TY - JOUR

T1 - Scalar-field dark energy nonminimally and kinetically coupled to dark matter

AU - Kase, Ryotaro

AU - Tsujikawa, Shinji

N1 - Funding Information:
R. K. is supported by the Grant-in-Aid for Young Scientists B of the JSPS No. 17K14297. S. T. is supported by the Grant-in-Aid for Scientific Research Fund of the JSPS No. 19K03854 and MEXT KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas “Cosmic Acceleration” (No. 15H05890).

PY - 2020/3/15

Y1 - 2020/3/15

N2 - We provide a general framework for studying the dark energy cosmology in which a scalar field φ is nonminimally and kinetically coupled to cold dark matter (CDM). The scalar-graviton sector is described by the action of Horndeski theories with the speed of gravitational waves equivalent to that of light, whereas CDM is treated as a perfect fluid given by a Schutz-Sorkin action. We consider two interacting Lagrangians of the forms f1(φ,X)ρc(nc) and f2(nc,φ,X)Jcμ∂μφ, where X=-∂μφ∂μφ/2, ρc and nc are the energy density and number density of CDM respectively, and Jcμ is a vector field related to the CDM four velocity. We derive the scalar perturbation equations of motion without choosing any special gauges and identify conditions for the absence of ghosts and Laplacian instabilities on scales deep inside the sound horizon. Applying a quasistatic approximation in a gauge-invariant manner, we also obtain the effective gravitational couplings felt by CDM and baryons for the modes relevant to the linear growth of large-scale structures. In particular, the nc dependence in the coupling f2 gives rise to an interesting possibility for realizing the gravitational coupling with CDM weaker than the Newton gravitational constant G.

AB - We provide a general framework for studying the dark energy cosmology in which a scalar field φ is nonminimally and kinetically coupled to cold dark matter (CDM). The scalar-graviton sector is described by the action of Horndeski theories with the speed of gravitational waves equivalent to that of light, whereas CDM is treated as a perfect fluid given by a Schutz-Sorkin action. We consider two interacting Lagrangians of the forms f1(φ,X)ρc(nc) and f2(nc,φ,X)Jcμ∂μφ, where X=-∂μφ∂μφ/2, ρc and nc are the energy density and number density of CDM respectively, and Jcμ is a vector field related to the CDM four velocity. We derive the scalar perturbation equations of motion without choosing any special gauges and identify conditions for the absence of ghosts and Laplacian instabilities on scales deep inside the sound horizon. Applying a quasistatic approximation in a gauge-invariant manner, we also obtain the effective gravitational couplings felt by CDM and baryons for the modes relevant to the linear growth of large-scale structures. In particular, the nc dependence in the coupling f2 gives rise to an interesting possibility for realizing the gravitational coupling with CDM weaker than the Newton gravitational constant G.

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U2 - 10.1103/PhysRevD.101.063511

DO - 10.1103/PhysRevD.101.063511

M3 - Article

AN - SCOPUS:85082747921

VL - 101

JO - Physical Review D

JF - Physical Review D

SN - 2470-0010

IS - 6

M1 - 063511

ER -