# K-essence and its observational imprints

## Summary

Scalar fields with a non-canonical kinetic energy (k-essence), became a
popular candidate for explaining dark energy and solving the cosmic
coincidence problem. K-essence has very peculiar properties concerning the
behavior of its equation of state and clustering. Therefore it could in
principle serve as a clustered dark energy component. In turn this can lead
to a distinctive features in the CMB power spectrum and influence the large
scale structure of the universe. In this project we will work out quantitative observational consequences of k-essence models. This should lead to constraints allowing to restrict the class of models. In particular, the speed of sound which parametrizes different k-essence models can act as a strong discriminator for dark energy.
The second part of the project concerns the microscopic justification of k-essence models. Although k-essence type lagrangians appear rather naturally in
low energy effective actions, their relevance for cosmological applications depends on their detailed structure which
still has to be clarified. The purpose of this part of the project is to obtain
concrete realizations of the cosmological k-essence in fundamental particle
theories and string theory in particular.
A class of stringy phenomena involving scalar fields with higher derivative terms and which is under reasonable control is the decay of unstable D-branes. These processes involve a small number of free parameters and therefore lead concrete predictions which can the be compared with the observational consequences obtained in the first part of the project.

### Role within the Transregional Collaborative Research Centre

A close interaction with the projects A1, A2, A3, B1, C3 and C4 is expected. We
will make predictions that can be tested in B3 and B4.

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