Dark Energy Constraints from Pantheon+ Ia Supernovae Data

Abstract: Measurements of the current expansion rate of the Universe, $H_0$, using standard candles, disagree with those derived from observations of the Cosmic Microwave Background (CMB). This discrepancy, known as the Hubble tension, is substantial and suggests the possibility of revisions to the standard cosmological model (Cosmological constant $\Lambda$ and cold dark matter - $\Lambda$CDM). Dynamic dark energy (DE) models that introduce deviations in the expansion history relative to $\Lambda$CDM could potentially explain this tension. We used Type Ia supernovae (SNe) data to test a dynamic DE model consisting of an equation of state that varies linearly with the cosmological scale factor $a$. To evaluate this model, we developed a new statistic (the $T_{\alpha}$ statistic) used in conjunction with an optimization code that minimizes its value to obtain model parameters. The $T_{\alpha}$ statistic reduces bias errors (in comparison to the $\chi^2$ statistic) because it retains the sign of the residuals, which is meaningful in testing the dynamic DE model as the deviations in the expansion history introduced by this model act asymmetrically in redshift space. The DE model fits the SNe data reasonably well, but the available SNe data lacks the statistical power to discriminate between $\Lambda$CDM and alternative models. To further assess the model using CMB data, we computed the distance to the last scattering surface and compared the results with that derived from the Planck observations. Although the simple dynamic DE model tested does not completely resolve the tension, it is not ruled out by the data and could still play a role alongside other physical effects.