Understanding Cosmological Redshift
Redshift occurs when light from distant galaxies is stretched to longer (redder) wavelengths as the universe expands. It's defined as z = (?_observed - ?_emitted) / ?_emitted. Positive z means redshift (moving away), negative means blueshift (approaching). For small z, recession velocity v ~= cz, where c is light speed. Most galaxies show redshift, confirming universal expansion discovered by Edwin Hubble. The cosmic microwave background has z ~= 1100, from when the universe was 380,000 years old.
Doppler vs Cosmological Redshift
Doppler redshift comes from objects moving through space. Cosmological redshift comes from space itself expanding, stretching light waves. For nearby galaxies (low z), they're equivalent. At high z, cosmological redshift dominates, and relativistic Doppler formulas are needed: z = sqrt[(1+v/c)/(1-v/c)] - 1. The most distant observed galaxies have z > 10, corresponding to when the universe was less than 500 million years old. Redshift is fundamental to measuring cosmic distances and studying universe expansion history.
Applications in Cosmology
Redshift is essential for studying the universe. Hubble's law relates redshift to distance: v = H?d, where H? is the Hubble constant (~= 70 km/s/Mpc). This allows distance measurement without direct methods. Redshift surveys map galaxy distributions in 3D. Observing distant galaxies at high z shows the early universe, allowing study of galaxy formation and evolution. Redshift measurements revealed dark energy by showing accelerating expansion. Understanding redshift is fundamental to cosmology, astrophysics, and our understanding of the universe's origin, evolution, and fate.
Quick Tips
- Always verify units are consistent
- Use scientific notation for very large/small numbers
- Results are approximations — real conditions may vary
Frequently Asked Questions
Cosmological redshift comes from universe expansion stretching light waves. Doppler redshift comes from motion through space. Gravitational redshift (less common) comes from light escaping gravitational fields. Most galaxy redshift is cosmological.
Hubble's law states that galaxy recession velocity is proportional to distance: v = H?d. The Hubble constant H? ~= 70 km/s/Mpc. This linear relationship holds for relatively nearby galaxies and indicates universal expansion.
Yes, many distant galaxies have z > 1, some over z = 10. At z = 1, wavelengths are doubled. At z = 10, they're 11 times longer. High redshift indicates very distant, early-universe objects.
The universe is expanding, so most galaxies recede from us, causing redshift. Only nearby galaxies in our local group (like Andromeda) show blueshift from gravitational attraction overcoming expansion. Cosmological expansion dominates at large scales.
Hubble's law converts redshift to distance: d = v/H? = cz/H? for small z. For large z, the relationship is more complex, involving cosmological models and parameters. Redshift provides a convenient distance indicator for distant galaxies.