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A new nonlinear instability for scalar fields

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arxiv 2107.14215 v2 pith:X2A5T7OI submitted 2021-07-29 astro-ph.CO cs.NAgr-qcmath.NA

A new nonlinear instability for scalar fields

classification astro-ph.CO cs.NAgr-qcmath.NA
keywords instabilitypartialappearbodycodecontextcosmologicaldark
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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In this letter we introduce the non-linear partial differential equation (PDE) $\partial^2_{\tau} \pi \propto (\vec\nabla \pi)^2$ showing a new type of instability. Such equations appear in the effective field theory (EFT) of dark energy for the $k$-essence model as well as in many other theories based on the EFT formalism. We demonstrate the occurrence of instability in the cosmological context using a relativistic $N$-body code, and we study it mathematically in 3+1 dimensions within spherical symmetry. We show that this term dominates for the low speed of sound limit where some important linear terms are suppressed.

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