons are not their own anti-particles. While neutrons have no electric charge, they are made up of quarks which do have charge. An anti-neutron is made up of quarks with opposite charges to those that make up a neutron.

    A photon, however, is a type of particle that is its own anti-particle. The reason this is so is a simple symmetry. Anti-matter is related to matter through a symmetry transformation. Take an electron, perform this symmetry transformation, and now that electron looks like a positron. If you perform the exact same transformation on a photon, you get the same particle right back. So, in this sense, a photon is its own anti-particle.

    Some other particles, such as certain mesons, have this same property, because they are made of a quark/anti-quark pair. The symmetry just swaps which part is the “quark” and which is the “anti-quark”, with the whole remaining the same (provided the two quarks are of the same type).

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  • Xerxes

    Grumble away. It is close enough and one tries not to get bogged down in too many technicalities when trying to write a semi-popular account.

    Ouch. Thanks for taking constructive criticism so well. 😛

    Seriously tho, I think this is an important distinction that is not too hard for people to understand. The lay public tends to think about energy as some kind of magical thing that permeates the universe and explains ghosts or psychic powers. Talking about “pure energy” reinforces that kind of view. People are certainly able to understand that mass is a property that objects have, and not something that exists by itself as “pure mass”. Energy should be thought of in the same sort of way, as a property that particles have. The public is probably never going to have a decent grasp of field theory, but energy is something I think they have a good shot at understanding if you talk about it in a clear and correct way.

  • adam

    Photons are easily enough created and destroyed/absorbed, though, that they are more disposable vehicles for energy than particles which are associated with additional conserved quantities (and mostly when we are interested in energy in these context, we are really interested in its exchange).

    Which, it seems to me, is why the phrasing makes a sort of intuitive sense even to people that do, as you might prefer, ‘know better’ (and, for that matter, I’ve heard it used as shorthand often enough by such others that I am not sure that there’s much point trying to change the usage). The picture you describe isn’t close to top of my personal list of ‘harmful conceptions of reality’, either, in any case, although that’s just a matter of taste.

  • Mark

    Hi Xerxes – it wasn’t meant to be a criticism of your point – you’re technically right, and I certainly don’t mind you making it (which is what I meant by “grumble away”). I just honestly think that one needs to pick and choose where to include technicalities in a post. Here I was already more technical than usual so pared it back where I didn’t think it necessary – this was one place.

    I did specifically refer to photons, and there is, of course, a fundamental difference between massless particles and massless ones that is somewhat relevant here. In fact, what ends up being relevant in calculations down the line is the baryon to entropy ratio, and most of the entropy is in photons.

  • FeralPhantom

    Ignorant layperson question – what justifies the expectation that there should have been equal amounts of matter and anti-matter in the early universe?

  • TK Tom

    I detect some kind of contradiction here.

    a) Most nucleosynthesis theories call for equal amount of matter and anti-matter at high energy condition of early universe because symmetry transformation that permits them to be created were more feasible.

    b) Later, with a lower energy universe, the symmetry was broken, halting further conversion of matter and anti-matter into each other.

    c) These then combine and all matter stuff should therefore be eliminated.

    Puzzle: How come we have so much matter and none anti-matter?

    Contradiction: The symmetry either does not exist or wrong. Because if one takes the current universe and run it backward in time to the big bang condition, you won’t get back the original universe in term of amount of energy.

  • Mark

    i can’t make any sense of your comment TK Tom – if you can make it clear I’ll try to explain better.

  • Saurabh Madaan


    Please pardon my ignorance. But why is it postulated that the early universe had nearly equal amounts of matter and anti-matter?


  • Lawrence B. Crowell

    Particles such as protons and neutrons have what is called a baryon number B = 1, and their anti-particle versions have a negative baryon number B = 1. So for a perfectly symmetric univeres we might expect that

    sum_i B_i~=~0,

    where the positive and negative baryon numbers cancel out in a sum over all particles. This means there is no net quantum number for baryons, which makes things nice. However, in our later universe this is no longer the case and the summation is positive. So somehow there is a net “creation” of these quantum numbers. It is also interesting to note that these numbers have some topological content as well. So by symmetry breaking these quantum numbers (B) were created with a net positive value, and the “lost” negative counter parts are in some ways either destroyed or buried away in a form we can’t observe.

