S8 Tension

S8 Tension: Recent cosmic-shear (gravitational lensing) surveys, including a deep sky survey by the University of Tokyo using the Hyper Suprime-Cam on the Subaru Telescope in Hawaii, have reported an S8 value of 0.747—aligning with earlier lensing-based estimates.

• About S8: S8 (Sigma Eight) is a cosmological parameter that measures how clumpy matter is on scales of ~26 million light-years. It reflects the degree of matter clustering, including both visible and dark matter, across the universe.

o The universe began nearly uniform after the Big Bang (~13.8 billion years ago), as seen in the Cosmic Microwave Background. Tiny density fluctuations later evolved into galaxies, clusters, and cosmic structures. 

• S8 Tension: There’s a growing discrepancy in the measured values of S8:

o Cosmic Microwave Background (CMB) Data (Planck mission): Suggests a higher S8 — more clumping.

o Cosmic Shear Surveys (gravitational lensing): Show a lower S8 (~0.747) — less clumping.

o This mismatch is called the S8 tension, challenging the prevailing ΛCDM model (Lambda Cold Dark Matter), which assumes a universe dominated by dark matter and dark energy.