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  • [ ♪ Intro ]

  • Although it might not seem obvious when you look at the night sky,

  • we live in a universe that's expanding faster by the instant.

  • Every day, stars fall over the horizon of what we can see,

  • as the space between us stretches faster than their light can reach us.

  • And we can never know what exists past that horizon.

  • So you might imagine, or you might have heard about, a far-off future, where space is stretching faster and faster,

  • and where all of the stars and galaxies are over that edge.

  • A future where Earth will be left with a dark, empty sky.

  • But luckily for us, or, at least, for hypothetical future earthlings, that's not actually the case.

  • Because the universe is expandingbut not all of it.

  • We've known that the universe is expanding since the 1920s,

  • but we only discovered that the expansion is accelerating in the 1990s, thanks to the Hubble Space Telescope.

  • Hubble was the first tool to measure really precise distances

  • to supernovas out near the edge of the observable universe.

  • And it showed us that out there, ancient galaxies, and the supernovas in them,

  • are zooming away from us faster than anywhere else.

  • In fact, astronomers realized that they were flying away even faster than expected.

  • Which, at first, didn't make sense.

  • At the time, we thought the universe was dominated by gravity, which pulls things together.

  • So seeing everything accelerate apart was weird.

  • It would kind of be like if you kicked a ball uphill and saw it speed up instead of coming back down to you.

  • Because of this, scientists concluded that there had to be something else going on,

  • something pushing these galaxies apart.

  • They came to call that thing dark energy.

  • Decades later, dark energy is still really mysterious,

  • and there's a lot we don't understand about it.

  • One explanation is that it's a property of empty space.

  • This means that space itself, with no stuff in it at all, has dark energy.

  • And that energy pushes space apart, creating new space, which in turn has dark energy,

  • which pushes space apart, creating new space, which in turn has dark energy, which

  • You get it.

  • If dark energy is a property of space, that also means you can't dilute it.

  • Its density will always be the same, no matter how much space expands.

  • Of course, that density is also pretty small.

  • If you borrow Einstein's “E=mc2” trick and express energy as mass,

  • it's equivalent to about one grain of sand in a space the size of the entire Earth.

  • But if you average that over the whole universe, which is mostly empty space,

  • there's more dark energy than anything else.

  • So it dominates, and the universe as a whole expands.

  • That's why the most ancient galaxies are also moving away the fastest:

  • It's taken a long time for their light to reach us, so the universe has had more time to stretch.

  • Now, this might all make dark energy seem super strong.

  • After all, it makes up more than two-thirds of all the stuff in the universe,

  • and it's pushing apart entire galaxies.

  • But it's only powerful because there's a lot of it.

  • Within small spaces, especially those full of planets and stars, dark energy is actually pretty weak.

  • Like, the gravity between the Sun and the Earth, or the Earth and the Moon,

  • is more than enough to overpower the repulsive dark energy between them.

  • In fact, most of the universe's mass is concentrated in galaxy clusters,

  • and these pockets of matter are completely immune to dark energy.

  • They're simply not expanding.

  • And I don't mean the expansion is negligible, like how technically your gravity pulls ever-so-slightly on Earth

  • but it's not enough to actually notice.

  • I mean that, as far as we know, dark energy is truly not stretching our galaxy at all.

  • This is because it's not a force like gravity, so it works a little differently.

  • To understand how, think about pushing on a heavy door.

  • If you push lightly, it won't open.

  • Push a little harder and it still won't.

  • But if you push hard enough, once you cross a certain threshold of pushing, it will open.

  • That door is gravity and, within a galaxy,

  • there's just not enough dark energy to push it open.

  • In other words, gravity is too strong.

  • So our galaxy will never expand, because if it can't stretch even a little,

  • then it can't create more space.

  • And that means the amount of dark energy inside will never grow.

  • Of course, this isn't something we've been able to directly observe,

  • like by looking at other galaxies.

  • But multiple observations have shown us what dark energy is like,

  • and they all suggest this should be true.

  • Eventually, in the really distant future, fewer and fewer galaxies will be visible from Earth.

  • And in 100 billion years or so, deep space will be almost empty.

  • But if Earth were still around by then, which, admittedly, is pretty unlikely,

  • we'd still have a beautiful night sky.

  • Even as the universe stretches, the glow of our galaxy will still be overhead,

  • and we'll have stars, constellations, and even a handful of galaxies bound by gravity to ours.

  • All because dark energy just can't get a foothold around here.

  • Of course, this will only last until the heat death of the universe

  • but that's another story.

  • Thanks for watching this episode of SciShow Space, especially to our patrons on Patreon!

  • Because of you, we're able to keep exploring big topics like dark energy,

  • along with all of the other amazing things that make our universe so cool.

  • If you'd like to support free space education online and help us make more content like this,

  • you can go to patreon.com/scishow.

  • [ ♪ Outro ]

[ ♪ Intro ]

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B1 dark energy dark energy universe space gravity

The Universe Is Expanding... But Not Everywhere

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    林宜悉 posted on 2020/03/30
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