# How much stuff is in the Universe?

“The most incomprehensible thing about the Universe is that it is comprehensible.” –Albert Einstein

The observable Universe is huge. Incredibly, mind-bogglingly huge. When we look out, in any direction, we see galaxies upon galaxies upon galaxies, stretching for billions and billion of light years.

This one picture, taken of a region of sky just one-tenth the size of the Moon, contains more than 10,000 unique galaxies. And there are maybe close to a hundred billion galaxies similar to ours in the Universe; each one contains billions and billions of stars and planets, along with huge molecular and atomic clouds of gas and dust.

All of these galaxies cluster together gravitationally: the places that start out with more matter pull on more and more galaxies, becoming clusters or giant clusters of galaxies, while the places that start out with less can’t attract them at all, and become great voids in space.

In fact, we’ve mapped out where the matter close to our neighborhood (within a few hundred million light years) lives, and here’s what we’ve found!

Pretty impressive, no?

But it gets better. Based on a whole host of observations, we can simulate, on large scales, what the matter in the Universe ought to look like! And what we find, spread out over a sphere 93 billion light years in diameter, is a Universe filled with stars, galaxies, and clusters like so.

All told, there are around a whopping 1080 atoms filling our observable Universe. This is a ridiculously huge number. If you and I were to each choose an atom in the Universe at random, the probability that we’d each choose the same atom is 1 in 1080, or about as likely as winning the powerball jackpot ten times. In a row.

But the Universe is also very large, and these atoms aren’t packed together very tightly. So I’ve got a couple of questions for you.

What if you took all of the atoms in the Universe and packed them together into a solid disk with the same radius as the observable Universe?

How thick do you suppose that disk would be, containing all of the matter in the Universe?

Let’s ask the second one: what if, instead of a disk, you packed all of the atoms in the Universe together into a solid cylinder whose length extended across the diameter of the Universe?

With all the atoms in the Universe packed in there, how thick would this cylinder be?

You’ve had your fun guessing; I’ve got the answers all figured out, but where’s the fun in a giveaway like that?!

Come on, Ethan, don’t be like that! GIMME THE ANSWERS!

Okay, okay already. If you compressed the Universe into a disk, it would be about 200 microns thick, or about the size of a paramecium.

But if you compressed the Universe into a cylinder, it would have the same diameter as Earth’s orbit around the Sun!

Pretty impressive? Not as impressive as this fact: the cylinder case would be so dense that it, itself, would be a black hole, with light unable to escape it! Just some food for thought on a Friday, and I hope you enjoyed thinking about it!

Update: I realized that the cylinder wouldn’t form a black hole, but an object from which no light could escape stretched across the entire Universe known as a cosmic string! WTF is a cosmic string? You’ll have to come back on Monday…