How do we know distant galaxies are not antimatter?

Seems to me the only things we are receiving from other galaxies are photons, gravity waves, and neutrinos. I also suspect that it is difficult to determine where the neutrinos you capture come from, nevermind neutrino conversion. And gravity waves obviously cannot differentiate between antimatter and matter even if we could detect them.

So, how do we know that distance galaxies are made up of conventional matter and not antimatter? Does anti-hydrogen have a different emission spectrum or something?

On further research, wiki seems to think it might be difficult to tell based on spectral emissions, so are scientists sure, and if so how?

Any antimatter galaxy or cluster of galaxies would have to be extraordinarily isolated from any and all conventional matter. Otherwise, the conversion energy would be visible.

I thought that all theories of universe formation require too much mixing of matter for any segment of the universe to be that isolated from all other matter. You’d have to come up with a scenario that takes an antimatter globule and have it sustain itself for the entire history of the universe without coming into contact with the rest of the universe. Seems doubtful to me.

The beginning of the universe certainly seems to pose a problem, but on the flip side there is also the lack of symmetry problem.

OTOH, there are HUUUUUGE spaces between some galaxy clusters. Do we know for a fact they are populated with hydrogen gas througout the mind-chillingly vast millions upon millions of lonely lightyears? If antimatter predominated in one section of the big bang, and matter in another, they might have been separated by inflation before they could annihilate each other completely. Whereas we already think that matter was mostly annihillated by antimatter.

Well, we know the Universe is expanding, and we know roughly how fast it’s doing that.

So we know that in the past, the matter that makes up distant galaxies was much closer together. If some of it were antimatter, it surely would have crashed into some matter very early on and been converted to energy.

Didn’t the expansion take place while all the particulates in the Universe were either photons, or quarks? Anti-matter and matter happened a bit later, if I remember correctly.


The expansion has always been happening and will continue to do so.

Does anti-matter produce photons, gravity waves, and neutrinos that is indistinguishable from that produced by normal matter?

I’m pretty sure the answer is yes.

Maybe the rest of the universe is matter and we’re the anti-matter… :wink:

I think “the expansion” here refers to the (generally accepted by still hypothetical) inflationary phase of the universe, not the comparatively slow expansion you’re thinking of that’s going on right now.

For the first two yes. Photons and gravitons (the (hypothetical) particles behind gravity waves as photons are behind light waves) are self-conjugate. That is, they are their own antiparticles. Antimatter emits antiphotons exactly like matter emits photons, but antiphotons are the same thing as photons.

Neutrinos, however, are a different story. Neutrinos are not self-conjugate, and (oddly enough) we can tell the difference because (almost?) all of the matter neutrinos spin one way as they fly and (almost?) all of the antimatter ones spin the other way.

The neutrinos produced by antimatter may or may not be distinguishable from those produced by normal matter: That’s currently a matter of some debate in particle physics. But the other two are the same for either, and we’ve yet to detect neutrinos from any further than the Magellanic Clouds, nor gravitational waves from anywhere.

However, even the great intergalactic voids aren’t a perfect vacuum. At the thinnest, you still have somewhere in the vicinity of one hydrogen atom per cubic meter, and galaxies tend to be arranged in a sort of foamy structure, so there would be “bridges” between them of denser matter than that. If you had a galaxy which was matter and one which was antimatter, you’d have to have some sort of transition region between them where it was half and half, and even at densities of one atom (or antiatom) per cubic meter, such a transition region would be detectable.

How about this,

If the inflationary epoch ended with the destruction of all the anti-matter/matter that was in astronomically close distances of each other, and the regions between the isolated regions of one or the other began to experience erosion between them that continued for billions of years, could there be portions of intergalactic space that are now void of matter/anti-matter for that very reason?

While the great regions between most galactic clusters are not void, the convoluted surface equidistant to the nearest objects of opposite type would be vastly less populated after billions of years during which chance collisions totally eliminate the matter/anti-matter particles in that region. The regions closer to either side may retain only slightly diminished populations, but they have no opposites to interact with. In fact, it seems to me, based on an intuitive assessment of geometry, the particles with the highest velocities relative to their same type primaries would have been the first to be destroyed, leaving those most likely to be recaptured by their original cluster behind. So, at some multi billion year period of dynamic equilibrium, you have isolation in distances large enough to make collisions very rare.

I suppose there might be some way to measure the density of the intergalactic medium with precision, but I don’t think it is discriminatory enough to detect a ten or one hundred light year thick region along the appropriate surface for any particular galactic cluster. Not that that proves the existence of antimatter, only that it describes as possible mechanism for concurrent existence on an astronomic scale.


Interesting stuff, Tris! I hope some lurking cosmologist will come along to throw in an opinion.

There’s no obvious reason that photons and gravitons from antimatter should be any different than those particles from matter, but it’s awfully difficult to make and hold more than a few (hundred) anti-hydrogen atoms – much less anything more complicated – and therefore we aren’t really sure.

Experiment is the arbiter of reality (insofar as we can access it at all), so until we have good antimatter measurements in controlled conditions, we can only apply the theoretical framework developed by our investigations into regular old matter and say that it shouldn’t matter: nothing changes but the charge.

On the bright side, Quantum Electrodynamics is a *good *theoretical framework, agreeing with the best measurements we have to one part in 10^14 or something ridiculous. So if we can ever make analogous measurements in antimatter, it will be powerful confirmation.

Triskadekamus, for that to work, you’d have to have not just voids, but two-dimensional expanses of voids, to separate the matter regions from the antimatter regions. And we can measure the density of matter in many different directions and at many different distances (based on absorbtion of light from quasars), and we see no evidence that it’s that thin even in the thinnest regions, much less in the denser bridges which would have to cross the “void walls”. It’s not unreasonable speculation, but it just doesn’t stand up to the observations.

Thanks! I had no idea our ability to measure densities in distant voids was that highly developed.