In simplest terms, if antiparticles interacted with the extra dimensions in a way any different from normal particles, then they would also have different electromagnetic interactions, since the familiar 1/r^2 behavior of electromagnetism is due to the three spatial dimensions. But the very first thing we were able to experimentally show about antimatter is that aside from the charge being flipped, it behaves just like normal matter, electromagnetically.
There’s also the issue that “matter” and “antimatter” aren’t actually well-defined categories. We conventionally say that the particles we’re made of are matter and their partner particles are antimatter. But what of particles that we aren’t made of? A positive pion and a negative pion are certainly antiparticles of each other, for instance, but which one is matter and which one is antimatter? Or, then, what of particles which are their own antiparticle, the most familiar example being the photon?
It wasn’t aimed at comprehension or even a full read but rather to just get a sense of how antimatter and matter are talked about in a more formal context, with phrases like “This solution is useful for showing the relation between anti-particle and particle” making appearances and with the sections “Four-spinor for particles” and “Four-spinor for antiparticles” showing marked parallels that might convey (if qualitatively) that the relationship between matter and antimatter is a deep one.
But this is a vanilla (3+1)-dimensional model that doesn’t have anything to do with your concept of hiding antimatter in some extra dimension(s).
It’s not that there aren’t new, sensible ideas out there to explain the baryon asymmetry. It’s a very active area of research, and some of the ideas (but not the one you linked to) even involve multiple dimensions (but not in the way you propose). The issue in this exchange is just that you can’t readily hide antimatter in extra dimensions just because you can hide other types of things in extra dimensions.
Essentially it seems they are considering a modification to the usual Dirac equation such that particles and their antiparticles have different mass, but by an amount which depends on the density of the universe – and so the difference is negligible today, but was non-negligible in the early universe. This is important; if you want to propose something that goes against well tested theory, you need to be able to say why your new theory would give the same results under the conditions for which the old theory was tested. (For example, Einstein’s special theory of relativity agrees with Newtonian mechanics for particles moving much slower than light speed.) If you can’t do that, no one will take you seriously.
Then they suggest that the early universe consisted of some hypothetical particles called archaeons, and equal numbers of anti-archeons. And that the mass difference between archaeons and anti-archeons caused the archaeons to decay mostly to normal stuff like protons and neutrons, and the anti-archaeons to decay mostly to some hypothetical dark matter particle. So the dark matter they propose is not normal anti-matter (it’s not the anti-particles of some known particle). Rather they identify it as anti-matter because it was produced fro the decays of hypothetical particles that were the antiparticles of the hypothetical particles that decayed into normal matter.
So in this theory dark matter is still a new kind of particle (not some known matter or antimatter particle), and moreover it still comes in particle and anti-particle varieties that are both equally dark. But they are suggesting a mechanism where the dark matter that exists in our universe ended up being all in the antimatter variety, counterbalancing the fact that the non-dark stuff ended up being all in the matter variety.
This is quite different than suggesting that the antiparticles of existing particles are dark matter. Their theory doesn’t predict that antiparticles in the present day universe behave in a different way than their positive matter equivalents. It also has nothing to do with extra dimensions (as Pasta noted).
Incidentally, one part I don’t understand is this:
If particles in the dark sector can couple to photons, in what sense are they dark? Perhaps they are proposing that the coupling is just very small.
Okay. Not the concept I imagine. And if I understood the math better I’d better understand why what I picture is already if not falsified, so incompatible with what else is known as to be out of any serious consideration.
One question about this. I recall being told in astrophysics class that by far most helium in the universe is “primordial” – created by the big bang, rather than by stars, for the simple reason that stars require a fuckton of hydrogen to work, but most of that is just mass to provide the pressure required to convert a tiny sliver to helium.
So couldn’t there be primordial antihelium nuclei zipping around out there? I.e. what makes antihelium, if detected by this experiment, so certain to be the product of an antistar rather than primordial? Is it just that it would be putzing about in our matter-dominated region with no obvious way of getting there except having been shat out of an antisupernova?
Yes, you are correct. The antihelium in antigalaxies could also be primordial. An anti-star (anti-supernova) is a common astrophysical phenomenon that can spew it our way, but indeed it could have just been bound up in the anti-star and not actually produced there. The main thing is just that you can’t make anti-helium in a matter galaxy even though you can make antiprotons and antideuterium at a high enough rate to make those isotopes unusable for this research.
“Bulk”–is that your nonce word or one that is commonly understood among physicists in this field when they speak?
A nice bumper sticker/net signature/mantra:
Given that my speculation regarding antimatter travelling in the bulk is pretty much shot out of the water, allow me some other questions -
Some models place some portion of n-dimensional fundamental objects in the bulk, correct?
Are there any serious models that hypothesize the objects, albeit apparently not antimatter, in the bulk, or predominantly in the bulk, could be a source for dark matter?
Things is that IF curled up dimensions exist, and IF fundamental objects are actually greater than 3n-spatially dimensional, then it seems odd that the bulk (so to speak) of those objects exist equally all within the same set of those extra spatial dimensions and travel simultaneously between them. I am still curious, as a thought experiment, what it would look like from the POV of something made up of a subset of those particles that moved through those extra curled up dimensions according to one set of behaviors to be interacting with particles that moved in a different group of ways. Just as a thought experiment, any thoughts?
Absolutely. Particles that live in the extra dimensions could gravitate but be otherwise invisible to us (or nearly so). Certain models predict a suite of field excitations (i.e., particles) in compact extra dimensions with a tell-tale spectrum of masses. The lightest of these could be stable and would make for a good dark matter candidate. ATLAS and CMS at the LHC are looking for such excitations, which would appear not as particles in the experiments (since they are invisible) but rather as unexplained gaps in the energy andmomenum tallies for the collision products.