Astrophysics Seminar

In the current era of precision cosmology, the persistence of cosmological tensions—most notably the Hubble tension and the growth tension—motivates a critical re-evaluation of the standard $\Lambda$CDM model. To reconcile these tensions via late-time modifications to expansion history, various features such as phantom crossing in the dark energy equation of state, a negative energy density at high redshifts, etc., are favoured. However, these features are difficult to accommodate within General Relativity (GR) without invoking theoretical pathologies. 

In this talk, I will discuss my recent works on exploring a dynamical dark energy scenario within the framework of Horndeski gravity, incorporating nonminimal coupling to gravity and self-interactions. We highlight that the model can exhibit novel features like phantom crossing and negative dark energy densities at high redshifts without introducing any instabilities. We perform a detailed analysis of both background dynamics and linear perturbations, with particular focus on observables such as the growth rate, matter power spectrum, and CMB temperature anisotropies. To test the viability of the model, we conduct a comprehensive MCMC analysis using a combination of BAO/$f\sigma_8$, Supernovae, and CMB data. While the model does not statistically outperform the standard $\Lambda$CDM framework in a combined likelihood analysis, there remains a preference for non-zero values of the model parameters within the data. This suggests that dynamical dark energy scenarios—especially those involving non-minimal couplings—warrant further investigation as promising alternatives to GR. Notably, such frameworks could also provide a phenomenological explanation of the dynamical features of dark energy as hinted in the recent DESI DR2 data release.