1. The 'Hubble tension' refers to which unresolved issue in cosmology?
- A) Disagreement over whether the universe is flat or curved
- B) Conflicting measurements of the universe’s expansion rate
- C) Uncertainty in the age of the Milky Way
- D) Variations in dark matter density across the universe
Answer: ✅ :alphabet-white-b: Conflicting measurements of the universe’s expansion rate
Different techniques—using Cepheid variables and supernovae nearby versus CMB observations from Planck—give expansion rates that disagree beyond expected errors. This mismatch is one of modern cosmology’s biggest puzzles.
2. The Sunyaev–Zel’dovich effect occurs when:
- A) CMB photons are gravitationally lensed by galaxy clusters
- B) CMB photons are scattered to higher energies by hot electrons in galaxy clusters
- C) Starlight is absorbed and re-emitted by interstellar dust
- D) Neutrinos interact with CMB photons, changing their energy distribution
Answer: ✅ :alphabet-white-b: CMB photons are scattered to higher energies by hot electrons in galaxy clusters
Inverse Compton scattering off energetic electrons in the intra-cluster medium boosts CMB photon energy, slightly distorting the CMB spectrum in that direction—an important tool for detecting distant clusters.
3. What does the Schwarzschild radius represent?
- A) The distance at which a star’s light becomes completely redshifted
- B) The radius defining the event horizon of a non-rotating black hole
- C) The point where general relativity and quantum mechanics break down
- D) The maximum radius of a stable neutron star
Answer: ✅ :alphabet-white-b: The radius defining the event horizon of a non-rotating black hole
Within this radius, the escape velocity exceeds the speed of light. It marks the boundary beyond which nothing—not even light—can escape.
4. The Eddington limit defines:
- A) The balance point where radiation pressure equals gravitational pull in a star
- B) The temperature at which hydrogen fusion begins in stellar cores
- C) The maximum luminosity of a supernova explosion
- D) The threshold mass for a black hole to emit Hawking radiation
Answer: ✅ :alphabet-white-a: The balance point where radiation pressure equals gravitational pull in a star
At this luminosity, the outward force from radiation on ionized gas matches gravity’s inward pull. Exceeding it can drive powerful stellar winds or instabilities.
5. In exoplanet or moon systems, 'mutual events' refer to:
- A) Collisions between planetary bodies
- B) Transits or occultations where one moon or planet passes in front of another
- C) Simultaneous eclipses of multiple planets across a star
- D) Orbital resonances causing synchronized rotations
Answer: ✅ :alphabet-white-b: Transits or occultations where one moon or planet passes in front of another
These events let astronomers measure orbital geometry and sizes with extreme precision, as light from one body partially blocks the other.