So, what is a vacuum, and why isn’t the universe truly a vacuum?
Firstly, let’s set aside the notion of a vacuum cleaner as a metaphor for the vacuum environment of the universe. A household vacuum cleaner is effective at cleaning because it creates suction by utilizing a pressure difference. However, it would be more accurate to call it a “suction cleaner” rather than a “vacuum cleaner.” The vacuum of the universe is quite the opposite. By definition, a vacuum means the absence of matter. The universe is a near-perfect vacuum because it is largely empty, not due to suction or a significant pulling force.
This emptiness gives rise to an extremely low-pressure environment. While we cannot replicate the vacuum of the universe on Earth, scientists can create partially vacuumed environments with extremely low pressures.
Even extrapolating from the idea of a household vacuum cleaner, “understanding the concept of a vacuum is nearly impossible as it contradicts how we exist,” says Jackie Faherty, a senior scientist in astrophysics at the American Museum of Natural History.
As humans, we are confined to a dense, crowded, and bustling sample of space within the vastness of the universe. It is challenging for us to truly grasp what it means to have nothing or to be in emptiness. However, in reality, what is normal for us on Earth is actually rare in the vast expanse of the universe, where most of space is nearly empty.
Gravity reigns supreme
Even without gravity, the universe would still be relatively empty. “It’s just that there aren’t many objects compared to the scale of the entire universe where we place those objects,” according to American astrophysicist Cameron Hummels.
According to NASA, the average density of the universe is approximately 5.9 protons (positively charged atomic particles) per cubic meter. However, gravity increases the emptiness in certain regions of the universe by causing matter to clump together.
Fundamentally, any two objects with mass are attracted to each other. This is the force of gravity. In other words, it’s “matter’s desire to be near other matter,” Faherty explains. In space, gravity pulls nearby objects closer together. When combined, their total mass increases, and with greater mass comes stronger gravitational forces, pulling more matter into their cosmic conglomerations.
In summary, the universe is often referred to as a vacuum due to its low density of matter in most regions. However, it’s important to recognize that the vacuum of the universe is not an absolute void but a relatively empty environment with extremely low pressures. Gravity plays a crucial role in gathering matter and creating regions of even greater emptiness within the vast expanse of space.