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  • [♪ INTRO]

  • Humans have been looking up at the stars for thousands of years, but it was only within

  • the past century that we learned there's more than one galaxy out there.

  • Then, ever since the 1920s, astronomers have been finding more and more,

  • farther and farther away.

  • But how many galaxies are there in the universe, really?

  • Well, we'll never have an exact number,

  • but astronomers have ways of coming up with pretty good estimates.

  • It doesn't seem like it'd be all that hard to calculate the total number of galaxies.

  • Just take a picture of the sky and count up all the ones you can see.

  • Right? Well, it's more complex than that.

  • How many galaxies your image will capture depends on how sensitive your equipment is,

  • and how long you leave it running to catch all the super dim stuff.

  • It also depends on what wavelengths of light it can detect.

  • Because the universe is expanding, light from distant galaxies gets stretched and shifted

  • toward the longer end of the electromagnetic spectrum.

  • This effect is known as redshift.

  • That means there could be galaxies whose light is redshifted so much that they're no longer

  • visible to your eyes or your camera.

  • So to see the oldest galaxies, with the most redshifted light,

  • you need special, infrared telescopes.

  • We've been using those for over a decade,

  • but we've been counting galaxies for much longer.

  • The first real step toward estimating the total number of them came back in 1996, when

  • NASA published the Hubble Deep Field Image.

  • Over a period of 10 days, the Hubble Space Telescope imaged a portion of the sky 1/30th

  • the diameter of the full Moon, using visible and ultraviolet light.

  • It was a portion you or I would say looked pretty darn empty.

  • But it definitely wasn't.

  • In fact, there were thousands of objects there, from pinpricks to large smudges.

  • Most were so faint that no telescope had ever seen them before.

  • Statistically speaking, the distribution of galaxies in the sky is uniform, so in each

  • speck of sky you have roughly the same number of galaxies.

  • Extrapolating from the Hubble Deep Field, that's 120 billion galaxies total.

  • A decade later, Hubble completed a new, deeper survey of a different part of the sky, producing

  • the Hubble Ultra Deep Field.

  • Thanks to the newer equipment, that upped the estimate to over 200 billion.

  • Altogether, with the various fields Hubble and other telescopes have provided, astronomers

  • estimated that there are between 100 and 200 billion galaxies in the observable universe.

  • And they've been working with that number for the past two decades.

  • But in 2016, a paper published in the Astrophysical Journal threw that number out the window.

  • It claimed a more accurate value was ten times that: 2 trillion galaxies.

  • To figure this out, the research team used deep space images from Hubble and other telescopes,

  • as well as work published by other astronomers about the very early universe.

  • See, nearby galaxies are relatively easy to count, because they're usually pretty bright.

  • But the farthest galaxies are mostly too dim to be detected.

  • In this study, the team found a way around that.

  • Because it takes light billions of years to travel from those galaxies to Earth, we can

  • only see them as they were in the distant past.

  • So by using data about the early universe, the scientists developed a new mathematical

  • model to estimate how many super far, dim galaxies are actually out there.

  • The model could infer the presence of those dim galaxies that couldn't be visually detected.

  • Even though they were too dim to show up in the image, the computer could tell they were

  • supposed to be there based on the number, mass,

  • brightness, and distance of the galaxies it could detect.

  • And that's where 2 trillion came from.

  • As they looked back in time and counted those distant galaxies, the team also noticed something

  • astronomers had seen before: The farther back they looked, the more galaxies there were.

  • In fact, when the universe was less than a billion years old,

  • there were way more galaxies than there are today.

  • That isn't actually that surprising.

  • We've thought for a while that, over time, early galaxies merged to make a smaller number

  • of physically larger galaxies.

  • This study just supported that.

  • But that also brings up a new problem: All those early galaxies don't actually exist anymore.

  • They likely merged billions of years ago, but because it's taken light so much time

  • to reach Earth, we can only see them from before those mergers happened.

  • So we don't really know how many galaxies there are in those parts of the universe.

  • This also means that there are galaxies so far away that their light will never reach

  • us, because the universe is also expanding.

  • They're beyond what astronomers call the cosmological horizon,

  • or outside the observable universe.

  • So the total number of galaxies in the entire universe is basically unknowable.

  • But that doesn't mean we won't stop learning,

  • and that 2 trillion number isn't necessarily our final count.

  • More powerful telescopes, like the James Webb Space Telescope, which will hopefully launch

  • in 2019, will allow astronomers to collect more accurate data.

  • The JWST is 100 times more powerful than the Hubble, and will be able to observe galaxies

  • in those longer wavelengths.

  • In the meantime, with at least 2 trillion galaxies out there, it's safe to say that

  • every point in the sky is completely covered by galaxies.

  • We definitely can't see all of them.

  • But it's kind of comforting to know they're out there, bathing us in invisible starlight.

  • Thanks for watching this episode of SciShow Space!

  • If you've ever wondered how many stars there are in the universe, we made a video about

  • that, too, and you can watch it on the main SciShow channel.

  • [♪ OUTRO]

[♪ INTRO]

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銀河の数は? (How Many Galaxies Are There?)

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    林宜悉 に公開 2021 年 01 月 14 日
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