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