This Christmas season, if you feel you’ve overeaten, drank too much, or simply don’t fancy the TV schedules, here’s a feast for the mind instead. Our observable Universe is thought by some theoretical cosmologists to begin and end in a repeating cycle of “Big Bangs” and “Big Crunches”. Could our “bauble”, or bubble, Universe co-exist with other universes that forever expand like Santa’s waistline?
A pair of scientists in Canada and Germany have brought these two theories together in order to solve some weird problems of theoretical cosmology, one of them being that intelligent observers should have arisen at a time when our Universe was hostile to life.
The concept of cyclic, bubble universes isn’t new. Such ideas were proposed by Neil Turok and Paul Steinhardt as part of the pre-existing Brane Theory. The idea is that our Universe came into being when two three-dimensional branes (short for ‘membrane’ – though they can have many spatial dimensions) floating in a higher-dimensional space were drawn to one another, collided, creating the ‘Big Bang’ and all of the matter and radiation that we see in our Universe today, and then ‘bounced’ away. Sometimes these branes come back together and the process repeats.
In our Universe, the Big Bang theory, which describes space as expanding at a certain rate since its origin, has trouble explaining a few things, such as why the Universe looks the same in all directions even though not enough time has passed for light to travel from one side of it to the other – not even Santa’s reindeer can exceed the speed of light. In 1980 physicist Alan Guth proposed Inflation. That is, the Universe experienced a super-fast period of expansion many times faster than light, which is possible because space itself can expand at any speed it likes, but then slowed to the rate we observe today (measured by redshifts of galactic spectra). Like a turbocharger, this would have given the early Universe enough of a boost to expand to the size we see today, but still be homogeneous.
Except that our Universe appears to show an accelerated expansion – explained by the repulsive effect of ‘Dark Energy’, an as-yet little-understood phenomenon. An eternally inflating parent universe, within which our bubble universe would reside, could explain both these phases of expansion.
The problem is that in an infinitely large, eternally inflating universe of infinite age, all possibilities are not only realised, but realised an infinite number of times. This makes meaningful predictions of models impossible, so what brane cosmologists do is restrict the region of space and time they look at. However, this presents major problems of its own: whatever region you choose may show you wildly different things that may agree or disagree with observations.
In their paper Cycles in the Multiverse (arXiv:1112.3360v1), Matthew Johnson (Perimeter Institute for Theoretical Physics, Canada) and Jean-Luc Lehners (Max-Planck Institute for Gravitational Physics, Germany) propose a model where eternal inflation perpetually seeds bubble universes, like bubbles growing in an expanding soufflé. Some of these universes will collide, expand, slow, contract, and collide again in a repeating cycle. Some will arise, then putter on forever, quite possibly seeding bubbles of their own. With the assumptions that Johnson and Lehners make in their hybrid cyclic-eternal scenario, which they admit is highly dependent on underlying physics, these problems melt away like Christmas snow.
Though the work remains highly theoretical, scientists such as Roger Penrose have proposed looking for ‘echoes’ of the Universe’s previous life (i.e. before the Big Bang) in the Cosmic Microwave Background; the remnant ‘fossil’ radiation of the Big Bang that permeates space. It remains to be seen whether Johnson and Lehners’ work will show the way forward.
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