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We've been trying to count the galaxies in the universe since the mid '90s, but our estimates change as our tools improve. So what does our current estimate really mean?

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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].