We investigate a dark energy scenario in which a canonical scalar field φ is coupled to the four velocity uµc of cold dark matter (CDM) through a derivative interaction uµc ∂µφ. The coupling is described by an interacting Lagrangian f(X, Z), where f depends on X = −∂µφ∂µφ/2 and Z = uµc ∂µφ. We derive stability conditions of linear scalar perturbations for the wavelength deep inside the Hubble radius and show that the effective CDM sound speed is close to 0 as in the standard uncoupled case, while the scalar-field propagation speed is affected by the interacting term f. Under a quasi-static approximation, we also obtain a general expression of the effective gravitational coupling felt by the CDM perturbation. We study the late-time cosmological dynamics for the coupling f ∝ X(2−n)/2Zn and show that the gravitational coupling weaker than the Newton constant can be naturally realized for n > 0 on scales relevant to the growth of large-scale structures. This allows the possibility for alleviating the tension of σ8 between low- and high-redshift measurements.
|Publication status||Published - 2019 Nov 5|
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