Cosmological perturbations in a generalised axion-like dark energy model

Understand

We analyse the cosmological evolution of a generalised axion-like field that drives the late-time acceleration of the Universe.

• This model can exhibit tracking behaviour, which alleviates the coincidence problem.
• The cosmological perturbations are carried within a multi-fluid approach where the scalar field is described by a non-adiabatic fluid, i.e., one whose speed of sound in the rest frame differs from the adiabatic one.
• The cosmological perturbations are solved since the radiation-dominated epoch, imposing initial adiabatic conditions for matter, radiation and the dark energy component, for modes well outside the Hubble horizon in the past.