It\u2019s easy to envisage other universes, governed by slightly different laws of physics, in which no intelligent life, nor indeed any kind of organised complex systems, could arise. Should we therefore be surprised that a universe exists in which we were able to emerge?<\/p>\n
That\u2019s a question physicists including me\u00a0have tried to answer<\/a>\u00a0for decades. But it is proving difficult. Although we can confidently trace cosmic history back to one second after the Big Bang, what happened before is harder to gauge. Our accelerators simply can\u2019t produce enough energy to replicate the extreme conditions that prevailed in the first nanosecond.<\/p>\n But we expect that it\u2019s in that first tiny fraction of a second that the key features of our universe were imprinted.<\/p>\n The conditions of the universe can be described through its ‘fundamental constants’<\/a>\u00a0\u2013 fixed quantities in nature, such as the gravitational constant (called G) or the speed of light (called C). There are about 30 of these representing the sizes and strengths of parameters such as particle masses, forces or the universe\u2019s expansion. But our theories don\u2019t explain what values these constants should have. Instead, we have to measure them and plug their values into our equations to accurately describe nature.<\/p>\n The values of the constants are in the range that allows complex systems such as stars, planets, carbon and ultimately humans to evolve. Physicists\u00a0have discovered<\/a>\u00a0that if we tweaked some of these parameters by just a few percent, it would render our universe lifeless. The fact that life exists therefore takes some explaining.<\/p>\n Some argue it is just a lucky coincidence. An alternative explanation, however, is that that we live in a\u00a0multiverse<\/a>, containing domains with different physical laws and values of fundamental constants. Most might be wholly unsuitable for life. But a few should, statistically speaking, be life-friendly.<\/p>\n What is the extent of physical reality? We\u2019re confident that it\u2019s more extensive than the domain that astronomers can ever observe, even in principle. That domain is definitely finite. That\u2019s essentially because, like on the ocean,\u00a0there\u2019s a horizon<\/a>\u00a0that we can\u2019t see beyond. And just as we don\u2019t think the ocean stops just beyond our horizon, we expect galaxies beyond the limit of our observable universe. In our accelerating universe, our remote descendants will also never be able to observe them.<\/p>\n What is the extent of physical reality? We\u2019re confident that it\u2019s more extensive than the domain that astronomers can ever observe, even in principle.<\/strong><\/p><\/blockquote>\n Most physicists would agree there are galaxies that we can\u2019t ever see, and that these outnumber the ones we can observe. If they stretched far enough, then everything we could ever imagine happening may be repeated over and over. Far beyond the horizon, we could all have avatars.<\/p>\n This vast (and mainly unobservable) domain would be the aftermath of ‘our’ Big Bang \u2013 and would probably be governed by the same physical laws that prevail in the parts of the universe we can observe. But was our Big Bang the only one?<\/p>\n The\u00a0theory of inflation<\/a>, which suggests that the early universe underwent a period when it doubled in size every trillionth of a trillionth of a trillionth of a second has genuine\u00a0observational support<\/a>. It accounts for why the universe is so large and smooth, except for fluctuations and ripples that are the ‘seeds’ for galaxy formation.<\/p>\n But physicists including\u00a0Andrei Linde<\/a>\u00a0have shown<\/a> that, under some specific but plausible assumptions about the uncertain physics at this ancient era, there would be an ‘eternal’ production of Big Bangs \u2013 each giving rise to a new universe.<\/p>\n String theory<\/a>, which is an attempt to unify gravity with the laws of microphysics, conjectures everything in the universe is made up of tiny, vibrating strings. But it makes the assumption that there are more dimensions than the ones we experience. These extra dimensions, it suggests, are compacted so tightly together that we don\u2019t notice them all. And each type of compactification could create a universe with different microphysics \u2013 so other Big Bangs, when they cool down, could be governed by different laws.