Seminar: ‘The tensions of LCDM: An update of observations and theoretical implications’

Title: ‘The tensions of LCDM: An update of observations and theoretical implications’

Speaker: Leandros Perivolaropoulos (Ioannina)

Venue&Time: Red Room / 15:00

Abstract: The standard LCDM cosmological model, despite its success in describing the large-scale structure of the Universe, is currently challenged by statistically significant discrepancies between early and late-universe observations. In this talk, I provide a comprehensive update on observational status and theoretical implications of three major tensions: the Hubble (H0) constant discrepancy, the DESI DR2 Phantom-Crossing Anomaly, and the S8 tension. First, I re-examine the H0 tension by classifying 88 Sound-Horizon-Free measurements. This analysis reveals that the discrepancy is not merely an “Early vs. Late” issue but is fundamentally driven by a clash between the Distance Ladder and the majority of other probes (reaching a significance of ~ 6.5\sigma). I will further discuss why simple late-time modifications to the expansion history fail to solve this tension due to constraints from unanchored Type Ia Supernovae (Pantheon+), effectively establishing a “no-go” theorem for such solutions.
Second, I address the implications of the recent DESI DR2 data, which favors a dynamical dark energy equation of state crossing the phantom divide (w(z) < -1). While this behavior is forbidden in simple quintessence models within General Relativity, I demonstrate how it can be naturally accommodated in reconstructed Scalar-Tensor theories. By allowing for an evolving effective gravitational constant (G_eff), these models can fit the phantom crossing while simultaneously alleviating the Hubble tension. Finally, I review the current landscape of the S8 tension. I discuss the shift toward a “Combined CMB” baseline (Planck + ACT + SPT) which reduces uncertainties, and analyze the dichotomy currently present in weak lensing surveys (e.g., DES Year 6 vs. KiDS Legacy). I conclude by summarizing which theoretical extensions of LCDM remain viable in the face of these combined precision constraints. (edited)