Where 13.8 billion years comes from: the CMB and the cosmic distance ladder together

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Hubble tension: two ways to measure the expansion rate differ by about 9 percent The universe is 13.8 billion years old and 93 billion light years wide: how both can be true Lookback time: the sunlight you feel now left the Sun eight minutes ago Strontium optical clocks would not lose a second in 15 billion years The Great Oxidation Event 2.4 billion years ago and how it rusted the oceans The cosmic microwave background is the oldest light we can ever see Uranium-lead dating in zircon: how a crystal the width of a hair dates the Earth at 4.54 billion years Distant Type Ia supernovae run slow by exactly their redshift factor The personal equation: 19th century astronomers' reaction times started experimental psychology Half-life is the same idea in radiocarbon dating and in pharmacology