Redshift

🌌 Redshift Explained Through a Simple Numerical Example

(A Beginner’s Journey from Light to the Universe)

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🌠 Introduction

Sometimes, the universe is not understood through long theories—but through one simple calculation.

In this post, I will not explain redshift in a complicated way.
Instead, I will walk through one real numerical example, step by step, exactly how astronomers think.

By the end, you will understand:

How a tiny shift in light tells us the speed, distance, and history of a galaxy

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🌊 Step 1 — Start with Light

Suppose we observe a distant galaxy.

We focus on a known spectral line (for example, hydrogen).

• Laboratory wavelength = 500 nm
• Observed wavelength = 535 nm

👉 The light has shifted toward red.

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🔴 Step 2 — Calculate Redshift

We use the formula:

z = \frac{\lambda_{observed} - \lambda_{original}}{\lambda_{original}}

Substitute values:

z = \frac{535 - 500}{500} = 0.07

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🧠 Meaning

The wavelength has increased by 7%

This is the first key signal from the universe.

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🚀 Step 3 — Convert Redshift into Velocity

For small redshift:

v = cz

Where:

• km/s

v = 300{,}000 \times 0.07 = 21{,}000 \text{ km/s}

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🧠 Meaning

The galaxy is moving away at 21,000 km/s

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📏 Step 4 — Convert Velocity into Distance

Now we use Hubble’s Law:

d = \frac{v}{H_0}

Take:

d = \frac{21{,}000}{70} = 300 \text{ Mpc}

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🧠 Meaning

The galaxy is 300 megaparsecs away

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🌌 Step 5 — Convert Distance into Light-Years

We know:

1 Mpc = 3.26 million light-years

300 \times 3.26 = 978 \text{ million light-years}

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🧠 Meaning

The light has traveled for ~1 billion years

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⏳ Step 6 — What Are We Actually Seeing?

This is the most beautiful part.

We are not seeing the galaxy as it is today.

We are seeing:

The galaxy as it was 1 billion years ago

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🔁 The Entire Flow (For Memory)

Measure wavelength → calculate redshift (z)
→ convert to velocity (v = cz)
→ find distance (d = v / H₀)
→ convert to light-years
→ interpret as lookback time

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🔵 A Quick Contrast — Blueshift Example

Not all galaxies are moving away.

Take the Andromeda Galaxy:

• Velocity ≈ −300 km/s

z = \frac{-300}{300{,}000} = -0.001

👉 Negative redshift = Blueshift

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🧠 Meaning

Andromeda is moving toward us, not away

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🌌 Final Reflection

From just one calculation, we discovered:

• How fast a galaxy moves
• How far it is
• How long its light traveled
• And how far back in time we are looking

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🔚 One-Line Summary

Redshift is not just a number—it is a bridge from light to the history of the universe.