How Do We Actually Know the Big Bang Happened?

ELI5/TLDR

The Big Bang is not a guess. It’s a chain of five observations, each building on the last. We figured out how to measure distances to stars, then realized the fuzzy blobs in the sky were other galaxies, then noticed their light was stretched as if they were running away from us, then realized the universe must have been smaller and hotter in the past, and finally went looking for the leftover glow from that hot phase — and found it, exactly where and in exactly the shape the theory predicted. The universe gave us a receipt.

The Full Story

The basic problem: how do you measure something you can’t touch?

Stand on a small planet. Look up. Some of those points of light are close, some are absurdly far. They all just look like dots. How do you tell them apart?

The first move is old. Around 2nd century BCE, a Greek astronomer named Hipparchus sorted stars by how bright they appeared — first magnitude for the brightest, down to sixth for the faintest the eye can see. In the 19th century, Norman Pogson made it mathematical. A difference of five magnitudes means a factor of 100 in actual light received. Which means each single magnitude step is a factor of about 2.512 (because 2.512 multiplied by itself five times equals 100).

This is where logarithms sneak in. Imagine you want to convert repeated multiplication into simple addition. That’s what a logarithm does. Multiplying by 10 becomes adding 1. So the equation Pogson landed on — a difference in magnitude equals -2.5 times the log of the flux ratio — is just a way to keep Hipparchus’s counting scheme while tying it to something you can actually measure with an instrument.

But there’s still a problem. A dim star nearby and a luminous star far away can look identical. Brightness as seen from Earth (“apparent magnitude”) doesn’t tell you how bright the thing actually is. You need distance.

First distance trick: parallax

Hold up your finger. Close one eye, then the other. Your finger jumps. That’s parallax.

The Earth does the same thing as it orbits the Sun — over six months, our viewpoint shifts by about 300 million kilometers. A nearby star appears to wobble against the background of more distant stars. Measure the wobble angle, do a little trigonometry, get the distance.

This gave astronomers a new unit: the parsec. A star one parsec away wobbles by one arcsecond (a tiny slice of angle, 1/3600 of a degree). One parsec is about 3.26 light years.

Parallax is extremely powerful… but it quickly breaks down for more distant ones and certainly for galaxies.

For anything past a few hundred light years, the wobble gets too small to measure. You need a new trick.

The woman who cracked the universe: Henrietta Swan Leavitt

Harvard, early 1900s. A team of women analyzed photographic plates under the astronomer Edward Pickering. One of them, Henrietta Swan Leavitt, became obsessed with a particular class of stars called Cepheid variables — stars whose brightness rises and falls on a regular cycle, because the star itself physically pulses, expanding and contracting.

Leavitt noticed something. Some Cepheids pulsed slow, some pulsed fast. They also had different brightnesses. Was there a relationship? You couldn’t tell from stars scattered around the sky, because distance messes everything up.

So Leavitt had a clever idea. She looked at Cepheids in the Small Magellanic Cloud, a companion system to our galaxy. The cloud is so far away that you can treat every star in it as being at roughly the same distance from Earth. Any brightness differences between its stars must be real, not an artifact of one being closer than the other.

She plotted 25 of them. Period on one axis, brightness on the other. A clean relationship emerged: the longer a Cepheid’s pulse period, the more luminous it actually was. Take the log of the period and the data falls on a straight line.

A remarkable relation between the brightness of these variables and the length of their periods will be noticed. Never has a discovery so profound been so understated.

Why did this matter? Because now, if you spot a Cepheid anywhere in the universe, time its pulse, and you immediately know its true luminosity. Compare true luminosity to how bright it looks from Earth, and you get its distance. Cepheids became a “standard candle” — a light bulb of known wattage. Shine one far away and the dimness tells you exactly how far.

Hubble settles the Great Debate

In 1920, astronomers were arguing about spiral-shaped fuzzy blobs in the sky. Were they gas clouds inside our own Milky Way? Or were they other galaxies, entire “island universes,” unimaginably far away? Nobody knew.

Enter Edwin Hubble — a guy whose father made him study law, who chucked the law after his father died, who declared “even if I were second rate or third rate, it was astronomy that mattered.”

October 1923. Hubble pointed the 100-inch Hooker telescope at the Andromeda Nebula. On the photographic plate, he spotted a new dot. At first he thought it was a nova. Then he realized it was pulsing. A Cepheid.

