What Are Cosmic Voids? The Biggest Nothings in the Universe

We talked about the biggest structures in the Universe, but what about the opposite? The biggest empty spaces in the Universe, the cosmic voids that separate the clusters of galaxies.

Check out our interview with Paul M. Sutter, a specialist on cosmic voids:

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Team: Fraser Cain – @fcain / [email protected]
Jason Harmer – @jasoncharmer
Chad Weber – [email protected]

Created by: Fraser Cain and Jason Harmer
Edited by: Chad Weber
Music: Left Spine Down – “X-Ray”

Clearly I need to learn to be more specific when I do these episodes. Everything time I open my mouth, I need to prepare for the collective imagination of the viewers.

We did a whole video about the biggest things in the Universe, and identified superclusters of galaxies as the best candidate. Well, the part of superclusters actually gravitationally bound enough to eventually merge together in the future.

But you had other ideas, including dark energy, or the Universe itself as the biggest thing. Even love? Aww.

One intriguing suggestion, though, is the idea of the vast cosmic voids between galaxies. Hmm, is the absence of something a thing? Whoa, time to go to art school and talk about negative space.

Ah well, who cares? It’s a super interesting topic, so let’s go ahead and talk about voids.

When most people imagine the expansion of the Universe after the Big Bang, they probably envision an equally spaced smattering of galaxies zipping away from one another. And that’s pretty accurate at the smallest scales.

But at the largest scales, like when you can see billions of light-years in a cube that fits on your computer screen, then a larger structure starts to take shape.

It looks less like an explosion, and more like a tasty tasty sponge cake, with huge filaments, walls, and the vast gaps in between. The gaps, the voids, the supervoids, are the point of today’s video, but to understand the gaps, we’ve got to understand why the Universe is clumped up the way it is.

Run the Universe clock backwards, all the way to the beginning, to a fraction of a second after the Big Bang. When the entire cosmos was compressed down into a tiny region of superheated plasma.

Although it was mostly uniform in density, there were slight variations – quantum fluctuations in spacetime itself. And as the Universe expanded, those differences were magnified. What started out as tiny differences in the density of matter at the smallest scale, turned into regions of higher and lower density of matter in the Universe.

Here we are, 13.8 billion years after the Big Bang, and we can see how the microscopic variations at the beginning of time were magnified to the largest scales. Instead of individual galaxies, we see huge walls containing thousands of galaxies; filaments of galaxies connect in nodes. These structures are huge; hundreds of millions of light-years across, containing thousands of galaxies.

But the gaps, the voids, between these clusters can be even larger.

Astronomers first started thinking about these voids back in the 1970s, when the first large-scale surveys of the Universe were made. By measuring the redshift of galaxies, and determining how fast they were speeding away from us, astronomers started to realize that the distribution of galaxies wasn’t even.

Some galaxies were relatively close, but then there were huge gaps in distance, and then another cluster of galaxies collected together.

Over the last few decades, astronomers have built sophisticated 3-dimensional models that map out the Universe in the largest scales. The Sloan Digital Sky Survey, updated in 2009, has provided the most accurate map so far. The Large Synoptic Survey Telescope, destined for first light in a few years will take this to the next level.

The largest void that we currently know of is known as the Giant Void (original, I know), and it’s located about 1.5 billion light-year away. It has a diameter of 1 billion to 1.3 billion light-years across.

To be fair, these regions aren’t really completely empty. They just have less density than the regions with galaxies. In general, they’ve got about a tenth the density of matter that’s average for the Universe.

Which means that there’s still gas and dust in these regions, as well as dark matter. There will still be stars and galaxies out in the middle of those voids. Even the Giant Void has 17 separate galaxy clusters inside it.