The Cosmic Stretch: Understanding Cosmological Redshift

Imagine you are drawing a wave on a piece of unstretched rubber band. If you pull the ends of the rubber band, making it longer, the wave you drew stretches out too! This is very similar to an incredible trick that happens to light as it travels across our universe. Scientists call this phenomenon cosmological redshift, and it is one of our greatest clues for understanding how the universe behaves.

To understand it, we first need to look at light. The light we see from stars and distant galaxies travels through space as waves. Just like waves in the ocean, light waves have wavelengths—the distance from the top of one wave to the top of the next. In visible light, different wavelengths look like different colors to our eyes. Short, squished-up wavelengths look blue or violet, while long, stretched-out wavelengths look red.

The Universe is Expanding!

When a distant galaxy sends light out into space toward Earth, that light has to travel for millions, or even billions, of years to reach us. While that light is traveling, the universe itself isn’t just sitting still—it is actually expanding!

As space expands, it literally stretches the light waves traveling through it. By the time that light finally hits an astronomer’s telescope on Earth, its wavelength has been lengthened. Because longer wavelengths shift toward the red end of the color spectrum, we call this a redshift.

It’s important to know that the galaxy itself isn’t necessarily racing away from us through space; rather, the space between us and the galaxy is stretching. The further away a galaxy is, the more space there is to stretch, and the redder its light becomes by the time it reaches us!

🔬 Try This with the Simulator Above!

Click the Animate Expansion button to watch light waves travel across the cosmos. Watch how a wave that starts out as a tight, energetic blue wave slowly stretches out into a long, relaxed red wave as the universe expands.

Can you spot the difference between light from a nearby galaxy versus light from a super-distant galaxy?

The Math of the Cosmos

Astronomers use a simple letter, $z$, to represent redshift. The formula they use to calculate how much the universe has stretched is beautiful and elegant:

$z = \frac{\lambda_{\text{observed}} - \lambda_{\text{emitted}}}{\lambda_{\text{emitted}}}$

In this equation:

• $z$ is the redshift value.
• $\lambda_{\text{observed}}$ (pronounced lambda) is the stretched wavelength we see on Earth.
• $\lambda_{\text{emitted}}$ is the original wavelength the galaxy sent out.

If a galaxy has a redshift of $z = 1$, it means the light waves have been stretched by $100\%$—doubling their original wavelength! This also tells us that the universe has doubled in size since that light was first emitted.

📏 Calculate and Test!

Let’s test the math out in the simulator! Scroll up to the input fields and try typing in a target redshift value.

• Enter 0.5 into the simulator. Notice how much the galaxy moves and how the color shifts.
• Now, enter 2.0. Notice how the light shifts completely out of the visible red spectrum and becomes invisible infrared light!
• Hint: Because the universe is expanding faster and faster, a higher redshift value ($z$) means you are looking at an object that is much further away and much older—allowing you to look back in time!

🚀 Cosmic Challenge!

Use the simulator’s presets to target different cosmic eras. Jump from a Nearby Galaxy to a Distant Quasar, and finally to the Cosmic Microwave Background (CMB)—the oldest light in the universe!

Observe how the redshift ($z$) skyrockets as you look further back in time. The CMB has a massive redshift of about $z = 1100$. This means the light waves have been stretched so much that they aren’t even light anymore; they have stretched all the way into microwaves!

By using the RedShiftCal simulator, you are calculating the history of our universe just like Edwin Hubble and the astronomers operating the James Webb Space Telescope. You are measuring the literal fabric of space stretching across time!