I don’t want to hijack that thread, so here we are. This one comment generated a number of questions - maybe I shoulda gone to school
This statement implies that there is matter that is not made of atoms - right? What are some examples of non-baryonic matter? Neutrinos? Are there others? Does non-baryonic matter form more complex structures similar to atoms, molecules, etc? Are quarks baryonic or non-baryonic? Any reading suggestions for the lay-person?
IANAPaleontologist, but yes, the baryonyx, whatever it is, was made out of baryonic matter. But then, so was the stegasaurus and the apatosaurus and the haplocanthosaurus.
The Universe is made up approximately of 70% dark energy (a weird sort of stuff which causes a gravitational repulsion, rather than attraction), 25% nonbaryonic dark matter, and 5% baryonic matter (most of which is also probably dark). Baryonic matter is, unsurprisingly, the form of matter with which we’re most familiar, since we can study big chunks of it in the lab. Dark energy is, for the present time, completely mysterious: We can deduce a few properties of it by observing its effects on the evolution of the Universe, but we know nothing else about it. The nonbaryonic dark matter is much more interesting: Any time any particle physicist proposes any new hypothetical particle, there’s always speculation that those particles might make up some or all of the nonbaryonic dark matter in the Universe. Of course, there’s no shortage of hypothetical particles, and if all of them existed with the supposed properties, we’d have solved the dark matter problem fifty times over (and thus, of course, produced an entirely different dark matter problem).
The current best candidate for nonbaryonic dark matter is the neutrino, which has the definite advantage that it’s known to exist. But neutrinoes probably can’t account for all of the dark matter. Other possibilities include primordial black holes (those dating back to the Big Bang; normal black holes are generally counted as baryonic matter, because they used to be made of baryons) and possibly some of the afore-mentioned hypothetical particles. None of the hypothetical nonbaryonic dark matter candidates form complex structures like atoms or molecules.
Baryons are particles like protons and neutrons, which are made of quarks, so baryonic matter is matter which gets most of its mass from baryons. Electrons are not baryons, but they’re so much lighter than protons and neutrons that they’re generally ignored. There are also other particles made of quarks (the mesons) which have masses comparable to or greater than protons and neutrons, but none of them are stable, so you can’t really have things made of them. Whether quarks themselves are baryons is a matter of definition, but it doesn’t matter anyway, since you can’t have quarks by themselves: They’re always combined into baryons, mesons, or (occasionally) more exotic combinations.
I think it’s been shown fairly convincingly that only a fraction (maybe around 30%) of the non-baryonic dark matter can be “hot dark matter” (so called because it moves really fast) like neutrinos, because all that motion wouldn’t allow the cosmic structures that we observe to form. That means that maybe around 70% of the dark matter must be “cold” dark matter, the composition of which is completely mysterious. All we know about it is that it moves around at well-under relativistic speeds, and it interacts with other matter and energy gravitationally.
Some people have theorized that there are really twice as many varieties of particles as the ones we observe. The idea is that there is really only one class of particles, but these particles can be rotated in a kind of extradimensional space such that they appear to have a quantum spin of half-integer or integer value, depending on how they are “oriented”. Hence you could take a photon (spin 1), and rotate it around to get a photino (spin 1/2). Or you could take a neutrino (spin 1/2) and rotate it around to get a sneutrino (spin 0). Pretty wild. I make no claim to understand this at all well, though I’ve tried (goodness knows). Anyhoo, this wonderful idea (called Supersymmetry, or SUSY for short), which helps tidy up many aspects of our current Standard Model of Particle Physics (and may help work gravity into the Standard Model), seems to be in trouble because we don’t observe any of these partner particles.
Or do we? Seems it’s possible to predict the properties of some of these particles. It’s hard to say for sure what the masses of these SUSY partners might be, but they could be very large. What’s almost definite is that there must be a lightest SUSY partner that all the other SUSY partners would decay into. Some of these lightest partner candidates (among the strongest being the neutralino) have pretty much exactly the properties one would expect cold dark matter to have. They’re heavy, they’re slow, and the only forces they would feel are gravity and the weak nuclear force. Billions of them could be passing through you every second, and you’d never even know it (as it is, you’ve got a similar number of solar neutrinos going through you right now in a similar fashion, only much faster!).
