Dark antimatter?
Published on Jan 26, 2007 at 12:56 pm.
8 Comments.
Filed under dark matter, physics.
Every particle of matter had a corresponding particle of antimatter. When a particle of matter and a particle of antimatter come together they annihilate one another, creating two gamma rays traveling in opposite directions. So, when an electron and a positron (an anti-electron) meet, they produce two gamma rays having 511keV of energy. The amount of energy in the gamma rays can be computed using Einstein’s famous equation: E=mc2.
What is antimatter like? In basically all appearances, antiparticles look and act like particles, with the exception that they have opposite electrical charge. An electron is negative, so a positron is the same mass, same spin, same other properties, but with positive charge. A proton is positive, so an antiproton is negatively charged. I find it rather interesting that these oppositely charged particles, having electric fields, annihilate to form electromagnetic radiation. I am not a theoretical physicist, but it makes me wonder whether or not if antimatter had negative mass if it would produce gravitational radiation upon annihilation.
The problem, though, comes from neutral particles. For example, a neutron has no electric charge. So, what distinguishes it from an antineutron? To answer that, you have to look deeper. Neutrons and protons are examples of a group of particles called hadrons. In fact, they are part of a subgroup of hadrons called baryons. Hadrons are particles composed of smaller particles called quarks. Baryons have three quarks, and eventually decay into protons. Talking about the subatomic particle zoo is a bit more than I want to do in this post. But, I wanted to mention this because neutrons are composed of an up quark and two types of down quarks. An up quark has a charge of 2/3 e, and down quarks have a charge of -1/3 e. Only quarks have fractional charges of the fundamental charge e. Antineutrons are composed of an anti-up quark and two anti-down quarks.  (We won’t really worry about how these baryons are really composed of lots of virtual quarks, too). So, what happens when a neutron and an antineutron come together is that the up and anti-up quarks annihilate, and the anti-down quarks annihilate withe the down quarks. But, things get a bit more complicated with neutrinos. You see, neutrinos are leptons, which are fundamental particles, themselves. They aren’t made of more basic things. So, neutral neutrinos are pretty much indistinguishable from antineutrinos. Indistinguishable means just that in particle physics. So, the neutrinos can act as their own antiparticles.
Now, we get to a couple of topics from a few of the speakers and poster sessions at the conference a couple weeks ago. They were talking about the gamma ray glow of the galaxy. It turns out that wherever you point a gamma ray telescope, you get a slight glow. And that glow seems to be brightest from the direction of the center of the galaxy. Other galaxies may also have this gamma ray halo. It is actually pretty much a mystery as to what it may be. Well, some people are now suggesting that perhaps this may be a sign of dark matter. In particular, they propose that perhaps dark matter has a dark antimatter counterpart. A particle of dark matter and dark antimatter would annihilate each other when running into one another, perhaps producing these gamma rays. Well, that might actually be a problem, because the gamma rays have the wrong energy from what theorists would suggest. Remember, the amount of energy is related to the energy of the gamma rays by Einstein’s mass-energy equivalence relationship. But, that works both ways. You can also create more particles and antiparticles from energy. So, very high energy gamma rays can also produce particle antiparticle pairs. And, some of those particles might be unstable, decaying into other particles that then produce the gamma rays that we see. Yeah, it seems like it may be a little of a stretch, but it is a very interesting idea, and it definitely deserves more work. Interestingly, these gamma rays do seem to be concentrated in regions where you’d expect dark matter to concentrate.
But, as I said, there is still a need for a lot of work. After all, it isn’t even clear what dark matter is, much less if it even has a dark antimatter type of symmetry. And, since dark matter does not interact via the electromagnetic forces, that means that it has no charge. What differentiates dark matter from dark antimatter? Is it composed of more fundamental dark things that do have charge, but only occur in pair or triplets like quarks? Hmm. This raises lots of questions. And, as I said, high energy physics isn’t really my thing. But, I do find it interesting.
-Astroprof
Anders Lerberg on April 11, 2007 at 4:13 pm: 1
Thanks for this great article. I found it very interesting and it was great help for my homework=) Could not find anything even close to it in my own language.
Susan Cartwright on July 20, 2007 at 5:01 am: 2
I just found your page googling for something else, and read a few articles out of interest. Keept up the good work! However, as a particle astrophysicist I thought this one did need some cleaning up - yes, high energy physics isn’t really your thing.
Firstly, the jury is still out as to whether neutrinos can act as their own antiparticles. Normally, one can definitely distinguish very clearly between neutrinos and antineutrinos: a neutrino will produce its related charged lepton when it interacts; an antineutrino will produce an antiparticle. For example, the electron-type neutrinos produced in solar fusion can interact with the heavy water in the Sudbury Neutrino Observatory to produce electrons, but never, ever, positrons, whereas the antineutrinos produced in nuclear reactors can make positrons, but never electrons. Neutrinos and antineutrinos are therefore observationally different.
