Abstract
Assuming that the universe contains a dark energy fluid with a constant linear equation of state and a constant sound speed, we study the prospects of detecting dark energy perturbations using CMB data from Planck, cross-correlated with galaxy distribution maps from a survey like LSST. We update previous estimates by carrying a full exploration of the mock data likelihood for key fiducial models. We find that it will only be possible to exclude values of the sound speed very close to zero, while Planck data alone is not powerful enough for achieving any detection, even with lensing extraction. We also discuss the issue of initial conditions for dark energy perturbations in the radiation and matter epochs, generalizing the usual adiabatic conditions to include the sound speed effect. However, for most purposes, the existence of attractor solutions renders the perturbation evolution nearly independent of these initial conditions.
Highlights
The biggest problem in cosmology today is the understanding of the accelerated expansion of the universe
We focus on an effective description which has already been studied by several authors: namely, a dark energy fluid with a linear equation of state p = wρ, a constant equation of state parameter w close to −1 and a constant sound speed defined in the range 0 ≤ c2s ≤ 1
We present for the first time the initial conditions fulfilled by the dark energy fluid in the synchronous gauge, when all other fluids have adiabatic primordial perturbations
Summary
The biggest problem in cosmology today is the understanding of the accelerated expansion of the universe. Other candidates (which may or may not alleviate the fine–tuning and coincidence problems of the cosmological constant) include, for instance, scalar field models, or effective descriptions in terms of a fluid with free parameters yet to be measured. In an effective fluid description, the sound speed is a free parameter, and dark energy clustering can be more efficient in the limit in which overdensities are not balanced by local pressure perturbations (cs → 0). The study of small perturbations could be used as a tool for discriminating between various models with a negative pressure component (cosmological constant, dark energy fluid, quintessence or k–essence fields, coupled dark energy, etc.) or a modified theory of gravity.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
