Cosmological tachyon condensation

Abstract

We consider the prospects for dark matter/energy unification in k-essence type cosmologies. General mappings are established between the k-essence scalar field, the hydrodynamic and braneworld descriptions. We develop an extension of the general relativistic dust model that incorporates the effects of both pressure and the associated acoustic horizon. Applying this to a tachyon model, we show that this inhomogeneous "variable Chaplygin gas" does evolve into a mixed system containing cold dark matter like gravitational condensate in significant quantities. Our methods can be applied to any dark energy model, as well as to mixtures of dark energy and traditional dark matter.

AdS braneworld with backreaction

“…At the linear level one expects a suppression of small-scale structure formation: initially growing modes undergo damped oscillations once they enter the co-moving acoustic horizon. Perturbation theory is not the whole story it would be worth studying the nonlinear effects, e.g., using the Press-Schechter formalism as in the pure tachyon model of [33].…”

“…Our fluid is a two component system with = 1 + 2 , ρ = ρ 1 + ρ 2 . As in [33] we start from the standard thermodynamic relation for each component = 1 2…”

“…The derivation of the so called adiabatic speed of sound follows the procedure described in Appendix A of [33] generalized to two dynamical fields Φ and Θ. Consider a Lagrangian of the form…”

Metric-Independent Spacetime Volume-Forms and Dark Energy/Dark Matter Unification

“…Since more than a decade a principal challenge in modern cosmology is to understand theoretically from first principles the nature of both "dark" species of the universe's substance as a manifestation of the dynamics of a single entity of matter. Among the multitude of approaches to this seminal problem proposed so far are the (generalized) "Chaplygin gas" models [10]- [13], the "purely kinetic k-essence" models [18]- [21] based on the class of kinetic "quintessence" models [14]- [17], and more recently -the so called "mimetic" dark matter model [22,23] and its extensions [24,25], as well as constant-pressure-ansatz models [26].…”

Constraints on the dark matter sound speed from galactic scales: The cases of the modified and extended Chaplygin gas

“…Hence, ΛCDM may be regarded as the simplest Unified Dark Energy (UDE) model, in which DM and DE are taken as different manifestations * Electronic address: pedro.avelino@astro.up.pt † Electronic address: vascoferreira1@gmail.com of a single unified DE fluid [14]. A broad class of UDE models, known as the Generalized Chaplygin Gas (GCG) [15], includes the original Chaplygin Gas [16,17] and ΛCDM as particular models. The GCG has been claimed to be essentially ruled out due to the late time oscillations or the exponential blow up of the DM power spectrum predicted using linear perturbation theory [18] (except for a tiny region of parameter space very close to the ΛCDM limit).…”