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The actual nature of the gravitational force between matter and anti-matter (attractive or repulsive) remains unsettled: See Are there experiments taking place right now that might show evidence for or falsify dark energy or dark matter? Assume for the sake of argument that the above cited experiment (ALPHA) (in my answer) reveals gravitational repulsion (GR) between matter and anti-matter (with the same coupling g that pertains for attraction). My questions are:

1) Could this repulsion between matter and anti-matter go all the way back to the Big Bang and provide the RG needed for the Inflationary Epoch?

2) If the answer to 1) is yes, Could the effects of RG be used to modify the remaining Big Bang time line to yield a Universe similar to what we observe today (with a dark matter web separated by voids and matter super-clusters strung out like beads on a string)? Note that an anti-matter super-cluster bound by gravity would have exactly the same electromagnetic spectral characteristics as a normal matter super-cluster and would have the same association with dark matter if the dark matter particle were its own anti-particle and were attracted equally by both normal matter and anti-matter via gravitation.

3) The equivalence principle (EP) of general relativity would not survive. Could a revised classical theory like general relativity with a reformulated EP be possible, or would a quantum field theory of general relativity be required? (Note, this has already been addressed in a comment by @CuriousOne)

4) Could this remove the need to modify the standard model to include baryon asymmetry (ie could the observed asymmetry be an artifact of the large distances between super-clusters)? We have increased the estimated size of our own super-cluster significantly in the very recent past and RG (as outlined here) could mean very little anti-matter in the local cosmic ray spectrum.

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  • $\begingroup$ None of what you call "generally accepted" is anything but speculation at the moment. Whether gravity between matter and antimatter is attractive is being tested at CERN, right now, but it wouldn't have any immediate consequences for your question, since a low energy, low density gravity experiment can't tell us anything about the early universe. $\endgroup$ – CuriousOne Jan 21 '16 at 3:44
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    $\begingroup$ @N.S.JOHN because the gravitational coupling constant is so small at the level of elementary particles with respect to all the other forces. hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html . Here is a question from four years ago physics.stackexchange.com/questions/5521/… $\endgroup$ – anna v Jan 21 '16 at 4:22
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    $\begingroup$ @N.S.JOHN: Because it is already very hard to make neutral antimatter. To make fast positrons is easy. To make room temperature positrons is a lot harder. To make room temperature antiprotons is a lot lot harder and to combine the two into a resting anti-hydrogen is really, really tough. Now start from the other end and try to make a precision atomic hydrogen gravity experiment... that in itself is very hard... and now "all" you have to do is to combine really, really tough with very hard and you know what the CERN folks are up against. $\endgroup$ – CuriousOne Jan 21 '16 at 4:26
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    $\begingroup$ Here is a recent proposal for testing antihydrogen gravity physics.purdue.edu/~robichf/papers/prl112.121102.pdf $\endgroup$ – anna v Jan 21 '16 at 4:31
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    $\begingroup$ Related question: physics.stackexchange.com/q/83378 $\endgroup$ – Mitchell Porter Jan 21 '16 at 23:17
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It's hard to see how gravitational repulsion between matter and antimatter would do any of those things, (1) because gravity is weak, and (2) because matter and antimatter are intermixed in the early universe, so the matter-antimatter repulsion would be competing with matter-matter attraction and antimatter-antimatter attraction.

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    $\begingroup$ The repulsion between matter and anti-matter could be much stronger than the attraction between matter and itself and anti-matter and itself. If you are breaking the laws of nature in a small way, why not go all the way and break them in a big way? If we forget about this being a cosmological hypothesis for a moment, it might actually be quite interesting to understand how such a two-phase system of liquids may actually behave while it separates. Would it, for instance, form string-like structures? $\endgroup$ – CuriousOne Jan 21 '16 at 6:34
  • $\begingroup$ @CuriousOne "Would it, for instance, form string-like structutes?" Precisely. As for breaking physical laws, the only one I am aware of is the Equivalence Principle. I doubt Einstein would have had any difficulty in reformulating it to accommodate RG had he known of that possibility. $\endgroup$ – Lewis Miller Jan 21 '16 at 16:11
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    $\begingroup$ @LewisMiller: Well, if you break the equivalence principle, then RG is out the window, entirely, and not just a little. You can only build a geometric theory if all matter behaves exactly the same, which is the only case that would allow us to "blame geometry" rather than consider a classical force type interaction. Your assumption that Einstein could have built it differently is therefor wrong. Having said that, there is nothing sacrosanct about Einstein's theories. If nature doesn't agree with them, then they get tossed out. At this moment there is no evidence for that, though. $\endgroup$ – CuriousOne Jan 21 '16 at 18:25
  • $\begingroup$ @CuriousOne I'm no expert in GR so I'll accept your word for it. But if ALPHA finds RG, what then? That in a nutshell is what my question is about. $\endgroup$ – Lewis Miller Jan 21 '16 at 18:53
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    $\begingroup$ @LewisMiller: Then GR is a dead theory and not just a little. Having said that, I don't know if they can find a strong violation of the equivalence principle without disagreeing with already established precision measurements of the equivalence principle. While it is customary to see "normal matter" as being made entirely of "normal matter", that's not the case. There is always an admixture of anti-matter, especially in the nucleus which contains a non-trivial density of (virtual) anti-quarks. This sets limits on the effect you are proposing, unless we change more than one sector of physics. $\endgroup$ – CuriousOne Jan 21 '16 at 19:07

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