How Big Is Space?

In an article at Popular Science Lauren Leffer explains a number of interesting facts about our universe. She talks about its size and shape, how we know it's expanding and how fast it's expanding.

On the question of the universe's size she quotes the astrophysicist Sara Webb who asserts that, “There is physically, absolutely zero way that we will ever know,” how big the universe is.

Leffer adds,

However, we do know that the universe is larger than 93 billion light-years across. This is the diameter of the sphere of the “observable universe” that we find ourselves at the center of. Our ability to look out and measure the stars is limited by the age of the universe and the speed of light.

The only light we can see is light that’s been able to travel to us in the time since the big bang, which happened about 13.8 billion years ago. Therefore, light that’s traveled 13.8 billion light-years is the oldest we can see.

However, the observable universe extends farther than 13.8 billion light-years in every direction because, for all the time space has existed, it’s also been expanding. That expansion means that light from 13.8 billion years ago has actually traversed 46.5 billion light-years to reach our eyes and telescopes.

“It means, in theory, that space is actually expanding faster than the speed of light, when we add it all up– which really conceptually hurts your brain,” says Webb. “The nothingness of space and time doesn’t really abide by the laws for matter and physical things.” And though we don’t have firm evidence of the universe’s total size, Webb thinks it’s quite possibly infinite. “There’s no reason that it should be bounded. There’s no reason why there should be an edge here or there,” she says.

About the universe's shape Leffer writes that,

....astrophysicists generally agree on the universe’s shape: it’s flat, though perhaps not in the way you’d imagine. Flat doesn’t mean our universe is two-dimensional (space-time exists in 4D, after all). However, it does mean that traveling forward without changing direction in the universe will never get you back to where you started. Instead of a doughnut, a sphere, or a Pringle, the universe is most probably a four-dimensional sheet of paper, says Webb.

Flatness is another concept that hurts the brain, but her explanation that unlike travel, say, on the earth where if you start from any point on the equator and fly in the same direction you'll eventually return to your starting point, in space no matter how long you travel, as long as you don't change your direction, you'll never return to your starting point.

There's more in Leffer's article that's interesting and her explanation of how we know the universe is expanding, and what's causing the expansion, is especially helpful. Check it out.