SSC Stenographer 2026 PYQ — Question 2

Solution: The correct answer is It assumes that dark matter is composed of hot, relativistic particles that decouple early in the universe's evolution.
Explanation:
☛ In the ΛCDM (Lambda Cold Dark Matter) model, the word "Cold" signifies that dark matter is composed of slow-moving, non-relativistic particles, not hot, relativistic particles.
☛ If dark matter were "hot" (relativistic particles like neutrinos), their high speeds would have smoothed out density fluctuations and prevented the formation of galaxies and smaller structures early in the universe.
☛ The Greek letter Lambda (Λ) represents the Cosmological Constant, associated with Dark Energy, which drives the accelerated expansion of the universe.
☛ According to this model, the total mass-energy budget of the universe is approximately 68% Dark Energy, 27% Cold Dark Matter, and only about 5% Ordinary (Baryonic) Matter.
☛ The model successfully accounts for the Cosmic Microwave Background (CMB) radiation and large-scale cosmic structure observed by missions such as Planck and WMAP.
Extra Exam-Relevant Facts:
☛ Standard Model of Cosmology: The ΛCDM model is widely recognized as the standard cosmological model of modern astrophysics.
☛ Accelerated Expansion: Discovered in 1998 by observing distant Type Ia supernovae, which earned the Nobel Prize in Physics in 2011.
☛ Baryonic Matter: Refers to familiar matter consisting of protons, neutrons, and electrons that make up stars, planets, and living organisms.
☛ Dark Matter Discovery: Swiss astronomer Fritz Zwicky first inferred the existence of unseen mass in the 1930s while studying the Coma galaxy cluster.
Important One-Liners for Upcoming Exams:
☛ What does the symbol Lambda (Λ) denote in modern cosmology? → The Cosmological Constant (Dark Energy)
☛ Who first proposed the concept of dark matter based on the Coma cluster observations? → Fritz Zwicky
☛ Which ESA space mission provided the most precise CMB map supporting the ΛCDM model? → Planck Satellite
☛ Approximately what percentage of the universe is composed of dark energy? → About 68%
☛ Which astronomical standard candles confirmed the accelerated expansion of the universe? → Type Ia Supernovae