Astrophysics Webinar

The nature of dark energy (DE) remains one of the most profound open questions in modern physics. In the standard ΛCDM framework, DE is attributed to the cosmological constant, a uniform energy density responsible for the universe’s accelerated expansion. However, recent high precision observations from the DESI collaboration, when combined with CMB and supernova data, suggest that DE may not be truly constant but could instead evolve over cosmic time. Remarkably, the data indicate that the DE equation of state (EoS) may have transitioned from a phantom-like regime (with EoS w < −1) in the past to a quintessence-like one (w > −1) today, implying a recent crossing of the phantom divide at w = −1.  Therefore, since the first DESI data release, there has been a surge of theoretical activity aimed at developing physically well-motivated models of decaying phantom dark energy.

In this talk, after providing a pedagogical introduction to DE and DESI observations, we examine a broad class of thawing/decaying scalar-field models evolving within a braneworld, where our (3+1)-dimensional Universe is embedded in a (4+1)-dimensional bulk spacetime. Such models arise naturally in higher-dimensional frameworks, including string theory. We show that they can successfully reproduce the observed phantom-divide crossing, yielding an evolution of the Hubble parameter and an effective DE EoS that closely matches the recent DESI DR2 observations. Our MCMC analysis further demonstrates that these models provide a statistical fit comparable to the widely used CPL parametrization, underscoring their excellent agreement with current data. We also emphasize that the resulting braneworld dark energy is only effectively phantom, avoiding physical instabilities or future Big Rip singularities, and we comment on the emergence and interpretation of a pole in the effective EoS of dark energy.