Imagine the entire universe compressed into a point smaller than an atom. All the stars, galaxies, planets, even time itself, packed into a single, infinitely dense moment. Then—BANG! The universe erupted into existence, setting off a cosmic expansion that continues to this very day. This event, known as the Big Bang, is not just an explosion; it’s the birth of everything.

In this lesson, we will travel back in time—13.8 billion years—to witness the birth of the universe. We will follow its journey from a tiny, blazing-hot point to the vast cosmos we observe today. But the Big Bang was just the beginning. The real story lies in what happened next: how the universe cooled, expanded, and began to form the building blocks of everything we see around us.

Video

Watch this video for an exciting journey through the Big Bang and the early universe. It covers topics such as cosmic inflation and the beginning of nucleosynthesis. Don't worry about understanding the Heisenberg uncertainty part of the video.

Did You Know? The term “Big Bang” was actually coined by a critic of the theory, Fred Hoyle, who intended to mock the idea. Ironically, the name stuck and became the most widely used term for the theory!

The Early Universe: From Blazing Heat to Subatomic Chaos

At the very moment of the Big Bang, the universe was an unimaginably hot and dense fireball. The temperature was so extreme that not even the particles that make up atoms could exist. But time moves quickly in these early moments—really quickly.

A Timeline of Chaos

At $10^-36$ seconds: The universe was smaller than a grain of sand but hotter than a billion trillion suns. At this point, the universe was expanding faster than the speed of light in what we call cosmic inflation. It was growing from something smaller than an atom to the size of a galaxy in a fraction of a second.
At $10^-12$ seconds: Things began to slow down—at least compared to the furious expansion of inflation. By now, the first particles were starting to form: quarks, the building blocks of protons and neutrons. But it was still far too hot for them to stick together.
At $10^-6$ seconds: The universe cooled just enough for quarks to bind together into hadrons—the particles that would eventually form atoms. This marks the beginning of the Hadron Epoch.
At 1 second: Electrons and neutrinos, tiny particles that would play key roles in the formation of matter, were now roaming freely.
At 3 minutes: Something incredible happened—nucleosynthesis. The temperature dropped low enough for protons and neutrons to fuse into atomic nuclei. The universe was now full of hydrogen and helium, the two lightest elements, along with a sprinkle of lithium.

It’s astonishing to think that in these first few minutes, the fundamental building blocks of everything—stars, planets, you and me—were set in motion.

Fun Fact: If you held a chunk of the early universe in your hand, it would be too hot to handle—but in only a few minutes, that searing chaos cooled down enough to start forming the atoms in your body!

Cosmic Inflation: The Universe’s Fastest Growth Spurt

To truly understand the Big Bang, we need to zoom in on a period that lasted less than a heartbeat: cosmic inflation. It’s one of the most extraordinary phases of the universe’s development. Imagine blowing up a balloon, except instead of gently inflating it, the balloon expands from the size of a molecule to something ten light-years across in the blink of an eye!

Solving Cosmic Mysteries

Cosmic inflation wasn’t just fast—it solved some of the biggest puzzles in cosmology. Have you ever wondered why the universe looks so uniform in every direction? It’s almost as if different parts of the universe “knew” how to match up with each other. Inflation explains this by suggesting that these regions were once tightly packed together before expanding rapidly.

Another mystery is the flatness problem. Why is the universe so geometrically flat? If the universe had expanded differently, it could have curled in on itself or expanded into strange shapes. But inflation flattened everything out, like smoothing out a wrinkled bedsheet.

Nucleosynthesis: The Universe’s First Elements

Once the universe cooled enough to allow atomic nuclei to form, the process of nucleosynthesis began. This was the universe’s first stab at creating the matter that makes up stars, planets, and eventually, life.

A Hydrogen-Helium Cocktail

During the first three minutes, temperatures were perfect for fusing protons and neutrons together. This created hydrogen and helium, the two simplest and most abundant elements in the universe today. But the universe didn’t have time to make much else. Heavier elements like carbon and oxygen would only be created later, inside the hearts of stars. So, for now, the universe was a mix of about 75% hydrogen and 25% helium, with just a dash of lithium thrown in.

