11-15-2025, 10:03 AM
Chapter 9 — Cosmology & the Expansion of the Universe
Cosmology is the study of the universe as a whole — its origin, structure, evolution, and fate.
It asks the biggest questions in science:
• Where did the universe come from?
• How is it changing?
• What is it made of?
• What will happen in the future?
This chapter introduces the fundamental ideas that shape modern cosmology.
---
9.1 The Universe Has a Beginning
Modern cosmology is built on the discovery that:
The universe is expanding.
This was first observed by Edwin Hubble in 1929.
He found that galaxies appear to be moving away from us, and the farther they are, the faster they recede.
This led to the Big Bang theory:
• ~13.8 billion years ago, the universe was extremely hot and dense
• Space itself expanded rapidly
• Matter, energy, and physical laws emerged
The Big Bang was not an explosion *in* space — it was an expansion *of* space.
---
9.2 Hubble’s Law
The relationship between distance and recessional velocity is:
v = H₀ × d
Where:
• v = recessional velocity
• d = distance
• H₀ = Hubble constant
The Hubble constant represents how fast the universe expands **per unit distance**.
This discovery confirmed that space is stretching everywhere.
---
9.3 Redshift — The Key to Measuring the Universe
As space expands, it stretches light.
This makes galaxies appear more “red” (longer wavelengths).
This is called:
Cosmological Redshift
It tells astrophysicists:
• how fast galaxies are moving away
• how far they are
• how old the universe was when the light left
Redshift allows us to see billions of years into the past.
---
9.4 Cosmic Microwave Background (CMB)
The CMB is the oldest light in the universe.
• Formed 380,000 years after the Big Bang
• When atoms first formed and the universe became transparent
• Now cooled to just 2.7 K above absolute zero
• Uniform, with tiny fluctuations that reveal early structure
The CMB is a major pillar of the Big Bang model.
---
9.5 What the Universe Is Made Of
One of the most surprising discoveries:
Most of the universe is invisible.
The approximate composition:
• 5% ordinary matter (stars, gas, planets)
• 27% dark matter
• 68% dark energy
Dark matter holds galaxies together.
Dark energy accelerates the expansion of the universe.
These two mysterious components dominate cosmology.
---
9.6 Dark Energy and Accelerating Expansion
In 1998, observations of distant supernovae revealed:
The universe is expanding faster over time.
This acceleration is driven by dark energy — a form of energy that fills space.
Dark energy acts like negative pressure, pushing space apart.
It determines the long-term fate of the cosmos.
---
9.7 The Fate of the Universe
Current evidence suggests several possible futures:
1. Heat Death (most likely)
• Expansion continues forever
• Stars burn out
• Galaxies drift apart
• Universe becomes dark and cold
2. Big Rip
• Expansion accelerates dramatically
• Galaxies, stars, planets, atoms are torn apart
3. Big Crunch (unlikely today)
• Expansion reverses
• Universe collapses back inward
• Potentially followed by a new Big Bang
The ultimate fate depends on the nature of dark energy — currently unknown.
---
9.8 Large-Scale Structure of the Universe
Galaxies form patterns across billions of light-years:
• Galaxy clusters
• Superclusters
• Filaments
• Walls
• Voids
These structures form the cosmic web, shaped by dark matter and gravity.
Cosmology connects small-scale physics — particles and fields — to this vast structure.
---
Chapter Summary
• Cosmology studies the origin, evolution, and fate of the universe.
• The universe is expanding, as discovered by Hubble.
• Redshift measures this expansion and reveals cosmic history.
• The Cosmic Microwave Background is the oldest observable light.
• Dark matter and dark energy dominate the universe’s mass-energy.
• Expansion is accelerating due to dark energy.
• Future scenarios include heat death, Big Rip, or Big Crunch.
• The universe forms a cosmic web of structures shaped by gravity.
---
Practice Questions
1. What does Hubble’s Law tell us about the universe?
2. How does cosmological redshift work, and what does it reveal?
3. Why is the Cosmic Microwave Background important?
4. What evidence shows that the universe’s expansion is accelerating?
5. Describe one potential long-term fate of the universe.
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive
Cosmology is the study of the universe as a whole — its origin, structure, evolution, and fate.
