In Annie Hall, young Woody Allen explains to his doctor that he won’t do his homework because the universe is expanding, so what’s the point? His mother exasperatedly tells him, “You’re here in Brooklyn. Brooklyn is not expanding!”
I post this because I’ve been reading Coming Of Age In The Milky Way by Tim Ferris, as good a summary of the state of cosmology between two covers as a person could ask for. Thinking about the horrifying enormousness and ancientness of the universe might have depressed Woody Allen, but it has a paradoxically calming effect on me. Reading books like Ferris’ is my favorite form of meditation.
Every single illustration you’ve ever seen of the Big Bang is wrong. They all show an explosion from a central point, expanding outwards into space. The Cartoon History Of The Universe shows the pre-Bang universe as a little blob floating against a white background. When you turn the page, the blob bursts forth in a gooey-looking explosion across a two-page spread. It works great as a graphic device, but it gives the wrong idea. At the moment of the Bang, there wasn’t a central point in the universe where everything was gathered up. The explosion happened everywhere in space. More accurately, the explosion created space.
Starting at the moment of the Bang, everything in the universe got further away from everything else. From anywhere in space it would have looked like everything was being blasted away from you. The universe still looks that way; anywhere you look, all the galaxies are racing away from us. The further away they are, the faster they’re receding.
It’s hard for me to wrap my head around the idea of space being smaller. Space never had a boundary, so far as anyone can tell, so how can it have a size? It helps me to think of an eighties video game like Pac-Man or Asteroids where the screen wraps around. When you go off the right side of the screen, you reappear on the left side. It’s as if the screen is on the surface of a cylinder, so really you’re just going around in a circle.
It’s harder to imagine three-dimensional space wrapping around this way, but not impossible. There was a moment right after the Bang when the universe had a radius of ten feet. If you had been there (and weren’t instantly vaporized), you’d see the back of your own head ten feet in front of you.
Tim Ferris’ book has a nice passage that helps visualize the Bang. He asks you to imagine a staircase going backwards in time. Each step is ten times shorter than the previous one. Step one is a billion years after the beginning of time. Step two is a hundred million years after the beginning of time. Step three is ten million years, step four is a million, step five is a hundred thousand, and so on. At step one, a billion years after the Bang, everything in space is younger, hotter and closer together. At step two, a hundred million years in, the universe is mostly dark. There are a few stars, but mostly it’s just a lot of hydrogen and helium gas swirling around. At step three, ten million years in, all of space is at room temperature. At step four, there are so many electrically charged particles rubbing against each other that the entire universe is flooded with blinding white light. Very biblical.
At step six, ten thousand years in, the entire universe is the same temperature as the surface of the sun, and totally dark, since there isn’t enough space between particles for the photons to get anywhere before whacking into something and being reabsorbed. At step eleven, the entire universe is hotter than the center of the sun. Between steps seventeen and eighteen, one second after the beginning of time, the entire universe is “denser than rock and as hot as the explosion of a hydrogen bomb.” Cool!
There are so many big mysteries. Why did the Big Bang happen at all? One school of thought says, well, if it hadn’t, then we wouldn’t be here to think about it. This makes logical sense but isn’t very satisfying. Another idea is that there’s a tiny but nonzero chance of a Big Bang happening anywhere and anytime as part of the random quantum fluctuations of the universe. By this thinking, Big Bangs could be happening all the time throughout the multiverse. Scientists don’t like this idea because there’s no way to test it; all the parallel universes would be mutually inaccessible and invisible to each other.
A more interesting question, because we might find an answer someday, is why the universe has stars, galaxies and other interesting structure to it. At the moment of the Bang, space was a single super hot energy field. As space expanded and cooled down, the energy converted itself into pairs of matter and antimatter particles: quarks and antiquarks, electrons and positrons and so on. Based on what we see in the world now, it would be logical to assume that the Bang would have produced equal numbers of particles and antiparticles. And yet, that isn’t what happened. The balance was close to equal, but not exactly. For every billion antiparticles, there were around a billion and one particles. As the pairs zapped each other out of existence, the leftovers congealed into galaxies, stars, planets and us.
A ratio of a billion to a billion and one is pretty improbable-seeming, and very lucky. If the matter and antimatter in the early universe been perfectly balanced, every particle-antiparticle pair would have mutually annihilated back into energy, resulting in a universe with no objects in it, just a photon every trillion cubic miles. If the ratio had been more unequal, the amount of matter (or antimatter) left over would have overwhelmed the expansion of space, collapsing instantly into a few humungous black holes. The very slight asymmetry between matter and antimatter production is the reason we’re here. So why did this extraordinarily lucky thing happen? Nobody knows. It might have something to do with muons and the weak nuclear force, but that doesn’t explain anything, it just locates the weirdness in a particular branch of particle physics.
Lee Smolin has an attractive hypothesis. He starts with the assumption that inside every black hole is a new Big Bang. As viewed from the inside, Smolin imagines the black hole’s violent collapse as looking like a violent explosion. This is a controversial idea among the physicists, but it appeals to my intuition. Smolin suggests that every new universe formed in a black hole has slightly different physical parameters, different basic physical constants and ratios. Most of these universes will have rules of physics totally unlike ours, and will be inconceivably different. Some universes will be kind of similar to ours. A very few will be nearly identical. The ones with delicate imbalances between matter and antimatter will have stars, planets and galaxies.
Here’s where Smolin’s idea gets really fun. Universes with more stars and galaxies are likelier to have more black holes, and will thus spawn more baby universes. So the multiverse could be selecting for improbable universes like ours, the way the conditions on the earth select for improbable lifeforms.
Another idea I find appealing is Paul Steinhardt and Neil Turok’s theory that our universe has an invisible twin out there in higher-dimensional space. We violently collide with this twin every few trillion years, an event which looks from our perspective like a Big Bang. I like to imagine the twin as being the Star Trek evil universe.
I have a lot of scientists in my family, professional and amateur. I was exposed to the Big Bang theory long before the biblical creation story. The idea that the universe just came into being for no real reason suits my intuition better than the idea that there’s a grand purpose to the whole thing, a purpose that specifically involves humans. The idea of the Big Bang as a meaningless, accidental event makes perfect sense to me.
My intuition has a harder time imagining the universe as it is now. Douglas Adams was right:
Space… is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is. I mean you may think it’s a long way down the road to the chemist, but that’s just peanuts to space.
My favorite illustration of Douglas’ point is the Hubble Ultra Deep Field, a photo taken by the Hubble Space Telescope:
It looks like a regular old photo of space, but it’s not. The Hubble was deliberately pointed at a boringly “empty” region of the sky. It imaged this region carefully over many exposures to reveal whatever objects there were that might be too faint to have been noticed. In the resulting picture, nearly every singly dot is a galaxy containing billions of stars. This is where the meditation exercise gets away from me. It’s still fun though.
I leave you with some wisdom from the Cartoon History Of The Universe: