Maybe we are in a "supersymmetric multiverse with vanishing cosmological constant" where the multiverse is the same as quantum mechanics idea of the universe being a range of probabilities.
Today, Leonard Susskind at Stanford University in Palo Alto and Raphael Bousso at the University of California, Berkeley, put forward the idea that the multiverse and the many worlds interpretation of quantum mechanics are formally equivalent.
But there is a caveat. The equivalence only holds if both quantum mechanics and the multiverse take special forms.
Could this theory be true? I have no idea. Neither do these guys at Berkeley and Stanford. But that isn't the point, is it? The point is that a multiverse would be really fun (though risky) if you could imagine to cruise around between universes.
One of my favorite fantasies: traveling between parallel universes in the multiverse. Imagine worlds that were causally identical to current Earth until 500 or 1000 or 2000 years ago. How might they differ starting with the smallest quantum fluctuation? Go back further. Imagine an Earth that split off from our Earth some millions of years ago that does not have humans on it. Would it be in an ice age now due to lower atmospheric CO2? The real estate would be very cheap.
If one could find worlds more technologically advanced one could use that tech (e.g. robots, AI, nanotech factories, rejuvenation tech using microfluidic devices) to an unpopulated world and create a luxurious community in a clean environment with no terrorism or pollution or resource depletion. On the other hand, if multiverse travel is possible how soon till multiverse invasions by hostile species?
Travel between stars looks so hard that theories about the multiverse become much more appealing. If we can' travel around at warp speed then we need a way to slide between universes.
The multiverse is so crowded that universes bump into each other. We need to move our universe out into a rural part of the multiverse so we can get some peace. Background cosmic microwave radiation provides evidence of whole universe collisions?
Now Stephen Feeney at University College London and a few pals say they've found tentative evidence of this bruising in the form of circular patterns in cosmic microwave background. In fact, they've found four bruises, implying that our universe must have smashed into other bubbles at least four times in the past.
We are just a bubble floating around in the multiverse?
Are there any causally identical universes which are ahead or behind us in time?
Enter loop quantum gravity, devised by Abhay Ashtekar of Pennsylvannia State University (PSU) in University Park and colleagues to reconcile general relativity with quantum mechanics. When Ashtekar's team created cosmological models inspired by LQG in 2006, these suggested the universe emerged from the remnants of an earlier universe that was crunched down to a tiny volume by gravity, not from the big bang
If that's the eventual fate of our own universe then what of all the events happening to us? "All those moments will be lost in time, like tears in rain."
The clock is ticking. We've got just 5 billion years to figure out out to break out of this pocket in the multiverse into a universe with more time left to run. The last restaurant at the end of the universe will be sitting its final guests just billion years hence. That restaurant should be located at the multiverse space port for a last meal before we escape.
"We could run into the end of time," Ben Freivogel tells a seminar at the Massachusetts Institute of Technology in Cambridge. Several colleagues seem nonplussed, and one Nobel laureate looks downright exasperated. "I'm aware that this sounds like a crazy conclusion," Freivogel admits, generating a round of what sounds like relieved laughter. But perhaps their relief is short-lived.
If there is an infinite number of universes in the multiverse then doesn't this mean that a universe identical to this one has already come into existence and winked out again? Didn't Kara Thrace say something to that affect too? In how many universes did she say it? Maybe there's a universe where she said it for real too.
Of course, if identical universe recur then similar universes probably recur as well. In a trip to some similar universe what do you most want to find is different?