88. What is Red Shift?

This guide explains the concept of red shift, how it provides evidence for the expansion of the universe, and how it relates to the Big Bang Theory.

1. Absorption Spectra

• The Concept: Stars emit a full spectrum of visible light. As this light passes through the star's atmosphere, certain chemicals (like hydrogen) absorb specific wavelengths.

• The Result: When the light reaches Earth, we see a pattern of dark lines where those specific wavelengths are missing. This is known as an absorption spectrum.

2. What is Red Shift?

When we analyze light from distant galaxies, we see the same pattern of dark lines, but they are all shifted toward the red end (longer wavelengths) of the spectrum.

• The Cause: This happens because the light waves are stretched as they travel through space toward Earth.

• The Meaning: Red shift tells us that distant galaxies are moving away from us. The further away a galaxy is, the faster it appears to be moving and the greater its red shift.

3. Expanding Space (The Balloon Model)

It is important to understand that galaxies aren't necessarily "traveling" through space away from us. Instead, space itself is expanding.

• The Model: Imagine a balloon with markers on it representing galaxies. As you blow up the balloon, the markers move further apart.

• The Reality: The markers aren't "walking" across the balloon; the surface between them is stretching. This is exactly what is happening to the space between galaxies in our universe.

4. The Big Bang Theory

If the universe is currently expanding, it stands to reason that it was smaller in the past.

• The Theory: The Big Bang Theory suggests that the entire universe began as a very small, unimaginably hot, and dense region.

• The Explosion: About 13.8 billion years ago, this region "exploded," and space began to expand. This expansion is still happening today, as evidenced by the red shift we observe.

5. Summary Table