11-15-2025, 10:13 AM
Chapter 17 — The Early Universe & The Big Bang
The Big Bang theory describes the origin and early evolution of the universe.
It explains how space, time, matter, and energy first emerged and how the universe expanded
from a hot, dense beginning into the vast cosmic structure we see today.
This chapter explores what happened in the first moments after the Big Bang, how the early universe evolved,
and the evidence that supports this model.
---
17.1 What Was the Big Bang?
The Big Bang was not an explosion in space —
it was the expansion of space itself.
Everything — matter, energy, space, and time — began in a hot, dense state around 13.8 billion years ago.
As the universe expanded:
• it cooled
• matter formed
• atoms emerged
• galaxies eventually appeared
The Big Bang describes the beginning of the universe's expansion, not the creation of something inside empty space.
---
17.2 The First Moments — Cosmic Timeline
0 seconds — The Beginning
Physics breaks down.
Quantum gravity needed to describe this moment.
10⁻³⁶ seconds — Inflation
A burst of exponential expansion.
The universe grew from subatomic size to cosmic size almost instantly.
Smoothed out the universe and created tiny quantum fluctuations.
10⁻³² seconds — End of Inflation
Energy transferred into matter and radiation.
10⁻⁶ seconds — Quark Era
The universe was a hot soup of quarks and gluons.
No atoms or protons yet.
1 second — Formation of Protons & Neutrons
Quarks combined as the universe cooled.
3 minutes — Nucleosynthesis
The first nuclei formed (hydrogen, helium, a little lithium).
380,000 years — Recombination
Electrons joined nuclei → atoms formed.
The universe became transparent.
Cosmic Microwave Background (CMB) was released.
200 million years — First Stars
Gravity pulled gas together to ignite the first stars.
1 billion years — First Galaxies
Galaxies formed and large-scale structure appeared.
13.8 billion years — Today
The universe is still expanding and cooling.
---
17.3 Cosmic Microwave Background (CMB)
The CMB is the oldest light in the universe —
released when the universe became transparent.
Key features:
• Temperature: 2.7 K
• Almost uniform
• Tiny fluctuations reveal early density variations
• Discovered in 1965 by Penzias & Wilson
The CMB is one of the strongest pieces of evidence for the Big Bang.
---
17.4 Evidence for the Big Bang
Astrophysicists rely on three major lines of evidence:
1. Cosmic Expansion
Galaxies are moving away from us.
Hubble’s Law shows space is expanding.
2. CMB Radiation
A perfect leftover glow from the early universe.
3. Element Abundances
The early universe created:
• 75% hydrogen
• 25% helium
• trace lithium
Exactly matching Big Bang predictions.
Together, this evidence strongly supports the Big Bang model.
---
17.5 Inflation — Solving Big Problems
The theory of inflation explains several mysteries:
Horizon Problem:
Why is the universe the same temperature everywhere?
Flatness Problem:
Why is the universe almost perfectly flat?
Monopole Problem:
Why don’t we see magnetic monopoles?
Inflation solves all three problems by rapidly stretching the early universe.
---
17.6 Formation of Structure
After recombination:
• dark matter collapsed first
• baryonic (normal) matter fell into dark matter “haloes”
• stars and galaxies formed
• galaxies grouped into clusters
• clusters formed the cosmic web
Tiny quantum fluctuations created during inflation grew into the structure we see today.
---
17.7 What Happened Before the Big Bang?
Science does not yet know.
Current ideas include:
• quantum gravity theories
• bouncing/cyclic universes
• multiverse scenarios
• emergent spacetime models
The Big Bang describes the evolution of the universe from the earliest moment we can model —
but not the ultimate “origin” itself.
---
17.8 The Future of the Universe
The fate of the universe depends on dark energy:
• Accelerating forever (heat death)
• Rapid acceleration (Big Rip)
• Slowdown or reversal (Big Crunch)
• Cycles of expansion and collapse (Big Bounce)
Current evidence suggests continued acceleration → heat death scenario.
---
Chapter Summary
• The Big Bang describes the expansion of the universe from a dense, hot state.
• Inflation rapidly expanded the universe in its earliest moments.
• The CMB is leftover light from 380,000 years after the Big Bang.
• Dark matter formed the first structures.
• The universe evolved into stars, galaxies, and the cosmic web.
• The ultimate origin — before the Big Bang — remains unknown.
• Dark energy controls the universe’s long-term fate.
---
Practice Questions
1. What is the Big Bang, and why is it not an explosion in space?
2. What happened during the inflationary era?
3. Why is the CMB important evidence for the Big Bang?
4. How did tiny early fluctuations grow into galaxies?
5. What are two possible futures for the universe?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive
The Big Bang theory describes the origin and early evolution of the universe.
