Short Answer
Nearly scale-invariant (but slightly red-tilted) primordial power spectrum predicted by inflation is the best answer.
Astrophysics questions often combine observation with first-principles physics. The winning move is to connect the measurement being made, such as luminosity, spectrum, redshift, or orbit, to the physical model behind it.
Inflation predicts n_s slightly less than 1 ('red tilt'); Planck measurements give n_s ≈ 0.965, consistent with slow-roll inflation.
Why This Answer Is Correct
This is a Hard-level question in Astrophysics & Cosmology. The prompt is really testing whether you can connect the concept to its defining physical relationship instead of picking a nearby-but-wrong term.
Good astrophysics reasoning always asks what the telescope actually measured and what physical quantity that measurement traces.
Choices At A Glance
- A. Nearly scale-invariant (but slightly red-tilted) primordial power spectrum predicted by inflation
- B. A perfectly scale-invariant Harrison–Zel'dovich spectrum
- C. Dark matter abundance
- D. Tensor-to-scalar ratio r
When similar options appear on an exam, eliminate the ones that break the core law, use the wrong units, or confuse a definition with a consequence.
Topic Snapshot
Topic: Astrophysics & Cosmology
Difficulty: Hard
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.