The reason the jury is still out, and has not returned a verdict of “not guilty”, is that it is possible that what distinguishes neutrinos from antineutrinos is not the particle, but the direction of spin. Because of quantum effects, the spin axis of a neutrino can point in only two ways: along the direction of travel (right-handed) or opposed to the direction of travel (left-handed). All neutrinos turn out to be left-handed; all antineutrinos are right-handed. So the question is: if we could flip the spin of a left-handed neutrino, would it act exactly like an antineutrino, or would it be some different beast, a not-found-in-nature right-handed neutrino? The current best guess is that it would probably look like an antineutrino (in the technical jargon, neutrinos are Majorana particles), but there is no convincing experimental evidence one way or the other. (Experiments on a process called double beta decay could settle this question: if they report a positive answer, the flipped neutrino definitely behaves like an antineutrino. A negative answer unfortunately means EITHER that the flipped neutrino isn’t an antineutrino (interesting) OR that the mass of the neutrino is lighter than some limit (less interesting)- and that’s the situation we have at the moment.)
Dark matter is a different breed of cat entirely. Firstly, a neutral object composed of charged constituents certainly does interact via electromagnetic forces (just think of atoms); a particle which does not interact electromagnetically AT ALL needs to be honestly neutral. Spin direction is only a good label for particles which (a) have spin (not all of them do) and (b) are very nearly massless (massive particles can flip their spins rather easily). That being the case, the natural inference is that for a massive, uncharged elementary particle, you would NOT be able to distinguish between particles and antiparticles - and indeed, in one of the two favoured particle-physics theories of dark matter, you can’t. If the dark matter is made up of “neutralinos”, the massive, weakly interacting particle expected in supersymmetry models (due to be tested next year when the Large Hadron Collider turns on), then it is its own antiparticle: any two neutralinos that come close enough together (VERY close, owing to the weakness of their interactions) can annihilate each other. It’s this property which sets the density of dark matter in the universe: in the first fractions of a second after the Big Bang, the neutralinos happily annihilate each other until the expansion of the universe drops the number of neutralinos per cubic centimetre down to the point at which the rate of interactions becomes negligible.
This is the model being used in the predictions of gamma ray luminosity you refer to above. In the Galactic centre, it is possible that the gravity of the Galaxy’s central black hole can concentrate the dark matter to a sufficient extent that annihilations take place at an observable rate. One likely product is high energy photons, i.e. gamma rays; others include antiprotons, positrons, high energy neutrinos and other exotica. (The Sun can also capture and concentrate neutralinos: a possibly observable indirect signature for dark matter would be a flux of high energy neutrinos - with energies about a million times higher than fusion neutrinos - from the Sun.)
So, really, “dark antimatter” is just a synonym of “dark matter”, and not all that interesting. The really interesting aspect of the matter/antimatter issue is why our Universe appears to be made of matter, and not a 50:50 matter/antimatter mix - which is what one would expect given the near-identical interactions of matter and antimatter. The imbalance is small (looking at the ratio of protons to photons in the present Universe suggests that the early Universe made about a billion and one protons for every billion antiprotons, so when everything annihilated one proton was left standing), but it’s key to the existence of galaxies, stars and us. This is one of the big open questions of modern particle physics.
TCSII on March 17, 2008 at 10:56 pm: 3
Actually, you can prove that dark matter and dark antimatter exist…as well as dark light (i.e., negative energy). It falls right out of the space-time invariance equation and also explains the matter-antimatter imballance. Want to know more? email me.
tcs361@yahoo.com
cheerio
Samuel john on August 19, 2008 at 11:58 am: 4
Nice…
Rob Wallace on August 23, 2008 at 9:00 pm: 5
In response to your question: ‘Is it composed of more fundamental dark things that do have charge, but only occur in pair or triplets like quarks?’, I think there is a good chance that the answer is yes. I’ve found a mathematical pattern, 40% of which which provides a sort of periodic table for particles of the standard model, and 60% of which could do the same for dark matter particles. If so, dark matter will turn out to be similar to neutrons, but composed of four quark-like particles (I call them varks). You can find the pattern described in ‘Advances in Applied Clifford Algebras’, Vol 18,1 pp 115-133.
Hasanuddin on March 22, 2009 at 5:40 am: 6
2nd try,
Sir,
Because of your interests I believe you will be highly interested in the antimatter-based cosmologic model being brought forward at hasanuddin org
Austin Truong on March 16, 2010 at 4:17 pm: 7
Call me a novice, imbecile, amateur, or whatever, but after hearing about a number of different theories, I would think that combining Regular matter and anti-matter would create nothing, for if they are combined in equal parts, no energy would be created, for the law of conservation of energy says that energy cannot be created.
Cru on June 22, 2010 at 2:14 am: 8
No, not created… Released. E= mc^2 remember?