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 .