How the Universe Became Clumpy
The universe began smooth after the Big Bang 13.8 billion years ago, as seen in the uniform cosmic microwave background (CMB).
Small early variations in density eventually led to clumps of matter like galaxies and clusters due to gravity.
The standard model of cosmology, Lambda Cold Dark Matter (ΛCDM), explains this evolution with dark matter and dark energy making up about 95% of the universe.
The S8 Tension
Sigma 8 (S8) measures how “clumpy” the universe is — higher S8 means more clumping; lower S8 means more uniform matter spread.
S8 is calculated by counting galaxies and cosmic structures within regions ~26 million light-years wide.
A mismatch exists: S8 from gravitational lensing (observing galaxy shapes) gives ~0.747, while CMB data gives a higher value.
This disagreement is known as the S8 tension and raises questions about the completeness of the ΛCDM model.
The Subaru Hyper Suprime-Cam (HSC) survey reaffirmed the lower S8 value using deep lensing observations.
New Findings and Their Implications
The Subaru survey showed that changes in ordinary matter (like gas) cannot alone explain the low S8 value — pointing to dark matter and dark energy.
The lensing surveys don’t resolve the tension but confirm that it’s not due to visible matter alone.
The Cosmic Background Explorer (1992) had detected ripples in the CMB, which seeded galaxy formation.
If the tension persists, it could mean either an issue with measurements or the need to revise cosmological theories.
One major uncertainty in these surveys is determining how fast galaxies are moving away — this is measured through redshift, which is hard to get from faint galaxies.
What’s Next in Cosmology
A 2023 study from the Dark Energy Spectroscopic Instrument (DESI) suggests dark energy may be weakening.
If true, the universe's expansion may slow, potentially reversing and leading to a “big crunch.”
The Rubin Legacy Survey of Space and Time (LSST) will start soon from the Vera C. Rubin Observatory in Chile.
LSST will offer the deepest and widest view of the universe ever and may provide answers to unresolved questions — or raise new ones.
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