How to Find a Galaxy

  • By Ari Daniel
  • Posted 01.28.16
  • NOVA

Margaret Geller set to work on a simple question that would yield a stunning answer—how are the galaxies in our universe distributed? She and her colleagues wound up discovering the largest patterns in nature, on a scale of hundreds of millions of light years. And the result tests the theory of how structure forms in the universe.

Running Time: 04:24


Ari: This story is as big as it gets.

Geller: It’s really the longest journey a human being can take. 

Ari: It’s a journey to the outer reaches of the universe. And here’s the question: how are the tens of billions of galaxies in this universe of ours distributed? Are they scattered randomly, or is there some kind of pattern?

Margaret Geller began her quest to map the universe in the 1970s.

Geller: We were limited to using a very small telescope. And it was a slow process: 25 minutes a galaxy.

Ari: 25 minutes to determine each galaxy’s 3D position. The latitude and longitude are easy. They come from the image that the telescope takes.

Geller: But galaxies don’t come with little tags saying how far away they are.

Ari: And so what’s an astrophysicist to do, but get clever, by measuring something called the redshift.

Geller: And we do that by spreading the light of a galaxy out into its colors, and then we can see features that tell us how rapidly it appears to move away from us.

Ari: The faster a galaxy’s moving away, the more we see its light getting stretched towards longer wavelengths, and the larger the redshift.

Geller: That’s proportional to the distance, so that’s how we know how far away a galaxy is.

Ari: Now, Geller couldn’t survey the entire sky. It’s the classic too much universe, too little time problem.

Geller: Let’s suppose you were going to see the Earth for the first time. And you wanted to know, does it have continents and oceans? Well, if you take a small patch, you won’t answer that question. It’ll land in the ocean, and it won’t tell you anything. But if you take a great circle in almost any orientation around the Earth, it’ll pass through continents and oceans, and it’ll tell you the Earth has two kinds of structures, both big. Simple question, simple answer. In the universe, of course, it’s a 3D place, so the analogy is you take an orange, and you cut a slice in it. So we observed galaxies in a slice of the universe.

Ari: And it worked. She and her colleagues plotted the locations of a thousand different galaxies up to 700 million light years away.

Geller: When we saw the data, it really was amazing because there was this glorious pattern. The galaxies are all in very thin structures, which surround, or nearly surround, vast, dark regions where there are very few, if any galaxies. And it was known that there were dark regions, it was known that there were clusters of galaxies, but what wasn’t known was, what was the general structure of the universe. This was the first time you could really see it—patterns that extend for hundreds of millions of light years.

Ari: In her subsequent maps—like this one—that went way deeper and contained tens of thousands of galaxies, the pattern held. Margaret Geller and her two colleagues had found the continents and oceans of our universe.

Geller: And here you are, the first three people ever to see this. For that moment, you’re the only three that know it. That’s it. Of the billions of people in the world, it’s yours. And so for that moment, you own the universe.

Ari: And then, Geller gave the universe to the world, helping reshape not just our understanding of how matter’s distributed in space, but also how that matter got there. You see, there’s a kind of faint radiation that fills outer space called cosmic microwave background. It’s a remnant of the early universe.

Geller: That radiation has little, tiny ripples in it.

Ari: Ripples in temperature.

Geller: Some regions are just a little bit hotter, or just a little bit cooler than others.

Ari: And those differences in temperature are related to differences in the matter density of the early universe.

Geller: So what happens is that gravity amplifies these tiny, little ripples in the matter distribution. And so in an expanding universe, two things happen: gravity likes to make lumps, and it also turns out that gravity likes to make holes. So if you start a hole, gravity will make the hole bigger. And that makes the structure that we observe today.

Ari: On cosmic scales, it’s all about gravity. But on human-sized scales, for Margaret Geller, it’s about something else.

Geller: The journeys that we take in science are journeys of the imagination. It’s a measure of our curiosity and our reach. It’s what makes us grand, in my opinion.



Production & Narration
Ari Daniel
Production Assistance
Ceri Riley
Greg Kestin
Original Footage
© WGBH Educational Foundation 2016


Additional Visuals
WorldWide Telescope (see more here)
American Astronomical Society
Igor Chilingarian
Dan Coe, Elinor Medezinski, & Mario Nonino
de Lapparent, Geller, & Huchra
Daniel G. Fabricant
Ho Seong Hwang & Margaret Geller
Michael Kurtz


(main image: Galaxy)
Daniel G. Fabricant

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