11-16-2025, 07:33 PM
? THE EXPANSION OF THE UNIVERSE — EXPLAINED SIMPLY & CLEARLY
A Lumin Archive Learning Thread — Cosmology & Universal Structure
✨ 1. What Does “Expansion” Actually Mean?
Most people imagine galaxies flying away through space.
That’s close… but not quite right.
Space itself is expanding.
Galaxies are not moving “through” space — the space between them is getting bigger.
A useful analogy:
• Imagine dots drawn on a balloon.
• When the balloon inflates, the dots don’t move — the surface stretches.
• Every dot sees every other dot moving away.
The universe behaves the same way.
? 2. The Key Evidence: How We Know Space Is Expanding
• Redshift:
Light from distant galaxies is stretched to longer (redder) wavelengths.
→ This happens when space expands while light travels through it.
• Hubble’s Law:
The farther a galaxy is, the faster it appears to be receding.
v = H₀ × d
• v = recession speed
• d = distance
• H₀ ≈ 70 km/s per megaparsec (current estimate)
So a galaxy 1 megaparsec away appears to recede at 70 km/s.
A galaxy 2 megaparsecs away appears to recede at 140 km/s.
This linear relationship is one of the strongest pieces of evidence we have.
⏳ 3. The Expansion Is NOT Slowing Down — It’s Accelerating
Astronomers expected gravity to slow the expansion.
Instead, they discovered:
The universe is expanding faster and faster.
This shocking discovery (1998) led to the idea of:
Dark Energy — a mysterious, repulsive component of the universe that:
• pushes space apart
• accelerates expansion
• dominates the universe today (~68%)
? 4. What Expanding Space Does *Not* Mean
• Atoms do not expand
• People do not expand
• Planets don’t drift apart
• The Solar System stays bound
• The Milky Way isn’t expanding
Gravity and electromagnetic forces overpower cosmic expansion at small scales.
Expansion only dominates on intergalactic distances.
? 5. The Observable Universe vs. The Entire Universe
Because space expands, distant light can never reach us if the expansion outruns the light.
This creates two important boundaries:
• Observable Universe: The part we can see (~46 billion light-years radius).
• Unobservable Universe: Everything beyond that, which may be infinitely larger.
Expansion means the observable universe grows in size…
but the total universe might be far bigger than we will ever observe.
? 6. Quick Summary — For Revision or Teaching
• Space itself expands
• Distant galaxies recede because of expanding space
• Redshift and Hubble’s Law confirm this
• Expansion is accelerating (dark energy)
• Local structures (galaxies, planets) are unaffected
• The observable universe grows, but we will never see all of reality
This thread gives the foundation for understanding dark energy, the cosmic fate, and modern cosmology.
Written for The Lumin Archive — Clear science for curious minds.
A Lumin Archive Learning Thread — Cosmology & Universal Structure
✨ 1. What Does “Expansion” Actually Mean?
Most people imagine galaxies flying away through space.
That’s close… but not quite right.
Space itself is expanding.
Galaxies are not moving “through” space — the space between them is getting bigger.
A useful analogy:
• Imagine dots drawn on a balloon.
• When the balloon inflates, the dots don’t move — the surface stretches.
• Every dot sees every other dot moving away.
The universe behaves the same way.
? 2. The Key Evidence: How We Know Space Is Expanding
• Redshift:
Light from distant galaxies is stretched to longer (redder) wavelengths.
→ This happens when space expands while light travels through it.
• Hubble’s Law:
The farther a galaxy is, the faster it appears to be receding.
v = H₀ × d
• v = recession speed
• d = distance
• H₀ ≈ 70 km/s per megaparsec (current estimate)
So a galaxy 1 megaparsec away appears to recede at 70 km/s.
A galaxy 2 megaparsecs away appears to recede at 140 km/s.
This linear relationship is one of the strongest pieces of evidence we have.
⏳ 3. The Expansion Is NOT Slowing Down — It’s Accelerating
Astronomers expected gravity to slow the expansion.
Instead, they discovered:
The universe is expanding faster and faster.
This shocking discovery (1998) led to the idea of:
Dark Energy — a mysterious, repulsive component of the universe that:
• pushes space apart
• accelerates expansion
• dominates the universe today (~68%)
? 4. What Expanding Space Does *Not* Mean
• Atoms do not expand
• People do not expand
• Planets don’t drift apart
• The Solar System stays bound
• The Milky Way isn’t expanding
Gravity and electromagnetic forces overpower cosmic expansion at small scales.
Expansion only dominates on intergalactic distances.
? 5. The Observable Universe vs. The Entire Universe
Because space expands, distant light can never reach us if the expansion outruns the light.
This creates two important boundaries:
• Observable Universe: The part we can see (~46 billion light-years radius).
• Unobservable Universe: Everything beyond that, which may be infinitely larger.
Expansion means the observable universe grows in size…
but the total universe might be far bigger than we will ever observe.
? 6. Quick Summary — For Revision or Teaching
• Space itself expands
• Distant galaxies recede because of expanding space
• Redshift and Hubble’s Law confirm this
• Expansion is accelerating (dark energy)
• Local structures (galaxies, planets) are unaffected
• The observable universe grows, but we will never see all of reality
This thread gives the foundation for understanding dark energy, the cosmic fate, and modern cosmology.
Written for The Lumin Archive — Clear science for curious minds.