Chapter 1
The History of the Universe

Where did everything come from? Big History's answer begins at the only logical place: the beginning. About 13.8 billion years ago, in a moment called the Big Bang, the Universe burst into existence. From that moment came all the ingredients for stars, planets, and, eventually, you. See how the cosmos builds, burns, and recycles itself. You'll learn that your atoms have been on quite the journey.

Chapter at a Glance

45 Minutes

3 Thresholds

6 Videos

1 Gallery

What Was the Big Bang?

Try to imagine something unimaginably tiny, impossibly dense, and unbelievable hot. Then suddenly, bang! Space, time, and all the matter in the Universe explodes outward at incredible speed. OK, it might have been a bounce...or a poof...or, like, a weird smush. You know what? Let's stick with bang!

Threshold 1: The Big Bang

Beginning at the beginning. As far as we know.

How Did the Universe Begin?

There's a lot we don't know about the Big Bang. Here's what we do know:

Within a few millionths of a second, the Universe expanded faster than light. Subatomic particles of matter and antimatter collided in a cosmic cage match, annihilating each other. By some impossible luck, a tiny bit of leftover matter survived. That's what everything is made of. You, your phone, your lunch, and some rock at the bottom of an alien ocean all come from that leftover debris.

One second after the Big Bang, things cooled to a refreshing 18 billion degrees Fahrenheit. To put that in perspective, if you drove your car into the center of the Sun, you would still be 17,973,000,000 degrees cooler than the early Universe. This primordial soup was hot, but as it cooled over the next few minutes, protons and neutrons began linking up to form hydrogen and helium, the first elements.

All this in the first three minutes of a 13.8-billion-year story.

What came before the Big Bang? (Podcast)

The Cosmic Microwave Background

After the Big Bang, the Universe hit the brakes. It cooled, it darkened, and nothing much happened. It took 380,000 years for things to cool to a brisk 5,000 degrees Fahrenheit. Hydrogen and helium nuclei snagged wandering electrons, creating the first stable atoms, shedding light on a dark Universe. We can still see their light today. We call it the cosmic microwave background (CMB): the Universe's baby picture.

Timeline of the universe over 13.77billion years, on the left is an oval-shaped map with colorful and scattered patterns. On the right is a diagram chronologically listing the evolution of the Universe — The cosmic microwave background reveals the slight temperature variations of the early Universe, about 13.77 billion years ago. Credit: CMB (left) and timeline of the expansion of the Universe (right), by NASA / WMAP Science Team, public domain.

Still, the Universe was mostly a dark place. A quiet expansion flung hydrogen and helium atoms unevenly across the Universe. There were tiny variations in distance and temperature. Those differences are still visible in the CMB today

This was our early Universe. There was matter, but it remained a simple, dark, and boring place. You'd rate it 1-out-of-10 stars on Cosmic Yelp.

That was about to change.

How Do We Know About the Big Bang?

Black and white photo of the Andromeda Galaxy on a glass plate. — A photo Hubble took of the Andromeda Galaxy. It helped him prove that it was a separate galaxy, which enabled him to measure distances and prove the Universe is expanding. © Carnegie Science.

Edwin Hubble's expanding Universe

When the theory of an expanding Universe was first proposed by Georges Lemaître in the 1920s, even the great physicist Albert Einstein didn't believe it. But in 1929, astronomer Edwin Hubble made observations from a powerful telescope that showed galaxies speeding away from each other. Still, many scientists dismissed the idea that our Universe was expanding. In 1998, two different teams of astronomers observed supernovae to prove that the expansion of the Universe was actually accelerating, powered by something they called dark energy. But dark energy is really just a placeholder name for whatever's doing that.

Black and white photo of the Holmdel Horn Antenna in a grass field. — The Holmdel Horn Antenna was used by Penzias and Wilson to discover the cosmic microwave background radiation (CMBR) that permeates the universe. Credit: NASA, public domain.

Cosmic microwave background

In 1964, two scientists in New Jersey—Arno Penzias and Robert Wilson—aimed a radio antenna at the night sky and listened. They were surprised to hear the same low, static hiss wherever they aimed it. A colleague at Princeton suggested the hiss might have something to do with the start of the Universe. He pointed them to mathematical calculations by astrophysicists that showed if the Universe did, in fact, begin with the Big Bang, it would have released a huge amount of energy in the same frequency they heard. They had discovered the CMB, and the first real evidence to confirm the Big Bang theory.

