How can space itself expand? Isn’t space the lack of matter, the area in between matter?

Question

How can nothing expand? Isn’t space made of nothing? You can’t have 5 pounds of space.

In: Physics

Answer 1

Imagine a blanket, with balls on it. The distance between the balls is empty space.

Now imagine the blanket is growing – getting larger. The balls start to get farther apart.

In our current models, and backed up by most observations, the universe is like the blanket, and everything is being pushed apart by the blanket expanding.

This gets hard to understand, because our day to day experience says it isn’t happening. My desk doesn’t get further away from my wall, cities don’t get further apart. So go back to the balls on the blanket. Individual objects have other forces keeping them together, that overcome the push from the growing blanket. So as the blanket grows, the balls don’t have enough force from the blanket to make individual balls expand, and they stay together as a ball in the same spot.

Now add gravity. Imagine the blanket is on a soft surface, and the balls push down on the surface. Other balls that are nearby may roll together, and overcome the speed of the blanket growing, so they seem to stay the same distance apart. That’s our solar system and Galaxy. There’s enough gravity to overcome the speed of the expanding universe, so it stays together.

But, if two objects are very far apart, the gravity keeping them together is less than the force of the blanket growing, and they move apart. That is what we observe when we look at galaxies that are far away.

And finally…. You’ll hear that the expansion is accelerating. We also observe this. Very slowly, so everything I’m about to to say is estimated to occur after the sun goes out and we’re all gone, billions of years in the future… But imagine if the blanket is growing, but going so fast, the balls start to roll really fast, and the blanket starts to push hard enough on the balls that they start to break apart from the force of the blanket expanding. That is what will eventually happen, as the speed of the universe expanding increases to the point that we could actually see our chair start to be further from the wall.

Obligatory not a physicist, so if anyone can correct what I understand, please make our thoughts experiment better!