    As for the equivalence principle, that is an interesting question. The Baryon number can be modelled in a simply spinor model. Experiments have demonstrated that photons with different helicities or spins will pass through certain material differently. The index of refraction is modified by a spin quantum Hall effect, and gravity has an optical analogue. Photons with different spin directions might actually become optically split! One might then wonder if there is an analogous physics where protons and anti-protons might have paths in curved spacetime which differ slightly.

    Lawrence B. Crowell

  • Mark

    FeralPhantom and Saurabh – that is a very good question actually. The point is that we know that the baryon asymmetry can be quantified rather precisely. If one starts the universe with that number then it is a boundary condition that we haven’t explained – the goal of baryogenesis is to find an explanation. But even if one were to set the known number as an initial condition, if inflation happened, then this would have been erased and when the universe repopulated with matter one would again have to figure out why it did so with precisely the right asymmetry.

    One possibility is that it does so because of the dynamics at the end of inflation (and never becomes matter-antimatter symmetric). A number of people (including me) have worked on various versions of this idea (and I may write about them at some point).

    But if baryogenesis doesn’t work this way, then in the absence of a chemical potential, eventually baryons and antibaryons will have abundances determined purely by their masses (since they will be in approximate thermal equilibrium). The CPT theorem then says that their masses are identical and therefore their abundances are identical. Thermal fluctuations of these numbers would mean a small random asymmetry at any given moment, but one far smaller than observations require.

    Hope this helps.

  • Saurabh Madaan

    Thanks, Mark. This helps — after reading some more related stuff from Wikipedia, I find myself even more curious, and puzzled!

  • FeralPhantom

    Helpful, thanks. Looking forward to more on this topic.

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  • ben wonders

    Forgive my ignorance. As I was not able to understand the sentence

    “In fact, this asymmetry between matter and antimatter can be made quantitative (for baryons such as protons and neutrons) through observations of the abundances of light elements in the universe (Big Bang Nucleosynthesis – BBN) and also from the pattern of anisotropies in the cosmic microwave background radiation (CMB)”

    I can still imagine a universe where zones of matter and antimatter are so far apart that we don’t experience the annihilation of these opposites. The possible symmetry would feel so much better.

    Or what if Dark matter is over proportionally Anti?

    Where should I look to find references on how to conclude that there must be an asymmetry?

    Thanks Ben

  • Mark

    Separated zones of matter and antimatter are not that easy to arrange it turns out. One can bound them by considering the observable effects of annihilations at their boundaries. A nice analysis here

    shows that:

    We ask whether the universe can be a patchwork consisting of distinct regions of matter and antimatter. We demonstrate that, after recombination, it is impossible to avoid annihilation near regional boundaries. We study the dynamics of this process to estimate two of its signatures: a contribution to the cosmic diffuse gamma-ray background and a distortion of the cosmic microwave background. The former signal exceeds observational limits unless the matter domain we inhabit is virtually the entire visible universe. On general grounds, we conclude that a matter-antimatter symmetric universe is empirically excluded.

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  • Jimbo

    Our current DOE secretary has measured g to the ppb level about a decade ago, so the technology is definitely in place to measure g for trapped and cooled anti-atoms as CERN will soon do.
    Baez is correct in advising caution, as nature often does present surprises. If the SN1987A neutrino & anti-neutrino fall times differed by only a few ppm, lets not get cocky. Anti-Hydrogen atoms weigh about one billion times what a neutrino weighs, and will pose pretty in the lab as their fall/rise times are measured.
    I predict anti-gravity will surprise and excite the physics community like nothing from the LHC will.

  • Hasanuddin

    Hi Jimbo,

    I wholeheartly agree, the potential opening of understanding caused through consideration of gravitational-repulsion between matter and antimatter (or “anti-gravity,” I hate that term) will be the next biggest thing, putting LHC on the back burner. I am very hopeful for the AEGIS experiment to prove the reality of gravitational-repulsion.

    Why? Because I am in process of advancing a deduction-produced new cosmologic model that is completely seamless from end to end. Unlike current assumptions, all verified/concrete data is compatible, i.e., there are no paradoxes between theory and the physical record.

    View, follow, and join the ongoing debate at:

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About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


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