Know that redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory

6.2.3 The Universe 🌌

What is Redshift?

When light travels through space, its wavelength can stretch or compress. If the wavelength stretches (moves toward the red end of the spectrum), we call this redshift. Think of a stretchable rubber band that gets longer as you pull it – the light’s “color” stretches too.

How Do We Measure It?

1. We look at a known spectral line, e.g. the hydrogen line at 656.3 nm.
2. In a distant galaxy, that line appears at a longer wavelength, say 700 nm.
3. The redshift \$z\$ is calculated as:

   \$ z = \frac{\lambda{\text{observed}} - \lambda{\text{rest}}}{\lambda_{\text{rest}}} \$

4. Using the Doppler formula for non‑relativistic speeds:

   \$ v \approx zc \$

   where \$c\$ is the speed of light.

Redshift as Evidence of an Expanding Universe 🚀

The farther a galaxy is, the larger its redshift. This is like hearing a long‑horned trumpet that gets lower in pitch as it moves away. The relationship is linear for nearby galaxies:

\$ v = H_0 \, d \$

where \$H_0\$ is Hubble’s constant and \$d\$ is distance. The table below shows typical values.

Why This Supports the Big Bang Theory 🌟

If the Universe is expanding now, it must have been expanding in the past. By reversing the expansion, we find that all galaxies were once very close together – a hot, dense “primordial soup.” This is the Big Bang. The redshift data give us a timeline: the farther we look, the further back in time we see.

Key Take‑aways 📚

• Redshift is the stretching of light’s wavelength.
• Measured using known spectral lines.
• Redshift increases with distance – evidence of an expanding Universe.
• Supports the Big Bang theory by showing the Universe was once compact and hot.