Chaplygin gas and other k-essence models exhibit emergent geometry, with perturbations propagating on an acoustic metric disformally related to the Einstein-frame metric. For superluminal sound speed, we identify the disformal metric as the “causal frame,” since choosing a finite causal-frame lapse yields hyperbolic equations of motion for fields propagating in either frame. We show that, with a phantom Chaplygin gas, the Einstein-frame lapse is forced to pass smoothly through zero and change sign, while the causal-frame lapse remains positive. As a result, Einstein-frame degrees of freedom (including the scale factor) undergo spontaneous time reversal, while the Chaplygin gas evolves monotonically, enforcing a robust nonsingular bounce even in the presence of additional matter canonically coupled to the Einstein frame.

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• Alternative gravity theories
• Cosmology
• Extensions of scalar sector
• General relativity