Observational Cosmology and Distance Measurements
The Cosmic Distance Ladder and Beyond
Distance indicators
Cosmology relies on multiple methods for measuring distance and expansion. Nearby distances can be measured geometrically, then calibrated standard candles and standard rulers extend the ladder to larger scales. Key examples include Cepheid variables, Type Ia supernovae, baryon acoustic oscillations, and gravitational lensing.
Common cosmological probes
| Probe | What it measures |
|---|---|
| Type Ia supernovae | Expansion history via luminosity distance |
| BAO | Standard ruler in galaxy clustering |
| CMB | Early-Universe parameters and geometry |
| Weak lensing | Growth of structure and matter distribution |
Why multiple probes matter
Different probes break parameter degeneracies, letting cosmologists test the same model from independent angles.
What is the main role of baryon acoustic oscillations in cosmology?
BAO provides a characteristic comoving scale that can be used to measure distances and the expansion history.
Correct answer: A standard ruler
Name one standard candle used in cosmology.
Type Ia supernovae are widely used because their peak luminosities can be standardized.
Correct answer: Type Ia supernovae
Standard candles vs standard rulers
Standard candles
- Use known intrinsic luminosity
- Infer distance from brightness
Standard rulers
- Use known physical size
- Infer distance from angular scale