WTF, Universe? You are too large for my puny human brain to comprehend
But let’s try anyway, shall we?
Welcome to the second post in our exploration of the weird as fuck Universe! In the first post, I talked about how distance is really the same as time and we can see into the past. This means we can almost see the Big Bang! This limits the size of our observable Universe — the part of the Universe that it’s even possible for us to observe. We can only see as far as light has had time to travel since the Big Bang. But don’t let that bum you out. At least we have dark nights!
Sidebar: If the Universe didn’t have a beginning, then all the light in the (perhaps infinite) Universe would have had an infinite amount of time to travel to us, and this light would fill our night sky. The Big Bang is a handy solution to this paradox that you probably didn’t even know was a paradox, unless you were smoking weed one night and wondered, “Why the fuck is the night sky even dark in an infinite Universe, dude?” Now you know the answer: the Universe had a beginning, and therefore our observable Universe is finite, not infinitely large.
Fuck infinity, though. Infinity is problematic. What we do know is that the Big Bang happened roughly 13 billion years ago, or equivalently*, 93 billion light-years away. But how far is that, really?
*It’s not 13 billion light-years away because 13 billion years ago, the observable Universe was smaller, and now it is much bigger, like 93 billion light-years big, because the Universe is getting bigger… but let’s deal with one incomprehensible fact at a time, shall we?
The Universe is YUGE. Bigly.
It turns out, your brain is not equipped to comprehend just how big the Universe is. Why should it be?
Nevertheless, it’s fun to try, and to see how far we can get before the extrapolation from our everyday experience loses all meaning completely.
Let’s start from the ‘everyday experience’ of a satellite floating in space. (At least we can imagine how big this satellite is, okay?) This video zooms out from the WMAP satellite all the way to the background radiation emanating from the edge of the observable Universe, 13 billion years ago, which is clever because WMAP measured that radiation.
As we zoom out on this “Journey to the Big Bang,” we pass some familiar planets until our Sun becomes a tiny dot. This is already super far away, according to scales we are used to thinking about: Pluto itself is 7.5 billion kilometers away, or 5.5 light-hours, which is a couple of pee breaks if you took a road trip to Pluto at the speed of light. But we can’t stop there — eventually we zoom out of our Milky Way Galaxy, which is about 100,000 light-years in diameter. Very quickly, the Milky Way becomes just one of 100 billion other galaxies in our observable Universe, too small to even bother with, really, until we reach the hot plasma of the cosmic background radiation, the sexy afterglow of the Big Bang.
One problem with conceptualizing something as huge as THE UNIVERSE is that we can only visualize a limited range of scales. Our eyes have a resolution limit. So, for example, you can look at a picture that shows very large scales in this image of a simulated universe, but you can’t resolve the smaller scales of our everyday experience.
At the same time, if you wanted to see things on tiny, miniscule — nay, negligibly small — scales, like this picture of the entire Earth, you are limited by your field of view. You have zoomed in on the Earth and can no longer see things on larger scales anymore.
But what if, instead of seeing things on a linear scale, we could see logarithmically? A logarithmic scale basically means that instead of counting by adding (10, 20, 30, …) you count by multiplying (10, 100, 1000, …), as explained in this lovely video by Vi Hart.
We can’t extrapolate from an everyday unit of length to cosmological scales because that would mean counting 10000000000000000000000 meters to get to 1 Megaparsec, the typical size of the largest structures in the Universe. But all that counting is literally impossible — it would take longer than the age of the Universe to count that high, unless you are a computer and can count super fast. (Are you? You would tell me, right?) On the other hand, on a logarithmic scale we only have to count to 22 to get that 1 Megaparsec is 3x10²² meters. Phew!
If you could see logarithmically, and you had a bird’s eye view of the Universe, for the kind of bird that could survive in the vacuum of space, and space-bird-you wanted to take a look at our Solar System, this might be what you would see:
In the center of this Universe is the Sun, which Copernicus knew all along, surrounded by planets placed conveniently close together in logarithmic space. Beyond the fuzzy Kuiper belt, Oort cloud, and Milky Way galaxy lie our immediate galactic neighbors and increasingly smaller and farther-away galaxies, which blend into the filamentary cosmic web of truly large-scale structure, at Megaparsec and Gigaparsec scales. Eventually we hit the wall of the “last scattering surface,” the farthest it is possible to see and the origin of the microwave background radiation, a mere 300,000 years after the Big Bang. The Big Bang itself is the edge of the image, beyond which our current theories break down. Possibly, it is the beginning of our Universe and of time itself. Or, possibly, it is the end of the progenitor universe that spawned ours. Who the fuck knows.
Next time, I will talk about how all this bigness is getting even bigger. Not just our observable patch of sky, which will continue to get bigger as long as time moves forward (because distance equals time) and as long as there are any observers left on Earth for the term “observable” to even make sense — so, a few decades then? — anyway, not just the observable Universe but the Universe itself is expanding. This expansion, while weird as fuck, all makes perfect sense in Einstein’s general relativity theory, so in a later post I will talk about how this expansion is accelerating, and so far we really have no clue why.