Glossary
Key terms of the cosmos — explained in one or two sentences, each linked to the full topic.
- Big Bang
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The hot, dense beginning of the universe about 13.8 billion years ago. Space and time themselves began with it, and space has expanded ever since.
Topic: The Big Bang & the universe → - Dark energy
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An unknown form of energy that accelerates the expansion of space. It makes up about 70 percent of the universe and is one of physics’ biggest puzzles.
Topic: The Big Bang & the universe → - Dark matter
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Invisible mass that holds galaxies together through its gravity. We know its effects but not its nature; it makes up about a quarter of the universe.
Topic: The Big Bang & the universe → - Entropy
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A measure of disorder that can also be read as missing information. The more we know about a system, the lower its entropy is for us.
Topic: Information as reality → - Event horizon
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The boundary of a black hole where the escape velocity exceeds the speed of light. Whatever crosses it can no longer escape.
Topic: Black holes → - Exoplanet
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A planet orbiting a star other than the Sun. More than 5,500 are confirmed; many are found through the tiny dimming of their star.
Topic: Exoplanets & life out there → - Habitable zone
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The distance from a star where liquid water is possible — neither too hot nor too cold. It is a key clue in the search for life.
Topic: Exoplanets & life out there → - Hawking radiation
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A tiny thermal glow at the edge of black holes, predicted by Stephen Hawking in 1974. Over enormous timescales it makes black holes shrink.
Topic: Black holes → - Holographic principle
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The idea that all the information of a region of space can be described on its surface — much as a hologram stores 3D data on a flat surface.
Topic: Information as reality → - Light-year
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The distance light travels in one year: about 9.46 trillion kilometers. A light-year is a distance, not a time.
Topic: Exoplanets & life out there → - Natural selection
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The engine of evolution: those better suited to their environment have more offspring on average. Over many generations, useful traits accumulate.
Topic: Evolution → - Quantum entanglement
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Two particles form one shared system. Measure one and the other’s result is instantly fixed — over any distance, but with no information faster than light.
Topic: Quantum mechanics → - RNA world
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The hypothesis that early life relied mainly on RNA. RNA can store information and drive reactions at once, solving a chicken-and-egg problem.
Topic: The origin of life → - Superposition
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A quantum particle can hold several states at once. Only measurement fixes a value; before that, the possibilities exist side by side.
Topic: Quantum mechanics →