Gravity and global symmetries.

@article{Kallosh1995GravityAG,
  title={Gravity and global symmetries.},
  author={Kallosh and Linde and Susskind},
  journal={Physical review. D, Particles and fields},
  year={1995},
  volume={52 2},
  pages={
          912-935
        }
}
There exists a widely held notion that gravitational effects can strongly violate global symmetries. If this is correct, it may lead to many important consequences. We argue, in particular, that nonperturbative gravitational effects in the axion theory lead to a strong violation of {ital CP} invariance unless they are suppressed by an extremely small factor {ital g}{approx_lt}10{sup {minus}82}. One could hope that this problem disappears if one represents the global symmetry of a pseudoscalar… 
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