Why is space a void?
Space is an almost perfect void, full of cosmic voids. And in short, gravity is to blame. But to truly understand the void in our universe, we have to take a moment to understand what a void really is – and what it is not.
So what is a void, and why isn’t space a real void?
First, forget about the vacuum cleaner as an analogy to the vacuum of space, Jackie Faherty, senior scientist in the Department of Astrophysics at the American Museum of Natural History in New York City, told Live Science. The household cleaning machine efficiently fills with dirt and dust sucked from your carpet. (That is, the vacuum cleaner uses differential pressure to create suction. The vacuum cleaner might be a better name than the vacuum cleaner). But the vacuum of space is the opposite. By definition, a vacuum is devoid of matter. Space is almost an absolute void, not because of aspiration but because it is almost empty.
This vacuum results in extremely low pressure. And while it’s impossible to mimic the space vacuum on Earth, scientists can create very low-pressure environments called partial vacuums.
Even without the vacuum cleaner analogy, “understanding the concept of a vacuum is almost foreign because it is so contradictory to the way we exist,” Faherty said. Our experience as humans is completely confined to a very dense, crowded and dynamic fraction of the universe. So, it can be difficult for us to truly understand nothingness or emptiness, she said. But in reality, what is normal for us on Earth, is in fact rare in the context of the universe, the vast majority of which is almost empty.
Gravity is king
On average, space would still be pretty empty even if we didn’t have gravity. “There just isn’t a whole lot compared to the volume of the universe you put these things in,” according to Caltech theoretical astrophysicist Cameron Hummels. The average density of the universe, according to NASA, is 5.9 protons (a subatomic particle) per cubic meter. But then gravity amplifies the vacuum in certain regions of the universe by bringing together matter in the universe.
Basically, two objects with mass will be attracted to each other. It’s gravity. In other words, “matter likes to be around other matter,” Faherty said. In space, gravity pulls objects that are close together. Together, their collective mass increases, and more mass means they can generate a stronger gravitational pull with which to draw even more matter into their cosmic cluster. Mass increases, then gravitational pull, then mass. “It’s a runaway effect,” Hummels said.
When these gravitational hotspots attract nearby matter, the space between them is evacuated, creating what is called a cosmic void, Hummels said. But the universe didn’t start out that way. After the big Bang, matter in the universe was dispersed more evenly, “almost like a fog,” he said. But over billions of years, gravity has gathered this matter into asteroids, planets, stars, solar systems and galaxies; and leaving between them the interplanetary, interstellar and intergalactic space.
But even the emptiness of space is not really pure. Between galaxies there are less than one atom in every cubic meter, which means that intergalactic space is not completely empty. It does, however, have far less matter than any vacuum that humans could simulate in a laboratory on Earth.
Meanwhile, “the universe continues to expand,” Faherty said, assuring that the cosmos would remain largely empty. “It seems so lonely,” she said.
Editor’s Note: This story was updated Oct 9 to correct the spelling of Cameron Hummels’ name.
Originally posted on Live Science.