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This is the weekly Q & A blog post by our Research Professor in Philosophy, Dr. William Lane Craig.

Question

Hi

In the news I notice that the BICEP2 project has released some data that measures the polarization of the cosmic background radiation due to gravitational waves in the very first instances of the universes existence. Physicists seem to be getting excited as they claim it supports the multiverse theory. I am not familiar with the mathematics that underpins cosmogony so I was wondering if you had any comments on a few of their claims:

Claim 1: "The gravitational waves are evidence of inflation." Could gravitational waves be compatible with some less popular alternative theory?

Claim 2: "Inflation proves the multiverse model." Andrei Linde (one of the original inflationary theorists) goes so far as claiming "if inflation is there, the multiverse is there". Is this claim well supported by mathematics? Do you think this claim is philosophically sound?

I would be really interested to know your views on this recent development.

Thanks

Pete

Dr. William Lane Craig’s Response

The recent news from the BICEP collaboration is reminiscent of the news last year concerning the discovery of the Higgs boson: the evidence confirmed what almost everyone already believed. The story is once again a wonderful illustration of the experimentalists’ discovering what the theorists had hypothesized. So there’s nothing revolutionary about this discovery (which is not to diminish in any way its significance!).

But once again, that hasn’t stopped some people from making irresponsible assertions. For example, I saw Lawrence Krauss respond to the discovery by repeating his tired, oft-refuted assertion that it shows how the universe could have come naturally into existence out of “nothing.” Never mind that the universe already existed prior to inflation! As we all know, Krauss is using the word “nothing” to refer to a physical state of the early universe out of which the universe evolved.

As for the claims you mention:

1. The cosmic microwave background is rather like a screen behind which we cannot look to see what happened in the earlier stages of the universe’s development. Nevertheless, what happened on the other side of that screen can leave imprints on the screen itself. By studying the patterns of those imprints astrophysicists can make inferences about what happened on the other side that left such patterns on the screen. The BICEP collaboration discovered that the cosmic microwave background bears the imprint not only of density fluctuations, which eventually evolved to become galaxies and stars, but also of something else, which imprinted on the background radiation a pattern of swirls in its polarization. The best explanation of that pattern is that it is the imprint of so-called gravitational waves, a phenomenon hitherto undetected by experimentalists, though predicted by the General Theory of Relativity. Gravitational waves are ripples in spacetime thought to arise normally by the mutual acceleration of masses, as in a system where two stars are revolving asymmetrically about a common center of mass. What caused the gravitational waves whose imprint has been left on the background radiation? The hypothesis that the universe underwent a brief moment of super-rapid or inflationary expansion predicts just such a pattern.

The team went to great lengths to ensure that the polarization pattern detected was not due to error in instrumentation or the influence of cosmic dust or galactic factors. They seem to have ruled out alternatives to gravitational waves as the source of the observed pattern. They did not discuss whether there might not be some other non-inflationary sort of model which could plausibly account for the primordial gravitational waves. I see that Paul Steinhardt, an inflation sceptic, has already admitted that if the results hold up, his model is untenable. Perhaps there are other models that could be devised, but the confirmation by experiment of a prediction made on theoretical grounds is powerful evidence for the truth of a theory.

2. As for Linde’s multiverse comment, inflation alone does not guarantee a multiverse. It all depends on the properties of the initial field responsible for inflation, about which we can only speculate. There may be other inflating universes, or there may not. What’s important to keep in mind is:

(i) Theology has no reason to deny that God may have created a wider reality than just our universe.

(ii) Inflationary models may be future-eternal (they will go on forever), but they cannot be past eternal (the multiverse itself had a beginning). Attempts to make the multiverse past-eternal (like Sean Carroll’s model) fail for a variety of reasons.

(iii) Multiverse scenarios face the troublesome Boltzmann brain problem. A finely-tuned universe like ours is incomprehensibly improbable on naturalism. The more you multiply worlds within the multiverse in order to make it likely that observers will appear somewhere in the multiverse of worlds, the more probable it becomes that we should be Boltzmann brains, isolated brains which have fluctuated into existence out of the quantum vacuum. For observable worlds like that are vastly more plenteous than worlds which are fine-tuned for embodied conscious agents. So if we were just random members of a multiverse of worlds, we ought to have observations like that. But we don’t; which disconfirms the multiverse hypothesis.

These points were all discussed in the recent Greer-Heard Forum on “.”

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