Baby universes = awesome

[steuard]

ricevermicelli recently posted an awesome quote by Sean Carroll: "...a universe could form inside this room and we’d never know." Sean is a cosmologist at Caltech whom I knew when we were both at Chicago, and if you ever have the chance to see him speak, do it: he's fantastic. (He's got a few talks online, some with video.) I offered to give a capsule summary of what he was talking about, but I figure it might be of broad enough interest to give it its own entry rather than burying it as a comment.

Because this is turning out to be less "a capsule" and more "three capsules and a blender", I'll put the details behind a cut.

To begin with, we need three pieces of physics background, each of which is pretty awesome in its own right:

• Inflation: The universe at the moment of the Big Bang may have been an unfathomably chaotic place fluctuating wildly from one point to the next, but our universe today is remarkably smooth and uniform. Our best understanding of how that came about is "inflation". Fractions of a second after the Big Bang some peculiar type of energy[1] caused our part of the universe to expand insanely quickly. The fabric of space-time stretched so fast that even light couldn't keep up, and a region of the primordial chaos small enough to be pretty much uniform ballooned in the blink of an eye to be our entire observable universe. Eventually, the "inflationary energy" dissipated into more familiar forms like light and matter, and the modern cosmos was born.


• Vacuum fluctuations: On scales much smaller than an atom, quantum mechanics tells us that "empty space" is actually a roiling sea of particles and fields that bubble into existence out of nothing only to vanish again without a trace. Particles and anti-particles appear from nowhere and just as quickly annihilate each other. Within the limits of Heisenberg's uncertainty principle, anything at all can happen (as long as it goes away fast enough).


• Black holes: A black hole is a clump of mass/energy so dense that not even light can escape if it gets too close. The surface where this happens is called the "event horizon", and what happens beyond that edge can never be seen from outside. But if vacuum fluctuations cause a particle to appear just outside the event horizon with its anti-particle just inside it, the particle that appeared out of nowhere can be free to escape into space, stealing away a little of the black hole's energy as it goes. That process speeds up as the black hole shrinks, eventually causing it to evaporate into nothing in a flash.

So what did Sean mean when he said "a universe could form inside this room and we’d never know"? There is a chance (very small but ever-present) that a vacuum fluctuation could create a bubble of that mysterious inflationary energy at any point in space. Usually it would immediately dissipate back into nothing, but if the conditions were just right more interesting things could happen. The energy density in that tiny region would be high enough to form an event horizon: it would be a tiny black hole hiding "between this molecule and the next" (as ricevermicelli put it).

From outside in the room, all we'd see would be a momentary burst of energy as the minute black hole evaporated back into the vacuum. But inside the horizon, the tiny bubble full of inflationary energy would take its cue from the primordial era and again stretch the fabric of space-time in an exponentially growing space of universal proportions. And once again, that energy would eventually stop inflating and seed the new universe with light and matter of its own. But because that burst of expansion occurred entirely within an unimaginably tiny bubble in our own universe (protected behind a black hole horizon where we could never see it at all), we would never know that a glorious child had been born.


There is a nice illustration of this process as part of an article in New Scientist entitled "Create your own universe" (which gives more details about some specific models and thoughts on making this happen in the lab, if you're interested; Sean has a long post on the general topic as well). If you're worried about conservation of energy, I sympathize completely: I objected to Sean on exactly those grounds when he first explained this story to me years ago. The answer is hidden in the subtle ambiguities that general relativity and curved space-time introduce into the definition of "total energy" in the first place: when viewed from outside, the enormous energy of a universe's worth of light and matter gets "canceled out" by the curvature of the bubble space-time as seen from our part of the universe. (Weird, but true.)

Sean's favorite proposal for the history (past and future) of our universe is based on these same ideas, in fact. As noted below, our own universe seems to be entering another fast-expanding phase and it seems likely that our eventual fate will be a vast and empty cosmos locked in eternal sterile growth. But because of that remnant of "dark energy" throughout space driving the new inflation, there will still be bubbles constantly forming in the vacuum. Over the unimaginable eras of emptiness, random chance will eventually lead to new baby universes (however unlikely they might be) and the cycle will begin again. Our own history might trace back to just such an event: one bubble out of nothing born of countless generations before and holding both death and infinite rebirth in its future.

Magical and philosophically profound indeed.


Update: I've added a comment that tries to condense all that into a single capsule of a couple short paragraphs. Maybe it even worked.


[1] Recent observations of supernovae and the cosmic microwave background indicate that there is still some energy of this mysterious sort throughout the universe today. Dubbed "dark energy", we have no clear idea what it is, but it is causing the expansion of the universe to accelerate again despite gravity and may eventually rip the galaxies forever apart. Spooky, eh?

Comments

[zathrus.livejournal.com]

Any suggestions for further reading for those who want a little bit more than "three capsules and a blender?" Preferably accessible to those who have totally forgotten what higher math they used to know? (I took Fields & Waves, and obviously DEs, but that was all 9+ years ago, and I really haven't used any of it since graduating from Mudd -- even grad school didn't require that much higher math from me (chemistry). These days, I'm back to arithmetic normally, although I fully expect that Chris and/or I will have to brush off our calculus skills, and possibly up through linear/DEs/discreet, in another decade or so.) I like having new ideas to play with, but don't really have time for playing with rigorous mathematics these days.

Newt

[steuard.livejournal.com]

I ought to be packing/cleaning for our trip at the moment, so I don't have time to go digging for good references, unfortunately. As a starting point, take a look at the New Scientist article and Sean Carroll blog entry that I linked to near the end of my post (Sean's blog entry includes a variety of other links, I believe). And I once again highly recommend looking at some of Sean's talks on his main website (as linked above; I watched a few minutes of his talk at the "Yearly Kos" meeting and it looked really good, and given the audience it had no advanced math at all). Those talks may not relate directly to this topic (this stuff comes up mostly in his talks touching on the "arrow of time"), but it's fascinating regardless.