Inflation is dead.  Long live inflation.

The cosmological inflation theory made Alan Guth famous back in 1981.  In case the enthralled didn’t get the message, it was a colossal failure, Amanda Gefter broke the news on New Scientist.  To set up Humpty Dumpty’s fall, she began with its seeming successes: “in one fell swoop,” it rescued big bang theory from the flatness problem and horizon problem  That was before cosmologists stopped admiring the “munificence” of inflation and starting thinking about its implications: it leads to nonsense:

The problem is that once inflation starts, it is nearly impossible to stop. Even in the tiny pre-inflation cosmos, quantum fluctuations ensured that the inflaton field had different energies in different places — a bit like a mountain having many balls balanced precariously at different heights. As each one starts rolling, it kicks off the inflation of a different region of space, which races away from the others at speeds above that of light. Because no influence may travel faster than light, these mini-universes become completely detached from one another. As the inflation continues its headlong descent in each one, more and more bits of space begin to bud off to independent existences: an infinite “multiverse” of universes is formed…

This is not good news for our hopes for cosmic enlightenment. In a single universe, an underlying theory of physics might offer a prediction for how flat the universe should be, say, or for the value of dark energy, the mysterious entity that seems to be driving an accelerated expansion of the universe.  Astronomers could then go out and test that prediction against observations.

That’s not possible in an infinite multiverse: there are no definite predictions, only probabilities. Every conceivable value of dark energy or anything else will exist an infinite number of times among the infinite number of universes, and any universal theory of physics valid throughout the multiverse must reproduce all those values. That makes the odds of observing any particular value infinity divided by infinity: a nonsense that mathematicians call “undefined”….

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