Cosmological Instabilities and the Role of Matter Interactions in Dynamical Dark Energy Models

Abstract

We consider cosmological models with a dynamical dark energy field, and study the presence of three types of commonly found instabilities, namely ghost (when fields have negative kinetic energy), gradient (negative momentum squared) and tachyon (negative mass squared). In particular, we study the linear scalar perturbations of theories with two interacting scalar fields as a proxy for a dark energy and matter fields, and explicitly show how canonical transformations relate these three types of instabilities with each other. We generically show that low-energy ghosts are equivalent to tachyonic instabilities, and that high-energy ghosts are equivalent to gradient instabilities. Via examples we make evident the fact that whenever one of these fields exhibits an instability then the entire physical system becomes unstable, with an unbounded Hamiltonian. Finally, we discuss the role of interactions between the two fields, and show that whereas most of the time interactions will not determine whether an instability is present or not, they may affect the timescale of the instability. We also find exceptional cases in which the two fields are ghosts and hence the physical system is seemingly unstable, but the presence of interactions actually lead to stable solutions. These results are very important for assessing the viability of dark energy models that may exhibit ghost, gradient or tachyonic modes.

11 pages. Updated to match published version

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Cited by in corpus (8)

• Matching current observational constraints with nonminimally coupled dark energy
• Robustness of dark energy phenomenology across different parameterizations
• The Hubble Tension, The $M$ Crisis of Late Time $H(z)$ Deformation Models and the Reconstruction of Quintessence Lagrangians
• Methodological Reflections on the MOND/Dark Matter Debate
• Foundations of ghost stability
• Cosmological Inflation and Meta-Empirical Theory Assessment
• On tachyonic stability priors for dark energy
• Some Classes of Interacting Two-Fluid Model of the Expanding Universe