S8 Tension

S8 Tension Latest News

The Subaru Telescope in Hawaii, using the Hyper Suprime-Cam, conducted a deep sky survey from the University of Tokyo, reported an S8 value of 0.747, consistent with prior lensing estimates.

What is S8?

• S8 (Sigma Eight) is a parameter used in cosmology to quantify the "clumpiness" or clustering of matter in the universe on a scale of about 26 million light-years.
• It helps determine how much matter (both visible and dark) is clustered in different cosmic regions.

Background

• The universe began with the Big Bang ~13.8 billion years ago, starting off highly uniform, as revealed by the Cosmic Microwave Background (CMB).
• Small fluctuations in density (1 in 100,000) eventually evolved into galaxies, star clusters, and filaments, forming the lumpy structure of the present universe.

What is the S8 Tension?

• The S8 tension refers to the mismatch in the value of S8 as obtained by:
  • CMB measurements (higher value).
  • Cosmic shear/lensing surveys (lower value).
• This discrepancy challenges the ΛCDM (Lambda Cold Dark Matter) model, the standard model of cosmology.

Cosmic Microwave Background (CMB)

• CMB is a sea of photons, the particles of light, present throughout the universe. They are left over from the Big Bang, its afterglow.
• Scientists have measured temperature changes in the CMB and studied its large-scale properties using complicated trigonometry.
• Based on these studies, cosmologists have estimated space to be expanding at around 68 kilometres per second per megaparsec ((km/s)/Mpc). That is, an object one megaparsec (3.26 million lightyears) away is moving away at 68 km/s)/Mpc.

Cosmic Shear & Gravitational Lensing

• Cosmic shear refers to the distortion in galaxy shapes due to gravitational lensing by intervening matter.
• This lensing helps map dark matter distribution and provides an indirect method to calculate S8.
• Lower S8 from lensing suggests less clumping than expected.

S8 Tension FAQs

Q1. What is the Cosmic Microwave Background (CMB)?
Ans. The Cosmic Microwave Background (CMB) is the residual thermal radiation from the Big Bang, uniformly filling the universe and acting as a snapshot of the universe when it was around 380,000 years old.

Q2. What type of radiation is the CMB?
Ans. The CMB is microwave radiation, specifically a type of electromagnetic radiation in the microwave spectrum, with a temperature of approximately 2.7 Kelvin.

Q3. Which missions have studied the CMB in detail?
Ans. Major space missions like COBE (1990), WMAP (2001), and Planck (2009) have studied the CMB, improving our understanding of cosmic inflation, dark matter, and dark energy.

Source: TH