A diagram showing the location of the Boötes Void in the sky
Very empty space

Richard Powell

Face it, the vast darkness of space is a little eerie. It’s no wonder we usually prefer to focus on the bright spots. But it’s in the void that we might find our best explanations of the cosmos.

In 1923, Edwin Hubble showed that the universe was far larger than expected by discovering that what we thought were swirls of gas on the edge of our own galaxy were actually galaxies in their own right: lonely “island universes” we could spot across an empty sea of black. That led to a comforting thought – we now know that even the darkest patch of sky, when seen through the telescope named after Hubble, is dotted with clumps of luminous stuff like our Milky Way.

But there’s another view of the universe, like the horror cliché of flipping an image to its photonegative. Since 1981, when astronomers found a vacant expanse called the Boötes void, we’ve also known that the universe has holes of cold, dark, lonely nothing that are larger than anyone expected. To truly understand the universe, we may have to gaze into the abyss.

A bubble in space

The Boötes void, which you will assuredly not see if you look at Boötes, the “ploughman” constellation adjacent to the Big Dipper, is a rough sphere about 280 million light years in diameter.

Galaxy-wise, it’s a ghost town. When we first saw the void, we found only one galaxy inside. Since then, we’ve detected only a few dozen more. By contrast, the Virgo Supercluster, a smaller region that includes the Milky Way, contains over 2000 galaxies.

As residents of the Milky Way, humans are able to see one large nearby galaxy, Andromeda, with our naked eyes. The proximity of Andromeda helped Edwin Hubble look at its individual stars to unlock the true scope of the universe. If our galaxy were in the Boötes void, our nearest peers would be much farther away – perhaps allowing us to fancy ourselves at the center of the cosmos for longer.

This is no statistical accident. At very large scales, the universe is often described as a cosmic web, with strands of invisible dark matter undergirding the universe’s luminous structure. It might be better here to think of it as cosmic foam, like soap bubbles in a bathtub. Just as it’s sudsy where bubbles intersect, galaxy clusters concentrate in walls, filaments and intersections. In between is mostly void.

Making peace with the vacuum

The problem was that the Boötes void was just too big. Voids grow because their dense edges have a much stronger gravitational pull than anything at their centres. But the universe wasn’t yet old enough to have inflated such a big bubble.

For an explanation, we had to wait until the 1998 discovery of dark energy:  a cosmic pressure that forces empty regions of space to expand as if someone was blowing air into each of the universe’s soap bubbles all at once.

Many astronomers, now in a boom of cataloging and mapping voids, think these spooky regions that expose the naked fabric of the universe could point to the next big discovery.

Soon, statistical analyses of their shapes may be able to help us measure dark energy, gravity and any mysterious new forces better than ever before. And in the process, perhaps, they will help us learn to embrace the emptiness.

Quelle: NewScientist