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Randomness

Bubble universes give new perspective to time's origin and its arrow

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Fans of T.S. Eliot are all aware that the world will end with a whimper. Fans of modern cosmology know that the world began with a bang, a big one. But the flow of time that transports the world from bang to whimper, from past to future, may itself neither begin nor end.

In any event, time’s possible birth and death remain open questions, debated among physicists groping to understand the cosmos. It seems that the spacetime bubble that humankind has grandly designated “the universe” did have a violent beginning 13.7 billion years ago, but that doesn’t mean the clock of ages began ticking only then. Many experts believe today’s universe is the offspring of a preexisting space. Like one of many baby soap bubbles sprouting from a bigger parent bubble, the “universe” is perhaps just one of countless siblings in a gigantic cosmic family. As each baby grows (universes expand, you know), it might become a parent as well, spawning new bubble universes that then spawn others, ad infinitum. Really, infinitely into the future. Forever.

If such an eternal multiverse of spacetime bubbles actually exists, a question arises about time’s origin: If time can go forward forever, how far into the past does it go? That is, does existence extend to the infinite past as well as to the infinite future? On this point, the experts do not agree. But Stanford physicist Leonard Susskind makes a strong case that an infinite past is the best way to bet.

In a reality with only one universe (similar to the one we live in), he notes, it seems that a beginning of time can be identified — the previously alluded to Big Bang. But with countless universes, all bets are off. Almost every universe, no matter how old, will have an even older parent. So suppose, just for fun, that there was an original parent, a mother of all universes. If you try to figure out how far back that original parent originated, Susskind argues, the logical answer is an infinitely long time ago.

Details of his argument engage some intense mathematics. But the basic idea is statistically straightforward. Among the multiple universes branching from the ancestral bubble, some go on to produce offspring of their own. Others have no heirs. Residents of the youngest bubbles might be able to trace their lineage back to the prime parent and determine that time did indeed have a beginning.

But the odds of that are slim. Among the baby bubbles will be some that also have babies that will in turn have babies of their own. So the vast majority of universes that will ever exist are many, many generations removed from the one original mother. If the process of bubbles inflating from other bubbles continues on and on eternally — as the current theory known as “eternal inflation” indicates — then statistically, universes close to the mother constitute a vanishingly small proportion of all possible places for people (or whatever) to live.

“In any kind of inflating cosmology,” Susskind writes, “the odds strongly favor the beginning to be so far in the past that it is effectively at minus infinity.”

So unless humans have a special status in the cosmos, all the physicists pondering these questions most likely occupy a bubble with a cosmic lineage much longer even than people (like me) who can trace their ancestry back to Charlemagne.

“If they think they are typical, observers should bet, with overwhelming confidence, that they cannot detect a beginning,” Susskind declares in a paper online.

Establishing the likelihood of an infinite past is not Susskind’s whole story. In related work, he and Stanford colleagues Daniel Harlow, Stephen Shenker and Douglas Stanford have explored the implications of the multiverse for another deep mystery about time: why it flows one way.

Equations describing the laws of physics work equally well no matter which direction time flows (the equations are “time-symmetric”), yet in real life eggs never de-scramble and popcorn never unpops. A common explanation for time’s direction relies on the concept of entropy, which says order tends to become disorder, and so all you need is a very low-entropy (highly ordered) condition in the distant past for time to proceed in its ordinary forward direction. Susskind doesn’t buy that. Instead, he and his collaborators contend that the key to time’s arrow is found in the statistics describing the multiverse. Their calculations suggest that if all bubbles have babies, time retains its past-future symmetry. But if some bubbles fail to reproduce, the math changes, introducing a distinction between past and future. For technical reasons, Susskind calls this new impetus behind time’s arrow a “fractal flow,” and he believes related ideas can resolve other cosmological conundrums, discussed in another recent paper online.

All this work involves very solid math, but it sits on a foundation infused with a fair amount of speculation. In the end, statistical inference based on eternal inflation may not turn out to be the best way to solve the deepest mysteries about eternity. But it beats whimpering about them.

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