He timed it — 31.4 days. Leavitt’s relationship told him the true luminosity. From there, the distance. His answer: 900,000 light years.

The Milky Way is about 100,000 light years across. Andromeda was nearly ten times farther than the entire Milky Way. It could not possibly be inside our galaxy. It was its own galaxy, one of many.

Hubble sent the result in a letter to Harlow Shapley, who had bet the other way. Shapley’s reported reaction:

Here is the letter that has destroyed my universe.

Why is everything running away from us?

Once you can measure distances to galaxies, you can start asking other questions. Like: what does their light actually look like?

When you pass starlight through a prism, you get a rainbow — but with dark lines cut through it at very specific wavelengths. Each element absorbs particular wavelengths, leaving a barcode-like fingerprint. Hydrogen has its own fingerprint. Helium has its own. This is how spectroscopy works — you can tell what a star is made of from 100 million light years away.

But astronomers noticed something odd. The barcodes from distant galaxies were shifted. Same pattern of lines, but stretched toward the red end of the spectrum (or, more rarely, the blue). Think of it like an ambulance siren. As the ambulance races away, the pitch drops — the sound waves get stretched. When it races toward you, the pitch rises — waves bunch up. Light does the same thing. Galaxies whose light is red-shifted are moving away; blue-shifted means moving toward us.

Starting in 1912, an astronomer named Vesto Slipher started measuring these shifts. By 1917, out of 25 galaxies, 21 were red-shifted. Almost everything was running away from us. Fast — hundreds, sometimes thousands of kilometers per second.

This was weird. If galaxies are huge clumps of matter, gravity should be pulling them together. Why was everything flying apart?

Hubble’s Law

Hubble again. He combined Slipher’s redshifts with his own distance measurements (using Cepheids). He plotted distance against recession velocity. A straight line emerged. The farther away a galaxy is, the faster it’s flying away. Twice as far, twice as fast. Ten times as far, ten times as fast.

This is Hubble’s Law: velocity equals a constant times distance. The constant, now called the Hubble constant, is the slope of that line.

Here’s the trick that messes with people’s heads. “Everything is running away from us” sounds like we’re at the center. We’re not. Imagine dots on a balloon. Inflate the balloon. From any dot’s perspective, every other dot is moving away, and dots twice as far away move twice as fast. There’s no center dot. The balloon itself is expanding. Every point is equally “the middle of nowhere.”

Either every place is the center of the universe, or no place is. And the only consistent interpretation is that there is no center at all.

Space itself is stretching. The galaxies aren’t flying through space — space is growing, and they’re along for the ride.

Run the movie backward

If everything is moving apart now, then in the past it was all closer together. Run the film backward far enough and everything collapses to a point. When?

There’s a quick estimate. Hubble’s Law says velocity equals constant times distance. Rearrange: distance divided by velocity equals one over the Hubble constant. Distance over velocity is a time — it’s how long the journey took. And because of the law’s proportionality, this comes out the same for every galaxy. Every galaxy gives the same answer to “how long since we were all at the same point?”

Hubble’s original numbers gave about 1.8 billion years. Which was awkward — the Earth was already known to be older than that. Something was off.

The fix came in 1952. Walter Baade realized there were actually two different types of Cepheid, with different luminosities, and everyone had been mixing them up. Distances were systematically too small. Fix that and everything stretches out. The modern Hubble constant is about 70 km/s per megaparsec, giving a universe roughly 14 billion years old — comfortably older than the 4.5-billion-year-old Earth.

Hotter in the past

Smaller universe means denser. Denser means hotter. Why?

Think of light stretching with space. As the universe expands, the wavelength of a photon stretches along with it. Run the film backward — wavelengths get shorter. Shorter wavelength means higher energy per photon. Higher photon energy means higher temperature.

Translate: the universe was once unimaginably hot. So hot that atoms couldn’t exist. Electrons couldn’t stay bound to nuclei — the radiation was too violent. Everything was a glowing plasma: free electrons, bare atomic nuclei, and a storm of photons all bouncing off each other constantly.

A plasma like that is opaque. Light can’t travel in a straight line because it keeps slamming into electrons. Imagine trying to see across a room full of pinballs.

But there’s a crucial property. When matter and radiation are interacting constantly, they settle into “thermal equilibrium” — a balance where the radiation takes on a very specific, predictable shape called a black body spectrum. Not just any random glow. A precise mathematical curve.