If cold dark matter is composed of the “lightest supersymmetric partner” particles, it might be possible to detect it using something other than its obvious gravitational effects, namely in the way it interacts with atomic nuclei. Such interactions would be exceedingly rare, however, given how weak the weak nuclear force is. Currently our most sensitive dark matter detectors have come up with zilch. Newer generations of dark matter detectors and particle accelerators might find something. Or not. I’ve read that if we don’t start detecting these particles pretty soon, SUSY might be in trouble as a theoretical framework.
But so far SUSY seems to be the best explanation we have for what cold dark matter might be.
Except that now that it’s known that neutrinos have mass, cold neutrinos are also a possibility. In fact, given current estimates of the neutrino mass, most of the neutrinos in the Universe, those of cosmological origin, probably are cold. But cold neutrinos are exceedingly difficult to detect.
Where does the limit of 5% of the universe’s mass being baryonic come from? I’ve heard this statistic quoted numerous times and can’t remember an explanation as to why it is. Or maybe I have but have forgotten. Anyway, please elucidate.
Yeah, I’ve heard of that. The best explanation for the solar neutrino problem (and I think this explanation has been pretty well proven lately) is that what we observe as the electron, mu, and tau neutrinos is really a hybrid of the “real” neutrinos (called, imaginatively enough, 1, 2, & 3) that are massive, propogate at different velocities, and hence are interfering with one another (think of two waves slightly out of phase, for those who haven’t heard of this…the two waves “beat” like two notes slightly out of tune). Since the observed mass of the observed flavors of neutrinos is just the difference in masses between these “real” neutrinos, the, I guess fundamental, neutrino masses could be, at the heaviest, around 1 eV. If this is true there could be neutrinos floating around that are heavy enough to be cold and clump.
But the thing is, the only way we have detected 1, 2, & 3 so far is by looking at their hybrids, the electron, mu, and tau neutrinos. These are hot; I think even the tau neutrino must be lighter than 0.1 eV, and hence still moves along at a good clip. So what are these heavy neutrinos? We only know of three generations of particles, so it seems if there is another flavor of neutrino, there must be another generation. From what I’ve read, there’s a fair amount of evidence indicating there are only three generations. I’ve been confused about all this since the first time I read up on it, and have never been able to come up with a way to make it all fit together based on what we know. Can you help me? I hope this isn’t too much of a hijack, as it relates back to the non-baryonic dark matter question ;).
Is it just that the wee little varieties of neutrinos we do know about have been able to slow down over cosmic time scales due to inertia? Are they heavy enough to do that?
I don’t know how to do the calculations myself, but knowing what we know about general relativity, the Standard Model of particle physics, and some known properties of nuclear particles, it’s possible to get a pretty good estimate of how much baryonic matter could have been formed during the Big Bang.
Basically, given an age of roughly 14 billion years, plus the stuff we know about its current state (given the theoretical framework mentioned above) you can run the clock backwards and get an idea of how hot and dense the universe was. Then you run the clock forwards and watch what happens.
I guess at around 1s after the bang (a.b.) the weak force was weak enough that neutrions no longer interacted with nucleons significantly, and hence the ratio of protons and neutrons became essentially fixed.
About 15s a.b., there was a brief re-heating due to most of the residual antimatter being used up in annihilations. About 3 minutes after that nucleons could stick together, forming nuclei heavier than a proton. After that, deuterons and helium nuclei and a small amount of other nuclei could form too, but that only continued for another thirty seconds before things had cooled down enough that no more nuclear reactions took place.
Given the ratios of nucleons predicted, the amount of time known to elapse from the beginning to the end of the nucleosynthetic period (the time when things got too cool for further nuclear reactions), and the way we know how those nucleons interact with each other, it is possible to predict how much hydrogen, deuterium, and helium there were shortly a.b. Throw in the smidgen of heavier elements you would expect from early stellar nucleosynthesis, and you can get a prediction of how much of these elements you should see now.
Turns out these predictions fit exceedingly well with what is observed. Using only theory and some known particle properties, you can predict the matter content of the present universe very, very well. This provides very strong evidence that the idea of a hot big bang is correct. Going in the other direction, using this info. taking into account the H/He ratio you see today, and what must be the total matter/energy of the universe (as indicated by the best measurement of the Hubble constant) it’s possible to say that baryonic matter makes up only 1/20 of the total mass of the universe.
