Recent progress seems to suggest that one must modify general relativity (GR) to stably violate the null energy condition and avoid the cosmological singularity. However, with higher-order derivative operators of the scalar field (a subclass of the degenerate higher-order scalar-tensor theory), we show that at energies well below the Planck scale, fully stable nonsingular cosmologies can actually be implemented within GR.
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Ye et al. (2019) studied this question.
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