Description

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Picture Courtesy: THE HINDU

Context:

Cosmologists are grappling with conflicting measurements of S8, a key parameter that reveals how clumpy the universe is.

What is the S8 Tension?

Cosmologists study how matter, like galaxies and stars, is spread across the universe. They use a term called S8 (Sigma 8) to measure how much matter clumps together in regions about 26 million light-years wide.

A higher S8 value means matter is more clustered (like dense galaxy clusters), while a lower S8 value means matter is more evenly spread out. The problem is that scientists are getting different values of S8 depending on how they measure it, and this disagreement is called the S8 tension.

The S8 tension suggests how the universe evolved from being smooth after the Big Bang to having galaxies, stars, and planets today. Scientists believe that small fluctuations in the early universe grew into the large-scale structures seen now due to gravity pulling matter together. But if the measurements of S8 don’t match, it could mean there’s something wrong with our understanding of the universe.

How Do Scientists Measure S8?

Scientists use two main methods to measure S8:

Cosmic Microwave Background (CMB):

The CMB is like a "baby picture" of the universe, showing what it looked like 380,000 years after the Big Bang.
By studying the ripples in the CMB, scientists predict a higher value of S8, meaning the universe should be clumpier than it appears.

Gravitational Lensing Surveys:

Gravitational lensing happens when light from distant galaxies bends as it passes through massive objects like galaxy clusters.
Scientists use this bending to map how matter is distributed in the universe today. These surveys give a lower value of S8, suggesting the universe is smoother.

The mismatch between these two methods creates the S8 tension .

The standard model of cosmology is called Lambda Cold Dark Matter (ΛCDM) . It explains the universe using dark matter, dark energy, and regular matter. However, the S8 tension suggests that the ΛCDM model might need updates. For example:

Some scientists think dark energy , which drives the universe's expansion, might be weakening over time.
If dark energy weakens, the universe’s expansion could slow down, and it might even collapse in a "big crunch."

Challenges in Solving the S8 Tension

One big challenge is measuring the redshift of galaxies. Redshift tells us how fast galaxies are moving away from us, but faint galaxies make this difficult. Without accurate redshift data, it is hard to confirm whether the S8 tension is real or caused by errors in measurement.

What’s Next?

To solve the S8 tension, scientists are looking forward to new tools like the Rubin Legacy Survey of Space and Time (LSST) . This survey will use the largest camera ever built to study the universe in unprecedented detail.

Source:

THE HINDU

PRACTICE QUESTION

Q. In the question given below, there are two statements marked as Assertion (A) and Reason (R). Mark your answer as per the codes provided:

Assertion (A) : Phosphine has been detected in the atmosphere of Venus.

Reason (R): This indicates possible biological activity on Venus.

Which of the options given below is correct?

A) Both A and R are true, and R is the correct explanation for A.

B) Both A and R are true, but R is not the correct explanation for A.

C) A is true, but R is false.

D) A is false, but R is true.

Answer: B

Explanation:

Assertion (A): Phosphine has been detected in the atmosphere of Venus. This statement is true. Multiple searches in your history from March and April 2025 confirm the detection of phosphine in Venus' atmosphere, referencing news from 2020 and later studies.

Reason (R): This indicates possible biological activity on Venus. This statement is also true. The detection of phosphine is significant because, on Earth, it is often produced by biological processes (specifically, anaerobic bacteria). Therefore, its presence on Venus raises the possibility of similar biological activity.

However, while both statements are true, Reason (R) is not the correct explanation for Assertion (A). The presence of phosphine suggests possible biological activity, but it doesn't definitively prove it or serve as the sole established explanation for its existence.