A fourth spatial dimension, like the dancing tesseract? That was Riemann, 1854.
(3+1)D, aka 4-dimensional spacetime? Einstein and Minkowsky, 1908.
All right, so how about (9+1)D, as in string theory, 1984?
Ah, but those were supposed to be our regular (3+1)D (space plus time) dimensions,
plus six spatial dimensions rolled up into tight cylinders of subatomic radius.

Nowadays, some of us (e.g. Itzhak Bars) are thinking grand scale:
that our (3+1)D universe is immersed in a (4+2)D spacetime
--that's right: four extensive space dimensions, plus two dimensions of time!
As foreshadowed by C.S. Lewis (1949):

You walk to the back of this closet and cross into Narnia; live there for fifteen years; re-enter the closet (from the Narnia side), thereby returning to your starting point, and find that only seconds have passed, or maybe a minute or two. Right. We normally think of a time line. If time were 2-dimensional, it would be like a sheet, and we would normally be advancing along one “axis” of time, say T1. If, upon crossing the portal, you inadvertently make a right-angle turn on that time sheet, then you might advance several years along T2 with no change in T1. When you eventually return to the portal's space coordinates and cross back into your own world, you undo the sharp turn, and find yourself back to traveling along the usual T1 direction.

The postulated (4+2)D spacetime might, in turn, be immersed in an even richer “bulk”, so, perhaps only one or two spatial dimensions are rolled up, or, maybe none of them are.

Whenever the (in)famous Large Hadron Collider finally gets into productive mode, we'll see if any “new physics” emerges. One idea that has almost reached consensus among physicists these days is that, of the forces known and measured by us humans, gravity, in particular, can leak out into the space that envelops our universe. This is taken as an explanation as to why gravity seems so much weaker than the other forces.

Allow me to opine: our non-detection of gravitational waves, despite several years' worth of exquisitely designed and deployed instrumentation to that effect --sensitive beyond belief-- and despite the fact that we can plainly “see” (from their diminishing frequency of rotation) that binary pulsars are losing gravitational energy, the non-detection of gravitational waves, I say, gives me the impression that such waves do, indeed, leak out into some large spatial dimension(s) external to our universe.

So, where does this leave Sipsi's Bubble? Mere whimsy?
Or, physically possible?
How about:

It's a bubble of false vacuum,
stabilized by a naked singularity of spacetime in dimension greater than 4.

Preposterous? Read for yourself,
Matthew C. Johnson and Magdalena Larfors (2008), p.1:

Domain wall spacetimes in certain two–field models contain naked singularities…
Such singularities prevent the formation of regions of a new phase inside of bubbles [of false vacuum] and might render the false vacuum stable.

But! If so, where would such naked singularities be coming from?
Depending on the structure of the spacetime that envelops our universe(s), they may be inevitable mathematically, in the same way that, for example, you can't comb a hairy ball flat without creating a cowlick. The center of the cowlick would be the singularity, the cowlick itself, Sipsi's bubble.

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This account/webspace I am devoting to two long-term projects:


Here's a Muu Naï executive summary.

Sipsi lives in a place SO far out it's not on any planet, and revolves around no sun!



P.S. If you feel like making a comment, please send me a note instead --uh; never mind: Zareste has convinced me to put the upcoming Visitor's Guide in the "Custom" module to the left of here. Thank you!