Advanced · ABOUT 2 MINUTES READING
The Expanding Universe
On large scales, distant galaxies show a systematic relationship between distance and redshift. Cosmology explains this pattern with an evolving scale of space.
Stretching wavelengths
Cosmological redshift means light’s wavelength grows as the universe expands during its journey. It is related to the change in cosmic scale factor between emission and observation. Local Doppler motion also contributes to observed redshifts, especially nearby, so not every shift should be assigned solely to cosmic expansion.
No privileged center is required
In a homogeneous expanding model, observers in different galaxies can all see sufficiently distant galaxies receding on average. This does not place each observer at a special central explosion site. Bound systems such as planetary systems and galaxies do not simply expand at the same rate as the large-scale cosmic background.
Measurements and uncertainties
Distances must be established independently of a naive redshift-to-distance assumption when measuring expansion. Different methods use different observations and models; disagreements can reveal calibration issues or new physics. The expansion history is not obtained by assuming one constant speed for every galaxy across all time.
Key takeaways
- Cosmic expansion stretches traveling light.
- The pattern does not select a special observer.
- Local gravitationally bound systems behave differently.
Make this knowledge yours
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Knowledge check
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