Describe redshift as an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies

6.2.3 The Universe: Redshift Explained 🌌

What is Redshift? 🔴

Redshift is the phenomenon where the light from a star or galaxy appears “shifted” toward the red end of the spectrum. This happens when the source is moving away from us, stretching the light waves and making their wavelength longer.

Why does the wavelength stretch? 📏

Think of a runner on a track. If the runner is moving away from you, each step you see takes a bit longer to reach you. In the same way, as a galaxy moves away, each crest of its light wave takes a little longer to arrive, so the wavelength increases.

The Doppler Effect Analogy 🚗

• 🚨 A police car with a siren drives toward you: the sound waves are compressed, making the pitch higher.
• 🚗 When the car drives away, the waves are stretched, and the pitch drops.
• 🔭 Light behaves similarly, but instead of pitch, we talk about wavelength.

Mathematical Description 📐

The key relationship is:

\$z = \dfrac{\Delta \lambda}{\lambda_{\text{emit}}}\$

where:

• \$\lambda_{\text{emit}}\$ = original wavelength emitted by the star or galaxy.
• \$\Delta \lambda = \lambda{\text{obs}} - \lambda{\text{emit}}\$ = increase in wavelength.
• \$z\$ = redshift value.

If a galaxy is moving away at a speed \$v\$ that is much less than the speed of light \$c\$, the redshift can be approximated by:

\$z \approx \dfrac{v}{c}\$

Examples from the Universe 🌠

Why Does It Matter? 🌍

Redshift tells us how fast galaxies are moving away from us, which in turn helps us understand the expansion of the Universe. The farther a galaxy is, the larger its redshift, showing that the Universe is stretching over time.

Quick Quiz 🎓

1. What happens to the wavelength of light when a galaxy moves away from us?
2. Write the formula that relates redshift \$z\$ to the change in wavelength.
3. Which galaxy in the table is moving away the fastest?