It explains how space, time, matter, and energy first emerged and how the universe expanded
from a hot, dense beginning into the vast cosmic structure we see today.
This chapter explores what happened in the first moments after the Big Bang, how the early universe evolved,
and the evidence that supports this model.
---
17.1 What Was the Big Bang?
The Big Bang was not an explosion in space —
it was the expansion of space itself.
Everything — matter, energy, space, and time — began in a hot, dense state around 13.8 billion years ago.
As the universe expanded:
• it cooled
• matter formed
• atoms emerged
• galaxies eventually appeared
The Big Bang describes the beginning of the universe's expansion, not the creation of something inside empty space.
---
17.2 The First Moments — Cosmic Timeline
0 seconds — The Beginning
Physics breaks down.
Quantum gravity needed to describe this moment.
10⁻³⁶ seconds — Inflation
A burst of exponential expansion.
The universe grew from subatomic size to cosmic size almost instantly.
Smoothed out the universe and created tiny quantum fluctuations.
10⁻³² seconds — End of Inflation
Energy transferred into matter and radiation.
10⁻⁶ seconds — Quark Era
The universe was a hot soup of quarks and gluons.
No atoms or protons yet.
1 second — Formation of Protons & Neutrons
Quarks combined as the universe cooled.
3 minutes — Nucleosynthesis
The first nuclei formed (hydrogen, helium, a little lithium).
380,000 years — Recombination
Electrons joined nuclei → atoms formed.
The universe became transparent.
Cosmic Microwave Background (CMB) was released.
200 million years — First Stars
Gravity pulled gas together to ignite the first stars.
1 billion years — First Galaxies
Galaxies formed and large-scale structure appeared.
13.8 billion years — Today
The universe is still expanding and cooling.
---
17.3 Cosmic Microwave Background (CMB)
The CMB is the oldest light in the universe —
released when the universe became transparent.
Key features:
• Temperature: 2.7 K
• Almost uniform
• Tiny fluctuations reveal early density variations
• Discovered in 1965 by Penzias & Wilson
The CMB is one of the strongest pieces of evidence for the Big Bang.
---
17.4 Evidence for the Big Bang
Astrophysicists rely on three major lines of evidence:
1. Cosmic Expansion
Galaxies are moving away from us.
Hubble’s Law shows space is expanding.
2. CMB Radiation
A perfect leftover glow from the early universe.
3. Element Abundances
The early universe created:
• 75% hydrogen
• 25% helium
• trace lithium
Exactly matching Big Bang predictions.
Together, this evidence strongly supports the Big Bang model.
---
17.5 Inflation — Solving Big Problems
The theory of inflation explains several mysteries:
Horizon Problem:
Why is the universe the same temperature everywhere?
Flatness Problem:
Why is the universe almost perfectly flat?
Monopole Problem:
Why don’t we see magnetic monopoles?
Inflation solves all three problems by rapidly stretching the early universe.
---
17.6 Formation of Structure
After recombination:
• dark matter collapsed first
• baryonic (normal) matter fell into dark matter “haloes”
• stars and galaxies formed
• galaxies grouped into clusters
• clusters formed the cosmic web
Tiny quantum fluctuations created during inflation grew into the structure we see today.
---
17.7 What Happened Before the Big Bang?
Science does not yet know.
Current ideas include:
• quantum gravity theories
• bouncing/cyclic universes
• multiverse scenarios
• emergent spacetime models
The Big Bang describes the evolution of the universe from the earliest moment we can model —
but not the ultimate “origin” itself.
---
17.8 The Future of the Universe
The fate of the universe depends on dark energy:
• Accelerating forever (heat death)
• Rapid acceleration (Big Rip)
• Slowdown or reversal (Big Crunch)
• Cycles of expansion and collapse (Big Bounce)
Current evidence suggests continued acceleration → heat death scenario.
---
Chapter Summary
• The Big Bang describes the expansion of the universe from a dense, hot state.
• Inflation rapidly expanded the universe in its earliest moments.
• The CMB is leftover light from 380,000 years after the Big Bang.
• Dark matter formed the first structures.
• The universe evolved into stars, galaxies, and the cosmic web.
• The ultimate origin — before the Big Bang — remains unknown.
• Dark energy controls the universe’s long-term fate.
---
Practice Questions
1. What is the Big Bang, and why is it not an explosion in space?
2. What happened during the inflationary era?
3. Why is the CMB important evidence for the Big Bang?
4. How did tiny early fluctuations grow into galaxies?
5. What are two possible futures for the universe?
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive