In the comments to another question's answer, I started wondering:

Assuming antimatter possessed negative gravitational mass§ (which is not proven impossible to date, though deemed unlikely), basically entire galaxies consisting of antimatter could form. Due to the repulsion of ordinary matter, they would rarely collide with usual ("pro-matter-")galaxies% and actually drift apart, maybe even partly contributing to inflation. But I don't want to speculate too much, so my questions is

Have entire galaxies consisting of negative-gravitational-mass antimatter been considered in any sound theory, model or simulation?

§ I.e. while such antimatter gravitationally repulsed conventional matter, it would still attract other antimatter.

% Unless there exists e.g. a (currently unknown) force that attracts antiparticles to their exact counterparts

  • $\begingroup$ Do you have ANY experimental evidence of ANY anti-gravitation observations or anti-matter galaxies ? Isn't this a science fiction question ? $\endgroup$
    – user26165
    Nov 6 '13 at 1:09
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    $\begingroup$ @GeorgeE.Smith Do you have ANY evidence of ANY "pro"-gravitation observations of antimatter? ;) I only rule out the impossible, not the improbable. Whether you consider that SciFi, specualtion, or a model (hey, have you seen any strings or superparticles so far?) is up to you, but it has not succinctly been proven impossible so far, so I do at least want to consider the possiblity $\endgroup$ Nov 6 '13 at 7:21
  • $\begingroup$ I'd appreciate the downvoter's feedback on what makes this a bad question - if you merely disagree with they notion of antigravity, please feel free to post an answer instead $\endgroup$ Sep 11 '14 at 5:57

Short answer: that depends on your definition of sound theory. For instance, it is possible to find peer-reviewed papers considering such possibilities.

The idea that antimatter can be gravitationally repulsed from ordinary matter is definitely not the most popular one. Nevertheless, some people do try to apply it in astrophysical context. Let us have a look at the works of M. Villata:

Villata, M. "CPT symmetry and antimatter gravity in general relativity." EPL (Europhysics Letters) 94.2 (2011): 20001. arxiv:1103.4937.

This one attempts to establish 'why' we can expect such repulsion. Subsequent works develop the cosmological model resulting from such repulsion. The latest one is:

Villata, M. "On the nature of dark energy: the lattice Universe." Astrophysics and Space Science (2013): 1-9. arxiv:1302.3515.

From the abstract of that one:

... Here we start from the recent theoretical results that come from the extension of general relativity to antimatter, through CPT symmetry. This theory predicts a mutual gravitational repulsion between matter and antimatter. Our basic assumption is that the Universe contains equal amounts of matter and antimatter, with antimatter possibly located in cosmic voids, as discussed in previous works. From this scenario we develop a simple cosmological model, from whose equations we derive the first results. While the existence of the elusive dark energy is completely replaced by gravitational repulsion, the presence of dark matter is not excluded, but not strictly required, as most of the related phenomena can also be ascribed to repulsive-gravity effects. With a matter energy density ranging from ∼5 % (baryonic matter alone, and as much antimatter) to ∼25 % of the so-called critical density, the present age of the Universe varies between about 13 and 15 Gyr. The SN Ia test is successfully passed, with residuals comparable with those of the ΛCDM model in the observed redshift range, but with a clear prediction for fainter SNe at higher z.

For the criticism of this approach from the theoretical grounds see:

Cross, Daniel J. "Response to" CPT symmetry and antimatter gravity in general relativity"." arXiv:1108.5117.

which states that

... This repulsion or anti-gravity is derived by applying the CPT theorem to general relativity. We show that this proposal cannot work for two reasons: 1) it incorrectly predicts the behavior of photons and 2) the CPT transformation itself is not consistently applied.

Another one rebuttal:

Cabbolet, Marcoen JTF. "Comment to a paper of M. Villata on antigravity." Astrophysics and Space Science 337.1 (2012): 5-7. arxiv:1108.4543

For experimental constraints on such antimatter hypothesis see

Ting, Yuan-Sen. "Experimental constraints on anti-gravity and antimatter, in the context of dark energy." arXiv:1310.6089.

Its abstract:

In a paper by Villata (2011), the possibility of a repulsive gravitational interaction between antimatter and ordinary matter was discussed. The author argued that this anti-gravity can be regarded as a prediction of general relativity, under the assumption of CPT symmetry. Stringent experimental constraints have been established against such a suggestion. The measurement of free-fall accelerations of various nuclei by the Eot-Wash group and searches for equivalence principle violation through the gravitational splitting in kaon physics consistently establish null results on any difference between the gravitational behaviour of antimatter and ordinary matter. The original arguments against antigravity were questioned by Nieto & Goldman (1991). In the light of new experiments as well as theoretical developments in the past 20 years, some of Nieto & Goldman's concerns have been addressed. While a precise measurement of the free-fall acceleration of antihydrogen will eventually lay this issue to rest, the purpose of this short letter is to argue that the substitution of dark energy with anti-gravity by antimatter, as suggested by Villata, is highly unlikely.

