Most of the Universe is empty space.
Duh. I mean, sure, that’s why we call it space. But what’s so very interesting about this is what you see when you look on very large scales; scales so huge that galaxies become mere dots. You might expect matter to be strewn evenly throughout the Universe, but it isn’t. Over these vast vistas, matter in the Universe is clumped, falling along huge filaments and sheets. These in turn are curved, closing in on themselves.
The Universe is foamy! It looks like a sponge, with matter clumping along the outside of the bubbles. Shortly after the Universe formed, dark matter clumped up, creating those filaments. These acted like gravitational scaffolding, its gravity attracting normal matter, which then fell onto the filaments like Spanish moss hanging from tree branches. This material formed galaxies and clusters of galaxies.
Matter is still falling into those filaments today. As it does, the voids—the bubbles of the sponge—get bigger. We can predict how they grow using Einstein’s Theory of Relativity, which describes how the Universe expands and how the matter and energy in it behave as it does. This video from the Max Planck Institute for Astrophysics shows that growth in a computer simulation:
But there’s more to this: Dark energy, the weird stuff pervading space that’s causing the Universe to expand faster every day, is also in there, inflating the voids. Relativity, as originally formulated, doesn’t include that. If we can measure just how voids grow we can use that as a test of relativity and also understand better how these gigantic pockets of nothing get bigger. To do this, traditionally, astronomer measure that growth by examining the galaxies along the bubble edges.
But that turns out to be hard, because galaxies aren’t just falling onto the filaments. They also have what’s called peculiar motion, sideways velocity which makes measuring their precise velocity difficult. What to do?
A group of astronomers did something clever: Instead of looking at the galaxies, they looked into the voids themselves. Using observational data that shows distances to galaxies in the Universe—and therefore the locations of voids—they compare how these voids change in shape over time to what’s expected by computer models using relativistic calculations.
Not surprisingly, they find relativity to be pretty robust. The behavior of the Universe appears to obey the rules laid down by relativity, which is reassuring. I’ll note relativity has been tested approximately a bazillion times, and always comes up looking good.
So while that’s not exactly shocking, what I find interesting about this is that by looking at the voids instead of the galaxies around them, the astronomers who did this work were able to improve on previous methods dramatically, with uncertainties (that is, statistical accuracies) four times smaller than previous models!
That’s pretty dang good. We’re still learning just how the Universe behaves on the very largest scaled; heck, dark energy was only discovered in 1998! Research like this will help us understand what the Universe itself is doing as it ages and grows, and that to me is simply stunning. Even if you don’t understand the details of the work or the math behind it, know this: Astronomers are trying to understand literally everything, across all space and time.
And they’re doing a pretty good job of it.