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.
Baryon asymmetry
The tiny excess of matter over antimatter in the early universe — roughly one survivor per billion pairs, and the reason anything material remains.
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.
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 — the energy density of empty space. Discarded, resurrected, and now the worst prediction gap and strongest anthropic datapoint in physics.
Dark matter
Matter inferred from gravity but invisible to light. It outweighs ordinary matter and supplied the early scaffolding in which galaxies formed.
Decoherence
The loss of visible quantum interference when a system entangles with its environment — how branches become effectively separate without a collapse rule.
Drake equation
Frank Drake's factor chain for counting communicative civilizations: half is now measured, half is terra incognita — the answer swings from one to millions.
Eternal inflation
Inflation that ends locally but continues elsewhere, continually producing pocket universes and turning a beginning mechanism into a multiverse.
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 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. 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.
Hoyle state
An excited level in carbon-12 that accelerates the triple-alpha reaction, allowing red giants to manufacture abundant carbon across cosmic history.
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.
Observer-selection effect
A bias created because only observations compatible with an observer enter the sample. The missing cases are not merely unseen; nobody exists there to see them.
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.
Reference class
The population you treat yourself as sampled from: all humans, intelligent observers, or observer-moments — a choice that can reverse the conclusion.
String landscape
The enormous set of metastable vacuum states allowed by string theory, each potentially presenting low-energy physics with different constants.
Tegmark levels
Max Tegmark's four-floor taxonomy of multiverses: repeated space, inflationary bubbles, quantum branches, all mathematics. Fine-tuning lives on floor two.
Typicality
The assumption that our observation is not an extreme draw from the relevant population — powerful enough to predict and dangerous enough to mislead.