What is the Level II Multiverse?
It has been described as a tree, as bubbles, as a block of Swiss cheese. The Level II multiverse is an implication of chaotic inflation. Max Tegmark described these universes as regions where inflation came to a stop sprouting bubble universes. The fundamental laws of physics may have been exactly the same to start with. Over time, however, the effective laws of physics would be manifestly different containing perhaps different constants, particles (and with that particle charges, spins, and masses), and dimensionality. An Einstein in one of those universes—assuming intelligent life exists, if any life at all (!)—he/she would come up with a different, though perfectly valid, set of field equations. In a nutshell, general relativity would look very different in such a universe.
So how can we envision a Level II multiverse? I’ve alluded to some examples above. So prior to fleshing out those examples, it’ll be useful to invoke an image first.
In the image above, if pictured as an axis, space would represent the x-axis whilst time would represent the y-axis. Max Tegmark puts it this way:
Any inflating region keeps expanding rapidly, but inflation eventually ends in various parts of it, forming U-shaped regions that each constitute an infinite Level I multiverse. This tree continues growing forever, creating an infinite number of such U-shaped regions—all of them together form the Level II multiverse. Within each such region, the end of inflation transforms the inflating substance into particles that eventually cluster into atoms, stars, and galaxies. Alan Guth likes to call each Level I multiverse, a “pocket universe,” because it conveniently fits into a small part of the tree.1
Briefly, a Level I multiverse can exist right here in our universe. Put simply, if you were to travel trillions of lightyears in some direction, you’d eventually find a solar system just like ours, with an Earth just like ours, and a copy of yourself, friends, and family members. That universe may be different, but the differences may be so subtle that they’re virtually negligible. Sounds crazy? Well, I’m not talking about the Level I multiverse…yet.
There is another way to imagine a Level II multiverse.
Behold our cosmic Swiss cheese! Admittedly, the holes in this swiss cheese can be a little exaggerated, but the image should be clear. The holes or in this case, the circles, represent one of Alan Guth’s “pocket universes.” The small spaces between the holes are places where inflation has continued. Brian Greene offered the following:
Think of the universe as a gigantic block of Swiss cheese, with the cheesy parts being regions where the inflaton field’s value is high and the holes being regions where it’s low. That is, the holes are regions, like ours, that have transitioned out of the superfast expansion and, in the process, converted the inflaton field’s energy into a bath of particles, which over time may coalesce into galaxies, stars, and planets. In this language, we’ve found that the cosmic cheese requires more and more holes because quantum processes knock the inflaton’s value downward at a random assortment of locations. At the same time, the cheesy parts stretch ever larger because they’re subject to inflationary expansion driven by the high inflaton field value they harbor. Taken together, the two processes yield an ever-expanding block of cosmic cheese riddled with an ever-growing number of holes. In the more standard language of cosmology, each hole is called a bubble universe (or a pocket universe). Each is an opening tucked within the superfast stretching cosmic expanse.
Don’t let the descriptive but diminutive-souding “bubble universe” fool you. Our universe is gigantic. That it may be a single region embedded within an even larger cosmic structure—a single bubble in an enormous block of cosmic cheese—speaks to the fantastic expanse, in the inflationary paradigm, of the cosmos as a whole. This goes for the other bubbles too. Each would be as much a universe—a real, gigantic, dynamic expanse—as ours.2
Briefly, an inflaton field is a field corresponding to a given inflationary model. Like the Higgs field corresponds to the Higgs boson and an electromagnetic field corresponds to electromagnetism, an inflaton field corresponds to inflation.
So, if cosmic Swiss cheese and eternally growing cosmic trees don’t float your boat, you can continue to think of the Level II multiverse as a plethora of floating bubbles. The issue there is that this gives the impression that these universes can somehow interact with one another—perhaps fissioning and fusioning during collisions with one another. This, to my knowledge, is wrong since each pocket universe would be separated by infinite space undergoing continuous inflation. Barring something extreme—perhaps an inflation busting wormhole—these universes wouldn’t be able to share information with one another.
Too hypothetical? Not exactly. Einstein’s general relativity suggested both an expanding universe and black holes long before there was evidence for either. Likewise, quantum mechanics (i.e. Everett’s Many Worlds Interpretation), string theory, and other equations suggest a multiverse. Some consider the multiverse the best explanation for the so called Fine-Tuning problem in physics. When mathematics strongly suggests that something is the case, it’s not long before we find out that that something is the case. It’s only a matter of time!
I strongly recommend Max Tegmark’s Our Mathematical Universe: My Quest for the Ultimate Nature of Reality and Brian Greene’s The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos.
1 Tegmark, Max. Our Mathematical Universe: My Quest For the Ultimate Nature of Reality, p. 133-134. New York: Alfred A. Knopf, 2014. Print.
2 Greene, B.. The Hidden Reality: Parallel Universes and The Deep Laws of the Cosmos, p.56-58. New York: Alfred A. Knopf, 2011. Print.
3 Image Credit: Tegmark, Max. Our Mathematical Universe: My Quest For the Ultimate Nature of Reality, p. 134. New York: Alfred A. Knopf, 2014. Print.
4 Image Credit: http://www.scientificamerican.com/article/multiverse-the-case-for-parallel-universe/