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Monday, March 17, 2014

Gravitational wave implications [updated]

A major physics discovery was announced this morning: a distinctive pattern in the cosmic microwave background suggesting gravitational waves that emerged from cosmic inflation--an extremely fast expansion of space in the very early universe. Here's the lead from Sciam's piece "Gravity Waves from Big Bang Detected":
Physicists have found a long-predicted twist in light from the Big Bang that represents the first image of ripples in the universe called gravitational waves, researchers announced today. The finding is direct proof of the theory of inflation, the idea that the universe expanded extremely quickly in the first fraction of a second after it was born. What’s more, the signal is coming through much more strongly than expected, ruling out a large class of inflation models and potentially pointing the way toward new theories of physics, experts say.
And here's from physicist Max Tegmark's gloss on it in the Huffington Post, "Good Morning, Inflation! Hello, Multiverse!":
Today is a great day for most scientists except multiverse skeptics -- at least in this particular universe. Alex Vilenkin, Andrei Linde, Alan Guth and others have shown that inflation generically predicts a space that is not merely large but infinite, teeming with duplicate copies of our civilization living out countless variations of our lives far far away. Now it's harder for skeptics to dismiss this by saying "inflation is just a theory": first they need to come up with another compelling explanation for BICEP2's gravitational waves. Today is also disappointing for the ekpyrotic/cyclic models that had emerged as the most popular alternative to inflation: they are ruled out by BICEP2's gravitational wave detection.
I hope subsequent coverage will provide more clarity as to how we get from A to B: what is the precise chain of logic--and assumptions--that leads from these gravity waves to "duplicate copies of our civilization living out countless variations of our lives far far away"? A trouble with much physics popularization, often by physicists, is the willingness to make such leaps with little reference to any details that might introduce some (boring?) uncertainty.

UPDATE 3/18: Recommended reading (from before the BICEP2 announcement but relevant to this post): "When does multiverse speculation cross into fantasy?" Also: Peter Woit in the Wall Street Journal, with bonus mention of Giordano Bruno; and at his own blog. One thing I'm not sure about is whether Tegmark's confident post of yesterday refers to a Level I or Level II multiverse, or both. Also recommended, while I'm on this subject: John Horgan's "Why I Still Doubt Inflation, in Spite of Gravitational Wave Findings" and his older "Is speculation in multiverses as immoral as speculation in subprime mortgages?"

One more UPDATE 3/18: Rod Dreher, proving that you can look at the cosmological data and see whatever you want to see. He does at least put a light touch on it in his update. For the record, I don't see how evidence for inflation provides any support for theism (if one already thinks the Big Bang supports theism, then inflation adds nothing to that). I do agree with Dreher that Paul Davies has written a lot of interesting and worthwhile material on cosmology and related topics.

Final UPDATE 3/18: Something for those who really want to delve into Tegmark duplicates.

OK, really final UPDATE 3/19: I've discovered to my surprise and his credit that Tegmark posted a Sciam blog post some weeks ago sketching out his assumptions and where they could be wrong. He sticks closely to physics/cosmology, which is understandable, but I'd add that further assumptions that he doesn't address could also upset his duplicate civilizations applecart. Those assumptions include: the origin of life as an event with a finite, if small, probability of occurring under the laws of physics (as I understand it, it could have zero probability and still occur given infinite opportunities to do so, but would occur a finite number of times); human consciousness being something with a finite, if huge, number of possible states, contrary to paper linked in update just above; universes not ever causally interacting in any way that throws off the endless replication; and super-intelligence not ever arising anywhere in the infinite multiverse in such a way as to substantially change the super-big picture.

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