Brian = bae. Definitely one of the more accessible cosmology writers of today. Any book discussing quantum field theory and string theory is going to require your undivided attention, of course, but it’s much more readable than many others, especially if you’re fond of analogies.
Turns out, there’s lots of theories that give rise to the possibility of alternate realities:
Theory 1, aka Quilted Multiverse: If the universe is infinite (it might not be, but if it is) then at some point in the distant, distant universe, there are worlds where there is a human named Annie just like me, typing on Goodreads, except she typed “GR” instead of “Goodreads.” And there’s another world with an Annie whose life is exactly like mine except she’s allergic to red wine (I would have to kill myself because there would be no point in living). In some, the worst things I’ve ever imagined happening have happened. In others, my wildest dreams have been fulfilled. (Somewhere, there is an Annie who can write like Kafka, DH Lawrence, and Kundera combined).
Theory 2 aka Swiss Cheese: If the universe is expanding (hint, it almost definitely is) then there would be places which expansion would create which would have to have lowered rates of expansion, and those are like holes in Swiss cheese. The number of cheese-holes, as the cosmos continues to expand, continues to increase, and each one of those is adorably termed a bubble universe, or pocket universe. Including our own. This also brings in the Quilted Multiverse idea: from inside a bubble universe, the bubble’s space is infinite, but from outside the bubble universe, it’s finite. What appears as endless time to an outsider appears as endless space, at each moment of time, to an insider. And if it’s infinite to an insider, then there have to be infinite worlds with infinite repetitions and variations, as in Theory 1.
Theory 3 aka String Theory aka Bread Loaf aka Cyclic Multiverse: [Strictly speaking there are five types of string theory, no one: Type I, Type IIA, Type IIB, Heterotic-O, and Heterotic-E, which all sound like sexually transmitted diseases to me, and anyway they all get subsumed into the grander M-theory]. String theory (particles are vibrating loops of filaments, and the way it vibrates determines what particle it is/how it behaves) and corresponding M-theory predict that there are ten dimensions of space and the eleventh dimension of time. Seven more than the ones we’re aware of: length, width, height, and time. But maybe that’s not an issue, because the four dimensions we’re used to extend over massive (maybe infinite) distances, but maybe the other seven are curled up at the other end of the cosmos and we can’t find them and they don’t extend far enough to reach into our reality. For analogy Greene gives the example of a straw, normal sized, except it’s as tall as the Empire State building. It’s three-dimensional up close, but from the other side of the river, it would just look like a vertical line (two-dimensional). Here comes the multiverse idea: pretend the world has just two dimensions (it’s a flat plane) called a two-braneworld. Line up a couple of these flat planes (slices of bread) and you can add on other parallel universes, other braneworlds, as many as you like (“just add slices to the cosmic loaf” as Greene memorably says). Of course, we have three spatial dimensions, but looking at 2 dimensions makes it easier to wrap our 3D heads around. It’s also possible that different braneworlds would bounce into each other and restart the cosmological clock, so that, rather than one big bang, things are a gigantic bumper-car game and universes are constantly (though very slowly, from our perspectives) getting remade and unmade and remade.
Theory 4 aka Landscape Multiverse: pretty much the idea that the cosmological constant (density of energy in space, creating certain gravitational values) might be necessary for life, and so what are the odds it happened to happen? Well, you’d need an infinite number of universes to make it likely that [at least] one of them just happened to have the exact necessary cosmological constant to make life happen.
Theory 5 aka the Quantum Multiverse: all the possibilities of [our] reality are realized in another reality, another multiverse. All possible histories and all possible futures are real. One of them is the reality we live in; the (basically infinite) rest exist in some way. True reality is more like a tree than a line.
Theory 6 aka Everything is a Hologram: The event horizon of a black hole (point of no return) is the surface of a black hole in some ways, and fluctuations in the surface of the event horizon contains all the information the black hole sucks in. Similarly, our 3D world may be a rendering of 2D-encoded information (on some cosmological event horizon-type-thing) which creates reality as we know it.
Theory 7 aka Virtual Reality: we can create other universes virtually and, someday, could probably create reactions in the (potentially sentient) actors in those universes which could make it feel like reality, every bit as much as it does to us. If everything’s infinite (see theory 1) with infinite numbers of beings creating infinite numbers of virtual universes whose inhabitants can also create infinite numbers of infinite universes and so forth, doesn’t it seem like odds are good that we’re just one of those games? Could we even tell if we were in a simulation? Depends on your Simulator and if they want to reveal themselves (are you there, God? It's me, Annie).
Other interesting things:
Dark energy might be the answer to Einstein’s unproven cosmological constant (that he retracted after he suggested it because he couldn’t account for it). There’s some… some shit out there, and we can’t see it, describe it, or know of its existence other than through a few methods of measurement.
The mind-boggling idea that if the universe is infinite, and you shrunk infinity, everything would be closer together but the universe would still be infinite. Rationally I understand that you can’t reduce infinity (infinity divided by two is still infinity) but my brain doesn’t want this to be a thing.
About 1 percent of the snow on a television that’s tuned to a non-working channel is due to reception of the big bang’s photons (aka background radiation).