We are told that there is a slight bias for matter in our universe over anti-matter. How do we know that it is not just a local thing, such as our local supercluster of galaxies. Do we know that distant galaxies are not anti-matter galaxies? How can we tell that very distant galaxies (or stars for that matter) are matter and not anti-matter? Wouldn’t they appear the same? Or do they send out anti-photons?
Matter and antimatter produces distinctive radiation when they meet and annihilate; we’d detect it if large masses of antimatter were nearby in cosmological terms. As for more distant objects, we don’t actually know for sure, it’s just our working assumption; I note from the Wiki page on antimatter that NASA is looking for evidence of annihilation events from far off galaxies.
I would think that antimatter and matter have slowly migrated to opposite sides of the universe. Most likley the majority of matter and anti matter have long been anilated. We will likely never know for sure. I don’t see how an imballance could occur unless they just moved away from each other.
Nitpick - matter doesn’t dominate our universe, and matter plus antimatter doesn’t either. It looks like dark energy is about 73% of all universal stuff. Another 22% is dark matter, which is presumed to be matter but is still mysterious. Only 4% is plain old fashioned matter and antimatter, as we used to think of them.
I think he means “in relation to anti-matter”, not “in relation to energy”, though I can see why one would interpret it that way.
I think the answer is that we would see comparatively huge amounts of radiation anywhere matter and antimatter meet, even if it was just a few sparse dust clouds. Since we don’t see that, anywhere, the observable universe must be predominately matter.
Now, beyond the observable universe is entirely open for debate, and I don’t think anyone can say with any certainty that there is or isn’t a balancing quantity of anti-matter elsewhere in the universe.
So basically, the answer to your question is “we don’t, we only know it dominates locally because we can tell by looking”.
Yes, in relation to anti-matter.
My question is how can astronomers tell if a far off galaxy is a galaxy of matter, or maybe if it is anti-matter? All or almost all of it. No more of the opposite around to collide with than here in our neck of the woods, so no huge tell tale explosions.
But the space between the galaxies is not really empty. There is always, at the lest, some very tenuous hydrogen. It may not be much, but, given the violent, very energetic nature of matter/anti-matter interaction, it is enough that if there were a region of anti-hydrogen adjacent to a region of hydrogen, we ought to be able to see signs of the interaction along the boundary. We don’t, so there isn’t. (That is assuming the regions, and thus the boundaries, were large, of course. If they were small, if matter and anti-matter were fairly well intermixed, they would have annihilated each other long ago.)
One example of what fills intergalactic space is matter shot out by plasma jets (link to a really impressive one).
These jets travel at a significant fraction of the speed of light (the linked article says 1/3 c but I seem to recall hearing higher speeds for other jets), and they stretch across millions of light years. If anything was going to highlight a matter/anti-matter boundary, this would have to do it. And yet… nothing.
Another important piece of information is the lack of light anti-isotopes (in particular, antihelium) in the primary cosmic ray flux. If there were antigalaxies, they would have antistars and antisupernovae blasting anti-isotopes outward. Antihelium is experimentally useful because it would be the most prolifically jettisoned isotope that is too hard to make in other ways. (Antiprotons, antideuterium, …, can occasionally show up through other means, but to make a whole antihelium pretty much requires an antistar.) All searches for antihelium have come up empty. The Alpha Magnetic Spectrometer, an experiment currently operating at the ISS, will push current limits down by another factor of 1000 if no antihelium is seen. If even a single antihelium nucleus is seen, that’ll be pretty solid evidence that a chunk of antimatter is out there somewhere.
There are other data pointing against a matter-antimatter symmetric universe that are somewhat less direct. For instance, the ratio of baryons to photons should be nine orders of magnitude smaller that what’s observed if things started out symmetric, at least according to our understanding of the early universe’s evolution (which actually isn’t half bad).
Pisser article, photo, and links to video.
That article appeared on-line today. Mysterious coincidence…?
That is what I was looking for. Thank you.
Just read this today. Big excerpt:
One step closer: Scientists help explain scarcity of anti-matter
December 27, 2012 by David Tenenbaum
A collaboration with major participation by physicists at the University of Wisconsin-Madison has made a precise measurement of elusive, nearly massless particles, and obtained a crucial hint as to why the universe is dominated by matter, not by its close relative, anti-matter. The particles, called anti-neutrinos, were detected at the underground Daya Bay experiment, located near a nuclear reactor in China, 55 kilometers north of Hong Kong. For the measurement of anti-neutrinos it made in 2012, the Daya Bay collaboration has been named runner-up for breakthrough of the year from Science magazine. Anti-particles are almost identical twins of sub-atomic particles (electrons, protons and neutrons) that make up our world. When an electron encounters an anti-electron, for example, both are annihilated in a burst of energy. Failure to see these bursts in the universe tells physicists that anti-matter is vanishingly rare, and that matter rules the roost in today’s universe. “At the beginning of time, in the Big Bang, a soup of particles and anti-particles was created, but somehow an imbalance came about,” says Karsten Heeger, a professor of physics at UW-Madison. “All the studies that have been done have not found enough difference between particles and anti-particles to explain the dominance of matter over anti-matter.” But the neutrino, an extremely abundant but almost massless particle, may have the right properties, and may even be its own anti-particle, Heeger says. “And that’s why physicists have put their last hope on the neutrino to explain the absence of anti-matter in the universe.”
“Last, best, hope.” Sounds desperate, physicists…
That looks pretty over-hyped to me. Antineutrinos are, if anything, even more routine than neutrinos, being produced in beta decay. And while it does seem possible that neutrinos and antineutrinos are actually the same thing, I don’t know of any mechanism for the matter-antimatter asymmetry that particularly involves neutrinos (though some proposed mechanisms depend on other particles, even lighter and more elusive than neutrinos).
Hmmm, so… all the anti-matter got dumped into becoming WIMPS and/or Dark Matter? The Universe is full of it, but it’s packaged in a way that it hardly interacts with matter.
The parameter of the Standard Model that the Daya Bay experiment (*) has measured relates to how neutrinos are quantum mechanically mixed with one another. It happens to be an angle, and its value is approximately 9 degrees. The previous upper limit only indicated that this angle was much smaller than its two sister angles, which had already been measured to reasonable precision. These three numbers together are free knobs in the Standard Model, but theorists have many speculative models that attempt to predict their values. Some of these assert that the smallest angle is actually zero degrees. (In some cases, the models subsequently break the underlying pattern causing the zero and would allow a non-zero small value.)
In any case, if this smallest angle were zero, then neutrinos would not be able to violate something called CP (charge-parity) symmetry, which relates to the decay and interaction rates of particles versus antiparticles. Quarks violate CP, but it is unknown whether neutrinos do.
What about the universe? To create the matter/antimatter imbalance discussed in this thread, one requires CP violation to be present at a sufficient level in the early universe. The known CP violation in the quarks is nowhere near large enough to pull it off, so a new source of CP violation is needed. Knowing that this newly measured angle is non-zero means that neutrinos can violate CP, too. It doesn’t mean they do; it just means that they can. The Daya Bay experiment cannot measure whether neutrinos actually do violate CP. A different type of experiment is needed for that.
<technical side note>
The neutrinos here are the usual ones. However, the relevant CP violation in the early universe would not come directly from these neutrinos but rather from heavy partner neutrinos that would all have decayed away by now and are too massive to create in the lab. However, if there is CP violation in the usual neutrinos, then it would be very weird not to have it in their heavy partners.
</technical side note>
(*) and several others, actually. Daya Bay just had the best precision among the lot. It was also a very elegantly designed experiment, making it rather immune to hidden experimental error.
Keyword: leptogenesis. (The wiki article is unhelpful and misleading, so I don’t link to it here.)
Ok, this is the second time I’ve read this in a post, so:
“knobs”?
And, is “angle” related to real-world physical angle in coupled neutrinos? (Somehow I doubt it, b/c that would be too intuitive…) 
By “knobs”, Pasta means “free parameters”. As in, the basic structure of the theory is the same no matter what values you put in for those values, so you can’t derive those numbers theoretically, but have to determine them experimentally. The mental image is of the theory as a machine that you have sitting on your desk in front of you, and you turn the knobs until it matches experiment, sort of like tuning an analog radio.
Oh, and thank you, Pasta. I wasn’t aware of any of that (well, almost any of that). Is the 1-2 mixing angle still believed to be (about) 45 degrees? That’s the most recent information I have on the topic.
Oh, and
You may be aware that there are three different kinds of neutrinos. Well, those three neutrinos can be regarded as the basis vectors of a 3-dimensional space (which is not the same space as the familiar length-width-depth space, though it has the same geometry). But there are many different sets of basis vectors possible, and it turns out that at least two of them are relevant: You’ve got the electron-mu-tau coordinate system, which is relevant for determining which particle reactions the neutrinos can be involved in, but you’ve also got the 1-2-3 coordinate system, for neutrinos with definite masses (1 is the lightest, 3 is the heaviest). These coordinate systems are not lined up with each other, so (for instance) an electron neutrino is not the same thing as a 1 neutrino, but a mix of mostly equal parts of 1 neutrino and 2 neutrino, and thus an average mass in between those two. The mixing angles are the angles between the axes of those two coordinate systems.
OR is oriented and travels within additional small curled up dimensions such that they in general do not interact with each other - gravity being the only force that can permeate all dimensions?
What would it look like to creatures of matter to have equal amounts of truly mulidimensionally symmetric matter and antimatter present in an n-dimensional universe but oriented such that they pass by each other most of the time?
What would it look like from the POV of creatures of matter able to see only a certain slice of the planes to see a small enough n-dimensional particle rotating around as it travels? We’d see only the portion that intersects with our space and as it rotated it would look like was changing forms or, flavors if you will. What would an antimatter particle perfectly symmetric to matter particle but oriented such that the portions of it that manifest in the three extended spatial dimensions are different than those portions of a matter particle that are in that cross section (and each having bits that extend indifferent curled up dimensions) look like. Hmmm. It might look like there were asymmetries, or CP violations, from that limited perspective. We might observe unexplained gravity.
Hypothetically.
The answer to the op is that WE DON"T. We know we observe less antimatter and we, so far, have assumed that the explanation for that lack of observation is that it does not exist. The* possibility* that it exists but where we cannot directly observe it most of the time (and then not observing it completely) has not so far been seriously considered.
You can’t get off the hook by assuming that the antimatter, for some reason, does not interact, and therefore makes up the dark matter. Even if something like that is the case, there’s still significantly more dark matter than ordinary matter, so you either have to posit an asymmetry the other way, in favor of antimatter, or you still have to find some other way to account for most of the dark matter.