  • $\begingroup$ Thanks for your exhaustive answer! So in short, aside from arXiv publications (which are fine, but still not as peer-reviewed as one of "The Big Journals") with very theoretical considerations, not much has happened yet and is unlikely to happen until enough antimatter has been produced on Earth to "once and for all" settle the question of antigravitating antimatter... $\endgroup$ Nov 6 '13 at 7:25
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    $\begingroup$ @TobiasKienzler it's not that arXiv is not "as peer reviewed as a big journal". It's not peer reviewed at all. They have some minimal standards about what can be posted (e.g. can't plagiarize), but there is no peer review process. $\endgroup$
    – Kyle Oman
    Sep 10 '14 at 16:20

Astrophysicists have been looking at electron positron annihilations in the cosmos

electron postiron annihilation galaxy

The Universe viewed trough INTEGRAL: the first complete map of the sky at the electron-positron annihilation energy (Credits J. Knödlseder - CESR - September 2005).

If there existed regions in the sky where antimatter was aggregating, the interface between matter and antimatter areas should be shining with electron positron and other annihilation. Antiproton proton have been considered. Antiproton annihilation in the galaxy have also been considered in explaining the gamma spectrum from the galaxy .

In the space between galactic clusters if one of them were of antimatter, the interface would be a diffuse source of gammas from electron positron and antiproton annihilations to $\pi_0$ ( 1/3 of particles into which antiproton proton annihilate).

I do not know if a specific experiment has been looking for this but we would have certainly heard about something so exciting if it had been seen.

  • $\begingroup$ That's a good point. Though I wonder if there would have to exist an actual interface or whether the antigravitational repulsion might suffice to obtain truly isolated (anti)galaxy(cluster)s with lots of vacuum around. The interface would, after all, have long annihilated... $\endgroup$ Sep 11 '14 at 10:05
  • $\begingroup$ well, within the BB model as it stands the separation would have happened after the quark gluon electrons plasma had coagulated to protons, in the normal history. If antimatter could coagulate it should happen then before the microsecond. en.wikipedia.org/wiki/File:History_of_the_Universe.svg . If we judge from gravitational attraction (weak otherwise galaxies would never appear separate) and the size of inhomogeneities, repulsion would also not be so strong . $\endgroup$
    – anna v
    Sep 11 '14 at 10:55

There are strong constraints on antigravitating antimatter, because it could, in principle, be used, to create a perpetual motion machine.

1) Use energy $E$ to create a particle/antiparticle pair at height $h_{i}$

2) Raise the particle/antiparticle pair to a height $h_{f}$. This takes zero work, because the antiparticle will be pushed up in the potential and the particle will be pushed down, and the forces will be equal, since they are a particle/antiparticle pair.

3) annhilate the particle/antiparticle pair, getting the energy $E$ back.

4) direct the resultant $N$ photons downward to the start point. During the trip down, they will be blueshifted by a factor $\sqrt{\frac{h_{i}\left(c^{2}h_{f}-2GM\right)}{h_{f}\left(c^{2}h_{i}-2GM\right)}}$

5) Now, you have photons that have $N\hbar f_{0}\left(\sqrt{\frac{h_{i}\left(c^{2}h_{f}-2GM\right)}{h_{f}\left(c^{2}h_{i}-2GM\right)}}-1\right)$ more energy than when you started, and you can use the remnant to restart the cycle.

  • $\begingroup$ Why does step 2 require zero work? If I understand it correectly the anti-gravity-particle will automatically go up, because of its anti-gravity nature. But why would the regular particle do the same? IMO it would fall down, seperating the two particles and not allowing them to annihilate in step 3. What do I misunderstand? $\endgroup$
    – user17574
    Sep 10 '14 at 15:20
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    $\begingroup$ @user17574: You're not letting them spontaneously do this. You're using a machine to transfer the energy gain from the anntiparticle's "fall" into the ascent of the particle. This is identically zero because the antiparticle would lose potential energy $mgh$, but the particle wants to fall down, so moving it up that same height would cost potential energy $mgh$, so the cost of rising the particle is exactly equal to the gain you get from letting the antiparticle fall up. $\endgroup$ Sep 10 '14 at 15:27
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    $\begingroup$ @user17574: perhaps an easier way to conceptualize this is to think of the particle-antiparticle pair, <i>when considered as a closed system</i>, would have zero weight. Gravity would be able to move one or the other, but you can freely move the center of mass of the system as much as you want. $\endgroup$ Sep 10 '14 at 15:37
  • $\begingroup$ Intriguing - though my gut feeling is something is missing: 1. conservation of momentum requires a third particle being present, the involvement of which will probably use up the gained energy to return to the original state, and 2. can one actually direct the photons? $\endgroup$ Sep 11 '14 at 3:50
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    $\begingroup$ @TobiasKienzler: that's actually the point. Their gravitational potential energy would have the opposite sign to the particles, so moving the antiparticle from point A to point B would give you an exactly opposite amount of energy from moving the particle from point a to point B. $\endgroup$ Oct 27 '14 at 1:12

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