ScienceVerseLearning path: Cosmic expansion
The concepts to learn before Cosmic expansion, in order. Each step links to its page and, where one exists, its quiz.
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Your path
- GalaxiesDirect prerequisite
A galaxy is an enormous system of stars, gas, dust and dark matter held together by gravity. Our Sun is one of about 100 billion stars in the Milky Way, a barred spiral galaxy, and beyond it lie countless other galaxies — a fact only proved in the 1920s. How fast galaxies spin reveals that most of their mass is invisible dark matter.
- Space-timeDirect prerequisite
Space-time is the single four-dimensional framework of three space dimensions and one time dimension in which every event happens. Einstein's general relativity explains gravity as the curving of space-time by mass and energy, which bends the paths of light and objects and makes clocks run at different rates. These predictions have passed every test so far, from the 1919 eclipse to the GPS satellites in your phone.
- Cosmic expansionYour goal
Cosmic expansion is the stretching of space that carries distant galaxies away from one another, so that the farther away a galaxy is, the faster it recedes (Hubble's law). Traced back, it implies a hot, dense beginning about 13.8 billion years ago, whose afterglow we still detect. Since 1998 we have known the expansion is speeding up, driven by an unexplained 'dark energy' — and today's best measurements of how fast it is expanding do not agree, an open problem called the Hubble tension.