As Loopydude stated, when we observe neutrinos via their interactions, we are only sensitive to three specific mixtures of the (in Loopydude’s terms) “real” neutrinos. Actually, neither set of neutrinos – the “flavor” particles ([symbol]n[/symbol][sub]e[/sub], [symbol]n[/symbol][sub][symbol]m[/symbol][/sub], [symbol]n[/symbol][sub][symbol]t[/symbol][/sub]) or the “mass” particles ([symbol]n[/symbol][sub]1[/sub], [symbol]n[/symbol][sub]2[/sub], [symbol]n[/symbol][sub]3[/sub]) – is more “real” than the other. It’s just that different aspects of nature prefer different combinations.
In particular, mass is only defined for the ([symbol]n[/symbol][sub]1[/sub], [symbol]n[/symbol][sub]2[/sub], [symbol]n[/symbol][sub]3[/sub]) particles. That is, there is no answer to, “What is the mass of the [symbol]n[/symbol][sub]e[/sub] (a.k.a the electron neutrino)?” It is pretty useful, though, to define effective masses for the “flavor” neutrinos in terms of the masses of their constituent “mass” neutrinos.
In neutrino oscillations (which is what the solar neutrino problem turned out to be), one measures differences in the squares of the masses of the neutrinos involved. Also, one unfortunately can’t measure an absolute scale for the masses. For example, if you measure a difference in squared mass of [symbol]D[/symbol]m[sup]2[/sup]=0.1 (eV/c[sup]2[/sup])[sup]2[/sup], you can’t tell if the two involved neutrinos have squared masses of
Upper limits on the actual masses of the neutrinos come from elsewhere. Only the electron neutrino has a respectable limit currently (m[sup]effective[/sup]<3 eV/c[sup]2[/sup]). The muon neutrino and tau neutrino have limits that are, respectively, 10[sup]5[/sup] and 10[sup]7[/sup] times larger.
Through cosmological observations, one can infer an upper limit on the sum of the masses of the neutrinos. This sets a rather strong but somewhat circuitous upper limit on the sum of the neutrino masses at 2.5 eV/c[sup]2[/sup]
Interestingly, the most commonly cited method for introducing neutrino masses into the standard model is via the colorfully named “see-saw” mechanism which requires a (possibly very) heavy partner for the neutrinos. (This could be a SUSY particle.)
Another possibility that’s hot in the neutrino world these days is “sterile” neutrinos – neutrinos that do not participate in the weak interaction but do participate in neutrino mixing (i.e., the “flavor particle” v. “mass particle” stuff originally mentioned by Loopydude above.) Massive sterile neutrinos would show up as dark matter.
The experimental evidence for three generations of neutrinos can be stated as: There are three “light” neutrinos that participate in the standard weak interaction, where light is defined to be <45 GeV/c[sup]2[/sup] (which is half the mass of the weak-interaction-mediating boson Z[sup]0[/sup].) Aside from the cosmological inferences, nothing rules out heavier neutrinos or lighter sterile ones.
Huh. Thanks, Pasta! Not that I understand it much, but it’s given me a term I didn’t know before, and can look up. “Sterile” neutrinos? What’s “sterile” supposed to mean? Just that they aren’t weakly interacting? Or that they’re “flavorless”? Or both?
The first one: “sterile” means they aren’t weakly interacting. This actually implies the second bit – that sterile neutrinos are flavorless – since a neutrino’s flavor just means which charged lepton (electon, muon, or tau) it is coupled to by the weak interaction.
I should note that the converse isn’t necessarily true. That is, you could have flavorless particles that do interact weakly. (This stems from the fact that the weak interaction comes in two forms: one that is manifestly flavorful – the “charged current”, mediated by the W particle – and one that isn’t – the “neutral current”, mediated by the Z.) For example, another popular non-baryonic dark matter candidate is WIMPs, or “weakly interacting massive particles,” which need not have flavor but, as their name implies, do interact weakly.
That may or may not be true, but in either case I’m happy to expound on whatever you might want clarified.
