Glossary
The recurring vocabulary, defined once and linked everywhere — every term hands you back to the essay or dial where the idea earns its keep.
Anthropic principle
The observation that a universe's features must be compatible with the observers who notice them — we could only have evolved where evolving was possible.
Big Crunch
If gravity wins, expansion halts and reverses: the universe falls back together and reheats into a hot, dense mirror of the Big Bang.
Big Rip
If dark energy's repulsion grows with time, acceleration strengthens without bound and tears apart clusters, galaxies, planets, and finally atoms — on a finite deadline.
Boltzmann brain
A lone brain with false memories, fluctuated briefly out of chaos — if your cosmology predicts more of those than evolved observers, the cosmology is wrong.
Copernican principle
The bet that we do not occupy a special position in the cosmos — astronomy's most productive assumption, and the anthropic principle's careful rival.
Cosmological constant
Einstein's lambda — an energy density of empty space itself. Discarded, resurrected, and now the worst prediction gap and strongest anthropic datapoint in physics.
Drake equation
Frank Drake's factor chain for counting communicative civilizations: half of it is now measured, half is terra incognita — the answer swings from one to millions.
Fermi paradox
The galaxy is old and enormous; life arose here fast; so where is everybody? The tension between a crowded sky on paper and a silent sky in fact.
Fine-tuning
The fact that small changes to the constants or initial conditions of physics leave a universe with no structure, no chemistry, no life — the windows are narrow.
Great Filter
Robin Hanson's name for whatever keeps the sky quiet: some step from dead matter to expanding civilization is nearly impossible. The question is whether it is behind us or ahead.
Habitable zone
The orbital band where a planet can keep liquid water on its surface — the Goldilocks region, and the smallest-scale tuning of all.
Heat death
The far-future state of maximum entropy: no temperature differences, no usable energy, nothing left to happen — the universe ends in equilibrium, not fire.
Multiverse
The proposal that our universe is one region among many, with the constants and laws varying from region to region — the pool that observer selection filters.
Rare Earth hypothesis
The thesis that simple life may be common but complex life needs a stack of rare accidents — stable star, giant moon, plate tectonics, lucky biochemistry.
Tegmark levels
Max Tegmark's four-floor taxonomy of multiverses: repeated space, inflationary bubbles, quantum branches, and all mathematics. The fine-tuning debate lives on floor two.