My first Magazine Sky & Telescope - 02.2019

irreversibly, even if temporary pockets of order can crop up here and there. If entropy has been increasing for all time, then it must have been very low at the origin of the universe. Things don’t naturally acquire low entropy, Carroll says. He realized that neither the Big Bang nor inﬂation offered an explanation. So he had to consider what came even earlier. Since the natural state of things is high entropy, he won- dered if a universe could be born from the highest-entropy- possible state of things. While it might seem counterintuitive, theories of quantum gravity point to empty space (with its formless vacuum energy) as the ultimate and ﬁnal equilibrium state of a once lively universe. That’s because empty space has more ways to be disorderly than a universe with structure, or even one ﬁlled with a dense, soup-like matter (see “Entropy,” facing page). Such a place would be very boring, most of the time. But the uncertainty principle of quantum mechanics allows interesting things to happen. Just as radioactive elements emit particles in a random way, Carroll says, there’s a probability that this empty space can give birth to the occasional baby universe. Such a newborn universe can pinch off to form a new and separate region of space and time. This doesn’t violate the second law, that they didn’t set out to transgress the bounds of experi- ment — the paradoxes of the Big Bang as originally formu- lated forced them to enter seemingly unobservable realms. New hope for empiricism came in 2014, when it appeared that a telescope called BICEP2, located at the South Pole, had picked up a polarization pattern in the cosmic microwaves. This pattern would have ruled out the collision theory and favored some versions of inﬂ ation over others. But it turned out the observers had misinterpreted their signal — what they saw had originated from mundane dust in our own galaxy (S&T: May 2015, p. 12). Then, in early September 2018, Xingang Chen, Avi Loeb, and Zhong-Zhi Xianyu (Harvard), announced another pos- sible test: a faint signal that could rule out either all the inﬂ a- tion-based scenarios or the contraction-then-bounce one. The test rests on the premise that there was a fraction of a second during which the space that would become our uni- verse existed before it became a hot, dense soup of particles — the so-called hot Big Bang phase. At this stage, space was permeated by high-energy ﬁ elds that oscillated at regular time intervals. The oscillations should have left different patterns in the density variations that gave rise to cosmic structure, depending on whether space was expanding exponentially, u B MODES If infl ation happened, then the resulting ripples in space- time should have left polarization patterns called B modes (three examples shown) in the cosmic microwave background. Conversely, a contraction and bounce would not leave such patterns. he says. “The entropy of the baby universe is small, but it’s a new addition to the entropy of the universe as a whole, which is therefore still increasing.” Others conjecture different mechanisms for generating an arrow of time — increasing complexity, for example, or grav- ity. These ideas assume that time is what’s called an emergent property — existing only under certain conditions, just as temperature and pressure as we experience them don’t exist independent of matter. But some, like physicist Lee Smolin (Perimeter Institute for Theoretical Physics, Canada), argue that time is a fundamental facet of reality that transcends the laws of physics. Looking for Fingerprints Inventing interesting, plausible theories, however, has become a lot easier than testing them with observations. The abun- dance of untested ideas has led to accusations that cosmology has left the path of empirical science. But cosmologists aver as in inﬂ ation, or contracting. And this, Chen says, might be observed in the hot and cold regions of the microwave back- ground, or in the distribution of galaxies across the sky. He and his colleagues posted the idea on arXiv.org, a site where not-yet-published scientiﬁ c papers go up for public review. If cosmologists can determine whether in this instant space was contracting or expanding, the result will point to vastly different pictures of a much larger, or even inﬁ nite, pre-existing physical reality. Thus, although these cosmo- logical theories might seem fanciful and far removed from observations, many scientists are not content to let them stay that way. As long as people keep thinking of tests that might work, these scenarios remain tethered to the great expansion of scientiﬁ c knowledge. ¢ FAYE FLAM is a columnist for Bloomberg Opinion. She writes on science, medicine, and the environment. Find more about her work at fayeﬂ amwriter.com. sk yandtele scope.com • FE B RUA RY 2 019 21

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