Quintom Model Perturbations

We build upon the work of Goh and Taylor 2025, in which we proposed a two scalar field quintom framework capable of naturally realising a phantom-to-quintessence transition in the dark energy equation of state. In this work, we derive the linear perturbation equations of our model and investigate its implications for large scale structure formation. We show that the quintom model is able to reproduce the phenomenological features of a w0waCDM cosmology with phantom-to-quintessence crossing, including suppressions in the matter power spectrum and enhancements to the late-time Integrated Sachs-Wolfe effect. We then perform a Bayesian analysis of the quintom framework with BAO, CMB and Type 1a supernovae data, finding that it is mildly favoured over the standard w0waCDM parametrisation, while successfully reproducing physical trends observed in the data. We further examine the parameter degeneracies inherent to the model and discuss prospective observational strategies for distinguishing quintom cosmologies from conventional dynamical dark energy models given current and future data precision.