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The Big Bang

Ingredients
Goldilocks Conditions
New Complexity

What key ingredient contributed to the Big Bang?
- Energy That is incorrect. Were you distracted by the blinding light of creation? Focus. The cosmos does not tolerate guesswork.
- Matter Incorrect. Matter showed up after the fireworks. Try again before the Universe cools down.
- We can only speculate Correct! Ignorance acknowledged. One of the few times in which “I don’t know” is the scientifically respectable position. Proceed before your neurons overheat.
What Goldilocks Conditions led to the Big Bang?
- Dramatic pressure imbalances Nope. Imbalances came later—along with gravity, electromagnetism, and wrong answers. Try again.
- Extremely high temperatures Incorrect. You’re skipping ahead in the script. Recalibrate.
- We honestly don't know Correct. Sensing a pattern here? Finally, an answer worthy of scientists everywhere: a shrug. Nicely done.
The Big Bang led to which form of "new complexity?"
- Matter and energy Yes. Matter and energy, the cosmic cousins that built everything, including your ability to guess correctly. We’re impressed.
- Organic life Whoa! Hold your horses! Horses—and all other forms of life—didn’t exist yet.
- Our Sun and Solar System Incorrect. You’re skipping a few chapters. The Universe may be vast, but there’s only one correct answer here.

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The Cosmic Dawn

The tiny ripples we see in the cosmic microwave background—those faint temperature and density variations—are the fingerprints of increasing complexity. Within 100-300 million years of the Big Bang, stars began to light up unevenly across the early Universe. The Universe just got a big glow-up.

Threshold 2: Stars Light Up

How stars are born.

Where Do Stars Come From?

Stars are born in nebulae, huge clouds of gas and dust that collect over millions of years, thanks to slight variations in gravity. These clouds can be hundreds of light-years across. Gas and dust clump together, drawing in more matter into denser clumps as gravity increases. As these compacted clumps of hydrogen and helium grow denser, they heat up to form a plasma, a sort of hot, swirling soup made up of free-floating atomic nuclei. At high enough temperatures, free-floating nuclei slam together with such intensity that they fuse into a new element. This process is called nuclear fusion.

Structure of the Universe

Detailed view of the Tarantula Nebula. — Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

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Stellar nursery

This image shows the Tarantula Nebula, which is 340 light-years across and is a massive star factory, containing tens of thousands of new stars. This region is in our Local Group—a collection of over 50 galaxies held together by gravity.

Hourglass shaped cosmic matter in vibrant orange and blue against the black void of space. — Credit: NASA, ESA, CSA, STScI; Image Processing: Joseph DePasquale (STScI), Alyssa Pagan (STScI), Anton Koekemoer (STScI), public domain.

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Gravity pulls a star together

A star is born—Protostar L1527. As the gravity of the protostar at the center of this hourglass cloud gathers in mass, its core will compress and heat up, eventually sparking nuclear fusion.

A detailed image of Stephan's Quintet, showing the grouping of five galaxies. — Credit: NASA, ESA, CSA, STScI, public domain.

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Galaxy group

A galaxy group typically contains fewer than 50 galaxies, bound together by gravity. These five galaxies form the Stephan's Quintet galaxy group. Our Milky Way Galaxy contains over 100 billion stars and is part of the Local Group.

Photo of numerous galaxies scattered across the black void of space. — Credit: NASA, ESA, CSA, STScI, public domain.

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Galaxy cluster

This image shows thousands of individual galaxies. They may look scattered across the sky, but collections of galaxy groups form galaxy clusters. These galaxies are part of the Galaxy Cluster SMACS 0723. The light from this cluster took 4.6 billion years to reach us.

Astronomical image of a sky filled with stars and galaxies against glowing deep purple haze. — Credit: NASA, ESA, C. Heymans (University of British Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M. Barden (Innsbruck), the STAGES collaboration, C. Wolf (Oxford University, U.K.), K. Meisenheimer (Max-Planck Institute for Astronomy, Heidelberg), and the COMBO-17 collaboration, public domain.

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Supercluster

Until the 1980s, astronomers considered galaxy clusters to be the largest structures in the Universe. But larger structures, called superclusters, were soon revealed. These strands of tens of thousands of galaxies link the Universe together in a cosmic web. This image of Supercluster Abell 901/902 uses a special filter to identify where astronomers believe dark matter is located. Dark matter is an invisible type of matter that we don't really understand, but astronomers believe it accounts for 85% of all mass in the Universe. Dark matter's gravity is the scaffolding that holds the superstructures of the Universe together.

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