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The Structure of the Universe — Expansion, Geometry & Dark Energy Explained - Leejohnston - 11-13-2025
The Structure of the Universe — Expansion, Geometry & Dark Energy Explained
Cosmology asks the biggest questions in science:
• What is the universe made of?
• How does it evolve?
• Why is it expanding?
• What is dark energy?
• What is the shape of spacetime itself?
This thread introduces the foundations of modern cosmology in a clear, accessible way.
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1. The Expanding Universe
In 1929, Edwin Hubble discovered that galaxies are moving away from us — and the further away they are, the faster they recede.
This tells us:
The universe itself is expanding, not galaxies moving through space.
This expansion is described by the metric in general relativity and encoded in the Friedmann–Lemaître–Robertson–Walker (FLRW) model.
Key features:
• expansion rate given by the Hubble constant
• space stretches uniformly
• no centre of the universe
• galaxies are carried with spacetime like raisins in rising dough
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2. Spacetime Geometry
The universe has three large-scale geometric possibilities:
• Flat — parallel lines stay parallel
• Open (negative curvature) — like a saddle
• Closed (positive curvature) — like a sphere
Current measurements (Planck satellite):
The universe is extremely close to flat.
This geometry affects:
• how light travels
• the fate of the universe
• gravitational lensing
• the distribution of galaxies
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3. The Role of Dark Energy
Dark energy is the mysterious force causing the universe’s expansion to accelerate.
We observe acceleration through:
• distant supernova measurements
• cosmic microwave background data
• large-scale structure surveys
Leading model:
ΛCDM — Lambda Cold Dark Matter
Dark energy (Λ) makes up about:
• 68% of the universe
Dark matter:
• 27%
Atoms and normal matter:
• only 5%
Dark energy could be:
• a cosmological constant Λ
• a dynamic field (quintessence)
• vacuum energy
• something entirely unknown
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4. The Cosmic Microwave Background (CMB)
The CMB is the oldest light in the universe — a snapshot of the cosmos 380,000 years after the Big Bang.
It tells us about:
• early universe density
• curvature
• composition
• primordial fluctuations
• seeds of galaxies
Temperature variations in the CMB reveal the universe’s structure at the earliest possible moment.
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5. The Large-Scale Structure of the Universe
Galaxies are not randomly scattered — they form:
• clusters
• superclusters
• filaments
• walls
• cosmic voids
This creates the cosmic web: a gigantic 3D network shaped by dark matter gravity.
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6. Ultimate Fate of the Universe
Depending on dark energy, the universe may:
• Expand forever
(becoming colder, emptier → “Heat Death”)
• Slow, then collapse
(Big Crunch — unlikely based on current data)
• Accelerate uncontrollably
(Big Rip — if dark energy gets stronger over time)
We don’t yet know which scenario is correct.
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7. Beginner Practice Questions
1. Why is the universe expanding?
2. How does the CMB help us understand the early universe?
3. What is the difference between dark matter and dark energy?
4. What does cosmic geometry tell us?
5. Why is the ΛCDM model so widely accepted?
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Summary
This post introduced:
• the expanding universe
• spacetime geometry
• dark energy
• the CMB
• cosmic structure
• the universe’s possible futures
Cosmology is the foundation of modern astrophysics — and a perfect starting point for deeper exploration here at The Lumin Archive.