Void Riders

The distribution of galaxies in the region around the Milky Way (in galactic coordinates). Each little dot represents a galaxy of typically 100 billion stars. The colors indicate the relative motions of galaxies with accurately measured distances, with shades of gree and blue indicating motions toward us, and shades of yellow to red indicating motions away from us. (For more information, see the IFA press release.

We are riding on the edge of a huge void in space, hurtling along at 600,000 miles per hour. Yes, it’s true. We’re moving along with the expansion of the universe AND with the coalescence of galaxies along filaments, in clusters at places where the filaments intersect. Yet, as we go about our daily lives, we’re largely (if not completely) unaware of the ride we’re taking through space and time as part of the Milky Way Galaxy. Yet, our motion tracks with the continuing evolution of the universe.

Did that get your attention? It’s a compelling story, and one that astronomer Brent Tully is telling in his latest research into the motions of galaxies in our neighborhood of the universe. In the past, our distance measurements to other galaxies could give us some very broad information about our galaxy’s motion through space. In addition, other measurements of the Cosmic Microwave Background Radiation tell about some aspects of our galaxy’s motion. But, there’s always been a part of our motion that was unexplained—until now. Tully and a team of colleagues have done extremely precise measurements of distances to galaxies around us. Those measurements tell a great story of motion and action of galaxies through time and helped them finger a largely unexplored component of space as the culprit: the Local Void. This is a gap in space that is 50 megaparsecs across. (A megaparsec is about 3,260,000 light-years, and a light-year is the distance that light travels in a year.)

Concentrations of matter (like the filaments and clusters) are aggregates of matter that is pulled together. Concentrations of matter have gravity; they PULL on things. The Local Void, on the other hand, is empty. It seems to PUSH on things, including our galaxy. Tully explains it as the absence of a pull. If an object is surrounded by matter in all directions, except for one empty sector, then the absence of a pull from that sector is actually the same as a push. And, it can have a large effect on our region of space, in effect turning us into void riders moving along the edge of the Local Void.

It’s quite fascinating that we (all of us in the Milky Way Galaxy) should live on the edge of such a huge gap of nothingness; and, we ride along not really feeling our motion across the depths of space and time.