Major Cosmological Theories

Why This Matters

Cosmology asks the biggest questions physics can pose: How did the universe begin? What is its ultimate fate? Will it end, or has it always existed? You're being tested not just on the names of theories but on the underlying mechanisms each proposes—singularities, expansion dynamics, dimensional frameworks, and cyclical versus linear time. Understanding these distinctions helps you evaluate how physicists approach problems when direct observation is impossible.

The theories in this guide fall into distinct categories based on what question they're trying to answer. Some address origins, others tackle unification of forces, and still others challenge our assumptions about the nature of reality itself. Don't just memorize definitions—know what problem each theory solves and how it differs from competing explanations. That's where exam questions live.

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Origin and Expansion Theories

These theories address the fundamental question: How did the universe begin, and how has it evolved? Each proposes a different mechanism for cosmic origins and the expansion of spacetime.

Big Bang Theory

• Singularity origin—proposes the universe began as an infinitely dense point approximately 13.8 billion years ago, then rapidly expanded
• Cosmic microwave background (CMB) radiation provides the strongest observational evidence, representing the "afterglow" of the early universe
• Galactic redshift confirms ongoing expansion; distant galaxies moving away faster supports an expanding universe model

Inflation Theory

• Exponential expansion—proposes space expanded faster than light in the first $10^{-36}$ to $10^{-32}$ seconds after the Big Bang
• Horizon problem solved—explains why distant regions of the universe have uniform temperature despite never being in causal contact
• Flatness problem addressed—accounts for why the universe's geometry is so precisely flat, a condition otherwise requiring extreme fine-tuning

Steady State Theory

• Continuous creation—proposes matter is constantly created to maintain constant density as the universe expands, meaning no beginning or end
• Perfect cosmological principle extends uniformity across both space and time, unlike the Big Bang's evolving universe
• Largely abandoned after CMB discovery in 1965; cosmic evolution observations contradicted the "unchanging universe" premise

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Cyclical Universe Models

Rather than a single origin, these theories propose the universe undergoes repeated phases of expansion and contraction. They challenge linear time and avoid the philosophical problem of "what came before."

Oscillating Universe Theory

• Big Crunch mechanism—gravity eventually halts expansion and reverses it, collapsing the universe back to a singularity
• Infinite cycles of Big Bang → expansion → contraction → Big Crunch → new Big Bang, with no ultimate beginning or end
• Thermodynamic challenges—entropy increase across cycles raises questions about whether true repetition is physically possible

Cyclic Model

• Periodic but not identical—each cycle features expansion and contraction, but conditions may vary between cycles
• Dark energy role—modern versions incorporate accelerating expansion, adjusting the collapse mechanism accordingly
• Avoids initial singularity problem by proposing the universe has always existed in some form, eliminating the "creation from nothing" question

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Higher-Dimensional Frameworks

These theories extend physics beyond four-dimensional spacetime, proposing that extra dimensions explain phenomena we can't account for otherwise. They attempt to unify gravity with quantum mechanics.

String Theory

• One-dimensional strings—replaces point particles with vibrating strings; different vibration modes produce different particles
• Unification goal—aims to reconcile general relativity (gravity) with quantum mechanics, the two incompatible pillars of modern physics
• Extra dimensions required—mathematically consistent only with 10 or 11 total dimensions; the additional 6-7 are compactified at scales too small to observe

Brane Cosmology

• Brane universe—proposes our 3D universe exists on a membrane ("brane") floating in a higher-dimensional "bulk" space
• Gravity leakage—explains gravity's relative weakness; gravitons may escape into extra dimensions while other forces remain confined to our brane
• Collision cosmology—brane interactions could trigger events resembling the Big Bang without requiring a singularity

Ekpyrotic Universe Theory

• Brane collision origin—proposes our universe formed when two parallel branes collided in higher-dimensional space
• Alternative to inflation—generates the universe's uniformity and flatness through collision dynamics rather than exponential expansion
• Dark energy implications—the separation and potential re-collision of branes may explain cosmic acceleration and predict future collisions

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Reality and Information Theories

These theories challenge fundamental assumptions about what the universe is—questioning whether three-dimensional space is fundamental or emergent. They connect quantum mechanics, gravity, and information theory.

Holographic Universe Theory

• Information on boundaries—proposes all information in a 3D volume can be encoded on its 2D boundary, like a hologram
• Black hole origins—emerged from studying black hole entropy, where information content scales with surface area ($S \propto A$), not volume
• Quantum-gravity bridge—suggests spacetime itself may be emergent from more fundamental quantum information processes

Multiverse Theory

• Infinite parallel universes—proposes our universe is one of countless others, each potentially with different physical constants and laws
• Anthropic principle connection—explains fine-tuning by suggesting we observe this universe because it's the one compatible with our existence
• Testability debate—raises philosophical questions about whether a theory predicting unobservable universes qualifies as scientific

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Quick Reference Table

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Self-Check Questions

1. Which two theories both propose cyclical cosmic evolution, and what distinguishes the modern version from the earlier one?

2. String Theory and Brane Cosmology both involve extra dimensions—what specific cosmological problem does Brane Cosmology address that pure String Theory doesn't?

3. Compare and contrast the Big Bang Theory and Ekpyrotic Universe Theory: What do they agree on about the universe's current state, and where do they fundamentally disagree about origins?

4. If an exam question asks you to identify a theory that was falsified by observational evidence, which theory provides the clearest example, and what observation contradicted it?

5. The Holographic Universe Theory and Multiverse Theory both challenge common-sense notions of reality. Explain how each theory's challenge is fundamentally different in nature.