The smoking gun

As the universe expanded, it cooled. At around 3,000 Kelvin (cool enough that electrons could finally stick to protons without getting knocked off), atoms formed. Suddenly the free electrons were gone. Light could finally travel freely. The universe went transparent.

The photons flying free at that moment had the temperature of a black body at 3,000K — meaning their wavelengths were mostly in the near-infrared range. But they’ve been traveling through an expanding universe for billions of years ever since. Their wavelengths stretched along with space. By now, they should be in the microwave range.

So here’s the prediction: if the hot Big Bang picture is right, the universe right now should be filled with faint microwave radiation, coming from every direction, with the precise mathematical shape of a black body spectrum, at a temperature of a few degrees above absolute zero.

That is a very specific prediction. It should either be there or it shouldn’t.

Pigeons at Bell Labs

Two radio astronomers at Bell Labs, Arno Penzias and Robert Wilson, were trying to calibrate a very sensitive antenna for satellite communication. Boring engineering work. They kept finding a background hiss they couldn’t get rid of. It was there day and night. Summer and winter. Every direction they pointed the antenna.

They tried everything. They even found pigeons nesting in the horn of the antenna and kicked them out, cleaning up what Penzias politely called “a white dielectric material.” The signal stayed.

When they quantified it, the signal corresponded to about 3 Kelvin. Microwaves, filling the sky uniformly. They had no idea what it was.

They weren’t looking for it. They stumbled into it. But the prediction already existed — Big Bang theorists had been saying for years that this radiation should be out there. Penzias and Wilson had accidentally detected the afterglow of the universe.

Kobe, WMAP, Planck

Finding microwave radiation wasn’t enough. The real test was whether it had the exact black body shape predicted. That came in 1989 with the COBE satellite, which measured the spectrum and found it matched the theoretical curve almost perfectly — one of the cleanest confirmations of a physical prediction ever made.

COBE also revealed tiny ripples in the background, about one part in 100,000 — early fluctuations that would eventually grow into galaxies. Later missions (WMAP in 2001, Planck in 2009) mapped those ripples in exquisite detail, and from that map cosmologists have pulled out the contents of the universe: how much ordinary matter, how much dark matter, how much dark energy, and the overall geometry of space.

From faint microwaves hitting Earth, we’ve reconstructed what the universe looked like when it was a few hundred thousand years old. That is the receipt.

Key Takeaways

Parallax measures nearby stars by watching them wobble against the background as Earth orbits.
Cepheid variables are pulsing stars whose pulse period reveals their true brightness, making them cosmic distance markers.
Henrietta Swan Leavitt’s 1912 period-luminosity relation is the foundation of all extragalactic distance measurement.
Edwin Hubble used Cepheids to prove Andromeda was a separate galaxy, ending the debate about whether other galaxies exist.
Spectroscopy — the barcode of light — lets us identify elements and measure redshift from any distance.
Redshift maps to velocity via the Doppler effect; galaxies are almost universally moving away.
Hubble’s Law (velocity proportional to distance) implies space itself is expanding, with no center.
Rewind the expansion and the universe was once tiny, dense, and hot — a plasma too energetic for atoms.
That hot phase must have left behind a cooled-down microwave glow with a specific black body shape.
Penzias and Wilson found exactly this by accident in 1964. COBE confirmed the shape in 1989. The Big Bang left a receipt.

Claude’s Take

This is one of the cleanest science explainers I’ve processed. Physics Explained is doing something rare — showing the full logical chain, each link justified before moving to the next, with actual math on screen. No hand-waving. The history is woven in because it is the argument — Leavitt, Hubble, Slipher, Penzias and Wilson are not color, they’re the rungs of the ladder.

The BS filter comes up almost empty. The one thing missing is a pause on what the Big Bang isn’t. A lot of people picture an explosion in space — a firework going off in a dark room. That’s wrong and it’s the default mental image. The video explains (correctly) that space itself is expanding, but it could have hit that misconception harder.

Also glossed: the current tension in the Hubble constant. The value measured from nearby supernovae (about 73) doesn’t match the value inferred from the CMB (about 67). That’s a real, unresolved problem in cosmology right now, and anyone who wants to follow the story further will run into it immediately.

Score is 9. Subtract one point for the modest oversimplifications near the end; everything else is rigorous, paced well, and earns its 67 minutes.