We consider solutions to the Einstein-massless-scalar field system with a positive cosmological constant, arising from sufficiently regular, near-FLRW (Friedmann-Lemaître-Robertson-Walker), initial data. We establish global existence in the future direction and derive their precise asymptotic behavior toward infinity. As a corollary, we infer that, unlike the FLRW background, the perturbed solutions do not describe a regular irrotational stiff fluid with a linear equation of state p = ρ for general asymptotic data at infinity. The reason for the breakdown of this interpretation is that the gradient of the scalar field stops being timelike at large times, eventually becoming null and then spacelike near infinity. Our results hold for open sets of initial data in Sobolev spaces without symmetries.

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50.

For simplicity, we an abuse notation by not referring to this identification of the initial data through the embedding in the rest of the paper.

51.

Compare (29) in Ref. 35, Theorem 2 with (1.16) and (1.17) in Theorem 1.2. The sharp decay for the variables ψψ, e0ψ in our model is e−2Ht, whereas Ref. 35, Theorem 2 gives eαHt decay for some α > 0. See also the asymptotic expansion (1.23) in Theorem 1.3.

52.

See, for example, Christodoulou’s breakthrough work10 on the formation of shocks for the relativistic Euler equations in Minkowski spacetime.

53.

Fluid stabilization, as an effect of an accelerated expansion, was first rigorously established by Brauer, Rendall, and Reula8 for Newtonian cosmological models.

54.

This is a parameter used in galaxy formation26 to indicate local enhancements in matter density. In the context of Ref. 33, it is equal to ∇aρ/ρ, where ∇a stands a spatial derivative.

55.

By virtue of the Sobolev inequality (4.1), the regularity class HNt), N ≥ 4, implies C0 control of up to two derivatives of the unknowns, which is more than enough for local existence of quasi-linear hyperbolic systems.

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