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FQXi BLOGS
April 24, 2014

CATEGORY: Blog [back]
TOPIC: Sean Carroll Fluctuates on Boltzmann Brains and Eternal Inflation [refresh]
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FQXi Administrator Zeeya Merali wrote on Jan. 9, 2014 @ 18:11 GMT
Sean Carroll
More audio from the FQXi meeting is now in. Those of you that read Ian Durham's summary of Day 2 will already know that cosmologist Sean Carroll, at Caltech, gave (in his own words) an "inflammatory" talk (with the deliberately boring title "Quantum Fluctuations in de Sitter Space'). With an eye to stirring up controversy, we present the talk for you to listen to, in its entirety.

Carroll has been thinking deeply about the what quantum fluctuations really are, and has come to the somewhat surprising (and contentious) conclusion that they are less fluctuation-y (that they fluctuate less) than we might have assumed. His argument (I think) roughly boils down to saying that even though we often talk about "quantum fluctuations" in a vacuum, those fluctuations happen because there's something out of equilibrium to generate them--essentially an observation.

This has implications for the Boltzmann brain problem: the idea that given long enough, fully formed conscious entities could pop out of the vacuum. This is an unsettling thought because, if Boltzmann brains become increasingly likely, it could be argued that we ourselves are more likely to be disembodied consciousnesses that have just been created, and--as Carroll says--we wouldn't then be able to trust our own thoughts about our history, or our conclusions about the workings of the universe. (I never trust my own thoughts, so I'm not too alarmed by this prospect.) But, at any rate, BBs would be more typical observers than normal humans, which calls into question a lot of our assumptions about cosmology.

But Carroll is now questioning whether this really is such a problem. Thinking specifically about de Sitter space, he argues that there won't be any quantum fluctuations in the vacuum to generate BBs: "If there are no Boltzmann brains now, you won't see Boltzmann brains tomorrow." He does clarify that you could see BBs pop into existence if you were there to look--your experimental BB-detecting-apparatus would be out of equilibrium with the vacuum and would interact with it, producing the fluctuations needed to generate BBs. But since you aren't out there observing, BBs shouldn't arise. Phew!

Of course, quantum fluctuations aren't always thought of as an enemy to cosmologists. Without them in the early universe, we wouldn't have the density perturbations needed to give rise eventually to structure in the cosmos. If Carroll apparently eradicates quantum fluctuations from the vacuum, then the obvious question (that Carroll asks and answers) is do we have to rethink the early history of the universe? No, and yes, is what he says. He claims that we *will* still get the required quantum fluctuations in the inflationary scenario. In standard cosmology, after this early burst of inflation--a rapid cooling expansion in the infant cosmos--inflation ends with a period of "reheating" (which does what it says on the tin, and makes the universe hotter). In Carroll's words this produces a "butt-load of entropy" (which I can't help thinking sounds like a very unpleasant affliction). This dramatic change acts like an observation of the universe, collapsing wavefunctions left, right and center, and generating the necessary perturbations in the cosmic microwave background.

However, it's not clear that, if Carroll is right, you would get the required quantum fluctuations needed to set up eternal inflation--the process that would create multiple big bangs, and give rise to a multiverse of neighbouring universes. More work needs to be done, etc. etc. to see what the implications are for eternal inflation.

Anyway, as Ian Durham blogged yesterday, it's by no means agreed that Carroll's correct to say that these quantum fluctuations are not happily fluctuating away as physicists have long assumed. But listen to Carroll in his own words on the podcast, and see what you think.

Suddenly popping into existence is not fun...


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Wilhelmus de Wilde wrote on Feb. 6, 2014 @ 15:35 GMT
Sean Carroll's idea is based on our causal way of thinking that in OUR universe everything must have a cause (reason). Maybe our universe is in "touch" with another "dimension" or is emerging from another source (field, maybe not measurable by us), and from this "source" emerge also quantum fluctuations.

It was also Sean Carroll who said : "Everything is Field".

Wilhelmus

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