How would we tell antimatter galaxies apart?

Given that antimatter galaxies are theoretically possible, how would they be distinguishable from regular matter galaxies?

That is, antimatter is equal in atomic weight and all properties, except for the opposite reverse charge of the particles, identical to regular matter. Hence a star composed of antimatter hydrogen would fuse to anti-helium in an analogous way to our own Sun, and it would emit light and radiation at the same wavelengths as any regular matter star and would cause the same gravitational forces for planetary systems to form as in any other star system.

Hence, what would be a telltale sign if you were observing a galaxy made up entirely of antimatter?

Also, is there any evidence for that half of all galaxies are not made of antimatter -- while general theories currently assume that there is an imbalance of matter over antimatter in the universe, then what is the rationale for not assuming that there is in fact an even balance between the two?

• An antigalaxy could have negative gravitational mass in theory - leading the question whether that has been modelled/simulated somewhere Nov 4 '13 at 15:08
• @TobiasKienzler You're mistaking antimatter and negative matter. Anti-matter, such as positrons is matter with opposite charge. Positrons annihilate neutrons on impact. Negative matter, on the other site, would be opposite of matter - with opposite gravity. It would annihilate matter without leaving anything behind. Nov 9 '15 at 15:10
• @TomášZato Positrons annihilate electrons, not neutrons... And an anti-neutron would still be electrically neutral but distinct from a neutron which it would annihilate with. And my proposal was rather hypothetical, though so far there hasn't been enough antimatter produced in labs to actually determine its gravitational weight's sign. Nov 9 '15 at 15:17
• @TobiasKienzler yeah, I meant electrons. Here's some reading on the difference of anti-matter (which is real) and negative matter (which is probably just sci-fi): askamathematician.com/2012/02/… Nov 9 '15 at 15:22

You're right - for isolated galaxies, there is no obvious way of discerning whether they are made of matter or antimatter, since we only observe the light from them. But if there are regions of matter and antimatter in the universe, we would expect to see HUGE amounts of radiation from annihilation at the edges of these regions. But we don't. You could also make the case that galaxies are well-separated in space, and there's not much interaction between them. But there are plenty of observed galaxy collisions even in our own small region of the universe, and even annihilation between dust and antidust in the intergalactic medium would (probably) be observable.

• The intergalactic medium is sparse by any terrestrial definition of matter, but given the large volume of it, there's a lot of it. Like Ben said, if there were any region of space dominated by anti-matter, the IGM would have to switch "polarity" as well. The boundary or transition area between matter and anti-matter regions would produce spectaaaaaaaaacular fireworks, which we just don't see anywhere in our visible universe. Jul 11 '11 at 14:44
• @Rogue We have yet to measure it directly, but every theory says that anti-Hydrogen should have indistinguishable spectral emissions and absorptions to ordinary Hydrogen (or for any other element/anti-element comparison, for that matter). Jul 12 '11 at 4:18
• Maybe much of the intergalactic dust in these transition regions has already annihilated, and the space between the matter/antimatter regions is getting larger due to the expansion of the universe. Jul 12 '11 at 6:01
• @TobiasKienzler: there are simple thought experiments that would let you build a perpetual motion machine if antimatter antigravitates, so we don't expect this to be the case. Another piece of evidence in favor of antimatter gravitating normally is that different nuclei have different parton functions, but they all gravitate identically. Sep 9 '14 at 20:06
• @dan Yes for solid matter, maybe yes for collisions between stars, and maybe yes for gases at atmospheric pressures, At the density of the intergalactic medium, though, it wouldn't happen, because the gammas from each annihilation would have very low odds of hitting anything else nearby. I'm not sure exactly where the cutoff would be; it might be worth asking a new question about that.
– zwol
Oct 28 '20 at 18:49

Let's assume that the anti-matter galaxy is well isolated from galaxies consisting of ordinary matter (you could assume that at the boundary the annihilation reactions would have proceeded very fast and matter and anti-matter don't come into contact at the time we see the anti-matter galaxy). Then the telltale sign would come from supernova neutrinos, although at present we can only detect such neutrinos from nearby galaxies.

When the gravitational collapse happens, the electrons of Iron atoms are pushed into the Iron nuclei and all the protons get converted to neutrons leading to an immediate burst of electron neutrino emissions.. In case of a star made out of anti-matter you would have a burst of electron anti-neutrino emissions, and this leads to a different detection signal in detectors on Earth. There are other processes that give rise to both neutrinos and anti-neutrinos, but the entire process is not symmetrical w.r.t. interchanging matter with anti-matter.

• That's generally the answer (other than the more observationally significant comment about the IGM). You have to observe some sort of process that's mediated by the weak force, since this is the lowest-energy force that violates C symmetry. Sep 9 '14 at 20:05

I concur with Count Iblis's answer that if we would detect an uptick in anti-neutrinos when a supernova occurs within said galaxy.

To understand this, let's take the case of what happens in our familiar 'matter' supernova. When the unfusable core of a star becomes so dense that the inward gravity pressure exceeds the outward electron degeneracy pressure, the whole core goes from normal density to neutron density in one second. To put this in perspective imagine the mass of the earth being compressed to a sphere no larger than Manhattan and this happens in one second. This is because when electron degeneracy pressure is breached, the electrons of all the atoms leave their quantum orbitals and go into the nucleus of the atoms where they get absorbed by the protons and become neutrons and emit neutrinos.

$$e^- + p \rightarrow n + \nu$$

An enormous amount of neutrinos are released in this way. The core contains no more individual atoms, just a super dense solid mass of neutrons. The sudden collapse of the core causes corresponding implosion of the entire star. The outgoing neutrinos collide with the implosion front and all exothermically fusible elements fuse all at once causing the supernova explosion. And indeed scientists have detected dramatic upticks in neutrino detection during the 1987 supernova.

For an anti matter supernova we have :

$$e^+ + \bar p \rightarrow \bar n + \bar\nu$$

where $$e^+$$ is positron, $$\bar p$$ is anti-proton, $$\bar n$$ is anti-neutron, $$\bar\nu$$ is anti-neutrino.

So if we see a supernova in a galaxy and detect an uptick in anti-neutrinos instead of neutrinos, that is how we would know the supernova is within an anti-matter galaxy. You may also notice a surge in anti-matter cosmic rays,ie positrons and anti-protons but these would come later due to speed difference.