text: 311,43,1072725398285377541,823277120944242689,Will Kinney,"New paper out today. Fun idea: cosmology where the speed of sound is faster than the speed of light. <LINK> <LINK> @RobJLow You would think, but remarkably enough the answer appears to be no, at least classically. Babichev, Mukhanov, and Vikman wrote a wonderful paper exploring this question about ten tears ago: <LINK> @RobJLow The trick is that these are not tachyons, but a superluminal ether. This means that it it not possible, for example, to build a tachyonic antitelephone. (Or, in less technical terms, to go back in time and kill baby Hilter.) <LINK> @RobJLow Yes, exactly! Theories of this type can always be re-written as equivalent bimetric theories. @RobJLow One of the very intriguing things about these solutions is that at the moment of the Big Bang singularity, the speed of sound diverges, and the effective metric becomes purely spacelike. That is, the 3+1 spacetime is reduced to an effective 3+0 spacetime. @RobJLow In this (somewhat narrow) sense, the time dimension of the spacetime is an emergent property. @RobJLow I'm actually not completely positive whether or not tachyacoustic theories obey the Dominant Energy Condition in all cases. But then cosmologists violate the DEC three times before breakfast these days anyway. It's the Null Energy Condition that's a killer.",https://arxiv.org/abs/1812.04447,"Recent studies show that there is tension between the de Sitter swampland conjectures proposed by Obeid, et al. and inflationary cosmology. In this paper, we consider an alternative to inflation, `tachyacoustic' cosmology, in light of swampland conjectures. In tachyacoustic models, primordial perturbations are generated by a period of superluminal sound speed instead of accelerating expansion. We show that realizations of tachyacoustic Lagrangians can be consistent with the de Sitter swampland conjectures, and therefore can in principle be consistent with a UV-complete theory. We derive a general condition for models with $c_S > 1$ to be consistent with swampland conjectures. ","Consistency of Tachyacoustic Cosmology with de Sitter Swampland