Axion as a Cold Dark Matter Candidate
J. Hwang, H. Noh & C-G Park
Convention:
no tensor-pert.
Spatial gauge condition:
anisotropic stress
Axion as a CDM
Scalar field:
Axion:
PLB 126, 179 (1991) Axenides et al
Axion:
PRD 44, 352 (1991) Ratra
Strictly ignore:
Ansatz:
EOM:
Fluid quantities:⇒
∴ Pressureless fluid!
Axion:
PLB 680, 1 (2009) JH, Noh
Axion-comoving gauge:
Ansatz:
Fluids:
EOM:
Jeans scale: (Khlopov, Malomed, Zeldovich, MN 1985; Nambu, Sasaki PRD 1990)
Sound speed:
Axion (K = 0):
PRD 86, 083535 (2012) Park, JH, Noh; PRL 103, 111301 (2009) Sikivie, Yang in zero-shear gauge
∴ CDM in ALL cosmological scales
Axion (K = 0):
Exact solution in mde with K=0=Lambda:
Large-scale asymptotic solution in mde with K=0=Lambda:
PLB 680, 1 (2009) JH, Noh
To second order
Axion (K = 0):
PLB 726, 559 (2013) JH, Noh
⇒
with
perturbed trace of extrinsic curvature
Low-mass Axion
Axion:
}⇒ Boltzmann eq.Tight coupling
PRD 86, 083535 (2012) Park, JH, Noh
Gij- …
T(a)c0;c
T(b)ci;c
T(b)c0;c
T(r)c0;c
T(r)ci;c
EOM
Axion:
PRD 86, 083535 (2012) Park, JH, Noh
Axion:Constant mass:
Evolving mass:
PRD 86, 083535 (2012) Park, JH, Noh
Low-mass axion as a CDM with SS cut:
PRD 86, 083535 (2012) Park, JH, Noh
Neutrino as a HDM:
PRD 86, 083535 (2012) Park, JH, Noh
Summary- Axion as a CDM, shown to second order pert.- Roles of low-mass axion as a CDM
with small-scale cut off in the power spectrum- Axion mass smaller than 1024 eV induces
a significant damping in the baryonic density power spectrum in cosmologically relevant scales
Future- Third order perturbation and NL power spectra- Axion as a Bose-Einstein condensate- Axion generated as isocurvature perturbation
The 6th KIAS Workshop onCosmology and Structure Formation
Nov. 3 - 7, 2014 KIAS, Seoul
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