c o s m o l o g y

Looking for Life in the Universes with different physical laws might still be habitable By Alejandro Jenkins and Gilad Perez

he typical Hollywood action hero skirts very little of the carbon and other elements that Key Concepts death for a living. Time and again, seem necessary to form planets, let alone life. If scores of bad guys shoot at him from the proton were just 0.2 percent heavier than it ■ ■ Multiple other univers- T multiple directions but miss by a hair. Cars ex- is, all primordial would have decayed es—each with its own plode just a fraction of a second too late for the almost immediately into neutrons, and no atoms laws of physics—may fireball to catch him before he finds cover. And would have formed. The list goes on. have emerged from the same primordial vacuum friends come to the rescue just before a villain’s The laws of physics—and in particular the that gave rise to ours. knife slits his throat. If any one of those things constants of nature that enter into those laws, happened just a little differently, the hero would such as the strengths of the fundamental forc- ■ ■ Assuming they exist, many be hasta la vista, baby. Yet even if we have not es—might therefore seem finely tuned to make of those universes may seen the movie before, something tells us that he our existence possible. Short of invoking a su- contain intricate struc- will make it to the end in one piece. pernatural explanation, which would be by tures and perhaps even In some respects, the story of our universe re- definition outside the scope of science, a num- some forms of life. sembles a Hollywood action movie. Several ber of physicists and cosmologists began in the ■ ■ These findings suggest physicists have argued that a slight change to 1970s to try solving the puzzle by hypothesiz- that our universe may not one of the laws of physics would cause some di- ing that our universe is just one of many exist- be as “finely tuned” for saster that would disrupt the normal evolution ing universes, each with its own laws. Accord- the emergence of life as of the universe and make our existence impos- ing to this “anthropic” reasoning, we might previously thought. sible. For example, if the strong nuclear force just occupy the rare universe where the right —The Editors that binds together atomic nuclei had been slight- conditions happen to have come together to

ly stronger or weaker, stars would have forged make life possible. FILMS SLIM

42 Scientific American © 2009 SCIENTIFIC AMERICAN, INC. January 2010 © 2009 SCIENTIFIC AMERICAN, INC. © 2009 SCIENTIFIC AMERICAN, INC. Amazingly, the prevailing theory in modern which our universe neither recollapsed into noth- What is the , which emerged in the 1980s, sug- ingness a fraction of a second after the big bang, Multiverse? gests that such “parallel universes” may really nor was ripped part by an exponentially acceler- Alternative universes have exist—in fact, that a multitude of universes would ating expansion. Nevertheless, the examples of al- now become a legitimate field incessantly pop out of a primordial vacuum the ternative, potentially habitable universes raise in- of study, in part because they way ours did in the big bang. Our universe would teresting questions and motivate further research may actually exist. According be but one of many pocket universes within a into how unique our own universe might be. to the prevailing cosmological theory, our universe was wider expanse called the multiverse. In the over- spawned from a microscopic whelming majority of those universes, the laws of The Weakless Way of Life region of a primordial vacuum physics might not allow the formation of matter The conventional way scientists find out if one by a burst of exponential as we know it or of galaxies, stars, planets and particular constant of nature is finely tuned