The first session on Saturday was given over to MIT’s Alan Guth and Stanford’s Andrei Linde, two inflationary behemoths. They both spoke about the development of inflation--the notion that the early universe went through a spurt of exponential expansion--and how that notion later morphed into “eternal inflation,” bringing with it the idea that our universe may be only one of many "pocket" or...

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The first session on Saturday was given over to MIT’s Alan Guth and Stanford’s Andrei Linde, two inflationary behemoths. They both spoke about the development of inflation--the notion that the early universe went through a spurt of exponential expansion--and how that notion later morphed into “eternal inflation,” bringing with it the idea that our universe may be only one of many "pocket" or "bubble" universes, which are continually popping into existence in a vast multiverse.

Alan Guth with Universe in Background, by Betsy Devine, Grand Cayman (http://betsydevine.com/blog)

The idea of a multiverse of universes has been seized upon to explain why our universe contains so many weirdly finely-tuned properties that are perfectly suited for life to have arisen. The argument goes that we shouldn’t be surprised to find ourselves in the one universe (out of many possible varieties of universe in a multiverse) with laws and physical constants that are well-suited for life to exist, because that’s the only place that life could have evolved and hence where we could find ourselves.

The only trouble is that when you try to ask how just how (un)likely any of these particularly fine-tuned conditions actually are, things can get sticky. As Guth put it, in a multiverse with an infinite number of bubble universes, “anything that can happen, will happen--an infinite number of times. So how do you know if some are more common than others?”

To put it another way, we instinctively feel that if we flip a coin, we’re unlikely to get 10 heads in a row. “But in the multiverse picture there are universes where you get 10 in a row--an infinite number of such universes,” explained Guth. So how do you count up the fraction of universes in which you get all heads (an infinite number of universes) out of all the universes in the multiverse (another infinite number of universes)?

The key is to find a recipe (known as the “measure”) for counting the relative number of events of different types in the multiverse. When cosmologists have it, they will be able to talk with more confidence about just how unlikely it really is for us to find ourselves in a universe with, say, the amount of dark energy that we have today.

Cosmologists aren’t there yet, largely because there’s more than one way to slice up the multiverse before taking a census. Many measures make somewhat bizarre predictions, for instance, suggesting that it’s far more likely that intelligent life should suddenly pop into the universe out of a quantum fluctuation, as a hallucinating disembodied brain (known as a Boltzmann Brain), than evolve slowly into human life. Since we aren’t Boltzmann Brains (as far as we know), measures that make those predictions can be ruled out. (There are more details in the pdf of Guth's talk, here.)

Different cosmologists favor different measures--Guth likes one called the “scale factor cutoff”, while his colleague Alex Vilenkin, of Tufts University, recently won an FQXi grant to investigate the measure problem further.

One of the reasons that inflationary cosmologists are so keen about chasing this measure is that it will help relate inflation to experimental measurements. (For instance, in the future, Guth hopes to be able to answer questions like: “Of all accelerators in the multiverse that resemble the LHC in some specified way, what is the fraction that (a) see the standard Higgs, (b) see no Higgs, or ( c) see a Higgs system of some other description?”)

As cosmologist Larry Krauss brought up at the AAAS conference a few weeks ago, the problem at the moment is that almost any observation can be used to support inflation and almost no observation can be used to disprove inflation. At this session, FQXi director Max Tegmark went a step further and said that until cosmologists have a measure that can predict the outcome of an experiment, then inflation can’t be called a theory.

String theorist Brian Greene asked if any observation would make Guth give up the idea of inflation. (Later in the day, the question would be turned back to him, regarding his own views on string theory.) Guth answered that no single observation would do that, but a better _theory_ might. He added that looking back at history, it’s naïve to say that one observation ever overthrows a theory. (For instance, “Newtonian physics didn’t yield to Einstein’s special relativity just because of the Michelson Morley experiment,” he said.)

At that point it was time to hand over to cosmologist (and also, as we discovered at dinner that evening, amateur magician and stage hypnotist) Andrei Linde, who picked up on the nature of different universes in the multiverse. Each universe could have different physical laws, or it could be the case that there is one yet-more-fundamental law of physics underlying the whole multiverse, “like a single genetic code,” but which could take a different form in various universes, “like H2O can be liquid, solid, or gas, which is still the same thing, but very different from the point of view of a fish!”

What's so special about our universe? (From Andrei Linde's talk)

For Linde, this genetic code of the multiverse is written by string theory, in particular, it’s encoded into the ways of wrapping up the extra dimensions of string theory. There are around 10^500 different ways of doing this, giving a corresponding number of possible variations of the universe. This huge number first appeared in calculations in the mid-1980s, but Linde lamented that for the best part of two decades he was one of only a few people that saw the possibility of a multiverse of universes as potentially a good thing. It wasn’t until much later that the multiverse began to transition from science fiction into science proper, following the discovery of the otherwise inexplicable acceleration of the universe and the mystery of dark energy. (There are more details on Linde's presentation slides.)

(He also added ruefully that “string theorists determine what is good and what is bad,” and so the multiverse idea had to wait for the seal of approval from that community.)

Interestingly, Linde also noted that when inflation was first set out, it seemed to be addressing questions that belonged in the realm of metaphysics (why is the universe so big?…) but which have now passed into the domain of physics. It’s this borderline area between metaphysics and physics that FQXi is interested in and Linde outlined future foundational questions that need to be addressed:

1) How can we compare infinites? (The measure problem)

2) How far away from us are our EXACT copies?

3) Can consciousness come out of nothing? (The Boltzmann Brain problem)

4) Can an unobserved universe exist?

5) Is physics a closed science that can describe everything, or does it need to be extended? (An answer from physicists in the audience: “Whatever it has to be extended into, we’ll just name “physics”?)

6) Can we test the multiverse theory?

In a partial answer to this last question, Linde added that the multiverse picture _does_ make some predictions, for instance, it predicts the probable nature of dark energy (that is, it predicts that dark energy is most likely vacuum energy rather than “quintessence”), and it increases the probability that dark matter is made up of particles known as axions, not supersymmetric WIMPs. The LHC and other experiments will be hunting dark matter. “If they find WIMPS, then this part will be over. If they find light axions, that’s a good vindication,” said Linde.

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