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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?
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(Tangentially, I now have the song "Baby Universe" from Katamari Damacy in my head.)
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Instead, your best bet would be to try to work out something like what's described in that New Scientist article: intentionally creating a baby universe in the lab. You'd somehow need to scale it up rather a lot (and do it with yourself inside), and you'd need a plan for surviving not just the initial universe-creation process but also the Big Bang that followed (and the billions of years waiting for galaxies, stars, and planets to form after the initial burst of energy runs out). But that's still far better than crossing your fingers that a sufficiently large baby universe happened to form right on top of you.
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Gosh, it's really bright in here.
To be precise, instead of just this one universe being inside some "black hole", I rather suppose this universe is only one out of many many thin hyperspherical shells (each a universe in its own right) that are nested within the "black hole". The mass+energy in each such shell is of course moving "down" towards the center of the "black hole": each of these shells appears to have the usual 4 spacetime dimensions despite being embedded in a higher-dimensional space because generating a force or momentum in the higher-dimensional space that is oriented "up" or "down" (as opposed to perpendicular to "down", i.e. staying within the same shell) is not physically possible. (Except perhaps for quantum scale fluctuations, which might randomly generate forces in the "up" or "down" directions?)
It's a nifty idea. The inflationary phase of our universe occurred "when" the "black hole" event horizon extended outwards past some bit of matter+energy. The remarkable cosmological flatness of our universe is a simple reflection of the fact that we're embedded in a really big hypersphere. Some of the modern theories about inaccessible extra dimensions that are "rolled up very tight", or about phyllo-pastry layers of neighboring universes, might have an alternative explanation as the nesting of nearby but forever unreachable iso-velocity shells within a "black hole".
One interesting question: Supposing we +are+ inside a "black hole" in some higher dimensions, a number of physical properties we currently take as constant might have very different (and perhaps predictable :-) values in the early beginnings or far future of our universe, as this shell has collapsed further and further into the "black hole". For example, the speed of light might change in predictable ways -- or there might be a predictable relationship between the apparent amount of mass+energy and the age of the universe -- or perhaps the effects of "dark matter" are a predictable effect of the mass+energy in the shells nearby. One nice thing about the "black hole" concept: if one just posits a differential geometry for the higher dimensional space, including an appropriate theory of gravitation, such that a hyperspherical shell inside a "black hole" has our beloved Einsteinian differential geometry and theory of gravitation -- well, working out all these other predictions is just a matter of a little geometry ;-) somewhat like showing that general relativity or quantum mechanics become Newtonian or Maxwellian at everyday scales.
Anyhow a month ago I looked forwards to boring you with these crackpot ideas at the ABL party, but within the last few weeks it has fallen out that I shall not be in attendance :-( Hence I am very happy to find you have given me an opportunity, here on your lj, to bore you as I had planned :-)
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In fact, if you accept the alternate translation of Genesis 1:2 using "became" instead of "was", it almost sounds like it is describing a "last Tuesday after lunch" sort of creation. Genesis 1:1 says, "In the beginning God created the heavens and the earth", which can be taken to refer to "space" and "matter" (perhaps even "planets"). That is, He created a universe that was already full of stuff (but still necessarily obeying physical laws more or less the same as those in our own universe, given that we were eventually going to emerge out of it: this first universe would probably have looked not too different from what we can see ourselves, so it may not have been obvious that it had just been created recently). After that, we get Genesis 1:2, which says, "Now the earth became formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters." So after creating one universe in the middle of its history, he waited for its stars to burn out, its matter to dissipate, and the final rapid expansion to stretch it into utter emptiness. Finally, Genesis 1:3-4, "And God said, "Let there be light," and there was light. God saw that the light was good, and He separated the light from the darkness." At this point, God creates a bubble of vacuum energy (we'll interpret "light" liberally here) which proceeds to pinch off from the previous space-time and inflate into the universe we know (technically, then, God didn't "separate the light from the darkness" but simply watched it separate naturally, but we can forgive some poetic license here). There are some awfully strange philosophical implications of all that: we're just an afterthought following up on God's primary creation, but at least that means when God pulled the "last Tuesday" prank He pulled it on someone else. (Personally, if I were a Christian I'd much rather read Genesis as some sort of allegory for God's love and care for us, or whatever Biblical scholars generally take it to mean.)
Re: Gosh, it's really bright in here.
As a recovering crackpot physicist myself, I'm very sympathetic to the impulse (I got way too excited about some half-baked ideas in high school, which lasted about as long as it took to talk to an actual physicist my parents knew). You've certainly got some interesting features here! My first instinct, though, is that this picture wouldn't hold up once you analyzed it in detail. For example, I think it would imply universal contraction rather than the expanding universe that we observe: the hypersphere radius and therefore its volume decreases as you approach the core of the black hole. Of course, you could try to set up a similar story for a white hole space-time (which would raise the spooky question of what would happen once our hypershell crossed outside the event horizon).
Still, your idea of reinterpreting "time" as some entirely different parameter of an enlarged system has gotten some high-profile attention recently, so you're in good company. One particularly odd idea is based on the fact that physical theories change in character as you observe them at different energy scales. There are some strong similarities between the changes in physical phenomena in a theory as you go from high energy to low energy as compared to the changes in physical phenomena in the universe as you go from early times to late times. So people have sometimes considered physics where they simply replace time with this "renormalization group flow" from high to low energy. Weird stuff.
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Newt
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Quantum mechanics tells us that empty space isn't just an empty stage on which the history of the universe plays out. Instead, it is a chaotic tapestry of tiny particles and bubbles of energy that flicker into existence and then evaporate again, too small and quick for us to notice at all. Together with evidence from our own universe's early history, this leads to a startling possibility with deep philosophical implications.
If the right sort of energy bubble happened to appear, the space inside it could stretch explosively into a vast new region. Hanging like a ballooning wart off the side of our own universe this space would experience its own Big Bang, expanding far too quickly for the expected evaporation back into nothingness to keep up. So even as the parts of the bubble that we could see vanished back into emptiness, the newborn universe would continue to grow and pinch off from our own. Whatever its future history or the dreams of its inhabitants to come, their lives and ours would never touch again.