<\/p>\n The ‘laws of nature’ may therefore, in this still grander perspective, be local by-laws governing our own cosmic patch.<\/p>\n If physical reality is like this, then there\u2019s a real motivation to explore ‘counterfactual’ universes \u2013 places with different gravity, different physics and so forth \u2013 to explore what range or parameters would allow complexity to emerge, and which would lead to sterile or ‘stillborn’ cosmos. Excitingly, this is ongoing, with recent reseach<\/a> suggesting you could imagine universes that are even more friendly to life than our own. Most ‘tweakings’ of the physical constants, however, would render a universe stillborn.<\/p>\n That said, some\u00a0don\u2019t like the concept of the multiverse<\/a>. They worry it would render the hope for a fundamental theory to explain the constants as vain as\u00a0Kepler\u2019s numerological quest<\/a>\u00a0to relate planetary orbits to nested platonic solids.<\/p>\n But our preferences are irrelevant to the way physical reality actually is \u2013 so we should surely be open minded to the possibility of an imminent grand cosmological revolution.<\/p>\n First we had the Copernican realisation that the Earth wasn\u2019t the centre of the Solar System \u2013 it revolves around the Sun. Then we realised that there are zillions of planetary systems in our galaxy, and that there are zillions of galaxies in our observable universe. So could it be that our observable domain \u2013 indeed our Big Bang \u2013 is a tiny part of a far larger and possibly diverse ensemble?<\/p>\n How do we know just how atypical our universe is? To answer that we need to work out the probabilities of each combination of constants. And that\u2019s a can of worms that we can\u2019t yet open \u2013 it will have to await huge theoretical advances.<\/p>\n So could it be that our observable domain \u2013 indeed our Big Bang \u2013 is a tiny part of a far larger and possibly diverse ensemble?<\/strong><\/p><\/blockquote>\n We don\u2019t ultimately know if there are other Big Bangs. But they\u2019re not just metaphysics. We might one day have reasons to believe that they exist.<\/p>\n Specifically, if we had a theory that described physics under the extreme conditions of the ultra-early Big Bang \u2013 and if that theory had been corroborated in other ways, for instance by deriving some unexplained parameters in the standard model of particle physics \u2013 then if it predicted multiple Big Bangs, we should take it seriously.<\/p>\n Critics sometimes argue that the multiverse is unscientific because we can\u2019t ever observe other universes. But I disagree. We can\u2019t observe the interior of black holes, but we believe what physicist\u00a0Roger Penrose<\/a>\u00a0says about what happens there \u2013 his theory has gained credibility by agreeing with many things we can observe.<\/p>\n About 15 years ago, I was on a panel at Stanford where\u00a0we were asked<\/a>\u00a0how seriously we took the multiverse concept \u2013 on the scale \u201cwould you bet your goldfish, your dog, or your life\u201d on it. I said I was nearly at the dog level. Linde said he\u2019d almost bet his life. Later, on being told this, physicist\u00a0Steven Weinberg<\/a>\u00a0said he\u2019d \u201chappily bet Martin Rees\u2019 dog and Andrei Linde\u2019s life\u201d.<\/p>\n Sadly, I suspect Linde, my dog and I will all be dead before we have an answer.<\/p>\n Indeed, we can\u2019t even be sure we\u2019d understand the answer \u2013 just as quantum theory is too difficult for monkeys. It\u2019s conceivable that machine intelligence could explore the geometrical intricacies of some string theories and spew out, for instance, some generic features of the standard model. We\u2019d then have confidence in the theory and take its other predictions seriously.<\/p>\n But we\u2019d never have the ‘aha’ insight moment that\u2019s the greatest satisfaction for a theorist. Physical reality at its deepest level could be so profound that its elucidation would have to await posthuman species \u2013 depressing or exhilarating as that may be, according to taste. But it\u2019s no reason to dismiss the multiverse as unscientific.<\/p>\n This article was originally published in <\/strong><\/em>The Conversation<\/strong><\/a>.<\/strong><\/em><\/p>\n