🚀 “Guys… can you see me? Oh no, why is everything red? Guys, why are you moving away?” 😱
Haha, do you think I’m rambling? Well, not really. This is actually about today’s topic — the expansion of the universe.
Before jumping into how it’s calculated, we need to ask something more basic:
👉 How do we even know the universe is expanding?
Let’s start there.
🔭 It all began with Edwin Hubble…
Yes, the man behind the famous Hubble Space Telescope. In 1929, while observing the universe, Hubble noticed something strange:
Galaxies were moving away from us.
But how did he figure that out?
Because the wavelength of the light from those galaxies was stretching, shifting toward the red end of the spectrum.
🌈 What does that mean?
Light comes in a variety of colors — even ones we can’t see with our eyes. When an object moves away from us, the light it emits stretches out, and its color shifts more toward red. This is called redshift.
(If you’re still a little confused about what redshift really is — don’t worry! Checkout this post: Redshift Explained)
So this “red light” Hubble observed?
💡 It wasn’t just pretty. It was a clue — and it revealed that the universe is expanding.
🌌 But wait… if the universe is expanding, does that mean galaxies are moving?
Not quite.
The galaxies themselves aren’t moving through space like rockets. Instead, the space between them is stretching — like a rubber band being pulled apart.
👉 If galaxy A and galaxy B are 10 cm apart, and the space between them expands, soon they’ll be 20 cm apart.
But they didn’t “travel” — the space itself grew.
That said, galaxies do have their own motion (called peculiar motion) — but that’s a separate thing from the overall expansion.
🤔 A Doubt You Might Have:
“What if galaxies are just moving away? Why assume the universe is expanding?”
Great question!
Astronomers did consider that. But here’s the thing: if galaxies were just flying around, we’d expect some to move toward us and others away. A random mix.
But that’s not what we observe.
Instead, almost all distant galaxies are moving away, and the farther they are, the faster they go.
That pattern is way too consistent to be random. It matches exactly what we’d expect if space itself is expanding.
📏 Now, let’s get to the nerdy part:
How do we calculate the universe’s expansion?
We use Hubble’s Law:
v = H₀ × d
Where:
- v = the velocity the galaxy is moving away
- H₀ = the Hubble constant (the expansion rate)
- d = the distance to the galaxy
Let’s break down how we figure out each of these:
1️⃣ Measure the galaxy’s redshift (z)
We observe the light from a galaxy and analyze its spectrum.
We use this formula:
z = (λ_observed – λ_rest) / λ_rest
Where:
- λ_observed = what we see
- λ_rest = what it would be if the galaxy weren’t moving
This tells us how much the light has stretched.
2️⃣ Convert redshift to velocity
For small redshifts (z ≪ 1):
v ≈ z × speed of light
So a redshift of 0.01 → galaxy is moving away at ~3000 km/s. 🚀
For larger redshifts (z > 1), we need to use Einstein’s relativity equations.
(P.S. — Don’t worry, we’ll get to that later! 😉)
3️⃣ Measure the distance
This is the hardest part.
One popular method uses Type Ia Supernovae as standard candles 🔥. These supernovae have a known brightness. By comparing how bright they should be vs how bright they look, we can calculate the distance.
4️⃣ Put it all together → Find H₀
We collect data from many galaxies, then plot:
- Distance on the X-axis
- Velocity on the Y-axis
The slope of this line = Hubble Constant (H₀).
🎯 But here comes the twist…
There’s a disagreement in the value of H₀ depending on how we measure it.
🌟 Using supernovae & local galaxies:
H₀ ≈ 73 km/s/Mpc
🌌 Using the early universe (CMB from Planck satellite):
H₀ ≈ 67.4 km/s/Mpc
Wait… what?! These values should be the same — but they’re not. This difference is known as the:
🧨 Hubble Tension
And it’s one of the biggest mysteries in modern astrophysics.
Maybe there’s a measurement error.
Or maybe… something else entirely is going on.
Some scientists think this tension could hint at new physics beyond what we currently understand.
🔍 In summary:
- The universe is expanding 🌌
- We know this thanks to redshift 🔴
- We calculate the expansion rate using Hubble’s Law
- But there’s a mysterious Hubble Tension — and no one knows exactly why it exists 🤯
What do you think is causing this tension?
Let’s talk theories, wild guesses, or even sci-fi explanations! 😄
Next time, we’ll dive into Redshift itself — what it really means and how it connects to our understanding of cosmic motion.
Until then, keep wondering. The universe has more secrets to unfold. 🌠
See you in the next post!
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