Why we think there's a Multiverse, not just our Universe

[n2doc]

Why we think there's a Multiverse, not just our Universe
Category: Gravity • Physics

Posted on: October 28, 2011 11:35 PM, by Ethan Siegel

"Every true, eternal problem is an equally true, eternal fault; every answer an atonement, every realisation an improvement." -Otto Weininger

The best measurements of the distant Universe -- out beyond our galaxy -- have led us to the current picture of exactly what our Universe is doing: expanding and cooling, with its galaxies progressively getting farther and farther apart.


(Image credit: Molly Read for the University of Wisconsin-Madison.)

But what does that mean for our past?

If we're expanding and cooling, that means our past was less expanded and less cooled, or as we like to think of it, denser and hotter.

Now, if you're thinking like a scientist, you don't just want to know what it's doing. You also want to know -- if it's expanding -- both what's causing the expansion, and by how much it's expanding. In other words, we'd like to determine the rate of expansion.

And the answer is actually straightforward: if general relativity is your theory of gravity, the Universe's expansion rate is determined by what type of energy dominates your Universe.

more
http://scienceblogs.com/startswithabang/2011/10/why_we_think_theres_a_multiver.php

[Jim__]

1. What's going on when only some regions of space are inflating?

In the diagram, the inflating (blue regions) and non-inflating regions (red Xs) are always the same size:



But, it seems like the non-inflating regions should be smaller than the inflating regions. If that's true, are there various neighboring-regions of space that are non-contiguous? Should the different regions be spherical rather than cubic?

[caraher]

2. You shouldn't intepret this literally

The shapes mean nothing - cubes are just easy to represent. I think this diagram is representing inflation by taking any region that is *still* expanding and showing it "splitting" into more cubes, some parts of which stop expanding and other which continue to expand. I don't think the red X regions are depicted any longer after their first appearances.

[Jim__]

3. Thanks. I'll have to go back and read the various links to get a better understanding..

[Ratty]

4. It certainly solves a lot of problems

With a multiverse you don't have to rely on weird quantum "observers" to create reality. It also sidesteps other strange phenomena that always seemed "tacked on" to make QM work, like entanglement. I really like the multiverse theory.

[caraher]

5. Maybe

Depends on what kind of multiverse theory you espouse - the parallel universes created this way aren't necessarily the Everett-style parallel universes.

Frankly, I don't understand why so many people think the parallel worlds interpretation solves anything. In particular, how do you cope with quantum measurements that have a continuous set of possible measurement outcomes? Or for that matter, even just two discrete outcomes whose probabilities have a ratio that is an irrational number? I can accept the consistency of a situation where, if outcome A has a 60% chance and outcome B a 40% chance, one says that upon measurement we create 3 universes in which the outcome is A and 2 in which the outcome is B. But this can never work if P(A)=sqrt(2) and P(B)=1-sqrt(2) without doing considerable violence to the notion that universes either exist or they don't. And you can't appeal to some notion of superposition because that's exactly what you were trying to explain away with parallel worlds in the first place!

Am I just missing something obvious here?