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  • The universe is expanding.

  • If you zoom out far enough, youll find that everywhere in the universe is moving

  • away from everywhere else.

  • But once you know the universe is expanding, you might wonder if it’s going to keep expanding

  • somehow, or eventually stop and crunch in on itself because of the attractive force

  • of gravity.

  • Well, all you have to do is point your telescope at a distant galaxy, measure how fast it’s

  • moving away from us, wait about a million years, and then do the same thing again to

  • see if the galaxy is moving faster or slower than before.

  • But if you don’t have a million years to spare, here’s what you do.

  • You look at different galaxies.

  • Because light takes time to reach us, when we look at a galaxy far, far away, we see

  • it as it was a long time ago.

  • So looking at a bunch of galaxies different distances away is kind of a rough proxy for

  • looking at the same one at different times.

  • The light from the stars in a distant galaxy can tell us two basic things.

  • First, its speed: light from anything moving away from you becomes redder in color (kinda

  • like how police sirens get lower in pitch when moving away).

  • This redshifting of light tells us exactly how fast stars are moving away from us.

  • If the light from a star is 5 percent redder than it should be, it means the star is moving

  • away from us at 5 percent the speed of light.

  • Light can also tell us how far away a star (and thus its parent galaxy) is.

  • When youre driving at night, you can tell how far away a motorcycle is by comparing

  • how bright its headlight seems to you, to how bright it would be if you were right in

  • front of it.

  • The same trick works with stars.

  • There’s a rare kind of supernova explosion that’s like a standard interstellar headlight

  • it’s about 5 billion times brighter than our sun, and, crucially, it’s very

  • consistent in its brightness.

  • These only happen a few times in a galaxy every thousand years, but there are enough

  • galaxies in the universe that, if you look closely at a wide enough patch of sky, it’s

  • possible to see a dozen of them go off in a month.

  • By comparing the brightness as seen here on Earth with how bright we know each of them

  • should be, we can work out how far away each supernova is.

  • The dimmer its light, the farther away it is.

  • So the color of a type Ia supernova tells us how fast it’s moving away from us, and

  • its brightness is like a ruler, telling us how far away it isor, because light takes

  • time to get here, how long ago were measuring its speed.

  • By measuring the speed of different supernovae at different distances – aka different

  • times in the pastwe can plot the expansion of the universe over time.

  • A straight line means the universe is expanding at a constant rate, a curve means it’s speeding

  • up or slowing down.

  • For a long time we thought the line was straight, or slightly curved in the slowing down direction.

  • But when astronomers finally looked carefully at supernova in super-distant galaxies, they

  • discovered, to their surprise, that the universe is expanding faster now than it was long ago.

  • The universe’s expansion is accelerating!

  • p.s.

  • We have a few basic ideas for why the universe is accelerating, but we still don’t really

  • know why it’s happening.

  • One of the simplest and most popular ideas is that there’s an everywhere-permeating

  • vacuum energy with negative pressure which weve never detected in any way other than

  • by measuring the expansion of the universe... so we call it dark energy.

  • Thanks to the Wide Field Infrared Survey Telescope project at NASA Goddard Space Flight Center

  • and the Space Telescope Science Institute for supporting this video.

  • WFIRST is planned to launch in the mid-2020s, and will, among other things, allow us to

  • see a much wider swath of the sky at Hubble-like resolutions and even farther distances, and

  • thus hunt for more supernovas many billions of light years away.

  • Thatll help us understand whether the acceleration of the universe is different in different

  • parts of the universe or whether it has changed over time, which should help us figure out

  • what exactly dark energy is.

The universe is expanding.

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宇宙が加速していることをどうやって知るのか? (How Do We Know The Universe Is ACCELERATING?)

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    林宜悉 に公開 2021 年 01 月 14 日
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