10 Greatest Ideas in the History of Science
The Universe Is Expanding

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About 13.8 billion years ago, the universe underwent a period of rapid expansion, known as cosmic inflation. Immediately after that was the Big Bang. (Yes, cosmic inflation occurred before the Big Bang; see this detailed explanation by our favorite astrophysicist, Ethan Siegel.) Ever since then, the universe has kept right on expanding.

We know the Big Bang occurred because of the telltale evidence it left behind: The cosmic microwave background radiation (CMB). As the universe expanded, the initial burst of light from the Big Bang got stretched. (Remember, light can be both a wave and a particle.) When light is stretched, the wavelength increases. Today, that light is no longer visible with the naked eye because it now inhabits the microwave range of the electromagnetic spectrum. However, you can still "see" it on old-school television sets with antennas; the static on "in-between" channels is partially due to the CMB.

But not only is the universe expanding, its rate of expansion appears to be accelerating due to dark energy. And the further away an object is from Earth, the faster it is accelerating away from us.

If you thought the universe was a lonely place now, just wait 100 billion years. Thanks to dark energy, we won't be able to see any stars beyond our own galaxy (which, at that time, will be a merger between the Milky Way and Andromeda galaxies).

Source: Galileo's Finger: The Ten Great Ideas of Science by Peter Atkins

Image: Gnixon/Wikimedia Commons


‹‹ Classical Mechanics Fails to Describe Small Particles Spacetime Is Curved by Matter ››

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