My first Magazine Sky & Telescope - 02.2019

Cosmic Reveries Inﬂ ation scenario Bounce scenario ? Inflationary expansion Current expansion Slow contraction Bounce Current expansion ingly common outcome of elementary particle theories. In Linde’s theory of chaotic inﬂ ation, patches of inﬂ ating space can emerge from an existing universe — one that’s much less orderly than our universe, a sort of hodgepodge of regions dense and sparse. Bouncing Universes to Baby Universes Despite such radical variations on the theme, inﬂ ation remains the mainstream view in cosmology. It ﬁ ts with new observational evidence, much of it from that same leftover radiation that helped conﬁ rm the original Big Bang theory. Over recent decades, scientists have studied this cosmic microwave background in ever-ﬁ ner detail from ground-based detectors and from the perch of a series of increasingly com- plex satellites. According to the theory, if inﬂ ation happened, then tiny density bumps in the early universe — inevitable thanks to the laws of quantum mechanics — would have grown large during the growth spurt and left an imprint of hot and cold spots in the cosmic microwave background. That imprint would survive as inﬂ ation ended and the hot Big Bang started, with the denser regions seeding the formation of galaxies. So far, observations show a pattern of hot and cold spots that matches those predictions. Still, other theorists found that they could explain these same observations with a very different scenario — a series 20 FE B RUA RY 2 019 • SK Y & TELESCOPE of “bouncing” phases of contraction and expansion. Back in 2001, a collaboration of physicists and cosmologists proposed such a scenario in which the Big Bang was really a collision of two existing universes ﬂ oating in a higher-dimensional space. The idea took the notion of higher dimensions from string theory, which predicts the existence of 11 dimensions — seven spatial ones beyond our familiar three of space and one of time. While string theory posits that the extra dimen- sions are curled up in a way that makes them impossible to observe, some physicists have proposed that one or more of these dimensions stretch out, so that our universe might ﬂ oat within a higher-dimensional space the way a sheet of paper might ﬂ oat through a three-dimensional room. One of the inventors of this scenario, Paul Steinhardt (Princeton University), says that in the past decade he and his collaborators have streamlined the bouncing universe idea so that they no longer need the collision or the extra dimen- sions. All that’s required is an existing universe, which con- tracts slowly until it “bounces” and starts expanding. This, he says, could happen once or in cycles. The contraction of an existing universe can solve all the puzzles that inﬂ ation ﬁ xes, Steinhardt argues. It smooths out variations and gives rise to the same large-scale unifor- mity and cosmic structure, all without requiring any more assumptions than inﬂ ation does. The cause of the contraction isn’t well understood, but neither is the cause of inﬂ ation. And the bouncing scenario has the advantage of not predict- ing an inﬁ nite number of universes, he adds. Caltech’s Sean Carroll favors another possible prequel to the Big Bang. He came to thinking about the origin of the universe while trying to answer a question about everyday life: Why do we have an apparent arrow of time? The laws of physics are symmetrical forwards and backwards, and yet, we can stir cream into coffee and scramble eggs, but can’t un-stir or unscramble them. Time streams along indifferently into the future, toward disorder, death, and decay. As Carroll explains in his 2010 book, From Eternity to Here, scientists of the 1800s ﬁ nally made some serious headway on this ancient problem when they discovered the second law of thermodynamics. A property called entropy, which is something like disorder, increases relentlessly and p INFLATION OR BOUNCE? To explain why the observable universe is geometrically fl at and its contents well mixed, astronomers think it began almost infi nitely compact, then grew in a brief, exponential spurt called infl ation before continuing to expand more slowly. However, it’s unclear what came before infl ation. One alternative idea is that instead of infl ating, the universe existed earlier in a contracting state, then bounced.

My first Magazine Sky & Telescope - 02.2019 | Page 22