It asks the biggest questions in science:
• Where did the universe come from?
• How is it changing?
• What is it made of?
• What will happen in the future?
This chapter introduces the fundamental ideas that shape modern cosmology.
---
9.1 The Universe Has a Beginning
Modern cosmology is built on the discovery that:
The universe is expanding.
This was first observed by Edwin Hubble in 1929.
He found that galaxies appear to be moving away from us, and the farther they are, the faster they recede.
This led to the Big Bang theory:
• ~13.8 billion years ago, the universe was extremely hot and dense
• Space itself expanded rapidly
• Matter, energy, and physical laws emerged
The Big Bang was not an explosion *in* space — it was an expansion *of* space.
---
9.2 Hubble’s Law
The relationship between distance and recessional velocity is:
v = H₀ × d
Where:
• v = recessional velocity
• d = distance
• H₀ = Hubble constant
The Hubble constant represents how fast the universe expands **per unit distance**.
This discovery confirmed that space is stretching everywhere.
---
9.3 Redshift — The Key to Measuring the Universe
As space expands, it stretches light.
This makes galaxies appear more “red” (longer wavelengths).
This is called:
Cosmological Redshift
It tells astrophysicists:
• how fast galaxies are moving away
• how far they are
• how old the universe was when the light left
Redshift allows us to see billions of years into the past.
---
9.4 Cosmic Microwave Background (CMB)
The CMB is the oldest light in the universe.
• Formed 380,000 years after the Big Bang
• When atoms first formed and the universe became transparent
• Now cooled to just 2.7 K above absolute zero
• Uniform, with tiny fluctuations that reveal early structure
The CMB is a major pillar of the Big Bang model.
---
9.5 What the Universe Is Made Of
One of the most surprising discoveries:
Most of the universe is invisible.
The approximate composition:
• 5% ordinary matter (stars, gas, planets)
• 27% dark matter
• 68% dark energy
Dark matter holds galaxies together.
Dark energy accelerates the expansion of the universe.
These two mysterious components dominate cosmology.
---
9.6 Dark Energy and Accelerating Expansion
In 1998, observations of distant supernovae revealed:
The universe is expanding faster over time.
This acceleration is driven by dark energy — a form of energy that fills space.
Dark energy acts like negative pressure, pushing space apart.
It determines the long-term fate of the cosmos.
---
9.7 The Fate of the Universe
Current evidence suggests several possible futures:
1. Heat Death (most likely)
• Expansion continues forever
• Stars burn out
• Galaxies drift apart
• Universe becomes dark and cold
2. Big Rip
• Expansion accelerates dramatically
• Galaxies, stars, planets, atoms are torn apart
3. Big Crunch (unlikely today)
• Expansion reverses
• Universe collapses back inward
• Potentially followed by a new Big Bang
The ultimate fate depends on the nature of dark energy — currently unknown.
---
9.8 Large-Scale Structure of the Universe
Galaxies form patterns across billions of light-years:
• Galaxy clusters
• Superclusters
• Filaments
• Walls
• Voids
These structures form the cosmic web, shaped by dark matter and gravity.
Cosmology connects small-scale physics — particles and fields — to this vast structure.
---
Chapter Summary
• Cosmology studies the origin, evolution, and fate of the universe.
• The universe is expanding, as discovered by Hubble.
• Redshift measures this expansion and reveals cosmic history.
• The Cosmic Microwave Background is the oldest observable light.
• Dark matter and dark energy dominate the universe’s mass-energy.
• Expansion is accelerating due to dark energy.
• Future scenarios include heat death, Big Rip, or Big Crunch.
• The universe forms a cosmic web of structures shaped by gravity.
---
Practice Questions
1. What does Hubble’s Law tell us about the universe?
2. How does cosmological redshift work, and what does it reveal?
3. Why is the Cosmic Microwave Background important?
4. What evidence shows that the universe’s expansion is accelerating?
5. Describe one potential long-term fate of the universe.
---
Written and Compiled by Lee Johnston — Founder of The Lumin Archive