Combined cosmological tests of a bivalent tachyonic dark energy scalar field model

TL;DR

This paper investigates a tachyonic scalar field dark energy model with a bivalent future, testing its compatibility with various cosmological observations and comparing its performance to the standard ΛCDM model.

Contribution

It introduces a novel tachyonic scalar field model with a bivalent future and tests its consistency with multiple cosmological datasets, including CMB, supernovae, and BAO.

Findings

Certain evolutions of the model fit all cosmological tests within 1σ confidence.

The model's CMB power spectrum aligns well with Planck data at high multipoles.

The model slightly overestimates power at low multipoles, similar to ΛCDM.

Abstract

A recently investigated tachyonic scalar field dark energy dominated universe exhibits a bivalent future: depending on initial parameters can run either into a de Sitter exponential expansion or into a traversable future soft singularity followed by a contraction phase. We also include in the model (i) a tiny amount of radiation, (ii) baryonic matter ($\Omega _{b}h^{2}=0.022161$, where the Hubble constant is fixed as $h=0.706$) and (iii) cold dark matter (CDM). Out of a variety of six types of evolutions arising in a more subtle classification, we identify two in which in the past the scalar field effectively degenerates into a dust (its pressure drops to an insignificantly low negative value). These are the evolutions of type IIb converging to de Sitter and type III hitting the future soft singularity. We confront these background evolutions with various cosmological tests, including the supernova type Ia Union 2.1 data, baryon acoustic oscillation distance ratios, Hubble parameter-redshift relation and the cosmic microwave background (CMB) acoustic scale. We determine a subset of the evolutions of both types which at 1$\sigma $ confidence level are consistent with all of these cosmological tests. At perturbative level we derive the CMB temperature power spectrum to find the best agreement with the Planck data for $\Omega _{CDM}=0.22$. The fit is as good as for the $\Lambda $CDM model at high multipoles, but the power remains slightly overestimated at low multipoles, for both types of evolutions. The rest of the CDM is effectively generated by the tachyonic field, which in this sense acts as a combined dark energy and dark matter model.