Interesting Side Note: The ratio of hydrogen to helium created during these first few minutes—roughly 3:1—remains the same to this day. Every star we see is mostly burning the same hydrogen fuel created during the Big Bang!

The Cosmic Microwave Background (CMB): The Universe’s Baby Picture

Now let’s fast forward a bit—about 380,000 years after the Big Bang. By this time, the universe had cooled enough for electrons to finally settle down with protons, forming neutral atoms. For the first time, the universe became transparent.

But what did the universe leave behind? A glowing fingerprint in the form of the Cosmic Microwave Background (CMB), a faint radiation that fills every corner of space.

A Discovery by Accident

In 1965, two scientists, Arno Penzias and Robert Wilson, accidentally discovered this relic of the Big Bang while working on a radio antenna. At first, they thought it was a mistake—perhaps some annoying noise from a nearby source. But they had stumbled upon the first direct evidence of the Big Bang itself.

The CMB gives us a snapshot of the universe when it was just a baby—about 380,000 years old. It reveals tiny fluctuations in temperature that would eventually grow into galaxies, stars, and planets. The temperature of the CMB today is about 2.7 K—barely above absolute zero, but it tells us so much about the early universe.

Cool Fact: The CMB is literally everywhere. The next time you turn on an old TV and see static, a tiny bit of that “noise” is actually the CMB!

Check Your Understanding

How does cosmic inflation solve the horizon problem in cosmology? Why was this expansion so critical for the universe we observe today?
What happened during the process of Big Bang nucleosynthesis, and why are hydrogen and helium the most abundant elements in the universe?
Why is the discovery of the cosmic microwave background (CMB) considered one of the strongest pieces of evidence for the Big Bang theory?
The current temperature of the CMB is about 2.7 K. What does this tell us about the universe’s history and expansion over time?

Conclusion: From the Big Bang to the Stars

We’ve journeyed through time, from the universe’s fiery birth to the formation of its first atoms. Along the way, we’ve seen how cosmic inflation smoothed out the universe, how nucleosynthesis created the first elements, and how the CMB gives us a glimpse into the distant past.

But this is only the beginning. As we continue to explore the cosmos, we’ll uncover how the first stars and galaxies formed, setting the stage for the universe we live in today.

5.1 The Big Bang and the Early Universe

https://www.youtube.com/watch?v=0L_MvPLspdg

Don’t worry about understanding the Heisenberg uncertainty part of the video.

Introduction to the Big Bang Theory

 - Introduction to cosmology as the study of the origin and development of the universe.
 - Brief history of the Big Bang theory and its development in the 20th century.
 - Address common misconceptions (e.g., Big Bang as a literal explosion) and clarify what actually occurred.

The Early Universe

 - Discussion of the extremely high temperatures and densities immediately after the Big Bang.
 - Timeline of the early universe, starting from 10^-36 seconds after the Big Bang.
 - Formation of subatomic particles: quarks, protons, neutrons, and electrons.
 - Symmetry breaking and the decoupling of the fundamental forces (gravity, strong, weak, electromagnetic).
 - The Hadron and Lepton Epochs.

Cosmic Inflation

 - Explanation of the theory of cosmic inflation, where the universe expanded exponentially within fractions of a second.
 - How inflation solved key cosmological problems, such as the horizon and flatness problems.
 - The rapid dispersal of energy and the creation of uniformity in the universe.
 - Transition from inflation to the formation of quarks, gluons, and photons.

Nucleosynthesis and the Formation of Elements

 - Formation of the first atomic nuclei (hydrogen, helium, and trace amounts of lithium) during the first few minutes of the universe.
 - Explanation of why hydrogen and helium dominate the universe’s composition today.
 - How the formation of elements set the stage for later star and galaxy formation.

The Cosmic Microwave Background (CMB)

 - Explanation of the CMB as the leftover radiation from the Big Bang, a critical piece of evidence for the Big Bang theory.
 - Penzias and Wilson's discovery of the CMB in the 1960s.
 - How the CMB provides a snapshot of the universe approximately 380,000 years after the Big Bang.
 - Basic calculations involving the CMB, such as the temperature of the universe and its significance in understanding early cosmology.