DBI-essence

TL;DR

This paper explores scalar field models with non-standard Dirac-Born-Infeld kinetic terms for dark energy, analyzing their attractor solutions, potential shapes, and observational viability, while discussing challenges and possible improvements.

Contribution

It provides a detailed analysis of DBI scalar field models for dark energy, including their attractor solutions, potential forms, and observational constraints, highlighting both advantages and shortcomings.

Findings

Scaling solutions are attractors in DBI models.

Potential shapes are exponential or power-law, similar to standard cases.

Large Lorentz factors at present can lead to small field values compared to Planck mass.

Abstract

Models where the dark energy is a scalar field with a non-standard Dirac-Born-Infeld (DBI) kinetic term are investigated. Scaling solutions are studied and proven to be attractors. The corresponding shape of the brane tension and of the potential is also determined and found to be, as in the standard case, either exponentials or power-law of the DBI field. In these scenarios, in contrast to the standard situation, the vacuum expectation value of the field at small redshifts can be small in comparison to the Planck mass which could be an advantage from the model building point of view. This situation arises when the present-day value of the Lorentz factor is large, this property being per se interesting. Serious shortcomings are also present such as the fact that, for simple potentials, the equation of state appears to be too far from the observational favored value -1. Another problem is that, although simple stringy-inspired models precisely lead to the power-law shape that has been shown to possess a tracking behavior, the power index turns out to have the wrong sign. Possible solutions to these issues are discussed.